JP7026758B2 - Magnetic powder, method for manufacturing magnetic powder, and magnetic recording medium - Google Patents

Description

The present disclosure relates to magnetic powder, a method for producing magnetic powder, and a magnetic recording medium.

As the magnetic powder used for the magnetic recording medium, hexagonal barium ferrite powder and the like are widely used. Hexagonal barium ferrite powder is plate-shaped and can achieve a high filling factor. However, in recent years, with the increase in performance of magnetic recording media, there is a demand for higher performance magnetic powder.
For example, ε-Fe ₂ O ₃ has a large anisotropic magnetic field (hereinafter, may be referred to as Hk) and can maintain magnetization even when smaller particles are used, so that it can be used as a next-generation magnetic material. Attention has been paid to a magnetic powder containing ε-Fe ₂ O ₃ and having an improved particle size distribution (see Patent Document 1).

As the magnetic powder containing ε-Fe ₂ O ₃ , the content of fine particles that do not contribute to the magnetic recording characteristics is small, the coercive force distribution is narrow, and the magnetic powder is specific as a magnetic powder suitable for increasing the recording density of the magnetic recording medium. Regarding the magnetic field-magnetism curve obtained by measuring the intensity of magnetism of the magnetic powder with respect to the magnetic field applied under the conditions, the zero magnetic field of the curve with respect to the intensity of the peak appearing on the high magnetic field side in the differential curve obtained by numerically differentiating the curve. In addition to the conditions of Patent Document 2, magnetic particles having an average particle diameter of 10 nm or more and 30 nm or less (see, for example, Patent Document 2) in which the ratio of the intensity of the section on the vertical axis is 0.7 or less, 2θ is 42 °. Background when X-ray diffraction measurement is performed when 2θ is 27.2 ° or more and 29.7 ° or less with respect to the maximum value of diffraction intensity excluding the background when X-ray diffraction measurement is performed at 44 ° or more. A magnetic particle (see, for example, Patent Document 3) in which the ratio of the maximum value of the diffraction intensity excluding the above is 0.1 or less has been proposed.

Japanese Unexamined Patent Publication No. 2017-24981 Japanese Unexamined Patent Publication No. 2016-174135 Japanese Unexamined Patent Publication No. 2016-130208

However, the magnetic particles obtained by simply narrowing the particle size distribution of the magnetic powder containing ε-Fe ₂ O ₃ or adjusting the magnetic properties of the fine particles to the range described in Patent Document 2 or Patent Document 3 can be used. It was found that when used as a magnetic recording medium, sufficient durability during repeated reproduction due to signal attenuation cannot be obtained. It is considered that this is because the control of particles that do not contribute to magnetization is not merely a problem of particle size control, but is due to the physical characteristics of the precursor of the magnetic powder.
Further, in the invention described in Patent Document 2 or Patent Document 3, as a means for controlling the physical property value of the magnetic powder, when producing a precursor of the magnetic powder, unnecessary ions are removed by ultrafiltration. There is. However, according to the study of the present inventor, focusing on the crystal structure, the magnetic powder formed by the conditions in the step of forming the metal powder containing iron by using the aqueous solution of the compound containing trivalent iron ions. In the precursor of the above, the amorphous portion of the metal powder increases, and even if unnecessary ions are removed after the precursor of the magnetic powder is formed, the precursor of the already formed magnetic powder is not present. It was found that the inclusion of crystallization eventually increases the amount of fine particles produced due to the amorphous portion, which may affect the magnetic properties.
That is, conventionally, attempts have been made to improve the magnetic properties of magnetic powder containing ε-Fe ₂ O ₃ . However, when the magnetic powder containing ε-Fe ₂ O ₃ is applied to a magnetic recording medium, signal attenuation is suppressed from the viewpoints of not only good magnetic properties but also durability when used for a long period of time. It is important to be done. In this regard, the magnetic recording medium containing ε-Fe ₂ O ₃ described in Patent Documents 1 to 3 in the magnetic layer does not sufficiently suppress signal attenuation and has a problem in durability when used repeatedly. It became clear by the examination of the present inventor that there is.

An object to be solved by one embodiment of the present invention is to provide a magnetic powder having good magnetic properties and suppressed signal attenuation when applied to a magnetic recording medium.
An object to be solved by another embodiment of the present invention is to provide a method for producing a magnetic powder having good magnetic properties and suppressed signal attenuation when applied to a magnetic recording medium.
An object to be solved by another embodiment of the present invention is to provide a magnetic recording medium in which signal attenuation is suppressed.

The means for achieving the above-mentioned tasks include the following aspects.
<1> A magnetic powder containing at least one epsilon-type iron oxide-based compound selected from the group consisting of ε-Fe ₂ O ₃ and a compound represented by the following formula (1) and having an average particle diameter of 8 nm to 25 nm. The following equation obtained by second-order differentiation of the magnetization M obtained from the magnetic field-magnetization curve obtained by measuring at a maximum applied magnetic field of 359 kA / m, a temperature of 296 K, and a magnetic field sweep rate of 1.994 kA / m / s with the applied magnetic field H. When the magnetic field where the value of (I) becomes zero is Hc'and the value of the magnetic field where the magnetization becomes zero in the magnetic field-magnetization curve is Hc, the value of Hc with respect to Hc'is 0.6 or more and 1.0 or less. A magnetic powder having Hc'satisfying the following formula (II).
d ² M / dH ² equation (I)
119kA / m <Hc'<2380kA / m formula (II)

In the formula (1), A represents at least one metal element other than Fe, and a satisfies 0 <a <2.
<2> The compound represented by the formula (1) is the magnetic powder according to <1>, which is a compound represented by the following formula (1-2).

In formula (1-2), A ¹ represents at least one trivalent metal element selected from the group consisting of Ga, Al, In, and Rh, and A ² is derived from Mn, Co, Ni, and Zn. Represents at least one divalent metal element selected from the group consisting of, and A3 represents at ^least one tetravalent metal element selected from the group consisting of Ti and Sn. x satisfies 0 <x <1, y satisfies 0 <y <1, z satisfies 0 <z <1, and x + y + z <2.
<3> The content of at least one iron oxide-based compound selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ is ε-Fe ₂ O ₃ and a compound represented by the formula (1). The magnetic powder according to <1> or <2>, which is 20% by mass or less with respect to 100 parts by mass of the content of at least one epsilon-type iron oxide-based compound selected from the group.
<4> The magnetic powder according to any one of <1> to <3>, wherein the content of the magnetic powder particles having a particle diameter of 7.5 nm or less with respect to the total magnetic powder is 5% by number or less.

<5> A step of mixing an aqueous solution of a compound containing trivalent iron ions, which is a raw material of magnetic powder, and an alkaline agent, a mixed solution obtained by mixing, and a polyvalent carboxylic acid aqueous solution are mixed and stirred. Then, the step of separating the solid component after stirring and the separated solid component are redistributed in a dispersion medium containing water, an alkaline agent is added, and a silyl compound having a hydrolyzable group is added to prepare a dispersion liquid. The step of obtaining, the step of mixing the obtained dispersion liquid with the component capable of aggregating the solid component by salting, and the step of separating the produced precipitate to obtain a precursor of the magnetic powder, and the step of obtaining the obtained magnetic powder. Production of magnetic powder, which comprises a step of heat-treating the precursor of the above in a temperature condition of 800 ° C. to 1400 ° C. Method.

<6> The step of mixing the aqueous solution of the compound containing trivalent iron ions and the alkaline agent further includes the step of adding at least one selected from polyvinylpyrrolidone and hexadecyltrimethylammonium bromide <5>. The method for producing magnetic powder according to.

<7> A magnetic recording medium having a non-magnetic support and a magnetic layer containing the magnetic powder according to any one of <1> to <4> on the non-magnetic support.

According to one embodiment of the present invention, it is possible to provide a magnetic powder having good magnetic properties and suppressed signal attenuation when applied to a magnetic recording medium.
According to another embodiment of the present invention, it is possible to provide a method for producing a magnetic powder having good magnetic properties and suppressed signal attenuation when applied to a magnetic recording medium.
According to another embodiment of the present invention, it is possible to provide a magnetic recording medium in which signal attenuation is suppressed.

Hereinafter, examples of specific embodiments of the present invention will be described in detail, but the following embodiments are merely examples and are not limited to the following description, and are appropriately modified within the scope of the object of the present invention. Can be added.

In the present specification, the numerical range indicated by using "-" indicates a range including the numerical values before and after "-" as the minimum value and the maximum value, respectively.
In the present specification, the amount of each component contained in the composition means the total amount of the plurality of substances when the composition contains a plurality of substances corresponding to each component, unless otherwise specified.
In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range described stepwise. .. Further, in the numerical range described in the present specification, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples.
In the present specification, the term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.

<Magnetic powder>
The magnetic powder of the present disclosure contains at least one epsilon-type iron oxide-based compound selected from the group consisting of ε-Fe ₂ O ₃ and a compound represented by the following formula (1), and has an average particle size of 8 nm to 25 nm. It is a magnetic powder, and the magnetization M obtained from the magnetic field-magnetization curve obtained by measuring at a maximum applied magnetic field of 359 kA / m, a temperature of 296 K, and a magnetic field sweep rate of 1.994 kA / m / s is second-ordered with the applied magnetic field H. When the magnetic field where the differentiated value of the following equation (I) becomes zero is Hc'and the value of the magnetic field where the magnetization becomes zero in the magnetic field-magnetization curve is Hc, the value of Hc with respect to Hc'is 0.6 or more. A magnetic powder having a value of 1.0 or less and Hc'satisfying the following formula (II).
d ² M / dH ² equation (I)
119kA / m <Hc'<2380kA / m formula (II)

In the formula (1), A represents at least one metal element other than Fe, and a satisfies 0 <a <2.
The magnetic powder composed of at least one epsilon-type iron oxide-based compound selected from the group consisting of ε-Fe ₂ O ₃ and the compound represented by the following formula (1), including the particles before the alignment treatment, is described below. , May be referred to as "specific magnetic powder".

In general, it is difficult to maintain magnetization with fine-sized metal particles, but since ε-Fe ₂ O ₃ has a large anisotropic magnetic field (Hk) as described above, even finer-sized particles can be used. Magnetization can be maintained. However, even if the magnetic powder contains ε-Fe ₂ O ₃ , if the average particle size is reduced, it tends to contain a superparamagnetic component having no magnetism. The superparamagnetic component does not contribute to magnetic recording, and may be easily reversed in magnetization due to thermal fluctuations, which causes a problem that signal attenuation of magnetic powder is likely to occur.
In the magnetic powder of the present disclosure, attention was paid to the magnetic characteristics of the magnetic powder, and the suppression of the signal attenuation was achieved by controlling the magnetic characteristics.

(Specific magnetic powder)
Examples of the iron oxide-based compound represented by the formula (1) that can be contained in the specific magnetic powder include the compound represented by the following formula (1-2), the compound represented by the formula (2), and the following formula. Examples thereof include a compound represented by (3), a compound represented by the following formula (4), a compound represented by the following formula (5), and a compound represented by the following formula (6). The compound represented by (1-2) is preferable.

In formula (1-2), A ¹ represents at least one trivalent metal element selected from the group consisting of Ga, Al, In, and Rh, and A ² is derived from Mn, Co, Ni, and Zn. Represents at least one divalent metal element selected from the group consisting of, and A3 represents at ^least one tetravalent metal element selected from the group consisting of Ti and Sn. x satisfies 0 <x <1, y satisfies 0 <y <1, z satisfies 0 <z <1, and x + y + z <2.

