Rare earth thin film magnet and method for producing same

Abstract

Provided is a rare earth thin film magnet having Nd, Fe and B as essential components, which is characterized in that a Nd—Fe—B base film is formed on a Si substrate having an oxide film formed on a surface thereof and has a composition in which the Nd content is higher than that of a stoichiometric composition and that a film (nano composite film) is formed on the base film and has a texture in which an α-Fe phase and Nd.sub.2Fe.sub.14B are alternately arranged and three-dimensionally dispersed. The rare earth thin film magnet provided is less susceptible to the occurrence of film separation and substrate breakage and exhibits favorable magnetic properties.

Claims

1. A rare earth thin film magnet having Nd, Fe and B as essential components comprising: a Nd—Fe—B base film on a Si substrate having an oxide film formed on a surface thereof; the Nd—Fe—B base film has a composition in which a Nd content is higher than that of a stoichiometric composition of Nd.sub.2—Fe.sub.14—B; nano composite film having a film thickness of 10 μm or more and 42 μm or less on the base film, and the nano composite film has a texture in which an α-Fe phase and Nd.sub.2Fe.sub.14B are alternately arranged and three-dimensionally dispersed.

2. The rare earth thin film magnet according to claim 1, wherein a film thickness of the base film is 0.5 μm or more.

3. The rare earth thin film magnet according to claim 2, wherein a film thickness ratio of the base film relative to the nano composite film is 1/10 or less.

4. The rare earth thin film magnet according to claim 3, wherein the oxide film on the surface of the Si substrate is a thermal oxide film.

5. The rare earth thin film magnet according to claim 4, wherein the rare earth thin film magnet has a residual magnetization of 0.70 T or more.

6. The rare earth thin film magnet according to claim 5, wherein the rare earth thin film magnet has a coercive force of 480 kA/m or more.

7. The rare earth thin film magnet according to claim 6, wherein the rare earth thin film magnet has a maximum energy product (BH).sub.max of 70 kJ/m.sup.3 or more.

8. The rare earth thin film magnet according to claim 1, wherein a film thickness ratio of the base film relative to the nano composite film is 1/10 or less.

9. The rare earth thin film magnet according to claim 1, wherein the oxide film on the surface of the Si substrate is a thermal oxide film.

10. The rare earth thin film magnet according to claim 1, wherein the rare earth thin film magnet has a residual magnetization of 0.70 T or more.

11. The rare earth thin film magnet according to claim 1, wherein the rare earth thin film magnet has a coercive force of 480 kA/m or more.

12. The rare earth thin film magnet according to claim 1, wherein the rare earth thin film magnet has a maximum energy product (BH).sub.max of 70 kJ/m.sup.3 or more.

13. A method of producing a rare earth thin film magnet, the method comprising: forming an oxide film on a Si substrate; forming a Nd—Fe—B base film having a composition in which a Nd content is higher than that of a stoichiometric composition of Nd.sub.2—Fe.sub.14—B on the Si substrate, as a first layer via pulsed laser deposition; forming a nano composite film having a film thickness of 10 μm or more and 42 μm or less and having a texture in which an α-Fe phase and Nd.sub.2Fe.sub.14B are alternately arranged and three-dimensionally dispersed on the first layer, as a second layer via pulsed laser deposition; and thereafter performing heat treatment thereto.

14. The method of producing a rare earth thin film magnet according to claim 13, wherein deposition is performed using a target made from Nd.sub.XFe.sub.14B (where X is a number that satisfies 2.1 to 2.7).

15. The method of producing a rare earth thin film magnet according to claim 14, wherein a laser intensity density satisfies 1 J/cm.sup.2 or more and less than 10 J/cm.sup.2 in forming the base film as the first layer and satisfies 10 J/cm.sup.2 or more and 1000 J/cm.sup.2 or less in forming the nano composite film having a texture in which an α-Fe phase and Nd.sub.2Fe.sub.14B are alternately arranged as the second layer.

16. The method of producing a rare earth thin film magnet according to claim 15, wherein the heat treatment is performed at 500° C. or higher and 800° C. or less.

