JP4599574B2 - Iron nitride magnetic powder - Google Patents

Description

  The present invention relates to an iron nitride magnetic powder suitable for constituting a magnetic layer of a high recording density medium.

  In recent years, higher recording density has been desired for magnetic recording media, and in order to achieve this, the recording wavelength has been shortened. For this reason, unless the size of the magnetic particle is extremely smaller than the length of the region for recording a short-wavelength signal, a clear magnetization transition state cannot be created, and recording becomes virtually impossible. Therefore, the magnetic powder is required to have a particle size sufficiently smaller than the recording wavelength.

Further, in order to increase the density, it is necessary to increase the resolution of the recording signal, and therefore it is important to reduce the noise of the magnetic recording medium. Noise often comes from the size of the particles, and the finer the particles, the more noise is reduced. Therefore, the magnetic powder for high recording density is required to have a sufficiently small particle size from this point.

  Further, the magnetic powder used in the magnetic recording medium corresponding to the higher density requires a higher coercive force (Hc) in order to maintain magnetism and ensure output in the high-density medium. Further, for the coercive force distribution (SFD), it is necessary for the high density recording to make the distribution as narrow and narrow as possible.

  Even if such a magnetic powder is obtained, if the magnetic layer is thick when applied in paint, the self-recording has not been a noticeable problem in the shortest recording wavelength region due to the long recording wavelength. Effects such as demagnetization loss and thickness loss due to the thickness of the magnetic layer are prominent, and a phenomenon in which sufficient resolution cannot be obtained occurs. Such a phenomenon cannot be overcome only by improving the magnetic properties with magnetic powder and improving the surface properties with medium manufacturing technology, and it is necessary to make the magnetic layer thinner. When the magnetic layer is thinned, as long as a conventional particle having a size of about 100 nm is used, there is a limit to reducing the thickness, and from this point, the particle size is required to be small.

  However, when the particle size is reduced to a certain level or more, the magnetic properties are significantly deteriorated due to thermal fluctuation, and when it is further reduced, superparamagnetism is exhibited and magnetism is not exhibited. Moreover, since the specific surface area increases as the particle size is reduced, there is a problem that the oxidation resistance deteriorates. Therefore, as a magnetic powder suitable for high-density recording media, thermal stability that can withstand this superparamagnetism even when micronized, that is, a large anisotropy constant, high Hc, high σs, low SFD, and oxidation resistance is realized. What you can do is needed, and the powder must be fine enough to be applied very thinly. However, magnetic materials having such characteristics have not yet been put into practical use.

Patent Document 1, high coercive force Hc, describes a large iron nitride-based magnetic material having a specific surface area as magnetic expressing high saturation magnetization [sigma] s, Fe ₁₆ N ₂ phase crystal magnetic anisotropy and powder of As a synergistic effect of increasing the specific surface area, it teaches that high magnetic properties can be obtained regardless of the shape.

Patent Document 2 describes a magnetic powder in which 10 to 90% of Fe ₁₆ N ₂ is generated as a magnetic material that expresses high saturation magnetization σ s at low cost, and the generation ratio of Fe ₁₆ N ₂ phase is particularly 60. %, The saturation magnetization is maximized.

Patent Document 3 proposes an essentially spherical or elliptical rare earth-iron-boron, rare earth-iron, rare earth-iron nitride magnetic powder as an improved magnetic powder from Patent Document 1, Excellent properties can be obtained when a tape medium is produced by using a rare earth-iron nitride magnetic powder mainly composed of Fe ₁₆ N ₂ phase, although it is a fine particle of about 20 nm. It is described that the magnetic force is as high as 200 KA / m or more, and since the BET specific surface area is small, the saturation magnetization is also high and the storage stability is good. By using this rare earth-iron nitride magnetic powder, a coating type magnetic recording medium is used. It is described that the recording density can be dramatically increased. The rare earth-iron nitride magnetic powder is produced by an ammonia nitriding method in which magnetite particles with a rare earth deposited on the surface are reduced to rare earth-iron magnetic powder and then nitriding with ammonia gas. is there. In Patent Document 3, it is said that a part of iron in the rare earth-iron nitride magnetic powder can be replaced with other transition metal elements. However, if a large amount of cobalt is added, it takes a long time for the nitriding reaction. Has been.