In formula ( ^1-2 ), in A1, ^A2 , and A3 ^, from the viewpoint of stabilizing the ε phase and magnetic properties, A1 is preferably ^a metal element selected from Ga and Al. , A ² is preferably a metal element selected from Co and Mn, and A ³ is preferably Ti.
In formula (1-2), in x, y, and z, x satisfies 0 <x <0.7 and y is 0 <y from the viewpoint of preferable magnetic properties for application to a magnetic recording medium. It is preferable that <0.4 is satisfied and z is 0 <z <0.4, 0.05 <x <0.4 is satisfied, 0.01 <y <0.2 is satisfied, and , 0.01 <z <0.2 is more preferable.
Specific examples of the compound represented by the formula (1-2) include, for example, ε-Ga _0.24 Co _0.05 Ti _0.05 Fe _1.66 O ₃ , ε-Al _(0.20). Co _(0.06) Ti _(0.06) Fe _(1.68) O ₃ , ε-Ga _(0.15) Mn _(0.05) Ti _(0.05) Fe _(1.75) O ₃ etc. Can be mentioned.

In formula (2), Z represents at least one trivalent metal element selected from the group consisting of Ga, Al, and In. z satisfies 0 <z <2.
In the formula (2), Z is preferably a metal element selected from Ga and Al from the viewpoint of stabilizing the ε phase.
From the viewpoint of magnetic characteristics, z preferably satisfies 0 <z <0.8, and more preferably 0.05 <z <0.6.
Specific examples of the compound represented by the formula (2) include, for example, ε-Ga _(0.55) Fe _(1.45) O3 and ε _- Al _(0.45) Fe _(1.55). O ₃ and the like can be mentioned.

In formula (3), X represents at least one divalent metal element selected from the group consisting of Co, Ni, Mn, and Zn, and Y represents at least one tetravalent selected from Ti and Sn. Represents the metal element of. x satisfies 0 <x <1, and y satisfies 0 <y <1.
In the formula (3), from the viewpoint of magnetic properties, X is preferably a metal element selected from Co and Mn, and Y is preferably Ti.
From the viewpoint of stabilizing the ε phase, x preferably satisfies 0 <x <0.4, and y preferably satisfies 0 <y <0.4, and 0 <x <0.2. Moreover, it is more preferable to satisfy 0 <y <0.2.
Specific examples of the compound represented by the formula (3) include, for example, ε-Co _(0.05) Ti _(0.05) Fe _(1.9) O ₃ , ε-Mn _(0.075). Examples thereof include Ti _(0.075) Fe _{( 1.85)} O3.

In formula (4), X represents at least one divalent metal element selected from the group consisting of Co, Ni, Mn, and Zn, and Z is selected from the group consisting of Ga, Al, and In. Represents at least one trivalent metal element. x satisfies 0 <x <1, and z satisfies 0 <z <1.
In formula (4), from the viewpoint of stabilizing the ε phase and magnetic properties, X is preferably a metal element selected from Co and Mn, and Z is a metal element selected from Ga and Al. Is preferable.
From the viewpoint of stabilizing the ε phase and the magnetic characteristics, it is preferable that x satisfies 0 <x <0.4 and z satisfies 0 <z <0.6, and 0 <x <0.2. And more preferably 0.05 <z <0.6.
Specific examples of the compound represented by the formula (4) include, for example, ε-Mn _(0.02) Ga _(0.5) Fe _(1.48) O ₃ , ε-Co _(0.02). Ga _(0.4) Fe _(1.58) O ₃ and the like can be mentioned.

In formula (5), Y represents at least one tetravalent metal element selected from Ti and Sn, and Z is at least one trivalent metal selected from the group consisting of Ga, Al, and In. Represents an element. y satisfies 0 <y <1 and z satisfies 0 <z <1.
In the formula (5), from the viewpoint of stabilization of the ε phase and magnetic properties, Y is preferably Ti, and Z is preferably a metal element selected from Ga and Al.
From the viewpoint of stabilizing the ε phase and the magnetic characteristics, it is preferable that y satisfies 0 <y <0.4 and z satisfies 0 <z <0.8, and 0 <y <0.2. And more preferably 0.05 <z <0.6.
Specific examples of the compound represented by the formula (5) include, for example, ε-Ti _(0.02) Ga _(0.5) Fe _(1.48) O ₃ , ε-Ti _(0.02). Examples thereof include Al _(0.5) Fe _{( 1.48)} O3.

In formula (6), X represents at least one divalent metal element selected from the group consisting of Co, Ni, Mn, and Zn, and Y represents at least one tetravalent selected from Ti and Sn. Represents the metal element of, and Z represents at least one trivalent metal element selected from the group consisting of Ga, Al, and In. x satisfies 0 <x <1, y satisfies 0 <y <1, z satisfies 0 <z <1, and x + y + z <2.
In formula (6), from the viewpoint of stabilizing the ε phase and magnetic properties, X is preferably a metal element selected from Co and Mn, Y is preferably Ti, and Z is Ga. , And Al are preferred.
In formula (6), in x, y, and z, x satisfies 0 <x <0.4 and y is 0 <y <0 from the viewpoint of preferable magnetic properties for application to a magnetic recording medium. It is preferable that 0.7 is satisfied and z is 0 <z <0.4, 0.01 <x <0.2 is satisfied, 0.05 <y <0.4 is satisfied, and 0 is satisfied. It is more preferable to satisfy 0.01 <z <0.2.
Specific examples of the compound represented by the formula (6) include, for example, ε-Ga _0.24 Co _0.05 Ti _0.05 Fe _1.66 O ₃ , ε-Al _(0.20) Co ₍ . _0.06) Ti _(0.06) Fe _(1.68) O ₃ , ε-Ga _(0.15) Mn _(0.05) Ti _(0.05) Fe _(1.75) O ₃ and so on. Be done.

The composition of the iron oxide compound and the method for confirming the crystal structure will be described in detail in Examples.

(Physical characteristics of magnetic powder)
As the physical characteristics of the magnetic powder of the present disclosure, it has been found that a preferable signal attenuation suppressing effect can be obtained in the magnetic powder by controlling the following physical characteristics obtained from the magnetic field-magnetization curve.
Specifically, the magnetization M obtained from the magnetic field-magnetization curve obtained by measuring at a maximum applied magnetic field of 359 kA / m, a temperature of 296 K, and a magnetic field sweep rate of 1.994 kA / m / s is second-ordered with the applied magnetic field H. When the magnetic field in which the differentiated value of the following equation (I) becomes zero is Hc'and the value of the magnetic field in which the magnetization becomes zero in the magnetic field-magnetization curve is Hc, the value of Hc with respect to Hc'is 0.6 or more. It is 1.0 or less, and Hc'satisfies the following formula (II).
d ² M / dH ² equation (I)
119kA / m <Hc'<2380kA / m formula (II)

The method for measuring (Hc / Hc') in the present specification will be described more specifically.
Using a vibration sample type magnetic flux meter (TM-TRVSM5050-SMSL type, manufactured by Tamagawa Seisakusho Co., Ltd.), the magnetic characteristics of the magnetic powder can be measured with a maximum applied magnetic field of 359 kA / m, temperature of 296 K, and magnetic field sweep speed of 1.994 kA / m / m. At s, the strength of the magnetization of the magnetic powder with respect to the applied magnetic field is measured. From the measurement results, a magnetic field (H) -magnetism (M) curve of the magnetic powder is obtained.

Based on the obtained magnetic field (H) -magnetization (M) curve, a magnetic field in which the value of the following equation (I) obtained by second-order differentializing the magnetization M with the applied magnetic field H is calculated, and this is calculated as Hc'. Is defined as. Hc'satisfies the following formula (II).
d ² M / dH ² equation (I)
119kA / m <Hc'<2380kA / m formula (II)
The value of the magnetic field (Hc') at which the value of the formula (I) becomes zero is equal to the value of the magnetic field when the value (dM / dH) obtained by differentiating the magnetization M with the applied magnetic field H becomes maximum.
Further, in the obtained magnetic field (H) -magnetization (M) curve, the value of the magnetic field H at which the magnetization M becomes zero is defined as Hc. Hc is a value representing the coercive force of the magnetic powder to be measured.
The ratio (Hc / Hc') of the value of the magnetic field (Hc) at which the magnetization becomes zero to the value (Hc') of the magnetic field at which the value of the equation (I) obtained here becomes zero was obtained. It is the magnetic property of the magnetic powder.

Although the action on the magnetic powder of the present disclosure is not clear, it is considered as follows.
That is, it is considered that the magnetic powder of the present disclosure is a magnetic powder in which Hc / Hc'is 0.6 or more and 1.0 or less, so that the amount of superparamagnetic components contained in the magnetic powder is reduced and the magnetic properties are improved. ..
The superparamagnetic component refers to primary particles having a particle size of 7.5 nm or less, and the content of iron oxide particles having a primary particle size of 7.5 nm or less is a certain amount or more, so that the magnetization force is increased. It is considered that signal attenuation occurs because the magnetic properties of the magnetic powder are deteriorated due to the influence of particles that are low, that is, the magnetization is easily reversed due to the surrounding environment, and it is difficult to maintain the magnetization. The above-mentioned Hc / Hc'of 0.6 or more and 1.0 or less is considered to be one of the indicators that the content of the fine magnetic powder is small, and by satisfying the above conditions, the magnetism of the magnetic powder It is considered that the characteristics are improved and the signal attenuation is effectively suppressed.

According to the present inventor, the value of Hc (Hc / Hc') with respect to Hc'obtained from the magnetic field-magnetization curve is not affected by the magnetization reversal magnetic field when affected by the supernormal magnetic component and the supernormal magnetic component. It was found that it represents the ratio of the magnetization reversal magnetic field in the case and is a parameter that indirectly indicates the amount of the supernormal magnetic component.
That is, the higher the value of (Hc / Hc') obtained by the above-mentioned method, the smaller the content ratio of the superparamagnetic component, which is a fine particle, and therefore, the signal attenuation caused by the superparamagnetic component is suppressed.
The value of (Hc / Hc') is 0.6 or more and 1.0 or less, preferably 0.7 or more and 1.0 or less, and more preferably 0.8 or more and 1.0 or less.

From the viewpoint of achieving the above magnetic properties, the content of the magnetic powder having a particle diameter of 7.5 nm or less with respect to the total magnetic powder is preferably 5% by number or less, more preferably 4% by number or less, and 3 pieces. % Or less is more preferable.

The particle shape of the specific magnetic powder is not particularly limited, and for example, a spherical shape, a rod-shaped shape, or the like is preferable, and a spherical shape is preferable from the viewpoint of better dispersibility and orientation.
The particle size of the specific magnetic powder has an average particle size of 8 nm to 25 nm, preferably in the range of 8 nm to 20 nm, and more preferably in the range of 10 nm to 17 nm.
When the average particle size of the specific magnetic powder is in the above range, it can be suitably used for a magnetic recording medium.

