JP6398911B2 - Permanent magnet and method for manufacturing the same - Google Patents

Description

  The present invention relates to a permanent magnet capable of improving coercive force and a method for manufacturing the permanent magnet.

  Although rare earth magnets using Nd and the like exhibit very excellent magnetic properties, there are resource risks because they use rare and expensive rare earth elements (particularly Dy). On the other hand, ferrite magnets are composed of abundant Fe oxides, so there is almost no resource risk, but magnetic properties (particularly coercive force) are insufficient.

  From such a viewpoint, various permanent magnets as alternatives to them have been studied, and one of them is a MnAl-based magnet. There is a description related to this in the following literature, for example.

JP 2001-217108 A

Applied Physical Letter, 86, 122509 (2005) .: SmCo nanocomposite magnet Journal of Alloys and compounds, 434-435, 611-613 (2007).

  Patent Document 1 proposes a magnet composition composed of Mn—B—Al, which has improved magnetic properties by mixing both phases of Mn—B phase and Mn—Al phase.

  Non-Patent Document 1 proposes an SmCo-based nanocomposite magnet as one of magnets in which a hard magnetic phase with high coercivity and a soft magnetic phase with high saturation magnetization are combined. However, there has been no report example that the coercive force has been actually improved by the composite of such different phases. As described above, improvement of the characteristics of the permanent magnet (particularly improvement of the coercive force) has been studied by making it into a composite (structure). However, there are still few practically effective proposals.

Non-Patent Document 2 has a description of a MnAl-based permanent magnet although it is not related to the composition of the permanent magnet structure. Specifically, MnAl based permanent magnet the main phase become L1 ₀ type MnAl phase (main phase) is the phase transformation to the non-magnetic phase of _Al 8 Mn ₅ phase (gamma-phase) and Mn phase (beta phase) In order to suppress this, it has been proposed to add a small amount of C. As can be seen from the description in Non-Patent Document 2, the Al ₈ Mn ₅ phase (γ phase) has been considered to be a non-magnetic material (a substance that exhibits neither ferromagnetism nor ferrimagnetism).

  The present invention has been made in view of such circumstances. That is, an object of the present invention is to provide a permanent magnet capable of improving magnetic characteristics (particularly, coercive force) by combining different from the conventional one and a manufacturing method thereof.

As a result of extensive research and trial and error, the present inventor has studied Al ₈ Mn ₅ phase (γ phase), which has been considered as a nonmagnetic material, but actually has a low magnetization level. However, it was discovered that the magnetic material exhibits ferromagnetism or ferrimagnetism. Then, by combining this Al ₈ Mn ₅ phase with a ferromagnetic material, the crystal magnetic anisotropy energy of the ferromagnetic material was increased, and the coercive force of the permanent magnet was actually greatly improved. By developing this result, the present invention described below has been completed.

"permanent magnet"
(1) the permanent magnet of the present invention, first of different _Al 8 Mn ₅ grains a first phase consisting of L1 ₀ type MnAl grains tetragonal of Mn and Al, the crystal grains of the first phase and two phases and are mixed one phase and said second phase said are both detected by X-ray diffractometry, the coercive force, characterized in der Rukoto least 13 kOe.

(2) The permanent magnet of the present invention is a conventional permanent magnet composed of a composite phase in which a first phase, which is a so-called magnetic material, and a second phase composed of Al ₈ Mn ₅ crystal grains are mixed, and at least only the first phase. It exhibits a higher coercive force than (a permanent magnet not including Al ₈ Mn ₅ crystal grains). Therefore, according to the present invention, in addition to improving the magnetic characteristics of conventional permanent magnets (rare earth magnets, ferrite magnets, etc.), it may be possible to propose new permanent magnets that can replace them.

