Rare earth thin-film magnet and method for producing same

Abstract

A rare earth thin-film magnet of a NdFeB film deposited on a Si substrate, wherein, when the film thickness of the rare earth thin film is 70 m or less, the Nd content satisfies the conditional expression of 0.15Nd/(Nd+Fe)0.25 in terms of an atomic ratio; when the film thickness of the rare earth thin film is 70 m to 115 m (but excluding 70 m), the Nd content satisfies the conditional expression of 0.18Nd/(Nd+Fe)0.25 in terms of an atomic ratio; and when the film thickness of the rare earth thin film is 115 m to 160 m (but excluding 115 m), the Nd content satisfies the conditional expression of 0.20Nd/(Nd+Fe)0.25 in terms of an atomic ratio. An object of the present invention is to provide a rare earth thin-film magnet having a maximum film thickness of 160 m and which is free from film separation and substrate fracture, and a method of producing such a rare earth thin-film magnet by which the thin film can be stably deposited.

Claims

1. A rare earth thin-film magnet of a NdFeB film formed by heat-treating an amorphous film deposited directly on a Si substrate, and having a film thickness of 10 m or more and 160 m or less, a composition consisting of Nd, Fe and B, and a structure consisting of Nd.sub.2Fe.sub.14B crystal grains mixed with crystal grains of a non-magnetic Nd-rich phase, wherein the composition has a Nd content which is larger than the stoichiometric content of Nd.sub.2Fe.sub.14B and satisfies a compositional condition depending on the thickness of the NdFeB alloy film such that, in a case where the thickness is 70 m or less, 70 m to 115 m excluding 70 m, or 115 m to 160 m excluding 115 m, a ratio of Nd/(Nd+Fe) in terms of atomic % is in a range of 0.15Nd/(Nd+Fe)0.25, 0.18Nd/(Nd+Fe)0.25, or 0.20Nd/(Nd+Fe)0.25, respectively.

2. The rare earth thin-film magnet according to claim 1, wherein the rare earth thin-film magnet has a coercive force of 1000 kA/m or more.

3. The rare earth thin-film magnet according to claim 2, wherein the rare earth thin-film magnet has a residual magnetization of 0.4 T or more.

4. The rare earth thin-film magnet according to claim 3, wherein the rare earth thin-film magnet has a maximum energy product of 40 kJ/m.sup.3 or more.

5. A method of producing the rare earth thin-film magnet according to claim 1, comprising: a step of depositing the rare earth thin film via pulsed laser deposition; a step of crystallizing the deposited rare earth thin film by performing heat treatment thereto; and a step of magnetizing the crystallized rare earth thin film to prepare the rare earth thin-film magnet.

6. The method of producing the rare earth thin-film magnet according to claim 5, wherein, in the step of depositing the rare earth thin film, pulsed laser power density is set to 0.1 to 100 J/cm.sup.2.

7. The method of producing the rare earth thin-film magnet according to claim 6, wherein, in the step of crystallizing the rare earth thin film, pulsed heat treatment is performed under the following conditions; namely, rated output of 2 to 10 kW and maximum output holding time of 1 to 3 seconds.

8. The method of producing the rare earth thin-film magnet according to claim 5, wherein, in the step of crystallizing the rare earth thin film, pulsed heat treatment is performed under the following conditions; namely, rated output of 2 to 10 kW and maximum output holding time of 1 to 3 seconds.

9. The rare earth thin-film magnet according to claim 1, wherein the rare earth thin-film magnet has a residual magnetization of 0.4 T or more.

10. The rare earth thin-film magnet according to claim 1, wherein the rare earth thin-film magnet has a maximum energy product of 40 kJ/m.sup.3 or more.

11. The rare earth thin-film magnet according to claim 1, wherein the film thickness is 67 m to 160 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 This is a diagram showing the NdFeB film separation mechanism.

(2) FIG. 2 This is a diagram showing the thermal expansion coefficient of the respective materials.

(3) FIG. 3 This is a diagram showing the relation of the Nd content and the film thickness that affect the separability of the NdFeB film.

(4) FIG. 4 This is a diagram showing the relation of the coercive force and the Nd content of the NdFeB film.

(5) FIG. 5 This is a diagram showing the relation of the residual magnetization and the Nd content of the NdFeB film.

(6) FIG. 6 This is a diagram showing the relation of the maximum energy product (BH).sub.max and the Nd content of the NdFeB film.

(7) FIG. 7 This is a schematic diagram showing the dicing method of the sample of Example 1 of the present invention.

(8) FIG. 8 This is a photograph showing the sample of Example 1 of the present invention after the dicing process.