17. The method of producing a rare earth thin film magnet according to claim 13, wherein the heat treatment is performed at 500° C. or higher and 800° C. or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 This is a schematic cross section showing an example of the Nd—Fe—B rare earth thin film magnet of the embodiment of the present invention.

(2) FIG. 2 This is a diagram showing the post-deposition X-ray diffraction pattern of Example 1.

(3) FIG. 3 This is a diagram showing a TEM observation photograph of the rare earth thin film magnet of Example 1.

(4) FIG. 4 This is a diagram showing the magnetic properties of the rare earth thin film magnet of Example 1.

(5) FIG. 5 This is a photograph showing the appearance of the diced thin film magnet of Example 1.

(6) FIG. 6 This is a diagram showing the magnetic properties of the rare earth thin film magnet of Comparative Example 4.

DETAILED DESCRIPTION

(7) The embodiment of the present invention is a rare earth thin film magnet having Nd, Fe and B as essential components, wherein a Nd—Fe—B base film is formed on a Si substrate having an oxide film formed on a surface thereof and has a composition in which a Nd content is higher than that of a stoichiometric composition, and a film (nano composite film) is formed on the base film and has a texture in which an α-Fe phase and Nd.sub.2Fe.sub.14B are alternately arranged and three-dimensionally dispersed. The schematic cross section of the rare earth thin film magnet of the embodiment of the present invention is shown in FIG. 1.

(8) A nano composite film has a structure in which an α-Fe phase (soft phase: having high saturation magnetization) and a Nd.sub.2Fe.sub.14B phase of a stoichiometric composition (hard phase: having high coercive force) are alternately arranged and three-dimensionally dispersed, and exhibits superior magnetic properties. Moreover, each crystal grain size is roughly 10 nm to 30 nm.

(9) The film thickness of a nano composite film is preferably 10 μm or more and 42 μm or less. When the film thickness is less than 10 μm, it is not possible to extract a sufficient magnetic field outside the film. Meanwhile, when the film thickness exceeds 42 μm, the ratio of thickness of the nano composite film relative to the thickness of the base film will increase, and there are cases where separation and other problems cannot be sufficiently suppressed due to the relationship of mechanical strength.

(10) A Nd—Fe—B base film has a composition in which a Nd content is higher than that of a stoichiometric composition; specifically, the atomic ratio of Nd and Fe satisfies the condition of 0.150≤Nd/(Nd+Fe). A base film containing a higher amount of Nd than a stoichiometric composition alleviates a difference of linear expansion coefficients between the Si substrate and the nano composite film, and contributes to suppressing film separation and substrate breakage. Furthermore, the crystal grain size of the Nd—Fe—B base film is roughly 200 nm to 300 nm, and this is clearly differentiated from a Nd—Fe—B phase in the foregoing nano composite film.

(11) The Nd—Fe—B base film preferably has a film thickness of 0.5 μm or more. When the film thickness is less than 0.5 μm, it becomes difficult to uniformly cover the entire substrate surface, and the effect of suppressing separation and other problems may deteriorate. Meanwhile, the film thickness of the Nd—Fe—B base film is preferably less than a film thickness ratio of 1/10 relative to the film thickness of the nano composite film. When the film thickness is any thicker, a notable two-jump configuration may arise in the M-H characteristics, and the magnetic properties may deteriorate.

(12) The expression “two-jump configuration” as used in the embodiment of the present invention refers to a phenomenon where a plurality of inflection points can be confirmed in a hysteresis loop of the M-H characteristics and, as shown in Comparative Example 4 described later, in the second quadrant of the hysteresis loop (excluding those on the X axis and the Y axis), in a case where a point indicating the residual magnetization on a magnetic polarization axis (Y axis) and a point indicating the coercive force on an external magnetic field axis (X axis) are connected with a straight line, refers to a phenomenon where such straight line and the M-H loop may intersect within the second quadrant.

(13) Furthermore, from the perspective of adhesion with the base film, the oxide film formed on the Si substrate is preferably a thermal oxide film. When a natural oxide film is used, there are cases where film separation occurs at the interface of Si and the rare earth thin film magnet. Meanwhile, when a thermal oxide film is used, based on the test result that the Si substrate itself will break, it is considered that the base layer adheres more favorably to a thermal oxide film in comparison to a natural oxide film.