Patent Document 4 describes a basic invention for obtaining an iron nitride magnetic powder composed of an Fe ₁₆ N ₂ phase by a low temperature nitriding method using ammonia gas.
JP 2000-277311 A JP 2001-176715 A International Publication WO 03/079332 A1 Publication Japanese Patent Laid-Open No. 11-340023

In Patent Documents 1 to 3, when to generate a Fe ₁₆ N ₂ phase that has a large crystal magnetic anisotropy, but good magnetic powder magnetic properties of fine particles is described to be obtained, the Fe ₁₆ N ₂ phase It is not described that a higher ratio is necessarily better. For example, Patent Document 2 describes that the highest σs can be obtained when about 60% of Fe ₁₆ N ₂ phase is formed. In Patent Document 3, the ratio of the Fe ₁₆ N ₂ phase and the α-Fe phase in the inner layer (core portion) of the magnetic powder need not be Fe ₁₆ N ₂ , and may be a mixed phase with α-Fe. It is stated that it can be easily set to obtain a desired coercive force by adjusting nitriding conditions. However, these are not significant problems when considering only the coercive force and the saturation magnetization, but the coercive force distribution (BSFD) of the powder and the coercive force distribution (SFDx) in the tape orientation direction (x direction). From the point of view, the coercive force differs greatly between the Fe ₁₆ N ₂ phase, the α-Fe phase, and the Fe ₄ N phase (which contains more nitrogen than the Fe ₁₆ N ₂ phase). It has a wide distribution with two or three local maximum points.

The wide coercive force distribution of magnetic powder means that particles with high or low Hc are mixed. Therefore, when such magnetic powder is coated and taped to form a high-density recording medium, Noise is likely to occur. In addition, if a low Hc component is present, there is a problem in terms of reliability because the particles may not retain magnetization due to thermal fluctuations and the recording may be erased. Therefore, the magnetic powder for the high recording density magnetic medium is preferably a magnetic powder that is essentially an Fe ₁₆ N ₂ phase and does not contain a mixed phase such as α-Fe phase or Fe ₄ N. Patent Document 4 describes a method of producing Fe ₁₆ N ₂ phase particles by a low temperature nitriding method, but there is no teaching regarding the crystal state, coercive force Hc, and coercive force distribution.

Regarding the oxidation resistance of the iron nitride-based magnetic powder composed of Fe ₁₆ N ₂ phase, for example, rare earth-iron-based magnetic powder described in Patent Document 3, for example, Fe ₁₆ N ₂ phase containing 5.3 atomic% Y and α A magnetic powder composed of a mixed phase of -Fe and having an average particle diameter of 20 nm and having Δσs of 12.6% is obtained. However, since the specific surface area is remarkably increased when finer particles are formed, further improvement in oxidation resistance is expected. As described above, a method for improving the oxidation resistance of the fine particle powder while achieving high Hc, high σs, and low SFD is effective at present in Fe nitride magnetic powder composed of Fe ₁₆ N ₂ phase. The reality is that no means are available.

Accordingly, an object of the present invention is to obtain an Fe ₁₆ N ₂ -based iron nitride powder capable of satisfying both excellent magnetic properties that can be used for a high recording density magnetic medium, particularly high Hc, low BSFD, and oxidation resistance. Is.