(9) FIG. 9 This is a diagram showing the magnetic properties (M-H properties) of the sample of Example 1 of the present invention before and after the dicing process.

(10) FIG. 10 This is a photograph showing the NdFeB film (sample) of Comparative Example 1 of the present invention.

DETAILED DESCRIPTION

(11) FIG. 1 shows the NdFeB film separation mechanism. When a NdFeB film is directly formed on a Si substrate, film separation and other problems will arise due to the strain caused by the difference in thermal expansion rate between the film and the substrate during the subsequent heat treatment. In many of the samples, it has been confirmed that film separation or substrate fracture occurs during the cooling process after the heat treatment, and one cause is considered to be the stress during the contraction. The difference in the thermal expansion rate during the rise in temperature is also considered to be one cause that generates stress, but the sample immediately after the deposition has an amorphous structure and crystallization based on heat treatment causes the sample to shrink, and therefore, the influence of the force that works when the once-crystallized sample shrinks; that is, the influence of stress that works during the fall in temperature, is considered to be greater in comparison to the stress that works during the rise in temperature.

(12) In light of the foregoing circumstances, as shown in FIG. 2, the present inventors focused on the fact that Nd has a thermal expansion coefficient intermediate between Nd.sub.2Fe.sub.14B and Ta, and discovered that, by directly depositing a NdFeB film having a Nd content that is greater than a stoichiometric composition on a silicon substrate, it is possible to alleviate the difference in thermal expansion rate between the Si substrate and the NdFeB film, and thereby prevent NdFeB film separation and Si substrate fracture.

(13) FIG. 3 shows the relation of the Nd content and the film thickness that affect the separability of the NdFeB film on a Si substrate when controlling the Nd content of the target and increasing the Nd content of the sample. Here, the Nd content is defined based on the atomic ratio Nd/(Nd+Fe). When the film thickness of the rare earth thin film is 70 m or less, the Nd content satisfies the conditional expression of 0.15Nd/(Nd+Fe)0.25 in terms of an atomic ratio. When the film thickness of the rare earth thin film is 70 m to 115 m (but excluding 70 m), the Nd content satisfies the conditional expression of 0.18Nd/(Nd+Fe)0.25 in terms of an atomic ratio. When the film thickness of the rare earth thin film is 115 m to 160 m (but excluding 115 m), the Nd content satisfies the conditional expression of 0.20Nd/(Nd+Fe)0.25 in terms of an atomic ratio.

(14) As shown in FIG. 3, when the film thickness is 70 m or less but the Nd content is Nd/(Nd+Fe)<0.15 in terms of an atomic ratio, NdFeB film separation and Si substrate fracture occur. Moreover, when the film thickness is thick at 70 m to 115 m (but excluding 70 m) but the Nd content is Nd/(Nd+Fe)<0.18, NdFeB film separation and Si substrate fracture occur. Furthermore, when the film thickness is even thicker at 115 m to 160 m (but excluding 115 m) but the Nd content is Nd/(Nd+Fe)<0.20, NdFeB film separation and Si substrate fracture occur. Meanwhile, if the Nd content is Nd/(Nd+Fe)>0.25 in terms of an atomic ratio, as described later, the residual magnetic flux density tends to decrease and, therefore, the Nd content is set to Nd/(Nd+Fe)0.25 in terms of an atomic ratio.

(15) Furthermore, the rare earth thin-film magnet of the present invention has a coercive force (iHc) of preferably 1000 kA/m or more, and more preferably 1300 kA/m or more. Note that the magnetic properties such as the coercive force and the residual magnetization and maximum energy product described later can be measured using a VSM (Vibrating Sample Magnetometer). The relation of the Nd content and the coercive force regarding the NdFeB rare earth thin-film magnet formed on a Si substrate is shown in FIG. 4.

(16) As shown in FIG. 4, pursuant to the increase in the Nd content in the rare earth thin-film magnet, the coercive force tends to increase, and favorable magnetic properties can be obtained. The reason for this is considered to be because magnetic separation of the Nd.sub.2Fe.sub.14B crystal grains occurs due to the increase in the volume of the non-magnetic Nd-rich phase in correspondence with the increase in the Nd content, which consequently increases the coercive force.

(17) Moreover, the rare earth thin-film magnet of the present invention has a residual magnetization (Br) of preferably 0.4 T or more. FIG. 5 shows the relation of the Nd content and the residual magnetization regarding the NdFeB rare earth thin-film magnet formed on a Si substrate. As shown in FIG. 5, hardly any change in the residual magnetization could be observed even when the Nd content in the rare earth thin-film magnet was increased. However, when the Nd content is Nd/(Nd+Fe)>0.25 in terms of an atomic ratio, the residual magnetization tended to decrease. The reason for this is considered to be because, as the volume of the Nd-rich phase increases in correspondence to the increase in the Nd content, the volume Nd.sub.2Fe.sub.14B phase and the saturation magnetization decreased, and consequently the residual magnetization also decreased.