  The reason why the coercive force can be greatly improved by the permanent magnet of the present invention is considered as follows. First, the coercive force of a permanent magnet usually depends strongly on the magnetocrystalline anisotropy energy (simply referred to as “magnetic anisotropy”) and saturation magnetization of the material (magnet alloy, etc.) constituting the permanent magnet. Magnetic anisotropy and saturation magnetization are inherently difficult to control physical values determined by the crystal structure and constituent elements. Therefore, it has been considered so far that it is very difficult to greatly improve the coercive force of existing permanent magnets.

However, although the mechanism is not necessarily clear, the second phase composed of Al ₈ Mn ₅ crystal grains that has not been noticed so far (also referred to as simply “Al ₈ Mn ₅ phase” as appropriate) is made of a magnetic material. It has been newly found that it acts strongly on one phase (also referred to simply as “magnetic phase” as appropriate) and induces induced magnetic anisotropy in the first phase in addition to the magnetic anisotropy inherent in the first phase. It was. As a result, a structure in which the magnetic phase and the Al ₈ Mn ₅ phase are mixed, coexisting, or composited (appropriately simply referred to as “composite phase”) exhibits a large magnetic anisotropy, which in turn increases the coercivity of the permanent magnet. It is thought that a significant improvement has become possible. Since the Al ₈ Mn ₅ phase causes induced magnetic anisotropy in the magnetic phase, it is appropriate to consider the Al ₈ Mn ₅ phase as a magnetic material rather than a non-magnetic material that has been recognized so far. It is.

<< Permanent Magnet Manufacturing Method >>
The present invention can be grasped not only as a permanent magnet but also as a manufacturing method thereof. For example, in the case of permanent magnet consisting MnAl alloy, the present invention is an alloy consisting of Mn and Al (appropriately referred. "MnAl alloy") and the preparation step of preparing a, L1 ₀ type from alloy MnAl It can also be grasped as a manufacturing method characterized by comprising a crystal grain and a production step for producing Al ₈ Mn ₅ crystal grain, and obtaining a MnAl-based permanent magnet.

<Others>
Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in various numerical values or numerical ranges described in the present specification can be newly established as a range such as “ab” as a new lower limit value or upper limit value.

It is a figure which shows the magnetization curve which concerns on sample A0 and sample A1. It is a figure which shows the magnetic torque curve concerning the sample 12 and the sample C0. It is a figure which shows the rotation hysteresis loss concerning the sample 12 and the sample C0. 3 is a diagram showing a magnetization curve related to a sample 14. It is a figure explaining the relationship between the magnetization curve of the 1st phase and the composite phase of the 1st phase and the 2nd phase, and the magnetization curve of the whole sample.

  The contents described in this specification can be applied not only to the permanent magnet of the present invention but also to the manufacturing method thereof. One or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. Constituent elements related to the manufacturing method are fixed (when it is impossible or impractical (impossible / unpractical circumstances), etc.) to directly identify the “product” by structure or characteristics It can also be a component related to “things” as a product-by-process. Note that which embodiment is the best depends on the target, required performance, and the like.

《First Phase》
The first phase is a magnetic phase composed of crystal grains that exhibit ferromagnetism or ferrimagnetism, and is usually the main phase. The type of the first phase is not limited as long as the coercive force of the permanent magnet mainly composed of the first phase is improved by the induced magnetic anisotropy due to the second phase (Al ₈ Mn ₅ phase). For example, the first phase, second phase and even better (tetragonal comprising Mn and Al) same type of MnAl system composed of an alloy L1 ₀ type MnAl grain. In addition, the first phase may be composed of various ferrite (iron oxide) or rare earth magnet alloy crystal grains.

《Second phase》
The second phase consists of Al ₈ Mn ₅ crystal grains, and of course, the composition and crystal structure are different from the first phase. As described above, the second phase is also a magnetic phase like the first phase, and is considered to exhibit a large magnetic anisotropy even if the magnetization level is small. As a result, the permanent magnet of the present invention can exhibit a large coercive force, but its effect is poor when the Al ₈ Mn ₅ crystal grains constituting the second phase are too small or too large. Therefore, the Al ₈ Mn ₅ crystal grains preferably have an average grain size of 10 nm to 5 μm, more preferably 50 nm to 1 μm. In addition, the average particle diameter as used in this specification is calculated by image-processing the microscope picture obtained with the transmission electron microscope (TEM). Specifically, for each Al ₈ Mn ₅ crystal grain present in the visual field (10 μm × 10 μm), it can be obtained as an arithmetic average value of the diameters of the circles corresponding to the area.