In order to solve the above-mentioned problems, the present inventors have made various tests and researches. The particles containing Fe ₁₆ N ₂ phase as the main phase by reacting Fe particles with a nitrogen-containing gas typified by ammonia gas. When the nitriding reaction is allowed to proceed under a pressure of 0.1 MPa or more, preferably 0.3 MPa or more when nitriding is performed, iron nitride-based magnetic powder consisting essentially of Fe ₁₆ N ₂ phase can be stably produced. I found out. That is, the production of a foreign phase such as α-Fe or Fe ₄ N is suppressed by the nitriding reaction under pressure, and a powder substantially consisting of an Fe ₁₆ N ₂ phase can be produced. Therefore, according to the present invention, the magnetic powder is composed of iron nitride particles mainly composed of Fe ₁₆ N ₂ phase, wherein the coercive force Hc is 200 KA / m or more and the powder coercive force distribution BSFD is 2 or less. An iron nitride magnetic powder is provided. This iron nitride-based magnetic powder is preferably composed of iron nitride particles containing Fe ₁₆ N ₂ phase in an XRD peak integration ratio of 80% or more, and the ratio of Hc 120 KA / m or less in the powder coercive force distribution is 15% or less. . The average particle size is preferably 50 nm or less. In addition, the iron nitride magnetic powder of the present invention comprises 5 to 30 at.% Al as an atomic percentage relative to Fe and 0.5 to 10 at.% Rare earth element (as an atomic percentage relative to Fe) as sintering inhibitors. Y in combination) or 1 to 10 at.% Si in atomic percent relative to Fe and 0.5 to 10 at.% Rare earth element inclusive relative to Fe (including Y) In combination.

Since the magnetic powder of the present invention is a fine powder of high Hc and low SFD consisting essentially of Fe ₁₆ N ₂ phase, it can be applied to a coating type magnetic recording medium to increase the recording density of the magnetic recording medium. Can be achieved. Therefore, it can contribute to the data storage field that backs up the amount of information expected to increase in the future.

The present inventors have continued research to apply this kind of iron nitride magnetic powder to a high recording density magnetic recording medium even after the proposal of a method for producing Fe ₁₆ N ₂ particles by a low temperature nitriding method in Patent Document 4. However, when nitriding the Fe particles, the nitriding reaction is allowed to proceed under a pressure condition of 0.1 MPa or more, preferably 0.3 MPa or more, compared with the case where no pressure is applied. The coercive force Hc was improved, and it was found that a high coercive force powder having Hc of 120 KA / m or more and a powder coercivity distribution BSFD of 2 or less was obtained. When nitriding is performed under pressure, Fe _16N2 single-phase particles with good crystallinity are likely to be formed. That is, this pressurization can suppress the generation of a different phase than before, and make it easier to generate the Fe ₁₆ N ₂ phase. Further, when nitriding is performed under pressure, it is found that even when a transition metal element such as Co or Ni is added, the nitriding reaction proceeds without requiring a long time for nitriding, and further Fe ₁₆ containing Co or Ni. It was found that the N ₂ phase-based magnetic powder showed good oxidation resistance. As described in Patent Document 4, the reaction temperature of nitridation is not so high and may be 200 ° C. or less.

In non-pressurized nitriding, the α-Fe phase remains when the temperature is low, and the Fe ₄ N phase is likely to be formed when the temperature is high. Therefore, the temperature for obtaining Fe ₁₆ N ₂ single phase particles It is difficult to control, and for this reason, it is difficult to produce a powder that has both high Hc and low BSFD. According to the present invention, an iron nitride magnetic powder having both high Hc and low BSFD can be obtained by setting the pressure during nitriding to 0.1 MPa or more, preferably 0.3 MPa or more. In order for the obtained magnetic powder mainly composed of Fe ₁₆ N ₂ phase to have Hc 120 KA / m or more and BSFD 2 or less, Fe ₁₆ N ₂ phase has an XRD peak integration ratio measured by X-ray diffraction method of 80% or more. , Preferably 90% or more.

Fe (α-Fe) particles may be used as a raw material for nitriding under pressure, but these Fe particle powders are goethite, hematite, magnetite, wustite, α-Fe, carbonyl iron, iron acetylacetonate. An Fe particle powder obtained by reducing or decomposing etc. may be used. The particle shape is not particularly limited and may be any shape such as a needle shape, a spindle shape, a spherical shape, and an elliptical shape, but the average particle size is preferably 50 nm or less, and more preferably 20 nm or less. By using Fe particles of 50 nm or less, a magnetic powder of Fe ₁₆ N ₂ single phase particles of 50 nm or less can be obtained, and this is a magnetic material suitable for short wavelength recording. Further, when the fine particles are 50 nm or less, a high recording density magnetic medium with good surface smoothness and less noise can be obtained.