(14) Moreover, a thermal oxide film is preferable in that its film thickness can be controlled more easily in comparison to a natural oxide film. Note that the thickness of a thermal oxide film is 380 to 600 nm, preferably 500 to 550 nm. This is differentiated from the thickness of a natural oxide film on a general Si substrate which is several nm (for instance, roughly 1 to 3 nm).

(15) The rare earth thin film magnet of the embodiment of the present invention exhibits superior magnetic properties, but in particular is able to achieve residual magnetization of 0.70 T or more, coercive force of 480 kA/m or more, and maximum energy product (BH).sub.max of 70 kJ/m.sup.3 or more. The embodiment of the present invention is particularly superior with respect to the point that it can maintain the foregoing favorable magnetic properties without any film separation or the like even when a thick film is produced.

(16) As the method of producing the rare earth thin film magnet, foremost, a Si substrate with an oxide film formed thereon is placed in a pulsed laser deposition (PLD) device, and then a Nd—Fe—B target is mounted so as to face the substrate. Next, the chamber is evacuated so that the degree of vacuum becomes 2×10.sup.−5 to 8×10.sup.−5 Pa, the target is subsequently irradiated with a laser through a condenser lens, and deposition is thereby started. As the laser, a Nd: YAG laser (oscillation wavelength: 355 nm, repetition frequency: 30 Hz) may be used.

(17) The target to be used shall be Nd.sub.xFe.sub.14B (X: 2.1 to 2.7) having a composition in which the Nd content is higher than that of a stoichiometric composition. When X is less than 2.1, the Nd concentration in the Nd—Fe—B base film decreases and the Nd-rich layer formed at the interface of the Si substrate and the base film becomes thin, and film separation or breakage within the Si substrate may occur after the heat treatment. Meanwhile, when X exceeds 2.7, the volume ratio of the α-Fe phase in the nano composite film will decrease, and Nd as an overabundant nonmagnetic component may remain and cause the residual magnetization to deteriorate.

(18) Subsequently, in a state where the focal distance is removed from the surface of the target (defocused irradiation), laser is irradiated by setting the laser intensity to a low energy density (1 J/cm.sup.2 or more and less than 10 J/cm.sup.2) to form a Nd—Fe—B base film having a composition containing a higher amount of Nd than a stoichiometric composition. When the laser intensity density is less than 1 J/cm.sup.2, numerous droplets are generated when the target is irradiated with a laser which causes the density to deteriorate, and this may consequently deteriorate the magnetic properties. Meanwhile, when the laser intensity density is 10 J/cm.sup.2 or more, it is not possible to obtain a Nd—Fe—B film with the intended compositional ratio in which the atomic ratio of Nd and Fe satisfies the condition of 0.150≤Nd/(Nd+Fe) and the Nd composition is higher than the stoichiometric composition. With this Nd-overabundant Nd—Fe—B base layer, a Nd-overabundant layer is formed directly on the Si substrate during the subsequent heat treatment process, and the effect of alleviating stress is exhibited.

(19) Next, the same Nd—Fe—B target is used and, by irradiating a laser by aiming the focal distance of the laser on the target material surface (by switching to a focused irradiation), deposition is performed at a laser intensity (10 to 1000 J/cm.sup.2) in which the energy density has been increased. When the laser intensity density is less than 10 J/cm.sup.2, a nano composite film cannot be stably formed. Meanwhile, when the laser intensity density exceeds 1000 J/cm.sup.2, a phenomenon where ablation is discontinued may occur. When deposition is performed under the foregoing focus conditions, even when a target material having a composition that is Nd-overabundant is used, it is possible to form a nano composite film from α-Fe and a Nd—Fe—B layer of a stoichiometric composition.

(20) The target surface irradiated with a laser as described above will undergo chemical reaction and melting reaction, and a plasma referred to as a plume is generated. As a result of this plume reaching the opposing substrate, it is possible to form an amorphous film on the substrate. The amorphous film deposited in the manner described above is subject to pulsed heat treatment after deposition under the following conditions; namely, a rated output of approximately 8 kW, and a maximum output holding time of roughly 3 seconds, in order to crystallize the Nd—Fe—B amorphous phase.