The average particle size of the specific magnetic powder can be calculated by the following method.
The specific magnetic powder is photographed using a transmission electron microscope (TEM) at an imaging magnification of about 50,000 to 80,000 times, and printed on photographic paper under the condition that the total magnification is 500,000 times to obtain a photograph of the particles constituting the powder. ..
Select the primary particle from the obtained photograph of the particle, trace the outline of the particle with a digitizer, and calculate the diameter of the circle (circular area phase diameter) of the same area as the traced area to determine the size of the particle (primary particle). Measure. Here, the primary particles refer to independent particles without agglomeration.
In the present specification, the above measurement is performed on 500 randomly selected particles, and the arithmetic mean of the obtained 500 particle sizes is taken as the average particle size of the powder.
As the transmission electron microscope, for example, a transmission electron microscope H-9000 manufactured by Hitachi can be used. Further, the particle size can be measured by using a known image analysis software, for example, an image analysis software KS-400 manufactured by Carl Zeiss. Scale correction at the time of image capture from a scanner and image analysis can be performed using, for example, a circle having a diameter of 1 cm.
The average particle diameter of not only the specific magnetic powder but also the magnetic powder and non-magnetic particles as an optional component can be measured by the same method as described above.

As for the content of particles having a particle diameter of 7.5 nm or less with respect to the total magnetic powder, the specific magnetic powder is photographed at a magnification of about 50,000 to 80,000 times using a transmission electron microscope (TEM) in the same manner as in the measurement of the average particle size. Take a picture and print it on printing paper under the condition that the total magnification is 500,000 times to obtain a picture of the particles that make up the powder. It can be obtained by counting the number of particles having a particle size of 7.5 nm or less and calculating the ratio to the total number of measured particles.

(Other components that can be contained in magnetic powder)
The magnetic powder of the present disclosure may further contain other components in addition to the magnetic powder of the present disclosure, if necessary.
Examples of other components include metal powders other than the magnetic powders disclosed in the present disclosure.
As the metal powder other than the magnetic powder of the present disclosure, for example, at least one iron oxide-based compound selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ (hereinafter, referred to as another iron oxide-based compound). There is). The content of at least one iron oxide-based compound selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ is 20 parts by mass or less with respect to 100 parts by mass of the magnetic powder content of the present disclosure. It is preferable to have.
That is, by replacing at least a part of the magnetic powder of the present disclosure with other iron oxide-based compounds described above that are generally used for magnetic powder, the magnetic properties can be adjusted and the cost can be reduced as long as the performance is not deteriorated. It can be alleviated.
On the other hand, the magnetic powder is preferably a single phase, that is, a phase containing no metal powder other than the specific magnetic powder, but at least selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ as unavoidable impurities. It may contain one iron oxide-based compound.
In any case, the content of the other iron oxide-based compound with respect to the content of 100 parts by mass of the magnetic powder of the present disclosure is preferably 20 parts by mass or less, and more preferably 5 parts by mass or less.

[Manufacturing method of magnetic powder]
The method for producing the magnetic powder of the present disclosure is not particularly limited, and the magnetic powder can be produced by a known production method. Further, in order to achieve the above-mentioned magnetic properties, a known method can be applied as a method for removing fine particles.
Among them, as a method for efficiently producing a magnetic powder having good magnetic properties, the method for producing the magnetic powder of the present disclosure described below can be mentioned.

The method for producing a magnetic powder of the present disclosure includes a step of mixing an aqueous solution of a compound containing trivalent iron ions, which is a raw material of the magnetic powder, with an alkaline agent [hereinafter, may be referred to as step (I)]. A step of mixing and stirring the mixed solution obtained by mixing and a polyvalent carboxylic acid aqueous solution, and separating the solid component after stirring [hereinafter, may be referred to as step (II)] and the separated solid. A step of redispersing the components in a dispersion medium containing water, adding an alkaline agent, and adding a silyl compound having a hydrolyzable group to obtain a dispersion liquid [hereinafter, may be referred to as step (III)]. In the obtained dispersion, a component such as ammonium sulfate that can aggregate solid components by salting out is contained, and the generated precipitate is separated to obtain a precursor of magnetic powder [hereinafter referred to as step (IV)). It may be referred to as], a step of heat-treating the obtained precursor of the magnetic powder at 800 ° C to 1400 ° C [hereinafter, may be referred to as step (V)], and a precursor of the heat-treated magnetic powder. The step of removing the silicic acid compound in the above using an alkaline aqueous solution [hereinafter, may be referred to as step (VI)] is included.
In the step of mixing the above-mentioned aqueous solution of the trivalent iron ion-containing compound with the alkaline agent [step (I)], at least one selected from polyvinylpyrrolidone and hexadecyltrimethylammonium bromide is further added. The step [step (I-2)] to be performed may be included.

[Step (I)]
In this step, an aqueous solution of a compound containing trivalent iron ions, which is a raw material for magnetic powder, and an alkaline agent are mixed. The mixing can be performed in an atmospheric atmosphere, that is, under normal temperature and pressure, and in the presence of air.
Examples of the compound containing trivalent iron ion include iron (III) nitrate 9 hydrate, iron (III) chloride hexahydrate and the like.
The aqueous solution of the compound containing trivalent iron ions includes metal elements other than iron contained in the magnetic powder, specifically, Ga, Al, In, Rh, Mn, mentioned in the above-mentioned formula (1-2). It can contain a metal compound containing at least one metal element selected from the group consisting of Co, Ni, Zn, Ti and Sn. Examples of the compound containing a metal element other than iron include gallium nitrate (III) octahydrate, cobalt nitrate (II) hexahydrate, titanium sulfate (IV), aluminum nitrate (III) nine hydrate, and nitrate. Indium (III) trihydrate, rhodium nitrate (III), cobalt chloride (II) hexahydrate, manganese nitrate (II) hexahydrate, manganese (II) chloride tetrahydrate, nickel nitrate (II) Examples thereof include hexahydrate, nickel chloride (II) hexahydrate, zinc nitrate (II) hexahydrate, zinc chloride (II), tin chloride (IV) pentahydrate and the like.
The phase of the obtained magnetic powder can be prepared by the content of the compound containing a metal element other than iron.
A compound containing trivalent iron ions and a compound containing a metal element other than iron to be contained if desired are dissolved in water and stirred to prepare an aqueous solution. A known method can be applied to the stirring, and examples thereof include stirring using a magnetic stirrer. While continuing stirring, the above-mentioned aqueous solution and the alkaline agent are mixed under the temperature condition of 5 ° C. to 80 ° C. in the atmospheric atmosphere. Examples of the alkaline agent include an aqueous ammonia solution of about 20% by mass to 30% by mass, an aqueous solution of an ammonium salt compound, an aqueous solution of sodium hydroxide (NaOH) of about 0.1% by mass to 1.0% by mass, and potassium hydroxide ( KOH) Aqueous solution and the like can be mentioned.

In step (I), in addition to a compound containing trivalent iron ions and a compound containing an element other than iron, which is optionally used in combination, polyvinylpyrrolidone (PVP) and hexadecyltrimethylammonium bromide are added to the aqueous solution. It may contain at least one compound of choice [step (I-2)]. By containing the above-mentioned compound, the particle size of the solid particles formed by adding the alkaline agent becomes more uniform.

[Process (II)]
In the step (II), the mixed solution obtained in the step (I) and the polyvalent carboxylic acid aqueous solution are mixed and stirred, and after stirring, the solid component is separated.
Examples of the polyvalent carboxylic acid include citric acid, tartaric acid, malic acid and the like, and citric acid is preferable from the viewpoint of making the particle size of the obtained particles more uniform.
The amount of the polyvalent carboxylic acid used is preferably in the range of 0.2 mol to 5.0 mol, more preferably in the range of 0.5 mol to 2.5 mol, with respect to 1 mol of Fe ion.
A polyvalent carboxylic acid is added to the mixed solution obtained in the step (I), stirring is continued for 10 minutes to 2 hours, and the solid component precipitated after stirring is separated.
As a method for separating the solid component, a method for centrifuging is preferably mentioned from the viewpoint of convenience of operation.
The separated solid component is washed with water or the like and dried at 60 ° C to 100 ° C.

In the method for producing a magnetic powder of the present disclosure, a step (III) described later, that is, a step of adding an alkaline agent and further adding a silyl compound having a hydrolyzable group and mixing them to form a precursor of the magnetic powder. As a prior treatment, the mixed solution obtained in step (I) and the polyvalent carboxylic acid aqueous solution were mixed to neutralize the mixed solution to obtain a solid component, and then solid-liquid separation was once performed. Perform step (II) of washing the solid components with water and drying them.
By going through the above-mentioned step (II), in the subsequent formation of the magnetic powder precursor, particles having a lower content of the amorphous component are formed, and when the magnetic powder precursor is fired. In addition, the formation of unwanted fine particles due to the presence of amorphous components is suppressed.

[Step (III)]
In the step (III), the solid component obtained in the step (II) is redispersed in a dispersion medium containing water, an alkaline agent is added, and a silyl compound having a hydrolyzable group is added to obtain a dispersion liquid.
The temperature of the dispersion liquid can be in the range of 15 ° C. to 80 ° C., and may be raised to 30 ° C. to 80 ° C.
As the dispersion medium, water is preferable, and pure water having few impurities, ion-exchanged water and the like are more preferable.
Examples of the silyl compound having a hydrolyzable group include triethoxysilane (TEOS) and trimethoxysilane, and TEOS is preferable.
The amount of the silyl compound having a hydrolyzable group to be used is preferably in the range of 0.5 mol to 30 mol and more preferably in the range of 2 mol to 15 mol with respect to 1 mol of Fe. ..
In this step, a precipitate containing a precursor of magnetic powder is obtained.

[Process (IV)]
In step (IV), a component such as ammonium sulfate that can aggregate solid components by salting out is added to the dispersion obtained in step (III), and the generated precipitate is separated to form a magnetic powder. Obtain a precursor.
In the step (IV), the dispersion obtained in the step (III) is continuously stirred while maintaining the temperature at 15 ° C to 80 ° C, and a component capable of aggregating the solid component by salting out is added dropwise. The mixing method can be taken.
Examples of the component capable of aggregating the solid component by salting out include ammonium sulfate, ammonium oxalate, and other salts having a relatively high valence and high solubility such as divalent and trivalent, and the salt is dropped as an aqueous solution. can do.
Here, the salt having high solubility means that when the salt is added to water at 25 ° C., it dissolves in an amount of 5% by mass or more.
The precipitate formed by mixing the above-mentioned dispersion liquid and the alkaline agent can be separated to obtain a precursor of magnetic powder.

[Step (V)]
In the step (V), the precursor of the magnetic powder obtained in the step (IV) is heat-treated at 800 ° C to 1400 ° C. The heat treatment can be performed in an atmospheric atmosphere, that is, under normal pressure and in an environment where air is present.
By applying the heat treatment, the precursor of the magnetic powder becomes magnetic. The heat treatment time is preferably 1 hour to 8 hours.

[Process (VI)]
In this step, the silicic acid compound in the precursor of the magnetic powder heat-treated in the step (V) is removed by using an alkaline aqueous solution. That is, the silicic acid compound derived from the silyl compound having a hydrolyzable group remaining in the heat-treated magnetic powder is removed by the alkaline aqueous solution.
The magnetic powder of the present disclosure can be obtained by washing the magnetic powder from which the silicic acid compound as an impurity has been removed with water or the like and drying it.