  In this starting material, if it is an amount that does not significantly inhibit nitriding, a sintering inhibitor for preventing sintering may be dissolved in the particles or may be deposited on the surface. Antisintering agents include those containing components such as rare earth elements including Al, Si, Cr, V, Mn, Mo, Ni, P, B, Zr, and Y. Is suitable for Al, which is easy to dissolve, and Si, which is easy to dissolve and deposit, is suitable for magnetite. Al and Si also have the effect of promoting nitriding, so adding them increases the effect of improving magnetic properties and productivity, and is more desirable.

  In particular, as a sintering inhibitor, it is preferable to use a combination of Al and rare earth elements (including Y) or a combination of Si and rare earth elements (including Y). In this case, for Al, the atomic percentage with respect to Fe is in the range of 5 to 30 at.%, And for Si, the atomic percentage with respect to Fe is in the range of 1 to 10 at.%. It may be combined with rare earth elements (including Y) in the range of 10 at.%. When the content of these sintering inhibitors is less than the above lower limit value, the sintering preventing effect is difficult to appear, and when the content is more than the above upper limit value, the ratio of non-magnetic components increases and the saturation magnetization σ s increases. Since it falls, it is not preferable.

In addition, one or two kinds of Co or Ni can be added to the starting material to improve the oxidation resistance of the iron nitride magnetic powder composed of the Fe ₁₆ N ₂ phase. The total atomic percentage is preferably 0.1 to 30 at.%, More preferably 10 to 30 at.%. When the total addition amount is less than 0.1 at.%, The effect of improving the oxidation resistance does not appear, and when it exceeds 30 at.%, The nitriding reaction takes a long time, which is not preferable. It is preferable to add one or two kinds of Co or Ni in a total amount of 0.1 to 30 at.%, Preferably 10 to 30 at.% In terms of oxidation resistance and nitriding reaction. According to the present invention, an iron nitride-based magnetic powder having an oxidation resistance index Δσs of 15% or less can be obtained.

Regarding the reduction treatment for obtaining α-Fe, any reducing agent may be used as long as the raw material powder becomes α-Fe by decomposition or reduction. In the case of reduction with hydrogen, hydrogen H ₂ is used. At this time, if the reduction is insufficient and oxygen remains, it is not preferable because the speed of nitriding is remarkably slowed. If the temperature at the time of reduction is too high, sintering between particles occurs, leading to an increase in average particle diameter and deterioration of dispersibility. Therefore, it is preferable to reduce at a temperature of 500 ° C. or lower, for example, 300 to 500 ° C.

  When nitriding the Fe particle powder according to the present invention, a nitrogen-containing gas typified by ammonia gas is used, and the pressure is 200 ° C. or lower under a pressure of 0.1 MPa or more, preferably 0.3 MPa or more. A nitriding treatment is preferably performed at a temperature for several hours to several tens of hours. This pressurization method is not particularly limited, and the furnace is constructed by flowing ammonia gas or a mixed gas containing ammonia using a furnace that can withstand pressurization and controlling the pressure on the upstream side and the downstream side. A method of adjusting the internal pressure is convenient. It is desirable that the amount of oxygen in the gas flowing into the furnace is several ppm or less.

As described above, according to the present invention, an iron nitride-based magnetic powder substantially consisting of an Fe ₁₆ N ₂ phase can be obtained, and this magnetic powder is suitable as a magnetic material for a high-density magnetic recording medium. That is, the average particle size can be 50 nm or less, Hc120 KA / m or more and BSFD can be 2 or less, and the ratio of Hc120 KA / m or less can be substantially 15% or less in the powder coercive force distribution. _Since a magnetic powder composed of ₁₆ N ₂ phase can be obtained, it can be a magnetic material for high-density magnetic recording media without the problem of thermal fluctuation and noise generation when taped. Further, those containing one or two kinds of Co or Ni are excellent in oxidation resistance and have high practical value.

  If the BSFD exceeds 2, that is, if the Hc distribution is wide and the ratio of the high Hc component and the low Hc component increases, it causes noise when taped, and the low Hc magnetic powder is caused by thermal fluctuations. The BSFD is preferably 2 or less because there is a risk that the record will be erased and a problem arises in reliability.