(21) Here, when heat treatment is not sufficiently performed, the crystallization of the Nd—Fe—B amorphous phase in the film will be insufficient and numerous amorphous phases may remain. Meanwhile, excessive heat treatment may coarsen the crystal grains, and deteriorate the magnetic properties. Accordingly, the pulsed heat treatment is preferably performed within a range of 500 to 800° C. Incidentally, pulsed heat treatment can promote the instant crystallization of the sample and realize the refinement of the crystal grains, by irradiating infrared rays for an extremely short time.

(22) Subsequently, by performing pulsed magnetization to the crystallized thin film at, for example, a magnetic field of 7 T, it is possible to prepare a rare earth thin film magnet. Note that the there is no particular limitation regarding the magnetization method in the embodiment of the present invention, and publicly known magnetization methods may be used. It is thereby possible to produce a Nd—Fe—B rare earth thin film magnet. This rare earth thin film magnet not only exhibits superior magnetic properties, but also is effective for preparing micro actuators of micro magnetic devices for MEMS because it is directly deposited on a versatile Si substrate.

EXAMPLES

(23) The embodiment of the present invention is now explained with reference to the Examples and Comparative Examples. Note that these Examples are merely illustrative, and the embodiment of the present invention shall in no way be limited thereby. In other words, various modifications and other embodiments are covered by the embodiment of the present invention, and the embodiment of the present invention is limited only by the scope of its claims.

Example 1

(24) A Nd.sub.2.5Fe.sub.14B target was prepared, and a monocrystal Si (100) having a thickness of 622 μm and 5 mm square was used as a substrate. Note that a thermal oxide film having a thickness of approximately 515 nm, which was formed by being heated in an oxygen atmosphere at a temperature of 800° C., has been formed on the Si substrate. Subsequently, the foregoing target and substrate were mounted at their predetermined positions in a pulsed laser deposition device, the inside of a chamber was thereafter evacuated to achieve a vacuum, and, after confirming that the degree of vacuum has reached 10.sup.−5 Pa, the target being rotated at approximately 11 rpm was irradiated with a Nd:YAG laser (oscillation wavelength: 355 nm) having a repetition frequency of 30 Hz, and the target substance was ablated and a film was deposited on the substrate. Here, the distance between the target and the substrate was set to 10 mm.

(25) Foremost, a Nd—Fe—B amorphous base film was deposited in a film thickness of 1.0 μm on the Si substrate by setting the laser intensity density on the target surface to roughly 2 J/cm.sup.2, the laser intensity density was thereafter increased to roughly 20 J/cm.sup.2, and an α-Fe/Nd.sub.2Fe.sub.14B nano composite amorphous film was continuously deposited in a film thickness of 23 μm on the base film. Note that, as the method of controlling the energy density, a method of moving the condenser lens of the laser and shifting the focal distance, and thereby changing the laser spot area was used.

(26) Here, the DF (defocus) Rate which quantifies the shift from the focal distance, is calculated from the formula of DF Rate=(TD−FD)/FD (TD: distance between the target and the condenser lens; FD: focal distance). Furthermore, upon depositing the Nd—Fe—B base film, the DF Rate was set to 0.3, and, upon depositing the nano composite film, the DF Rate was set to 0 (zero).

(27) The post-deposition X-ray diffraction pattern is shown in FIG. 2. In FIG. 2, in addition to the peak of the α-Fe crystal grains being confirmed, similar results as the X-ray diffraction pattern of a nano composite film formed on a conventional Ta substrate are obtained. Next, pulsed heat treatment was performed at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds to crystallize the Nd—Fe—B-based amorphous phase. FIG. 3 shows the transmission electron microscope (TEM) cross section photograph of the film after heat treatment. In the TEM photograph of FIG. 3, similar results as the nano composite film formed on a conventional Ta substrate are obtained. Subsequently, pulsed magnetization was performed at a magnetic field of 7 T to prepare a rare earth thin film magnet. In order to check the separation characteristics of the film, a 5 mm×5 mm sample was subject to cutting work via dicing in order to split the sample into quarters each having a size of 2.5 mm×2.5 mm. Consequently, as shown in FIG. 5, it was confirmed that the samples could be processed without any mechanical damage. Next, the magnetic properties of the diced samples were measured using a VSM (Vibrating Sample Magnetometer). As shown in Table 1, the results were as follows; specifically, residual magnetization was 0.87 T, coercive force was 570 kA/m, and (BH).sub.max was 86 kJ/m.sup.3, and favorable magnetic properties were obtained. FIG. 4 shows the M-H loop.