Further, if the second phase (Al ₈ Mn ₅ phase) is too small, the effect is poor, and if it is excessive, the magnetization (magnetic flux) as a permanent magnet can be lowered. Therefore, the total permanent magnet is preferably 100% by volume, and the Al ₈ Mn ₅ phase is preferably 5% by volume or less.

If the permanent magnet is made of an MnAl-based alloy, Al: 39 to 46%, Al: 40 to 44%, and Al: 41 when the whole is 100 atomic% (simply referred to as “%”). ˜43%, the balance being Mn and an arbitrary small amount (for example, 3% or less, further 2% or less) of modifying elements (for example, interstitial elements such as B, C, N, O, Fe, Ni, etc.) Etc.) or impurities. In this case, L1 ₀ type MnAl first phase consisting of crystal grains (as appropriate, simply referred to as "L1 ₀ type MnAl phase".) And a high coercive force of a composite phase of the second phase consisting of _Al 8 Mn ₅ grains Permanent magnets can be obtained. Incidentally, the composite phase and L1 ₀ type MnAl phase occupying most, if small amounts of _Al 8 Mn ₅ phase consists finely dispersed state of the tissue adjacent to the L1 ₀ type MnAl phase preferred.

<< Permanent Magnet Manufacturing Method >>
The manufacturing method of the permanent magnet of the present invention is not limited. For example, if it is a permanent magnet made of an MnAl alloy, it can be obtained by performing the following preparation process and generation process.

(1) Preparation process A preparation process is a process of forming an alloy layer on a base material (board | substrate) by sputtering with respect to the target raw material prepared according to the desired composition of the permanent magnet, for example. The composition of this alloy layer may be within the range as described above, for example. What is necessary is just to adjust the thickness of an alloy layer, the number of lamination | stacking, etc. suitably. When laminating a plurality of alloy layers, the alloy composition for each layer may be different as long as it has a desired composition as a whole. When laminating alloy layers, the thickness of each layer may be adjusted within a range of, for example, 5 to 500 nm in order to obtain a homogeneous permanent magnet.

The preparation of the MnAl-based alloy is preferably performed under heating at 560 to 750 ° C, further 580 to 720 ° C. This is because the first phase is a L1 ₀ type MnAl and second phases in which _Al 8 Mn ₅ above 560 ° C. For crystal formation of phase heating is required, also the Al evaporation becomes more than 750 ° C. This is considered to be conspicuous and difficult to control the composition. In the case of forming a MnAl-based alloy layer, the substrate temperature may be set within the above range. Note that the preparation step is preferably performed in an oxidation-preventing atmosphere (vacuum atmosphere, inert gas atmosphere, nitrogen gas atmosphere, or the like).

(2) generating step generating step, from MnAl alloy obtained in the preparation step, crystallization, precipitation, by crystal growth or the like, comprising L1 ₀ type MnAl crystal grains comprising a first phase and a second phase Al _This is a step of obtaining ₈ Mn ₅ crystal grains. Crystallization or precipitation of crystal grains can be performed by cooling the high-temperature MnAl-based alloy after the preparation step, or by additionally performing heat treatment.

If the case of obtaining an L1 ₀ type MnAl grain or Al ₈ Mn ₅ grain by crystal growth, to select the base material (substrate) such as crystal grains the crystal orientation is aligned in a specific direction by the epitaxial growth is obtained, the substrate It is preferable to form the alloy layer described above. The substrate is preferably selected so that the crystal lattice constant, thermal expansion coefficient, and the like of the substrate side (or underlayer) and the alloy layer side are close to each other. Examples of such a substrate include an MgO single crystal substrate made of a single crystal of magnesium oxide (MgO), a single crystal substrate of W, Mo, Cu, and Si.