  Examples of the iron nitride magnetic powder according to the present invention will be described below. First, a test method for evaluating the characteristics will be described. Not only the examples but also the characteristic values described in the present specification are those evaluated according to the following test methods.

[Evaluation method of powder characteristics]
・ Measurement of particle size: Prepare multiple photographs with magnification of 100,000 times or more with a transmission electron microscope, and measure the longest part of each particle for 400 or more particles. Is used.
Measurement of magnetic properties of powder: Measure with an externally applied magnetic field of 796 KA / m at maximum using VSM (manufactured by Digital Measurement Systems).
・ Measurement of powder coercive force distribution (BSFD): Using the above-mentioned VSM, first, an external magnetic field of 796 KA / m is applied in one direction (this is the positive direction) and then 7.96 KA up to an external magnetic field of 0. The hysteresis curve is created by applying the voltage in increments of 7.m / m and then applied in the reverse direction (negative direction) every 7.96 KA / m, and the differential curve (coercivity distribution curve) of the hysteresis curve drawn in the negative direction is created. The half-width for the peak is BHa, and BSFD is calculated by the following formula.
BSFD = BHa / Hc
Calculation of the ratio of Hc120KA / m or less in the powder coercive force distribution: 120KA when the area obtained by integrating the above coercive force distribution curve in the range of 0 to −796 KA / m is defined as 100% of the entire coercive force distribution. It is the ratio of the area integrated in the range of / m or less. That is, it is calculated by the following formula.
Ratio of 120 KA / m or less = 100 × (area of 120 KA / m or less of coercive force distribution curve) / (area of entire coercive force distribution curve)
Measurement of oxidation resistance (Δσs) of magnetic powder: After storage for 1 week at 60 ° C. and 90% RH in a constant temperature and humidity chamber, the change% of the saturation magnetization value σs before and after storage is calculated according to the following formula.
100 × (saturation magnetization value before storage−saturation magnetization value after storage) / saturation magnetization value before storage / specific surface area measurement: measured by BET method.

[Evaluation method of tape properties]
(1) Preparation of magnetic paint Weigh 0.500g of magnetic powder and put it into a pot (inner diameter 45mm, depth 13mm). Leave for 10 minutes with the lid open. Next, vehicle [PVC resin MR-110 (22wt%), cyclohexanone (38.7wt%), acetylacetone (0.3wt%), n-butyl stearate (0.3wt%), methyl ethyl ketone (MEK, 38.7wt%) ] Is collected with a micropipette and added to the pot. Immediately add 30 g of steel balls (2φ) and 10 nylon balls (8φ) to the pot, close the lid and let stand for 10 minutes. After that, set this pot on a centrifugal ball mill (FRITSCH P-6), slowly increase the rotation speed, adjust to 600 rpm, and disperse for 60 minutes. After the centrifugal ball mill has stopped, remove the pot and use a micropipette to add 1.800 ml of a pre-mixed MEK and toluene mixture at 1: 1. Set the pot on the centrifugal ball mill again, disperse for 5 minutes at 600 rpm, and complete the dispersion.

(2) Preparation of magnetic tape After the above dispersion was completed, the pot lid was opened, the nylon ball was removed, the paint was put together with the steel ball into the applicator (55 μm), and the support film (polyethylene film manufactured by Toray Industries, Inc .: Product name 15C-B500: film thickness 15 μm). Immediately after coating, place it in the center of the coil of 5.5 kG aligner, orient the magnetic field, and then dry it.

(3) Tape property evaluation test and magnetic property measurement: The coercive force Hcx, SFDx and SQx are measured with an externally applied magnetic field of 796 KA / m at maximum using the obtained tape.
・ Measurement of oxidation resistance (ΔBm) of tape: After storage for 1 week at 60 ° C. and 90% RH in a thermo-hygrostat, the% change in Bm before and after storage is calculated.