Examples 2 to 11

(28) In Examples 2 to 11, the same method as Example 1 was used to deposit a Nd—Fe—B base film on a Si substrate and then deposit an α-Fe/Nd.sub.2Fe.sub.14B nano composite film thereon, and thereafter perform pulsed heat treatment to obtain a crystallized film. Here, in each of Examples 2 to 11, the target composition, film thickness of the Nd base film, film thickness of the nano composite film, and laser intensity density were changed as shown in Table 1. Each of the thin films was subject to pulsed magnetization to prepare a rare earth thin film magnet, and the film separation and magnetic properties of the obtained rare earth thin film magnet were checked in the same manner as Example 1. Consequently, as shown in Table 1, all Examples 2 to 6 is free from any occurrence of film separation and substrate breakage and exhibits favorable magnetic properties.

Comparative Example 1

(29) The same method as Example 1 was used to deposit a Nd—Fe—B base film on a Si substrate and then deposit an α-Fe/Nd.sub.2Fe.sub.14B nano composite film thereon, and thereafter perform pulsed heat treatment to obtain a crystallized film. However, the target composition was set to be Nd.sub.2Fe.sub.14B. This thin film was subject to pulsed magnetization to prepare a rare earth thin film magnet. The thus obtained rare earth thin film magnet was subject to cutting work via dicing in order to split the 5 mm×5 mm sample into quarters each having a size of 2.5 mm×2.5 mm based on the same method as Example 1, but film separation occurred and it was not possible to check the magnetic properties. The cause is considered to be that the amount of Nd in the Nd—Fe—B base film was so small that it was not possible to faun a Nd-rich layer which could play the role of alleviating the strain at the interface of the Si substrate.

Comparative Example 2

(30) The same method as Example 1 was used to deposit a Nd—Fe—B base film on a Si substrate and then deposit an α-Fe/Nd.sub.2Fe.sub.14B nano composite film thereon, and thereafter perform pulsed heat treatment to obtain a crystallized film. However, the film thickness of the base film was set to be 0.45 μm. This thin film was subject to pulsed magnetization to prepare a rare earth thin film magnet. The thus obtained rare earth thin film magnet was subject to cutting work via dicing in order to split the 5 mm×5 mm sample into quarters each having a size of 2.5 mm×2.5 mm based on the same method as Example 1, but film separation occurred and it was not possible to measure the magnetic properties. The cause is considered that because the Nd—Fe—B base film was thin, it was not possible to uniformly cover the entire surface of the Si substrate surface.

Comparative Example 3

(31) The same method as Example 1 was used to deposit a Nd—Fe—B base film on a Si substrate and then deposit an α-Fe/Nd.sub.2Fe.sub.14B nano composite film thereon, and thereafter perform pulsed heat treatment to obtain a crystallized film. However, the film thickness of the nano composite film was set to be 43 μm. This thin film was subject to pulsed magnetization to prepare a rare earth thin film magnet. The thus obtained rare earth thin film magnet was subject to cutting work via dicing in order to split the 5 mm×5 mm sample into quarters each having a size of 2.5 mm×2.5 mm based on the same method as Example 1, but film separation partially occurred and it was not possible to measure the magnetic properties. Incidentally, while the film thickness ratio of the base film relative to the nano composite film was 0.07 in Comparative Example 3 and fell within the range of the technical feature (0.1 or less) capable of inhibiting the magnetic properties of the base film from influencing the nano composite film, because the nano composite film was too thick, the mechanical strength deteriorated and it was not possible to inhibit film separation and substrate breakage, and the magnetic properties could not be measured.