  Moreover, you may form the base layer which has a crystal structure consistent with the crystal plane by the side of an alloy layer on a base material (base layer formation process). Such an underlayer includes a seed layer and a buffer layer. The seed layer is a layer that promotes crystal growth of the buffer layer, and the buffer layer is a layer that serves as a foundation for promoting the formation of the alloy layer. Examples of such a base material include Mo, Ta, W, Ti, Cr, V, Nb, Pd, Pt, Ag, and Au. Note that the underlayer can also be formed by sputtering.

Furthermore, the alloy surface after the preparation process or the L1 ₀ type MnAl crystal grains and Al ₈ Mn ₅ grains surface after generation step, it is preferable to form a protective layer for suppressing the oxidation (protective layer forming step). The protective layer can also be formed by the sputtering described above. As a target at this time, a simple substance such as Cr, Ag, Au, Pd, Pt, Mo, Cu, Ti, Ta, Ru, V, Hf, W, and Ir, an alloy, a compound, or the like can be used. Usually, it is sufficient to perform this sputtering at room temperature.

"permanent magnet"
The permanent magnet of the present invention can be used for various electromagnetic devices regardless of its application. For example, the permanent magnet of the present invention can be arranged in a rotor or stator of an electric motor to constitute a field. In addition, the permanent magnet of the present invention can be manufactured not only by a thin film method but also by a melting method, a sintering method, or the like.

To produce a variety of sample by thin film method, and characteristics of the _Al 8 Mn ₅ phase itself comprising a second phase, _Al 8 Mn ₅ phase, L1 ₀ type MnAl phase is a magnetic phase (particularly an example a first phase ) Was evaluated. The present invention will be described more specifically based on these examples.

<Thin film method>
An MgO single crystal substrate (also simply referred to as “substrate” as appropriate) was prepared. This MgO single crystal substrate is processed so that the (001) plane becomes a film-forming surface and polished to reduce the surface roughness. Unless otherwise specified, the film was formed directly on the (001) plane by sputtering.

Unless otherwise specified, sputtering in the examples was performed based on a magnetron sputtering method with an ultimate vacuum before film formation of 5 × 10 ⁻⁸ Pa or less and a film formation shape of φ8 mm. Each film thickness was calculated from the product of the film formation speed and the film formation time. Incidentally, the film formation rate was 0.4 to 1 cm / s in this example.

  The substrate after film formation was cooled to room temperature (35 ° C. or lower), and a Ta layer (protective layer) for preventing oxidation was formed on the film surface in the room temperature region (protective layer forming step).

<< First Example >>
(1) Manufacture of sample In order to investigate the characteristics of Al ₈ Mn ₅ itself, sample A0 in which only the Fe layer was directly formed on the substrate, and the Al ₈ Mn ₅ layer directly formed on the substrate, Sample A1 on which an Fe layer was laminated was prepared. The thickness of each Fe layer was 5 nm, and the thickness of the Al ₈ Mn ₅ layer was 45 nm. The substrate temperature at the time of forming the Fe layer of sample A0 and Al was 50 ° C. or less, and the substrate temperature at the time of forming the Al ₈ Mn ₅ layer of sample A1 was 650 ° C.

(2) Measurement Using each sample thus obtained, the magnetization in the direction perpendicular to the film surface was measured with a vibrating sample magnetometer (VSM). The magnetization curve concerning each sample obtained at this time is also shown in FIG.

(3) Evaluation First, the following can be understood from the magnetization curve of sample A0. Fe has a small magnetocrystalline anisotropy energy, but the Fe layer (thin film) according to sample A0 is strongly influenced by shape anisotropy, and its easy magnetization direction is a direction parallel to the film surface (appropriately “plane direction” "). For this reason, in the Fe layer, the direction orthogonal to the film surface being measured (referred to as “the direction perpendicular to the plane” as appropriate) is the magnetization difficult direction. Therefore, the magnetization curve obtained by measuring the magnetization in the perpendicular direction is as shown in FIG. When the magnetic field at which the magnetization is saturated is read from the magnetization curve related to the Fe layer, the anisotropic magnetic field of the Fe layer related to the sample A0 can be estimated to be about 20 kOe.