[Example 1]
Starting powder made of magnetite particles having an Si and Y oxide layer on the surface and an average particle size of 27 nm (Si and Y contents are 4.7 atomic% and 1.0 atomic%, respectively, in terms of atomic percentage relative to Fe) It was. This powder was charged into the furnace, heated up, and reduced by flowing hydrogen gas at 500 ° C for 1 hour, then cooled to 100 ° C. At that stage, the flow gas was switched from hydrogen gas to ammonia gas, When the temperature was raised again and reached 165 ° C., the pressure in the furnace was controlled by adjusting the exhaust gas outlet pressure, and the nitriding treatment was performed for 11 hours under this condition. .

  After the nitriding treatment, the exhaust gas outlet pressure is returned to atmospheric pressure and the pressurization is stopped. Then, the exhaust gas is cooled to 80 ° C. and switched to nitrogen gas, and this nitrogen gas has an oxygen concentration of 0.01 to 2 vol.%. After adding a small amount of air, the surface of the powder to be treated was gradually oxidized, and the powder was taken out into the atmosphere.

The average particle size of the obtained powder was 25 nm, and the specific surface area by the BET method was 43 m ² / g. As a result of the evaluation of the magnetic properties, Hc = 224 KA / m, σs = 111 Am ² / Kg, BSFD = 1.41, the oxidation resistance Δσs of the powder was 19.8%, and the Hc120KA / The ratio of m or less was 11.6%. FIG. 1 shows hysteresis curves and differential curves measured for the magnetic powder obtained in this example.

  As a result of the characteristic evaluation of the tape prepared using the magnetic powder obtained in this example, Hcx = 251KA / m, SFDx = 0.66, SQx = 0.73, and the oxidation resistance ΔBm of the tape is 9. It was 8%.

[Example 2]
Example 1 was repeated except that the pressure inside the furnace was controlled under a pressure of 0.3 MPa by adjusting the outlet pressure of the exhaust gas.

The average particle diameter of the obtained powder was 25 nm, and the specific surface area by the BET method was 44 m ² / g. As a result of the magnetic property evaluation, Hc = 239 KA / m, σs = 97 Am ² / Kg, BSFD = 1.31, the oxidation resistance Δσs of the powder is 23.7%, and Hc120KA / in the coercive force distribution of the powder. The ratio of m or less was 9.3%. As a result of tape characteristic evaluation, Hcx = 265 KA / m, SFDx = 0.56, SQx = 0.75, and the oxidation resistance ΔBm of the tape was 11.8%.

Example 3
Example 1 was used except that goethite with an average particle size of 20 nm containing 9.4 at.% And 1.9 at.% Of atomic percentages of Fe and Al as the sintering inhibitor was used as a starting material. Repeated.

The average particle diameter of the obtained powder was 15 nm, and the specific surface area by BET method was 69 m ² / g. The results of magnetic property evaluation are Hc = 214 KA / m, σs = 67 Am ² / Kg, BSFD = 1.77, the oxidation resistance Δσs of the powder is 35.3%, and the Hc120 KA / m in the powder coercive force distribution. The following proportion was 13.2%. As a result of tape characteristic evaluation, Hcx = 233KA / m, SFDx = 0.71, SQx = 0.70, and the oxidation resistance ΔBm of the tape was 16.8%.

Example 4
As a starting material, a goethite with an average particle diameter of 20 nm containing Co at an atomic percentage of Fe of Fe of 3.0 nm (provided Al = 9.1 at.%, Y = 1.0 at.% As a sintering inhibitor) Example 1 was repeated except that was used.

The average particle diameter of the obtained powder was 15 nm, and the specific surface area by the BET method was 66 m ² / g. As a result of the magnetic property evaluation, Hc = 210 KA / m, σs = 71 Am ² / Kg, BSFD = 1.80, the oxidation resistance Δσs of the powder is 14.5%, and Hc120KA / m in the powder coercive force distribution. The following ratio was 13.5%. As a result of tape characteristic evaluation, Hcx = 228KA / m, SFDx = 0.73, SQx = 0.70, and the oxidation resistance ΔBm of the tape was 8.0%.

Example 5
As a starting material, a goethite with an average particle size of 25 nm containing Co at an atomic percentage of Fe of 20 at.% (But containing Al = 9.1 at.%, Y = 1.0 at.% As a sintering inhibitor) Example 1 was repeated except that was used.