[Other processes]
The method for producing a magnetic powder of the present disclosure may include other steps in addition to the steps (I) to (VI).
For example, the magnetic powder obtained through the steps (I) to (VI) is dispersed in an aqueous solution of a water-soluble polymer, and then solid-liquid separation by centrifugation is performed to remove undesired fine particles. Can include steps.
First, by dispersing the magnetic powder in an aqueous solution of a water-soluble polymer, the dispersibility of the magnetic particles is further improved, and then by centrifuging, classification (that is, removal of undesired fine particles) is more efficient. You can do it well.
Examples of the water-soluble polymer include polyvinyl alcohol (PVA), hydroxymethyl cellulose (HEC), polyvinylpyrrolidone (PVP) and the like.
As a method for solid-liquid separation, a method for centrifugation is preferable from the viewpoint of simplicity.
By performing a surface treatment with a water-soluble polymer prior to the solid-liquid separation, undesired fine particles are efficiently removed, and the magnetic properties of the specific magnetic powder are further improved.

<Magnetic recording medium>
The magnetic powder of the present disclosure described above is suitably used for a magnetic recording medium.
The magnetic recording medium of the present disclosure is a magnetic recording medium having a non-magnetic support and a magnetic layer containing the magnetic powder of the present disclosure described above on the non-magnetic support.
The magnetic recording medium of the present disclosure has a non-magnetic support and a magnetic layer containing magnetic powder and a binder on at least one surface of the non-magnetic support. That is, the magnetic recording medium has a non-magnetic support as a base material and a magnetic layer as a magnetic recording layer, and may have other layers depending on the purpose.
Since the magnetic recording medium of the present disclosure contains the magnetic powder of the present disclosure having good magnetic properties, it has a magnetic layer with good durability in which signal attenuation is suppressed when used repeatedly for a long period of time, and has good durability. It is a magnetic recording medium.
Examples of other layers that the magnetic recording medium may have include a non-magnetic layer and a back coat layer. Other layers will be described later.

[Non-magnetic support]
The non-magnetic support refers to a support having no magnetism. Hereinafter, the non-magnetic support may be simply referred to as a "support".
"Without magnetism" means that at least one of the conditions that the residual magnetic flux density is 10 mT or less and the coercive force is 7.98 kA / m (100 Oe) or less is satisfied.

Examples of the non-magnetic support include a base material formed of a material having no magnetism, for example, a resin material containing no magnetic material, an inorganic material having no magnetism, and the like. The material for forming the support can be appropriately selected and used from materials that satisfy the physical characteristics such as moldability required for the magnetic recording medium and the durability of the formed support.
The support is selected according to the usage pattern of the magnetic recording medium. For example, when the magnetic recording medium is a magnetic tape, a flexible disk, or the like, a flexible resin film can be used as the support. When the magnetic recording medium is a hard disk or the like, the support may be a disk-shaped body, a resin molded body harder than the support for a flexible disk, an inorganic material molded body, a metal material molded body, or the like.

Examples of the resin material forming the support include polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), polyolefins such as polyethylene and polypropylene, and amide resins such as polyamide, polyamideimide and aromatic polyamides containing polyaramid. Examples thereof include resin materials such as polyimide, cellulose triacetate (TAC), polycarbonate (PC), polysulfone, and polybenzoxazole. A support can be formed by appropriately selecting from the resin materials described above.
Among them, polyester, amide resin and the like are preferable, and polyethylene terephthalate, polyethylene naphthalate, and polyamide are more preferable from the viewpoint of good strength and durability and easy processing.

When the resin material is used for a support such as a magnetic tape, the resin material is formed into a film. As a method for molding the resin material into a film, a known method can be used.
The resin film may be an unstretched film or a stretched film such as uniaxially stretched or biaxially stretched. For example, when polyester is used, a biaxially stretched polyester film can be used in order to improve dimensional stability.
Further, a film having a laminated structure of two or more layers can also be used depending on the purpose. That is, for example, as shown in Japanese Patent Application Laid-Open No. 3-224127, two different layers of films are laminated for the purpose of changing the surface roughness between the surface forming the magnetic layer and the surface having no magnetic layer. A support or the like can also be used.

When the magnetic recording medium is a hard disk, a resin molded body obtained by molding the above-mentioned resin material into a disk shape, an inorganic material molded body obtained by molding an inorganic material such as glass or a metal material such as aluminum into a disk shape, is used as a support. Can be used as.

For example, for the purpose of improving the adhesion to the magnetic layer provided on the surface of the support, the support is subjected to surface treatment such as corona discharge, plasma treatment, easy adhesion treatment, heat treatment, etc. in advance, if necessary. May be done. Further, in order to prevent foreign matter from entering the magnetic layer, the support may be subjected to surface treatment such as dustproof treatment.
Each of the above-mentioned surface treatments can be carried out by a known method.

The thickness of the support is not particularly limited and can be appropriately set according to the use of the magnetic recording medium. The thickness of the support is preferably 3.0 μm to 80.0 μm. For example, when the magnetic recording medium is a magnetic tape, the thickness of the support is preferably 3.0 μm to 6.5 μm, more preferably 3.0 μm to 6.0 μm, and 4.0 μm to 5 It is more preferably .5 μm.

The thickness of each layer of the non-magnetic support and the magnetic recording medium described below is determined by exposing the cross section of the magnetic recording medium in the thickness direction by a known method such as an ion beam or a microtome, and then scanning the exposed cross section. Cross-section observation is performed with an electron microscope, and it can be obtained as the thickness at one location in the thickness direction in the cross-section observation, or as the arithmetic average of the thickness obtained at two or more randomly selected locations (for example, two locations). can.

[Magnetic layer]
The magnetic layer is a layer that contributes to magnetic recording. As the magnetic material, the magnetic layer is preferably a layer containing a ferromagnetic powder containing the above-mentioned magnetic powder of the present disclosure and a binder which is a film-forming component, and may further contain an additive depending on the purpose.

(Ferromagnetic powder)
The ferromagnetic powder in the magnetic layer includes the magnetic powder of the present disclosure.
As described above, the SNR of the magnetic recording medium is significantly improved when the magnetic layer contains the specified magnetic powder of the present disclosure.
When the magnetic layer contains a ferromagnetic powder (other ferromagnetic powder) other than the magnetic powder of the present disclosure, the content of the other ferromagnetic powder with respect to 100 parts by mass of the magnetic powder of the present disclosure is 20 parts by mass or less. Is preferable.

The method for measuring the average primary particle size of the magnetic powder and other ferromagnetic powders of the present disclosure is as described above. The method for measuring the average primary particle size of the magnetic powder will be described more specifically in Examples described later.

The sample powder of the magnetic powder or other ferromagnetic powder for measuring the average primary particle size may be a raw material powder or a sample powder collected from a magnetic layer.
Examples of the method for collecting the magnetic component from the magnetic layer as sample powder include the following methods.
1. 1. The surface of the magnetic layer is surface-treated with a plasma reactor manufactured by Yamato Scientific Co., Ltd. for 1 to 2 minutes to incinerate and remove organic components (binder components, etc.) on the surface of the magnetic layer.
2. 2. A filter paper soaked with an organic solvent such as cyclohexanone or acetone is attached to the edge of the metal rod, and then the above 1. The surface of the magnetic layer after the treatment is rubbed, and the magnetic layer component is transferred from the magnetic recording medium to the filter paper and peeled off.
3. 3. Above 2. The magnetic layer component peeled off in step 1 is shaken off into an organic solvent such as cyclohexanone or acetone, the organic solvent is dried, and the peeled component is taken out from the filter paper. As a method of shaking off, a method of putting the entire filter paper in a solvent and shaking it off with an ultrasonic disperser can be taken.
4. Above 3. The magnetic layer component scraped off is placed in a thoroughly washed glass test tube, and about 20 ml of n-butylamine is added to the magnetic layer component in the glass test tube to seal the glass test tube. It is preferable to add n-butylamine in an amount capable of decomposing the residual binder without incineration.
5. The glass test tube is heated at 170 ° C. for 20 hours or more to decompose organic components contained in the magnetic layer such as a binder and a curing agent component.
6. Above 5. The precipitate after decomposition is thoroughly washed with pure water and then dried, and the powder is taken out.
By the above steps, the sample powder can be collected from the magnetic layer and used for measuring the average primary particle size.

The content (filling rate) of the ferromagnetic powder in the magnetic layer is preferably 50% by mass to 90% by mass, and more preferably 60% by mass to 90% by mass with respect to the dry mass of the magnetic layer. When the filling rate of the ferromagnetic powder in the magnetic layer is 50% by mass or more with respect to the dry mass of the magnetic layer, the recording density can be improved.
The magnetic powder of the present disclosure is preferably a magnetic powder having a surface-treated layer which is surface-treated with a silane coupling agent under specific conditions. Therefore, although the particle size is smaller than that of the general magnetic particles used conventionally, the dispersibility is good, the filling rate can be further improved, and the recording density is further improved.

(Binder)
The binder is selected from the film-forming resins useful for forming the magnetic layer containing the above-mentioned ferromagnetic powder.
The resin used as the binder is not particularly limited as long as it can form a resin layer satisfying various physical properties such as desired strength and durability. It can be appropriately selected from known film-forming resins according to the purpose and used as a binder.

The resin used as the binder may be a homopolymer or a copolymer. The resin used as the binder may be a known electron beam curable resin.
Resins that can be used as a binder include acrylic resins obtained by (co) polymerizing polyurethane, polyester, polyamide, vinyl chloride resin, styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetal. , Polyvinyl butyral and the like, and examples thereof include resins selected from polyvinyl butyral resins and the like. As the resin used as the binder, the above-mentioned resins may be used alone or in combination of two or more. Of these, polyurethane resin, acrylic resin, cellulose resin and vinyl chloride resin are preferable.

The resin as a binder preferably has a functional group, for example, a polar group, which can be adsorbed on the powder surface, in the molecule in order to further improve the dispersibility of the ferromagnetic powder contained in the magnetic layer. Preferred functional groups that the resin as a binder may have include, for example, -SO ₃ M, -SO ₄ M, -PO (OM) ₂ , -OPO (OM) ₂ , -COOM, = NSO ₃ M, =. Examples thereof include NRSO ₃ M, -NR ¹ R ² , -N ⁺ R ¹ R ² R ³ X-, ^and the like. Here, M represents a hydrogen atom or an alkali metal atom such as Na or K. R represents an alkylene group, and R ¹ , R ² and R ³ independently represent a hydrogen atom, an alkyl group or a hydroxyalkyl group, respectively. X represents a halogen atom such as Cl or Br.
When the resin as a binder has the above functional groups, the content of the functional groups in the resin is preferably 0.01 meq / g or more and 2.0 meq / g or less, and 0.3 meq / g or more and 1.2 meq / g or less. More preferred. When the content of the functional group in the resin is within the above range, the dispersibility of the ferromagnetic powder or the like in the magnetic layer becomes better, and the magnetic density is further improved, which is preferable.

Of these, polyurethane containing an —SO ₃ Na group is more preferable as the resin used as the binder. When the polyurethane contains a —SO ₃ Na group, the —SO ₃ Na group is preferably contained at 0.01 meq / g to 1.0 meq / g with respect to the polyurethane.
As the binder, a commercially available resin can be appropriately used.

The average molecular weight of the resin used as the binder can be, for example, 10,000 or more and 200,000 or less as the weight average molecular weight.
The weight average molecular weight in the present disclosure is a value obtained by converting a value measured by gel permeation chromatography (GPC) into polystyrene. The following conditions can be mentioned as the measurement conditions.
GPC device: HLC-8120 (manufactured by Tosoh)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mm ID (Inner Diameter) x 30.0 cm)
Eluent: Tetrahydrofuran (THF)
Sample concentration: 0.5% by mass
Sample injection volume: 10 μl
Flow velocity: 0.6 ml / min
Measurement temperature: 40 ° C
Detector: RI detector

The content of the binder in the magnetic layer can be, for example, in the range of 1 part by mass to 30 parts by mass, and preferably in the range of 2 parts by mass to 20 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.