Next, the following can be understood from the magnetization curve of the sample A1. The Fe layer according to sample A1 is also the same as the Fe layer according to sample A0 in that the perpendicular direction is the direction of magnetization difficulty. However, the Fe layer according to the sample A1 is adjacent to the Al ₈ Mn ₅ layer on the lower layer side, and the anisotropic magnetic field is reduced to about 10 kOe. This indicates that the Fe ₈ layer is easily magnetized in the perpendicular direction by the Al ₈ Mn ₅ layer. In other words, it can be seen that the magnetic anisotropy of the Fe layer is induced in the perpendicular direction by the Al ₈ Mn ₅ layer.

As described above, Al ₈ Mn ₅ induces induced magnetic anisotropy in the magnetic material in the adjacent region or the adjacent region. Therefore, whether Al ₈ Mn ₅ is a ferromagnetic material or a ferrimagnetic material. It can be said that it is a magnetic material. Further, according to the present embodiment, the counterpart material (first phase) on which the Al ₈ Mn ₅ phase (second phase) acts is not limited to a hard magnetic material (for example, a MnAl-based magnetic material described later), but soft magnetism. It became clear that wood could be used.

<< Second Example >>
(1) Manufacture of a sample The alloy layer which changed various alloy compositions or the substrate temperature at the time of film-forming was formed into a film on the board | substrate by the thin film method mentioned above. Thus, various samples shown in Table 1 were obtained. Sample C0 is obtained by forming a Cr layer (underlying layer) on the (001) surface of the substrate by sputtering (underlying layer forming step) and then forming a Mn—Al alloy layer on the Cr layer. Incidentally, the film thickness of each alloy layer as a whole was 10 nm of Cr layer and 45 nm of MnAl layer.

<< Observation and measurement of sample >>
(1) an alloy layer of tissue to each sample, by performing X-ray diffraction method (XRD), the presence or absence of generation of L1 ₀ type MnAl crystals (first phase) and _Al 8 Mn ₅ crystals (second phase) confirmed. Table 1 also shows the presence or absence of phase formation along with the XRD diffraction intensity. “◯” in Table 1 means that phase is being generated, and “×” means that phase is not being generated.

(2) Magnetic properties Based on the results shown in Table 1, the magnetic properties (saturation magnetization and coercivity) at room temperature (23 ° C.) of a sample in which a composite phase composed of both phases is generated are measured using a vibrating sample magnetometer (VSM ). The results are also shown in Table 1.

  FIG. 2 shows the change in magnetic torque when the applied magnetic field is 27 kOe for Sample 12 and Sample C0. Moreover, the change with respect to the applied magnetic field H of the rotation hysteresis loss Wr which concerns on those samples was shown in FIG. Furthermore, the magnetization curve concerning the sample 14 is shown in FIG. 4A.

<Evaluation>
(1) The following can be seen from FIG. The undulations of the magnetic torque curve shown in FIG. 2 indicate magnetic anisotropy, and in the case of the sample C0, the magnetic torque changes from positive to negative at 0 ° (360 °) and 180 °. Therefore, the sample C0 is seen that one of the easy axis of magnetization caused by the L1 ₀ type MnAl phase (see Table 1) is present. On the other hand, in the case of the sample 12, a new undulation different from the case of the sample C0 occurs around 180 °. This is due to the _Al 8 Mn ₅ phase (see Table 1), from the original magnetic anisotropy of L1 ₀ type MnAl phase separately, the new magnetic anisotropy (induced magnetic anisotropy) is added It is thought that it was because of.