The average particle diameter of the obtained powder was 21 nm, and the specific surface area by the BET method was 55 m ² / g. As a result of magnetic property evaluation, Hc = 221 KA / m, σ s = 104 Am ² / Kg, BSFD = 1.52, the oxidation resistance Δσ s of the powder is 8.7%, and Hc 120 KA / m in the coercive force distribution of the powder. The following proportion was 11.8%. The results of tape characteristic evaluation were Hcx = 244KA / m, SFDx = 0.68, SQx = 0.71, and the oxidation resistance ΔBm of the tape was 4.6%.

Example 6
As a starting material, a goethite with an average particle diameter of 25 nm containing 10 atomic% of Ni with respect to Fe (however, Al = 9.1 at.% And Y = 1.0 at.% Are included as sintering inhibitors) Example 1 was repeated except that was used.

The average particle diameter of the obtained powder was 20 nm, and the specific surface area by BET method was 57 m ² / g. As a result of magnetic property evaluation, Hc = 218 KA / m, σ s = 102 Am ² / Kg, BSFD = 1.62, the oxidation resistance Δσ s of the powder is 9.0%, and Hc 120 KA / m in the powder coercive force distribution. The following proportion was 12.1%. The tape characteristic evaluation results were Hcx = 230 KA / m, SFDx = 0.69, SQx = 0.71, and the oxidation resistance ΔBm of the tape was 4.9%.

[Comparative Example 1]
Example 1 was repeated except that the inside of the furnace was not pressurized by releasing the exhaust gas outlet pressure to atmospheric pressure (furnace pressure = 0.01 MPa or less).

The average particle diameter of the obtained powder was 25 nm, and the specific surface area by the BET method was 45 m ² / g. The results of magnetic property evaluation are Hc = 158 KA / m, σs = 117 Am ² / Kg, BSFD = 2.71, oxidation resistance Δσs is 25.2%, and Hc120 KA / m or less in the powder coercive force distribution. The proportion was 21.1%. FIG. 2 shows hysteresis curves and differential curves measured for the magnetic powder obtained in this example. As a result of tape characteristic evaluation, Hcx = 172KA / m, SFDx = 1.65, SQx = 0.63, and the oxidation resistance ΔBm of the tape was 13.0%.

  Table 1 shows the manufacturing conditions in each of the above examples, the bulk characteristics and magnetic characteristics of the obtained magnetic powder, and the tape characteristics.

As is clear from these examples, when nitriding was performed under pressure as in Examples 1 and 2, the magnetic properties were the same even though the average particle size and specific surface area were the same as in Comparative Example 1 under no pressure. In particular, the coercive force Hc is increased to 120 KA / m or more, and the coercive force distribution BSFD of the powder is also 2 or less. This feature also appears in the tape characteristics, which is high Hcx and low SFDx. Of particular note is that the ratio of the low Hc component in Examples 1 and 2 is significantly lower than that in Comparative Example 1. This embodiment 1-2 in Fe ₁₆ N ₂ is less heterogeneous phases other than phase, shows that the powder consisting of substantially Fe _16N2 phase was obtained. Further, it can be seen that the powders containing appropriate amounts of Co and Ni as in Examples 4 to 5 have a reduced Δσs of the powder, and accordingly, the ΔBm of the tape is also lowered, and the oxidation resistance is improved.

Example 7
The iron nitride magnetic powder obtained in Example 1 was subjected to a production test of a magnetic tape having a multilayer structure of a magnetic layer and a nonmagnetic layer, and an electromagnetic conversion measurement and storage stability evaluation were performed. For the production of the magnetic coating material, the following materials were blended in a proportion of the following composition with respect to 100 parts by weight of the iron nitride magnetic powder. Moreover, in preparation of a nonmagnetic coating material, the following materials were blended at a ratio such that the following composition was added to 85 parts by weight of the nonmagnetic powder. All the mixtures were kneaded and dispersed using a kneader and a sand grinder. The obtained coating liquid for forming a magnetic layer and the coating liquid for forming a nonmagnetic layer (lower layer) are each formed on a base film made of an aramid support and have a target layer thickness of 2.0 μm and a magnetic layer thickness of 0.20 μm. While the magnetic layer was in a wet state, it was oriented by applying a magnetic field, dried and calendered to produce a multi-layered magnetic tape.