(Other additives)
In addition to the above-mentioned ferromagnetic powder and binder, the magnetic layer can contain various additives depending on the purpose as long as the effect of the magnetic layer is not impaired.
Examples of the additive include an abrasive, a lubricant, a dispersant, a dispersion aid, a fungicide, an antistatic agent, an antioxidant, carbon black and the like. Further, as the additive, colloidal particles as an inorganic filler and a curing agent can also be used, if necessary.
As the additive, a commercially available product can be appropriately used depending on the desired properties.

-Abrasive-
The magnetic layer can contain an abrasive. Since the magnetic layer contains an abrasive, it is possible to remove deposits adhering to the head during use of the magnetic recording medium.
Examples of the abrasive include α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide titanium carbide, and oxidation having an pregelatinization rate of 90% or more. It is preferable that known materials having a Mohs hardness of 6 or more, such as titanium, silicon dioxide, and boron nitride, are used alone or in combination. Further, the above-mentioned composite of abrasives (abrasive surface-treated with another abrasive) may be used.
The abrasive may contain a compound or element other than the metal compound particles which are the main components described above, but if the main component is 90% by mass or more, the effect does not change.
Further, as the polishing agent, a material obtained by surface-treating the particles may be used.
As the abrasive, a commercially available product can be used as appropriate.
Specifically, commercially available abrasives include AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT20, HIT-30, HIT-55, HIT60A, and HIT70 manufactured by Sumitomo Chemical Co., Ltd. , HIT80, HIT100; ERC-DBM, HP-DBM, HPS-DBM manufactured by Reynolds; WA10000 manufactured by Fujimi Abrasive Co., Ltd .; UB20 manufactured by Uyemura Kogyo Co., Ltd .; TF100, TF140 manufactured by Toda Kogyo Co., Ltd .; Beta Random Ultra Fine manufactured by Ibiden Co., Ltd .; B-3 manufactured by Showa Mining Co., Ltd .; and the like.

The particle size of the abrasive is preferably 0.01 μm to 2 μm, more preferably 0.05 μm to 1.0 μm, and even more preferably 0.05 μm to 0.5 μm.
In particular, in order to enhance the electromagnetic conversion characteristics, it is preferable that the abrasive has a narrow particle size distribution. Further, in order to improve the durability, it is possible to combine abrasives having different particle sizes as necessary, or to widen the particle size distribution even with a single abrasive to obtain the same effect. The tap density of the abrasive is preferably 0.3 g / ml to 2 g / ml, the water content is preferably 0.1% to 5%, the pH is preferably 2 to 11, and the BET ratio. The surface area (S _BET ) is preferably 1 m ² / g to 30 m ² / g.
The shape of the abrasive may be needle-shaped, spherical, or cubic, but particles having corners in a part of the shape are preferable because of their high abrasiveness.

When the magnetic layer contains an abrasive, the content is preferably in the range of 1 part by mass to 10 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.

-lubricant-
The magnetic layer can contain a lubricant.
When the magnetic layer contains a lubricant, for example, the running durability of the magnetic recording medium can be improved.
As the lubricant, a known hydrocarbon-based lubricant, a fluorine-based lubricant, or the like can be used.
As the lubricant, a commercially available product may be used as appropriate.

As the lubricant, known hydrocarbon-based lubricants, fluorine-based lubricants, extreme pressure additives and the like can be used.

Carboxylic acids such as stearic acid and oleic acid; esters such as butyl stearate; sulfonic acids such as octadecylsulfonic acid; phosphoric acid esters such as monooctadecyl phosphate; stearyl alcohol and oleyl alcohol. Alcohols such as; carboxylic acid amides such as stealic acid amides; amines such as stearylamine; and the like.

Examples of the fluorine-based lubricant include a lubricant in which a part or all of the alkyl groups of the above-mentioned hydrocarbon-based lubricant are replaced with a fluoroalkyl group or a perfluoropolyether group.
Examples of the perfluoropolyether group include perfluoromethylene oxide polymer, perfluoroethylene oxide polymer, perfluoro-n-propylene oxide polymer (CF ₂ CF ₂ CF ₂ O) _n , and perfluoroisopropylene oxide polymer (CF). (CF ₃ ) CF ₂ O) _n or a copolymer thereof.

Further, a compound having a polar functional group such as a hydroxyl group, an ester group or a carboxyl group in the terminal of the alkyl group of the hydrocarbon-based lubricant or in the molecule is suitable because it has a high effect of reducing the frictional force.
Further, the molecular weight of the lubricant is 500 to 5000, preferably 1000 to 3000. By setting the value to 500 to 5000, volatilization can be suppressed and deterioration of lubricity can be suppressed. In addition, it is possible to prevent the viscosity from increasing, and the slider and the disc are easily adsorbed, so that running stop and head crash can be prevented.
Specific examples of the perfluoropolyether that can be used as a lubricant are commercially available under trade names such as FOMBLIN manufactured by Audimond and KRYTOX manufactured by DuPont.

Extreme pressure additives include phosphate esters such as trilauryl phosphate; thiophosphite esters such as trilauryl phosphite; thiophosphoric acid esters such as trilauryl trithiophosphate; dibenzyl disulfide. Sulfur-based extreme pressure agents such as;

When the magnetic layer contains a lubricant, the lubricant may be used alone or in combination of two or more.
When the magnetic layer contains a lubricant, the content of the lubricant is preferably in the range of 0.1 part by mass to 5 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.

-Non-magnetic filler-
The magnetic layer can include a non-magnetic filler. The non-magnetic filler is preferably colloidal particles from the viewpoint of dispersibility and surface roughness.
As the colloidal particles, inorganic colloidal particles are preferable from the viewpoint of availability, and inorganic oxide colloidal particles are more preferable. Examples of the inorganic oxide colloidal particles include the above-mentioned inorganic oxide colloidal particles, which include SiO ₂ / Al ₂ O ₃ , SiO ₂ / B ₂ O ₃ , TIO ₂ / CeO ₂ , SnO ₂ / Sb ₂ O ₃ , SiO ₂ / Al ₂ O ₃ / TiO ₂ , TiO ₂ / CeO ₂ / SiO ₂ and the like. Preferred examples thereof include inorganic oxide colloidal particles such as SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , and Fe ₂ O ₃ , and silica colloidal particles are available from the viewpoint of easy availability of monodisperse colloidal particles. (Coloidal silica) is particularly preferable.

When the magnetic layer contains a non-magnetic filler, the non-magnetic filler may be used alone or in combination of two or more.
As the non-magnetic filler, a commercially available product can be appropriately used.
When the magnetic layer contains a non-magnetic filler, the content is preferably in the range of 1 part by mass to 10 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.

The thickness of the magnetic layer can be optimized according to the saturation magnetization amount of the magnetic head used, the head gap length, the band of the recorded signal, and the like. The thickness of the magnetic layer is preferably 10 nm to 150 nm, more preferably 20 nm to 120 nm, and further preferably 30 nm to 100 nm from the viewpoint of high-density recording.
The magnetic layer may be at least one layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a known configuration relating to a multi-layer magnetic layer can be applied. With respect to the multi-layered magnetic layer, the thickness of the magnetic layer means the total thickness of a plurality of magnetic layers.

Hereinafter, the non-magnetic layer and the backcoat layer, which are arbitrary layers in the magnetic recording medium, will be described.

[Non-magnetic layer]
The non-magnetic layer is a layer that contributes to thinning the magnetic layer. The non-magnetic layer is preferably a layer containing a non-magnetic powder as a filler and a binder which is a film-forming component, and may further contain an additive depending on the purpose.

The non-magnetic layer can be provided between the non-magnetic support and the magnetic layer. The non-magnetic layer includes a non-magnetic layer and a substantially non-magnetic layer containing a small amount of ferromagnetic powder as an impurity or intentionally.
Here, "non-magnetic" means that the residual magnetic flux density is 10 mT or less and the coercive force is 7.98 kA / m (100 Oe) or less, as described in the above-mentioned "non-magnetic support". Satisfying at least one of them.

(Non-magnetic powder)
The non-magnetic powder is a non-magnetic powder that functions as a filler. The non-magnetic powder used for the non-magnetic layer may be an inorganic powder or an organic powder. In addition, carbon black or the like can also be used. Examples of the inorganic powder include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like. The non-magnetic powder can be used alone or in combination of two or more. The non-magnetic powder is available as a commercial product and can also be produced by a known method.

Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr _{2O 3} , α-alumina with an pregelatinization rate of 90 to 100%, β-alumina, γ-Alumina, α-iron oxide, gateite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide , Titanium oxide and the like can be used alone or in combination of two or more. Preferred are α-iron oxide and titanium oxide.

The shape of the non-magnetic powder may be needle-shaped, spherical, polyhedral, plate-shaped or the like. The crystallite size of the non-magnetic powder is preferably 4 nm to 500 nm, more preferably 40 nm to 100 nm. When the crystallite size is in the range of 4 nm to 500 nm, it is preferable because it does not make dispersion difficult and has a suitable surface roughness. The average particle size of the non-magnetic powder is preferably 5 nm to 500 nm, but the same effect can be obtained by combining non-magnetic powders having different average particle sizes as necessary, or by widening the particle size distribution even with a single non-magnetic powder. It is also possible to have. A particularly preferable average particle size of the non-magnetic powder is 10 nm to 200 nm. The range of 5 nm to 500 nm is preferable because the dispersion is good and the surface roughness is suitable.

The content (filling rate) of the non-magnetic powder in the non-magnetic layer is preferably in the range of 50% by mass to 90% by mass, and more preferably in the range of 60% by mass to 90% by mass.

The "binder" and "additive" in the non-magnetic layer are synonymous with the "binder" and "additive" described in the section of "magnetic layer", and the preferred embodiments are also the same.

The thickness of the non-magnetic layer is preferably 0.05 μm to 3.0 μm, more preferably 0.05 μm to 2.0 μm, and even more preferably 0.05 μm to 1.5 μm.

[Back coat layer]
The back coat layer is a layer that contributes to running stability and the like. The backcoat layer is preferably a layer containing a non-magnetic powder as a filler and a binder which is a film-forming component, and may further contain an additive depending on the purpose.

The backcoat layer can be provided on the surface of the non-magnetic support opposite to the magnetic layer side.

The “non-magnetic powder” in the backcoat layer is synonymous with the “non-magnetic powder” described in the section “Non-magnetic layer”, and the preferred embodiment is also the same. Further, the "binder" and "additive" in the backcoat layer are synonymous with the "binder" and "additive" described in the section of "magnetic layer", and the preferred embodiments are also the same.

The thickness of the backcoat layer is preferably 0.9 μm or less, more preferably 0.1 μm to 0.7 μm.

<Manufacturing method of magnetic recording medium>
The method for producing the magnetic recording medium of the present disclosure is not particularly limited, and a known production method can be applied.
As a method for producing a magnetic recording medium, for example, a step of preparing a composition for forming a magnetic layer (step (A)), a composition for forming a magnetic layer by imparting the composition for forming a magnetic layer on a non-magnetic support. The step of forming the layer (step (B)), the step of magnetically orienting the formed magnetic layer forming composition layer (step (C)), and drying the magnetically oriented composition layer for forming the magnetic layer. A manufacturing method including a step of forming a magnetic layer (step (D)) can be mentioned.
The method for manufacturing a magnetic recording medium further includes, if necessary, a step of calendering a non-magnetic support having a magnetic layer, a step of forming an arbitrary layer such as a non-magnetic layer and a backcoat layer, and the like. be able to.
Each step may be divided into two or more steps.