(18) Furthermore, FIG. 6 shows the relation of the Nd content and the maximum energy product (BH).sub.max regarding the NdFeB rare earth thin-film magnet formed on a Si substrate. While the (BH).sub.max tends to decrease pursuant to the increase in the Nd content in the rare earth thin-film magnet, the rare earth thin-film magnet of the present invention can attain a maximum energy product (BH).sub.max of 40 kJ/m.sup.3 or more.

(19) The rare earth thin-film magnet of the present invention can be produced, for example, as follows.

(20) Foremost, a target having a composition of Nd.sub.2.6Fe.sub.14B is mounted on a pulsed laser deposition device. Next, the inside of the chamber is evacuated until the vacuum degree becomes 2 to 810.sup.5 Pa, and the target is thereafter irradiated with a laser through a condenser lens. As the laser, a Nd:YAG laser (emission wavelength: 355 nm, repetition frequency: 30 Hz) may be used.

(21) The power density of the laser is preferably set to be 0.1 to 100 J/cm.sup.2. When the laser power density is less than 0.1 J/cm.sup.2, a large amount of droplets may arise when the target is irradiated with the laser, and it causes the density to deteriorate and then causes the magnetic properties to deteriorate. Meanwhile, when the laser power density exceeds 100 J/cm.sup.2, etching of the target caused by the laser irradiation may occur considerably, and undesirable phenomena, such as the discontinuation of the ablation phenomena, may arise.

(22) On the target surface that was irradiated with a laser as described above, a chemical reaction and a melting reaction will occur, and a plasma referred to as a plume is generated. As a result of the plume reaching the opposing substrate, it is possible to form a thin film of a NdFeB-based amorphous phase. Subsequently, in order to crystallize the thus deposited NdFeB-based amorphous film, pulsed heat treatment is performed after the deposition under the following conditions; namely, rated output of 2 to 10 kW and maximum output holding time of 1 to 3 seconds in order to crystallize the NdFeB-based amorphous mother phase.

(23) Here, if the heat treatment is not sufficiently performed, the crystallization of the NdFeB-based amorphous phase in the film will be insufficient, and the amorphous phase will remain excessively. Meanwhile, excessive heat treatment will cause the coarsening of the Nd.sub.2Fe.sub.14B crystal grains, and the magnetic properties may deteriorate. Accordingly, the pulsed heat treatment is preferably performed under the conditions within the foregoing range. Note that, by extremely short irradiation of infrared rays, the pulsed heat treatment can promote the instantaneous crystallization of the sample to realize the refinement of crystal grains.

(24) Subsequently, by performing, for instance, pulsed magnetization to this crystallized thin film at a magnetic field of 7 T, it is possible to prepare a rare earth thin-film magnet. Note that there is no particular limitation in the magnetization method in the present invention, and a publicly known magnetization method may be used. It is thereby possible to produce the rare earth thin-film magnet of a NdFeB film, which is free from film separation and substrate fracture and has a large Nd content, on a Si substrate. Furthermore, not only does this rare earth thin-film magnet yield superior magnetic properties, but can be directly deposited on a versatile Si substrate, and therefore, the rare earth thin-film magnet of the present invention is effective in preparing micro actuators of micro magnetic devices for MEMS and others.

EXAMPLES

(25) The present invention is now explained based on the following Examples and Comparative Examples. Note that these Examples merely illustrate certain examples of the present invention, and this invention is not limited in any way by these Examples. In other words, the present invention is limited only by the scope of its claims, and covers the various modifications other than the Examples that are included in this invention.

Example 1

(26) A Nd.sub.3.0Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the inside of a chamber was evacuated to a vacuum and, after confirming that a vacuum degree of 10.sup.5 Pa has been attained, the target being rotated at approximately 11 rpm was irradiated with a Nd:YAG laser (emission wavelength: 355 nm) at a repetition frequency of 30 Hz to subject the target material to ablation. Here, the distance between the target and the substrate was set to 10 mm, and the laser power density on the target surface was set to be roughly 4 J/cm.sup.2. As the substrate, a (100) monocrystal Si of 5 mm square having a thickness of 622 m was used. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.18 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 113 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(27) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds.