[Composition of magnetic paint]
Iron nitride magnetic powder 100 parts carbon black 5 parts alumina 3 parts vinyl chloride resin (MR110) 15 parts polyurethane resin (UR8200) 15 parts by weight stearic acid 1 part by weight acetylacetone 1 part by weight methyl ethyl ketone 190 parts by weight cyclohexanone 80 parts by weight 110 parts by weight of toluene

[Composition of non-magnetic paint]
Nonmagnetic powder α-Fe ₂ O ₃ 85 parts by weight Carbon black 20 parts by weight Alumina 3 parts by weight Vinyl chloride resin (MR110) 15 parts by weight Polyurethane resin (UR8200) 15 parts by weight Methyl ethyl ketone 190 parts by weight Cyclohexanone 80 parts by weight Toluene 110 parts by weight

  The magnetic characteristics and electromagnetic conversion characteristics (C / N, output) of the obtained magnetic tape were measured. Of these, the C / N ratio was obtained by attaching a recording head to a drum tester and recording a digital signal at a recording wavelength of 0.35 μm. At that time, the reproduction signal was measured using an MR head, and the noise was measured as modulation noise. In the evaluation, the output when the iron nitride magnetic powder obtained in Comparative Example 1 was used, and C / N was displayed as 0 dB. The evaluation results are shown in Table 2.

[Examples 8 to 12]
Example 7 was repeated except that the iron nitride magnetic powder obtained in Examples 2 to 6 was used, and the same evaluation as in Example 7 was performed. The results are shown in Table 2.

[Comparative Example 2]
Example 7 was repeated except that the iron nitride magnetic powder obtained in Comparative Example 1 was used, and the same evaluation as in Example 7 was performed. The results are also shown in Table 2.

  From the results shown in Table 2, when Comparative Example 2 is compared, Examples 7-12 multilayer tapes using the iron nitride-based magnetic powders of Examples 1-6 have higher output, noise, and C than those of Comparative Example 2. / N is improved, indicating that a good magnetic recording medium was obtained.

It is a hysteresis curve and a differential curve obtained by measuring the iron nitride magnetic powder (of Example 1) according to the present invention. It is a hysteresis curve and a differential curve obtained by measuring the iron nitride-based magnetic powder of Comparative Example (Comparative Example 1).

Claims (7)

A magnetic powder mainly composed of Fe ₁₆ N ₂ phase having an average particle diameter of 20 nm or less obtained by nitriding under a pressure of 0.3 MPa or more, and having a coercive force Hc of 200 KA / m or more and a powder coercive force distribution BSFD Is an iron nitride-based magnetic powder, wherein   2. The iron nitride magnetic powder according to claim 1, wherein the proportion of Hc120KA / m or less in the powder coercivity distribution is 15% or less.   One or two kinds of Co or Ni are added in a total percentage of 0.1 to 30 at. The iron nitride magnetic powder according to claim 1 or 2, comprising: The iron nitride-based magnetic powder according to claim 3, wherein Δσs, which is an oxidation resistance index, is 15% or less. However, Δσs is% calculated according to the following equation from the saturation magnetization value σs before and after storage after the sample was stored in a constant temperature and humidity chamber at 60 ° C. and 90% RH for one week.
100 × (saturation magnetization value before storage−saturation magnetization value after storage) / saturation magnetization value before storage The iron nitride based magnetic powder according to any one of claims 1 to 4, which contains Fe ₁₆ N ₂ phase in an XRD peak integral ratio of 80% or more.   As a sintering inhibitor, an atomic percentage of 5 to 30 at. % Al and atomic percentage with respect to Fe, 0.5 to 10 at. 6. The iron nitride-based magnetic powder according to claim 1, wherein the iron nitride-based magnetic powder contains a rare earth element (including Y) in combination.   As a sintering inhibitor, 1 to 10 at. % Si and atomic percentage with respect to Fe, 0.5 to 10 at. 6. The iron nitride-based magnetic powder according to claim 1, wherein the iron nitride-based magnetic powder contains a rare earth element (including Y) in combination.

Images