[Step (A)]
The method for producing the magnetic powder preferably includes a step (step (A)) of preparing a composition for forming a magnetic layer.
Step (A) involves adding and dispersing a ferromagnetic powder, a binder and, if necessary, an additive to the solvent.

All raw materials such as the ferromagnetic powder, the binder, the non-magnetic powder, and the additive of the present disclosure may be added at any time in the present step (A).
The individual raw materials may be added at the same time, or may be added in two or more portions. For example, the binder can be added in the dispersion step and then further added for viscosity adjustment after dispersion.

For the dispersion of the raw materials of the composition for forming the magnetic layer, a known dispersion device such as a batch type vertical sand mill or a horizontal bead mill can be used. As the dispersed beads, for example, glass beads, zirconia beads, titania beads, steel beads and the like can be used. The particle size (bead diameter) and filling rate of the dispersed beads can be optimized and used.
Further, the raw material of the composition for forming a magnetic layer can be dispersed by using, for example, a known ultrasonic device.
Further, before dispersion, at least a part of the raw materials of the composition for forming a magnetic layer can be kneaded using, for example, an open kneader.

The raw materials of the composition for forming a magnetic layer may be mixed after preparing a solution for each raw material. For example, a magnetic liquid containing a ferromagnetic powder and a polishing liquid containing an abrasive can be prepared and then mixed and dispersed.

(Composition for forming a magnetic layer)
The "ferromagnetic powder", "binder", and "additive" for preparing the composition for forming a magnetic layer are the "ferromagnetic powder", "binder", and "binding agent" described in the section of "magnetic layer". It is synonymous with "additive", and the preferred embodiment is also the same.

The content of the ferromagnetic powder in the composition for forming a magnetic layer is preferably 5% by mass to 50% by mass, more preferably 10% by mass to 30% by mass, based on the total mass of the composition for forming a magnetic layer.
The content of the binder in the composition for forming a magnetic layer is preferably in the range of 1 part by mass to 30 parts by mass, and more preferably in the range of 2 parts by mass to 20 parts by mass with respect to 100 parts by mass of the ferromagnetic powder. ..

-solvent-
The solvent is a medium for dispersing the ferromagnetic powder, the binder and, if necessary, the additives.
The solvent may be only one kind, or may be a mixed solvent of two or more kinds. As the solvent, an organic solvent is preferable.
Examples of the organic solvent include ketone compounds such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, alcohol compounds such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and methylcyclohexanol. Ester compounds such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ether compounds such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene, cresol, chlorbenzene, etc. Use chlorinated hydrocarbon compounds such as aromatic hydrocarbon compounds, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorbenzene, N, N-dimethylformamide, hexane and the like. Can be done. Preferred organic solvents include methyl ethyl ketone, cyclohexanone, and mixed solvents containing them in any proportion.

In order to improve the dispersibility, a solvent having a strong polarity to some extent is preferable, and a solvent having a dielectric constant of 15 or more is preferably contained in an amount of 50% by mass or more based on the total mass of the solvent. Further, the solubility parameter is preferably 8 to 11.

-Hardener-
The composition for forming a magnetic layer can contain a curing agent.
When the composition for forming a magnetic layer contains a curing agent, a crosslinked structure is formed in the binder contained in the magnetic layer when the magnetic layer is formed, and the film strength of the formed magnetic layer is further improved.
As the curing agent, an isocyanate compound is preferable. Examples of the isocyanate-based compound include tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate and the like. Examples thereof include the above-mentioned products of the isocyanate-based compound and the polyalcohol, and bifunctional or higher polyisocyanate produced by the condensation of the isocyanate-based compound can be used.

As the curing agent, a commercially available product can be used as appropriate. Commercially available isocyanate compounds include, for example, Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Co., Ltd., Takeda D-102, Takeda D-110N. , Takeda D-200, Takeda D-202, Death Module L manufactured by Sumitomo Bayer, Death Module IL, Death Module N, Death Module HL and the like.

When the composition for forming a magnetic layer contains a curing agent, one type of curing agent may be used alone, or two or more types may be used.
When the composition for forming a magnetic layer contains a curing agent, the content of the curing agent may be, for example, 0.1 part by mass to 10 parts by mass added to 100.0 parts by mass of the ferromagnetic powder. From the viewpoint of improving the strength, preferably 1 part by mass to 10 parts by mass can be added.
If necessary, the curing agent can be contained in the composition for forming another phase for the purpose of forming the film strength of the other layer at the time of forming the other layer.

[Step (B)]
In the method for producing a magnetic recording medium of the present disclosure, after the composition preparation step, a step of applying a magnetic layer forming composition on a non-magnetic support to form a magnetic layer forming composition layer (step (B)). ) Is preferably included.
This step (B) can be performed, for example, by applying the composition for forming a magnetic layer on a non-magnetic support under running in an amount having a predetermined film thickness. The preferable film thickness of the magnetic layer is as described in the section of "magnetic layer".

As a coating method for applying the composition for forming a magnetic layer on the surface of the support, an air doctor coat, a blade coat, a rod coat, an extrusion coat, an air knife coat, a squeeze coat, an impregnation coat, a reverse roll coat, a transfer roll coat, etc. Known methods such as gravure coat, kiss coat, cast coat, spray coat, spin coat and the like can be used. For the coating method, for example, "Latest Coating Technology" (May 31, 1983) published by General Technology Center Co., Ltd. can be referred to.

[Step (C)]
The method for producing a magnetic recording medium of the present disclosure preferably includes, after the composition layer forming step, a step (step (C)) of applying a magnetic field orientation treatment to the formed magnetic layer forming composition layer.

When the support is on a film such as a magnetic tape, the formed composition layer for forming a magnetic layer is oriented in a magnetic field with respect to the ferromagnetic powder contained in the composition for forming a magnetic layer by using a cobalt magnet or a solenoid. Can be processed. If the support is a support for a hard disk, sufficient isotropic orientation may be obtained even if it is not oriented without using an alignment device. It is preferable to use a known random alignment device such as by applying a magnetic field. Further, it is also possible to impart isotropic magnetic characteristics in the circumferential direction by using a known method such as a magnet facing different poles and making the orientation vertical. In particular, vertical orientation is preferable for high-density recording. It can also be circumferentially oriented using spin coating.
The magnetic field orientation treatment is preferably performed before the formed magnetic layer forming composition layer is dried.

The magnetic field alignment treatment can be performed by a vertical alignment treatment in which a magnetic field having a magnetic field strength of 0.1 T to 1.0 T is applied in a direction perpendicular to the surface of the formed magnetic layer forming composition layer.

[Step (D)]
In the method for producing a magnetic recording medium of the present disclosure, after the step (C) of performing the magnetic field orientation treatment, the step (step (D)) of drying the composition layer for forming the magnetic layer subjected to the magnetic field orientation treatment to form the magnetic layer. ) Is preferably included.

In the drying of the composition layer for forming a magnetic layer, the drying of the composition layer for forming a magnetic layer can be controlled by controlling the temperature, the air volume, and the coating speed of the dry air. For example, the coating speed is preferably 20 m / min to 1000 m / min, and the temperature of the dry air is preferably 60 ° C. or higher. Also, the composition can be moderately pre-dried before applying a magnetic field.

[Calendar processing process]
The method for producing a magnetic recording medium of the present disclosure is a non-magnetic method having a magnetic layer after forming a magnetic layer on a support through steps (A), steps (B), steps (C) and steps (D). It is preferable to carry out a step of calendering the support.

The non-magnetic support having a magnetic layer can be once wound up by a take-up roll and then unwound from the take-up roll and subjected to a calendar treatment. The calendar treatment further improves the surface smoothness, eliminates the pores created by the removal of the solvent during drying, and improves the filling rate of the ferromagnetic powder in the magnetic layer, so that the electromagnetic conversion characteristics are higher. A magnetic recording medium can be obtained. The calendar treatment is preferably performed while changing the calendar treatment conditions according to the smoothness of the surface of the magnetic layer.

For the calendar processing, for example, a super calendar roll or the like can be used.
As the calendar roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyamide-imide can be used. It can also be processed with a metal roll.

As the calendar treatment conditions, the temperature of the calendar roll can be, for example, in the range of 60 ° C to 120 ° C, preferably in the range of 80 ° C to 100 ° C, and the pressure can be, for example, 100 kg / cm to 500 kg / cm (98 kN /). It can be in the range of m to 490 kN / m), preferably in the range of 200 to 450 kg / cm (196 to 441 kN / m).

[Step of forming an arbitrary layer such as a non-magnetic layer and a back coat layer]
The method for producing a magnetic recording medium of the present disclosure can include, if necessary, a step of forming an arbitrary layer such as a non-magnetic layer and a backcoat layer.
For the non-magnetic layer and the back coat layer, after preparing the composition for forming each layer, the same steps as in the steps (B), (C), and (D) in the formation of the magnetic layer are performed. Can be formed.
As described in the sections of "non-magnetic layer" and "backcoat layer", the non-magnetic layer can be provided between the support and the magnetic layer, and the backcoat layer is a magnetic layer of the support. It can be provided on the surface opposite to the holding side.

The composition for forming the non-magnetic layer and the composition for forming the back coat layer can be prepared with the components and contents described in the sections of "non-magnetic layer" and "back coat layer".

Hereinafter, the magnetic powder of the present disclosure, a method for producing the same, and a magnetic recording medium will be described in more detail based on examples, but the present disclosure is not limited to the description of the following specific examples and deviates from the gist of the present invention. Unless otherwise, various modifications are possible.
Unless otherwise specified, "%" and "part" in the examples are based on mass.

[Example 1]
In 90 g of pure water, 8.3 g of iron (III) nitrate 9 hydrate, 1.3 g of gallium nitrate (III) octahydrate, 190 mg of cobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and , 1.5 g of polyvinylpyrrolidone (PVP) was dissolved, and 4.0 g of a 25 mass% aqueous ammonia solution was added under the condition of 25 ° C. in an air atmosphere while stirring with a magnetic stirrer. The mixture was stirred for 2 hours under the temperature condition of 25 ° C. [Step (I)]. To the obtained solution, a citric acid solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added, and the mixture was stirred for 1 hour [Step (II)]. The powder precipitated after stirring was collected by centrifugation, washed with pure water, and dried at 80 ° C.
800 g of pure water was added to the dried powder, and the powder was dispersed in water again to obtain a dispersion liquid. The temperature of the obtained dispersion was raised to 50 ° C., and 40 g of a 25% aqueous ammonia solution was added dropwise with stirring. After stirring for 1 hour while maintaining the temperature of 50 ° C., 14 mL of tetraethoxysilane (TEOS) was added dropwise, and the mixture was stirred for 24 hours [Step (III)]. 50 g of ammonium sulfate was added to the obtained reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried at 80 ° C. for 24 hours to obtain a precursor of magnetic powder [Step (IV)]. ..