(28) Pulsed magnetization was thereafter performed at a magnetic field of 7 T to prepare a rare earth thin-film magnet. In order to examine the separability of the NdFeB film, cutting work via dicing was considered. Dicing was performed, as shown in FIG. 7, by quartering a sample of 5 mm square into a size of 2.5 mm2.5 mm. However, air-blowing and alcohol immersion were performed during the cutting work so that there will be no residual moisture after the cutting work. A photograph of the samples after dicing is shown in FIG. 8. In the case of a NdFeB-based thin-film magnet using a Ta strain buffer layer, mechanical damage occurred during the dicing process. However, by directly depositing the thin film on a Si substrate without a strain buffer layer, processing was enabled even after the dicing process without any mechanical damage.

(29) FIG. 9 shows the measurement results of the magnetic properties of the samples before and after dicing as measured with a VSM (Vibrating Sample Magnetometer). The broken line shows the properties before dicing, and the solid line shows the properties after dicing. Before the dicing process, the properties were as follows; specifically, coercive force (iHc) of 1163 kA/m, residual magnetization (Br) of 0.50 T, and maximum energy product (BH).sub.max of 41.9 kJ/m.sup.3. Meanwhile, after the dicing process, the properties were as follows; specifically, coercive force (iHc) of 1144 kA/m, residual magnetization (Br) of 0.55 T, and maximum energy product (BH).sub.max of 53.1 kJ/m.sup.3; and no notable deterioration in the magnetic properties due to the processing could be observed.

Example 2

(30) A Nd.sub.2.6Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the target material was subject to ablation under the same conditions as Example 1. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.15 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 67 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(31) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds. Pulsed magnetization was thereafter performed at a magnetic field of 7 T to prepare a rare earth thin-film magnet. Subsequently, dicing was performed in the same manner as Example 1, but no mechanical damage could be acknowledged. As a result of evaluating the magnetic properties of this rare earth thin-film magnet with a VSM (Vibrating Sample Magnetometer), the properties were as follows; specifically, coercive force (iHc) of 1144 kA/m, residual magnetization (Br) of 0.55 T, and maximum energy product (BH).sub.max of 53.0 kJ/m.sup.3; and favorable results were obtained.

Example 3

(32) A Nd.sub.3.5Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the target material was subject to ablation under the same conditions as Example 1. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.23 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 160 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(33) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds. Pulsed magnetization was thereafter performed at a magnetic field of 7 T to prepare a rare earth thin-film magnet. Subsequently, dicing was performed in the same manner as Example 1, but no mechanical damage could be acknowledged. As a result of evaluating the magnetic properties of this rare earth thin-film magnet with a VSM (Vibrating Sample Magnetometer), the properties were as follows; specifically, coercive force (iHc) of 1200 kA/m, residual magnetization (Br) of 0.51 T, and maximum energy product (BH).sub.max of 44.0 kJ/m.sup.3; and favorable results were obtained.

Comparative Example 1

(34) A Nd.sub.2.0Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the target material was subject to ablation under the same conditions as Example 1. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.119 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 67 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(35) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds. The photograph of the NdFeB film (sample) after the heat treatment is shown in FIG. 10. In cases where the film thickness is 67 m, when the NdFeB film is of a stoichiometric composition, film separation and substrate fracture occurred as shown in FIG. 10.

Comparative Example 2

(36) A Nd.sub.2.8Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the target material was subject to ablation under the same conditions as Example 1. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.17 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 110 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(37) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds. In cases where the film thickness is relatively thick at 110 m, even when the Nd content is slightly high, film separation and substrate fracture occurred.

Comparative Example 3

(38) A Nd.sub.2.8Fe.sub.14B target having a purity of 99.9% (3N) and a relative density of 99% was mounted on a pulsed laser deposition device. Subsequently, the target material was subject to ablation under the same conditions as Example 1. A NdFeB amorphous film having a Nd content of Nd/(Nd+Fe)=0.19 in terms of an atomic ratio was thereby deposited on a Si substrate at a thickness of 150 m. Note that the film thickness was evaluated using a micrometer, and the chemical composition was analyzed using EDX (Energy Dispersive X-ray spectroscopy).

(39) Subsequently, the NdFeB-based amorphous phase was crystallized by pulsed heat treatment (heat treatment temperature: approximately 500 to 800 C.) at a rated output of 8 kW and a maximum output holding time of approximately 3 seconds. In cases where the film thickness is thick at 150 m, even when the Nd content is slightly high, film separation and substrate fracture occurred.

(40) The present invention yields a superior effect of being able to stably deposit a NdFeB film, which is free from separation, on a Si substrate via pulsed laser deposition up to a maximum film thickness of 160 m. The NdFeB rare earth thin-film magnet of the present invention is effective in applications to magnetic devices being applied in energy sectors such as energy harvest, and medical device sectors. The present invention is particularly effect in preparing micro actuators of micro magnetic devices for MEMS.