Comparative Example 4

(32) The same method as Example 1 was used to deposit a Nd—Fe—B base film on a Si substrate and then deposit an α-Fe/Nd.sub.2Fe.sub.14B nano composite film thereon, and thereafter perform pulsed heat treatment to obtain a crystallized film. However, the film thickness ratio of the base film relative to the nano composite film was set to be 0.11. Next, this thin film was subject to pulsed magnetization to prepare a rare earth thin film magnet. The thus obtained rare earth thin film magnet was subject to cutting work via dicing in order to split the 5 mm×5 mm sample into quarters each having a size of 2.5 mm×2.5 mm based on the same method as Example 1. Incidentally, it was confirmed that the samples could be processed without any mechanical damage. Next, the magnetic properties of the diced samples were measured using a VSM (Vibrating Sample Magnetometer). Residual magnetization was 0.78 T, coercive force was 488 kA/m, and (BH).sub.max was 68 kJ/m.sup.3, but as shown in FIG. 6, an extreme two-jump configuration was observed in the M-H characteristics. Here, the expression “two-jump configuration of the M-H characteristics” refers to a phenomenon where a plurality of inflection points of an M-H loop can be confirmed as though magnetic materials having different magnetic properties coexist. In the present invention, the cause of such two-jump configuration is considered to be as follows; specifically, as a result of the film thickness of the base film increasing relative to the film thickness of the nano composite film, the influence of the magnetic properties of the base film has increased, and the respective magnetization processes of the base film and the nano composite film have been reflected.

(33) TABLE-US-00001 TABLE 1 Nano Laser energy density composite Target (J/cm2) NdFeB base film Film Resid- compo- Nano Film Film thick- ual Coer- Two-jump sition compos- thick- thick- ness magneti- cive configu- Nd × Base ite ness ness ratio zation force (BH)max ration Fe14B film film (μm) (1) (μm) (2) (1)/(2) (T) (kA/m) (kJ/m3) of loop Film adhesion Example 1 X = 2.5 2 20 1.0 23 0.04 0.87 570 86 None No film separation, No substrate breakage Example 2 X = 2.5 2 20 0.50 21 0.02 0.90 540 87 None No film separation, No substrate breakage Example 3 X = 2.5 2 20 2.0 42 0.05 0.85 530 86 None No film separation, No substrate breakage Example 4 X = 2.5 2 20 1.8 18 0.10 0.82 550 84 None No film separation, No substrate breakage Example 5 X = 2.1 2 20 1.0 20 0.05 0.95 480 89 None No film separation, No substrate breakage Example 6 X = 2.7 2 20 1.5 20 0.08 0.70 650 70 None No film separation, No substrate breakage Example 7 X = 2.5 1 10 1.1 19 0.06 0.86 565 84 None No film separation, No substrate breakage Example 8 X = 2.5 5 70 1.5 23 0.07 0.89 542 85 None No film separation, No substrate breakage Example 9 X = 2.5 7 100 1.3 17 0.08 0.92 539 82 None No film separation, No substrate breakage Example 10 X = 2.5 8 300 0.9 18 0.05 0.91 545 83 None No film separation, No substrate breakage Example 11 X = 2.5 9 800 1.4 21 0.07 0.93 542 81 None No film separation, No substrate breakage Comparative X = 2.0 2 20 2.0 30 0.07 None Film separation, Example 1 Substrate breakage Comparative X = 2.5 2 20 0.45 22 0.02 None Film separation, Example 2 Substrate breakage Comparative X = 2.5 2 20 3.0 43 0.07 None Film separation, Example 3 Substrate breakage Comparative X = 2.5 2 20 2.0 19 0.11 0.78 488 68 Yes No film separation, Example 4 No substrate breakage

(34) The embodiment of the present invention enables to form a Nd—Fe—B base film on a Si substrate and thereafter continuously deposit α-Fe/Nd—Fe—B nano composite film by using only one type of target and simply changing the intensity of the laser with which the target is irradiated, which significantly improves the productivity of rare earth magnets. Furthermore, the obtained rare earth thin film magnet is free from any occurrence of film separation and substrate breakage and exhibits favorable magnetic properties. The Nd—Fe—B rare earth thin film magnet of the embodiment of the present invention is effective for use as a magnetic device to be applied in the energy sector such as energy harvest as well as in the field of medical appliances. Moreover, the embodiment of the present invention is particularly effective for use in preparing micro actuators of micro magnetic devices for MEMS.