The following can be seen from FIG. The rotational hysteresis loss Wr related to the sample C0 decreases toward an anisotropic magnetic field _HA corresponding to a magnetic field showing saturation magnetization. On the other hand, the rotational hysteresis loss Wr related to the sample 12 shows only an increasing tendency at least in the same magnetic field. From this, it can be seen that the anisotropic magnetic field _HA of the sample 12 composed of the composite phase is considerably larger than the anisotropic magnetic field _{HA of the} sample C0 composed of the single phase.

(2) Table 1 shows the following. When producing the alloy layer in the thin film method described above, when the overall composition and the substrate temperature is in the desired range, it can be seen that tends composite phase is formed consisting of L1 ₀ type Mn phase and Al ₈ Mn ₅ phase .

Further, since the coercive force of the L1 ₀ type MnAl single phase as in Sample C0 is about 4 kOe, by complex phase as in Sample 11 to 25 are formed, the coercive force than the case of single-phase It can also be seen from Table 1 that it increases significantly.

Further, L1 from XRD diffraction intensity according to the _type-0 MnAl phase and _Al 8 Mn ₅ phase, even to the extent the _Al 8 Mn ₅ phase slightly exists for L1 ₀ type MnAl phase, coercive force as a whole significantly It can be seen that it increases. Therefore, the permanent magnets made of a composite phase L1 ₀ type MnAl phase and _Al 8 Mn ₅ phase coexist, it can be seen that is compatible with high magnetization and high coercive force by L1 ₀ type MnAl phase.

(3) This can also be seen from the magnetization curve (FIG. 4A) of the sample 14 shown as an example. That is, in the magnetization curve, two-stage magnetization reversal occurs, and it can be seen that there are two types of magnetic phases (composite phases). And with high magnetization is expressed by L1 ₀ type MnAl phase (first phase), high coercive force by complexation with Al ₈ Mn ₅ phase (second phase) is expressed. With particular attention to the coercivity, coercivity of L1 ₀ type MnAl phase is about 4 _kOe, the presence of Al 8 _{Mn 5} phase, the coercivity of the composite phase is far beyond the 40 kOe. The magnetization curve shown in FIG. 4A was not measured in a fully magnetized state because the upper limit of the applied magnetic field at the time of measurement was 50 kOe. Nevertheless, it can be sufficiently confirmed that the generation of the Al ₈ Mn ₅ phase greatly increases the coercive force of the composite phase regardless of the amount of generation.

  Thus, the coexistence of the first phase exhibiting high magnetization and the second phase exhibiting high magnetic anisotropy makes it possible to obtain a permanent magnet composed of a composite phase having both high characteristics. This mechanism is schematically shown in FIG. 4B with reference to the magnetization curve of FIG. 4A. That is, it can be seen that the magnetization curve of the permanent magnet composed of the composite phase has a form in which the magnetization curve of the first phase and the magnetization curve of the composite phase are fused, and can have a form in which both high magnetization and high coercive force can be achieved.

Claims (6)

A first phase consisting of tetragonal there L1 ₀ type MnAl grain Mn and Al,
The second phase consisting of Al ₈ Mn ₅ crystal grains different from the crystal grains of the first phase is mixed ,
The first phase and the second phase are both detected by X-ray diffraction;
Permanent magnet coercive force, characterized in der Rukoto least 13 kOe. 2. The permanent magnet according to claim 1, wherein the second phase is 5% by volume or less based on 100% by volume of the entire permanent magnet. Al ₈ Mn ₅ grains, the permanent magnet according to claim 1 or 2 having an average particle diameter of 10 nm to 5 [mu] m. The whole when 100 atomic% (referred to simply as "%".) And, Al: 39~46%, Mn: permanent magnet according to any one of claims 1 to 3, the balance is a MnAl alloy. A preparation step of preparing an alloy comprising Mn and Al;
And a generating step of generating an L1 ₀ type MnAl crystal grains and _Al 8 Mn ₅ grains from the alloy,
A method for producing a permanent magnet, wherein the permanent magnet according to claim 4 is obtained.   The said preparation process is a manufacturing method of the permanent magnet of Claim 5 made at 560-750 degreeC.