The obtained magnetic powder precursor was loaded into a furnace and heat-treated at 1030 ° C. for 4 hours in an air atmosphere [step (V)].
From the precursor of the heat-treated magnetic powder, the precursor of the heat-treated magnetic powder was put into a 4 mol / L aqueous solution of sodium hydroxide (NaOH), and the liquid temperature was maintained at 70 ° C. and stirred for 24 hours. The silicic acid compound, which is an impurity, was removed [Step (VI)].
After that, the magnetic powder from which the silicic acid compound was removed was collected by centrifugation and washed with pure water to obtain iron oxide nanomagnetic particles (specific magnetic powder) which are single phases of ε-Fe ₂ O ₃ phase. Obtained.
When the composition of the obtained magnetic powder of Example 1 was confirmed by ICP-OES (high frequency inductively coupled plasma emission spectroscopic analysis), it was found to be ε-Ga _0.24 Co _0.05 Ti _0.05 Fe _1.66 O ₃ . there were. Further, by confirming the peak of the powder XRD (X-ray diffraction) pattern, the magnetic powder does not contain at least one iron oxide-based compound selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ ε-Fe. It was confirmed that it was a _2O3 single _- phase magnetic powder.

As the ICP-OES measuring device, ICPS-8100 manufactured by Shimadzu Corporation was used.
The measurement was performed by the following method.
A container (beaker) containing 12 mg of sample powder and 10 mL of 4 mol / L hydrochloric acid was held on a hot plate at a set temperature of 80 ° C. for 3 hours to obtain a solution.
After adding 30 mL of pure water to the obtained solution, the mixture was filtered using a 0.1 μm membrane filter. The obtained filtrate was analyzed by an ICP-OES measuring device to determine the content of each metal atom.

As the powder XRD (X-ray diffraction) apparatus, X'pert Pro manufactured by PANalytical Co., Ltd. was used, and the measurement was performed under the following measurement conditions.
X-ray source: Cu Kα ray (wavelength 1.54 Å (0.154 nm)), (output: 40 mA, 45 kV)
Scanned range: 20 ° <2θ <70 °
Scan interval: 0.05 °
Scan speed: 0.75 ° / min
When observed by the method described later, the shape of the magnetic powder was spherical. The average particle size calculated by the method described later was 13.5 nm.
Further, as confirmed by the method described above, the content of particles having a particle size of 7.5 nm or less in the total magnetic powder was 3.9% by number.

(Calculation of average particle size of metal powder and magnetic powder)
A powder such as magnetic powder is photographed with a transmission electron microscope (TEM: Hitachi, Ltd., transmission electron microscope H-9000 type) at a magnification of 80,000 times, and a printing paper with a total magnification of 500,000 times. A photograph of the particles constituting the powder was obtained by printing on.
Primary particles were selected from the obtained photographs of the particles, and the contours of the particles were traced with a digitizer to measure the size of the particles (primary particles). Here, the primary particles refer to independent particles without agglomeration. The particle size was measured by tracing the contour of each particle with a digitizer and calculating the diameter of a circle (circle area phase diameter) having the same area as the traced area. As the image analysis software, Carl Zeiss's image analysis software KS-400 was used.
The above measurements were performed on 500 randomly sampled particles.
The arithmetic mean of the 500 particle sizes thus obtained was taken as the average particle size of the powder. The results are shown in Table 1 below.

(Evaluation of magnetic characteristics)
The magnetic properties of the magnetic powder were evaluated by the following method.
Using a vibration sample type magnetometer (TM-TRVSM5050-SMSL type, manufactured by Tamagawa Seisakusho Co., Ltd.), the maximum applied magnetic field: 4.5 kOe (kilomersted: 354 kA / m), temperature: 296 K, magnetic field sweep speed: 25 Oe. At / s (1.994 kA / m / s), the strength of the magnetization of the magnetic powder with respect to the applied magnetic field was measured. From the measurement results, a magnetic field (H) -magnetism (M) curve of the magnetic powder was obtained.

For the obtained magnetic field (H) -magnetization (M) curve, the magnetic field was calculated by differentiating the magnetization with the applied magnetic field to the second order and the value of the following equation (I) became zero, and this was defined as Hc'.
d ² M / dH ² equation (I)
However, 119 kA / m <Hc'<2380 kA / m.
The value of the magnetic field (Hc') at which the value of the formula (I) becomes zero is equal to the value of the magnetic field when the value (dM / dH) obtained by differentiating the magnetization with the applied magnetic field becomes the maximum.
Further, in the obtained magnetic field (H) -magnetization (M) curve, the value of the magnetic field at which the magnetization becomes zero is defined as Hc. Hc is a value representing the coercive force of the magnetic powder to be measured.
The ratio (Hc / Hc') of the value of the magnetic field (Hc) at which the magnetization becomes zero to the value (Hc') of the magnetic field at which the value of the equation (I) obtained here becomes zero was obtained. The magnetic characteristics of the magnetic powder were used.
A magnetic powder having a magnetic property (Hc / Hc') of 0.6 or more and 1.0 or less and an Hc' of more than 119 kA / m and less than 2380 kA / m is the magnetic powder of the present disclosure.

[Examples 2 to 6]
In the production of the magnetic powder of Example 1, the temperatures at the time of adding the PVP added when producing the precursor of the magnetic powder or adding the aqueous ammonia solution before adding the aqueous citric acid solution are shown in Table 1 below. The magnetic powders of Examples 2 to 6 were prepared in the same manner as in Example 1 except that the conditions were changed, and evaluated in the same manner as in Example 1.

[Example 7]
1 g of the magnetic powder obtained in Example 1 was added to 30 g of a 5 mass% aqueous solution of polyvinyl alcohol (PVA: average degree of polymerization 500, fully saponified type, Wako Pure Chemical Industries, Ltd.). Zirconia (Zr) beads having a diameter of 100 μm were added to the obtained liquid, and the mixture was shaken at room temperature (25 ° C.) for 6 hours with a shaker to sufficiently disperse the magnetic powder in the above-mentioned PVA aqueous solution. The obtained dispersion was treated with a centrifuge at a centrifugal force of 200,000 G (1,961,330 m / s ² ) for 45 minutes. The precipitate settled by centrifugation was washed with pure water and dried at 80 ° C. for 24 hours to obtain the magnetic powder of Example 7 and evaluated in the same manner as in Example 1.

[Example 8]
The magnetic powder of Example 8 was obtained in the same manner as in Example 7 except that the conditions for centrifugation were changed to 30 minutes with a centrifugal force of 1,961,330 m / s ² , and the evaluation was carried out in the same manner as in Example 1. did.

[Example 9]
The magnetic powder of Example 9 was obtained in the same manner as in Example 7 except that the conditions for centrifugation were changed to 20 minutes with a centrifugal force of 1,961,330 m / s ² , and the evaluation was carried out in the same manner as in Example 1. did.

[Comparative Example 1]
In 90 g of pure water, 8.3 g of iron (III) nitrate 9 hydrate, 1.3 g of gallium nitrate (III) octahydrate, 190 mg of cobalt (II) nitrate hexahydrate, and 150 mg of titanium (IV) sulfate are dissolved. While stirring the solution using a magnetic stirrer, 4.0 g of a 25% aqueous ammonia solution was added under the condition of 40 ° C. in an air atmosphere, and the mixture was stirred for 2 hours while maintaining the temperature at 40 ° C. .. Further, a solution prepared by dissolving 480 mg of citric acid in 4.5 g of water was added to the mixture, and then 5.7 g of a 10% aqueous ammonia solution was added, and the mixture was stirred for 1 hour.
The obtained dispersion was ultrafiltered with an ultrafiltration membrane (molecular weight cut off: 50,000) until the electrical conductivity of the filtrate became 50 mS / m or less.
To the liquid obtained by ultrafiltration, an amount of water having a total volume of 120 mL was added, and then 6.5 g of 25% ammonia was added while stirring at 30 ° C. Further, 16 mL of TEOS was added to the mixture, and stirring was continued for 24 hours while maintaining the temperature of 30 ° C.
Then, a solution in which 3 g of ammonium sulfate was dissolved in 4.5 mL of pure water was added. The precipitated powder was collected by centrifugation, washed with pure water, and dried at 80 ° C. for 24 hours to obtain a precursor of magnetic powder. The magnetic powder of Comparative Example 1 was obtained in the same manner as in Example 1 except that the obtained precursor of the magnetic powder was used, and the evaluation was carried out in the same manner as in Example 1.
When the composition of the obtained magnetic powder of Comparative Example 1 was confirmed in the same manner as in Example 1, the shape of the magnetic powder was spherical, and the composition was ε-Ga _0.24 Co _0.05 Ti _0.05 Fe ₁ . It was _{.66 O3} .

[Comparative Example 2]
In 90 g of pure water, 5.5 g of iron (III) chloride 9 hydrate, 1.3 g of gallium nitrate (III) octahydrate, 155 mg of cobalt (II) chloride hexahydrate, and 120 mg of titanium (IV) chloride are dissolved. While stirring the solution using a magnetic stirrer, 4.0 g of a 25% aqueous ammonia solution was added under the condition of 10 ° C. in an air atmosphere, and the mixture was stirred for 2 hours while maintaining the temperature of 10 ° C. ..
Further, a solution prepared by dissolving 480 mg of citric acid in 4.5 g of water was added, and then 5.7 g of a 10% aqueous ammonia solution was added, and the mixture was stirred for 1 hour. The obtained solution was ultrafiltered with an ultrafiltration membrane (molecular weight cut off 50,000) until the electrical conductivity of the filtrate became 50 mS / m or less. Then, the obtained slurry was hydroheat-treated at 160 ° C. for 6 hours using an autoclave.
To the slurry after the hydrothermal treatment, an amount of water having a total volume of 120 mL was added, and then 6.5 g of 25% ammonia was added while stirring at 30 ° C. Further, 16 mL of TEOS was added, and stirring was continued for 24 hours while maintaining the temperature of 30 ° C.
Then, a solution in which 3 g of ammonium sulfate was dissolved in 4.5 mL of pure water was added. The precipitated powder was collected by centrifugation, washed with pure water, and dried at 80 ° C. for 24 hours to obtain a precursor of magnetic powder. The magnetic powder of Comparative Example 2 was obtained in the same manner as in Example 1 except that the obtained precursor of the magnetic powder was used, and the evaluation was carried out in the same manner as in Example 1.
When the composition of the obtained magnetic powder of Comparative Example 2 was confirmed in the same manner as in Example 1, the shape of the magnetic powder was spherical, and the composition was ε-Ga _0.24 Co _0.05 Ti _0.05 Fe ₁ . It was _{.66 O3} .

[Preparation of magnetic recording medium]
(Prescription of magnetic liquid)
ε-Fe ₂ O ₃ powder (magnetic powder of the above-mentioned Example or Comparative Example) 100.0 parts Oleic acid 2.0 parts Vinyl chloride copolymer (Zeon Corporation, MR-104) (Table 1) Amount stated)
SO ₃ Na group-containing polyurethane resin (amount shown in Table 1)
(Weight average molecular weight 70,000, SO ₃ Na group: 0.07 meq / g)
Amine-based polymer (amount shown in Table 1)
(DISPERBYK-102 manufactured by Big Chemie)
Methyl ethyl ketone (solvent) 150.0 parts Cyclohexanone 150.0 parts

(Prescription of abrasive liquid)
6.0 parts of α-alumina (BET specific surface area 19 m ² / g, Mohs hardness 9)
SO ₃ Na group-containing polyurethane resin 0.6 parts (weight average molecular weight 70,000, SO ₃ Na group: 0.1 meq / g)
2,3-Dihydroxynaphthalene 0.6 part Cyclohexanone 23.0 part

(Prescription of non-magnetic filler liquid)
2.0 parts of colloidal silica (average particle size 120 nm)
Methyl ethyl ketone 8.0 parts

(Prescription of lubricant and curing agent liquid)
Stearic acid 3.0 parts Stearic acid amide 0.3 parts Butyl stearate 6.0 parts Methyl ethyl ketone 110.0 parts Cyclohexanone 110.0 parts Polyisocyanate (Nippon Polyurethane Coronate (registered trademark) L)
3.0 copies

(1. Preparation of composition for forming a magnetic layer)
The composition for forming a magnetic layer was prepared by the following method.
Various components described in the above magnetic liquid formulation were dispersed for 24 hours using zirconia beads (first dispersed beads, density 6.0 g / cm ³ ) having a bead diameter of 0.5 mmφ by a batch type vertical sand mill. (First step), then the dispersion A was prepared by filtering using a filter having an average pore size of 0.5 μm. The amount of zirconia beads used was 10 times the amount of the magnetic powder containing ε-Fe ₂ O ₃ on a mass basis.

Then, the obtained dispersion liquid A was dispersed for 1 hour by a batch type vertical sand mill using diamond beads (second dispersion beads, density 3.5 g / cm ³ ) having a bead diameter of 500 nmφ (second step). ), A dispersion liquid (dispersion liquid B) from which diamond beads were separated using a centrifuge was prepared and used as a magnetic liquid.

As the polishing agent liquid, various components described in the above-mentioned polishing agent liquid formulation are mixed, zirconia beads having a bead diameter of 0.3 mmφ are added, and the mixture is placed in a horizontal bead mill disperser, and the bead volume / (volume of the polishing agent liquid +) The bead volume) was adjusted to 80%, a bead mill dispersion treatment was performed for 120 minutes, the treated liquid was taken out, and an ultrasonic dispersion filtration treatment was performed using a flow-type ultrasonic dispersion filtration device. In this way, the abrasive liquid was prepared.

The prepared magnetic liquid and polishing agent liquid, as well as the above-mentioned non-magnetic filler liquid, lubricant and curing agent liquid are introduced into a dissolver stirrer, stirred at a peripheral speed of 10 m / sec for 30 minutes, and then by a flow type ultrasonic disperser. A composition for forming a magnetic layer was prepared by treating with 3 passes at a flow rate of 7.5 kg / min and filtering with a filter having a pore size of 1 μm.

(Prescription of composition for forming non-magnetic layer)
Non-magnetic inorganic powder α-iron oxide 100.0 parts (average particle size 10 nm, BET specific surface area 75 m ² / g)
Carbon black 25.0 parts (average particle size 20 nm)
18.0 parts of SO ₃ Na group-containing polyurethane resin (weight average molecular weight 70,000, SO ₃ Na group content 0.2 meq / g)
Stearic acid 1.0 part Cyclohexanone 300.0 part Methyl ethyl ketone 300.0 part

(2. Preparation of composition for forming non-magnetic layer)
Each component described in the formulation of the above non-magnetic layer forming composition was dispersed for 24 hours using zirconia beads having a bead diameter of 0.1 mmφ by a batch type vertical sand mill, and then an average pore size of 0.5 μm was obtained. A composition for forming a non-magnetic layer was prepared by filtering using a filter having the same.

(Prescription of composition for forming back coat layer)
Non-magnetic inorganic powder α-iron oxide 80.0 parts (average particle size 0.15 μm, BET specific surface area 52 m ² / g)
Carbon black 20.0 parts (average particle size 20 nm)
Vinyl chloride copolymer 13.0 parts Sulfonic acid base-containing polyurethane resin 6.0 parts Phosphonate 3.0 parts Cyclohexanone 155.0 parts Methyl ethyl ketone 155.0 parts Steaic acid 3.0 parts Butyl stearate 3.0 parts Poly 5.0 parts of isocyanate 200.0 parts of cyclohexanone

(3. Preparation of composition for forming backcoat layer)
After kneading and diluting each component described in the formulation of the above backcoat layer forming composition except for the lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone with an open kneader. Using a horizontal bead mill disperser, zirconia beads having a bead diameter of 1 mmφ were used, the bead filling rate was 80% by volume, the rotor tip peripheral speed was 10 m / sec, and the 1-pass residence time was 2 minutes, and the beads were subjected to 12-pass dispersion treatment.
Then, the remaining components described above were added and stirred with a dissolver, and the obtained dispersion was filtered using a filter having an average pore size of 1 μm to prepare a composition for forming a backcoat layer.

(4. Preparation of magnetic tape)
On the surface of a polyethylene naphthalate support (non-magnetic support) having a thickness of 5.0 μm, the composition for forming a non-magnetic layer prepared above is applied and dried in an amount such that the thickness after drying becomes 100 nm. A non-magnetic layer was formed.
The composition for forming a magnetic layer prepared above was applied onto the surface of the formed non-magnetic layer in an amount such that the thickness after drying became 70 nm to form a coated layer.
While the coated layer of the formed magnetic layer forming composition is in a wet (undried) state, a magnetic field having a magnetic field strength of 0.15 T is applied in a direction perpendicular to the surface of the coated layer of the magnetic layer forming composition. It was subjected to vertical orientation treatment.
Then, the coating layer of the composition for forming a magnetic layer subjected to the vertical alignment treatment was dried to form a magnetic layer.
On the surface of the non-magnetic support opposite to the surface on which the non-magnetic layer and the magnetic layer are formed, the composition for forming the back coat layer prepared above is applied in an amount such that the thickness after drying becomes 0.4 μm. It was applied and dried to obtain a laminate.
A pair of laminates having a non-magnetic layer and a magnetic layer on one side of the non-magnetic support and a back coat layer on the surface opposite to the side having the magnetic layer of the non-magnetic support, composed of only metal rolls. The calendar roll is subjected to calendar treatment (surface smoothing treatment) at a speed of 100 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a surface temperature of the calendar roll of 100 ° C. did.
The heat-treated laminate was slit to a width of 1/2 inch (0.0127 m) to obtain a magnetic tape as a magnetic recording medium.

<Evaluation of magnetic recording medium>
The obtained magnetic tape as a magnetic recording medium was evaluated for signal attenuation during repeated reproduction by the following method.

(Evaluation of Signal Decay)
As a Signal Decay, in this evaluation, a decrease in the reproduction output, that is, a reproduction output attenuation rate was evaluated when the magnetic recording medium on which the signal was recorded was repeatedly reproduced. The smaller the value of the reduction rate, the more the output attenuation during reproduction is suppressed.
A recording head [MIG (Metal-in-Gap), gap length 0.15 μm, track width 1.8 μm] and a GMR (Giant magnetoresistive) head for reproduction (reproduction track width 1 μm) were attached to a loop tester to form a reproduction device. ..
After recording a signal with a line recording density of 200 kfci on each of the magnetic tapes of Examples and Comparative Examples, the recorded signal is repeatedly reproduced by the above-mentioned reproduction device, and the reproduction output is attenuated with respect to the time from immediately after recording to reproduction. The rate was measured. The results are shown in Table 1 below. In addition, when the attenuation of the reproduction output was less than the detection lower limit (-0.5% / decline), it was described as "<-0.5%".

In Table 1, the "temperature of the mixed solution" indicates the temperature of the mixed solution when the aqueous ammonia solution in the above-mentioned step (I) is mixed, and the "additive" is optionally contained in the step (I). The names of the additive compounds are shown.
"Centrifugation time after post-treatment" indicates the centrifugation treatment time in solid-liquid separation after the magnetic powder of Example 1 is dispersed in an aqueous PVA solution.
In addition, PVP in Table 1 is polyvinylpyrrolidone described above, and CTAOH is hexadecyltrimethylammonium bromide.

From the evaluation results shown in Table 1, the magnetic tapes using the magnetic powders of Examples 1 to 9 in which Hc / Hc'is within the range specified in the present disclosure have a large reproduction output attenuation rate. It can be seen that the durability is good. Further, from the comparison between Example 1 and Examples 7 to 9, the magnetic powder formed by forming the magnetic powder and then redispersed by adding polyvinyl alcohol has a larger Hc / Hc'and attenuates the reproduction output. It can be seen that the rate is higher and the durability is further improved.
On the other hand, as a pretreatment for forming particles under alkaline conditions, polyvalent carboxylic acid is added, and no further treatment for centrifugal separation surface is performed. After formation of particles (precursors of magnetic powder) under alkaline conditions, impurities are subjected to ultrafiltration. The magnetic powders of Comparative Example 1 and Comparative Example 2 subjected to the treatment for removing the ions as Hc / Hc'have a small value of Hc / Hc', and the magnetic tape obtained by using these magnetic powders has a higher reproduction output attenuation rate. It is small and it can be seen that the durability is lower than that of the magnetic tape using the magnetic powder of the example.

By performing the pretreatment described above in the method for producing a magnetic powder of the present disclosure, the amorphous component is removed before the precursor of the magnetic powder is formed, and the impurities are removed after the precursor of the magnetic powder is formed. It is considered that the formation of undesired fine particles due to the amorphous component was suppressed as compared with the method of performing the treatment.

Claims (5)

It contains at least one epsilon-type iron oxide-based compound selected from the group consisting of ε-Fe ₂ O ₃ and a compound represented by the following formula (1).
It is a magnetic powder with an average particle diameter of 8 nm to 25 nm.
The content of the magnetic powder having a particle diameter of 7.5 nm or less with respect to the total magnetic powder is 5% by number or less.
The following equation (2nd-order differentiation of the magnetization M obtained from the magnetic field-magnetization curve obtained by measuring at a maximum applied magnetic field of 359 kA / m, a temperature of 296 K, and a magnetic field sweep speed of 1.994 kA / m / s with the applied magnetic field H ( When the magnetic field where the value of I) becomes zero is Hc'and the value of the magnetic field where the magnetization becomes zero in the magnetic field-magnetization curve is Hc, the value of Hc with respect to Hc'is 0.6 or more and 1.0 or less. A magnetic powder whose Hc satisfies the following formula (III).
d ² M / dH ² equation (I)
269 kA / m ≤ Hc ≤ 310 kA / m Equation (III)

In the formula (1), A represents at least one metal element other than Fe, and a satisfies 0 <a <2. The magnetic powder according to claim 1, wherein the compound represented by the formula (1) is a compound represented by the following formula (1-2).

In formula (1-2), A ¹ represents at least one trivalent metal element selected from the group consisting of Ga, Al, In, and Rh, and A ² is derived from Mn, Co, Ni, and Zn. Represents at least one divalent metal element selected from the group consisting of, and A3 represents at ^least one tetravalent metal element selected from the group consisting of Ti and Sn. x satisfies 0 <x <1, y satisfies 0 <y <1, z satisfies 0 <z <1, and x + y + z <2. The content of at least one iron oxide-based compound selected from α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ is from the group consisting of ε-Fe ₂ O ₃ and the compound represented by the above formula (1). The magnetic powder according to claim 1 or claim 2, wherein the content of at least one selected epsilon-type iron oxide-based compound is 20 parts by mass or less with respect to 100 parts by mass. The magnetic powder according to any one of claims 1 to 3, wherein the content of the magnetic powder having a particle diameter of 7.5 nm or less with respect to the total magnetic powder is 4 % by number or less. A magnetic recording medium having a non-magnetic support and a magnetic layer containing the magnetic powder according to any one of claims 1 to 4 on the non-magnetic support.