Rare-earth nanocomposite magnet

A film with the structure illustrated schematically in FIG. 1 ( 1 ) was formed by sputtering on a thermally-oxidized film (SiO 2 ) of a Si single crystal substrate. The conditions for film forming were as follows. In FIG. 1 ( 1 ) “NFB” stands for Nd 2 Fe 14 B.

<Film Forming Conditions>

A) lower Ta layer: formed at room temperature

B) Nd 2 Fe 14 B layer: film formation at 550° C.+annealing at 600° C. for 30min

C) Ta spacer layer (intercalated layer)+α-Fe layer+Ta cap layer: film formation between 200 to 300° C.

wherein the Nd 2 Fe 14 B layer of B) is a hard magnetic phase, the Ta spacer layer of C) is an intercalated layer between a hard magnetic phase and a soft magnetic phase, and the α-Fe layer of C) is a soft magnetic phase.

A TEM micrograph of a cross-sectional structure of the obtained nanocomposite magnet is shown in FIG. 1 ( 2 ).

<Evaluation of Magnetic Properties>

The magnetization curve of the nanocomposite magnet produced in the current Example is shown in FIG. 2 .

The directions of an applied magnetic field are vertical (plotted as filled circles in the Figure) and parallel (plotted as filled squares in the Figure) to the surface of a formed film.

Coercive force of 14 kOe, residual magnetization of 1.55 T, and maximum energy product of 51 MGOe were obtained in the vertical direction to the formed film surface. The magnetic properties were measured by a VSM (Vibrating Sample Magnetometer). The same holds for other Examples and Comparative Example.

A film with the structure illustrated schematically in FIG. 3 ( 1 ) was formed by sputtering on a thermally-oxidized film (SiO 2 ) of a Si single crystal substrate. The conditions for film forming were as follows. In FIG. 3 ( 1 ) “NFB” stands for Nd 2 Fe 14 B.

<Film Forming Conditions>

A) lower Ta layer: formed at room temperature

B′) Nd 2 Fe 14 B layer+Nd layer: film formation at 550° C.+annealing at 600° C. for 30min

C) Ta spacer layer (intercalated layer)+α-Fe layer+Ta cap layer: film formation between 200 to 300° C.

wherein the Nd 2 Fe 14 B layer of B′) is a hard magnetic phase, the Ta spacer layer of C) is an intercalated layer between a hard magnetic phase and a soft magnetic phase, and the α-Fe layer of C) is a soft magnetic phase.

The Nd layer formed on the Nd 2 Fe 14 B layer was diffused and infiltrated into a grain boundary phase of a Nd 2 Fe 14 B phase during annealing.

A TEM micrograph of a cross-sectional structure of the obtained nanocomposite magnet is shown in FIG. 3 ( 2 ).

<Evaluation of Magnetic Properties>

The magnetization curve of the nanocomposite magnet produced in the current Example is shown in FIG. 4 .

The directions of an applied magnetic field are vertical (plotted as filled circles in the Figure) and parallel (plotted as filled squares in the Figure) to the surface of a formed film.

Coercive force of 23.3 kOe, residual magnetization of 1.5 T, and maximum energy product of 54 MGOe were obtained in the vertical direction to the formed film surface.

In the current Example, a higher coercive force compared to Example 1 could be obtained by diffusion of Nd into a grain boundary phase of a Nd 2 Fe 14 B phase. As a diffusing component, in addition to Nd, also a Nd-Ag alloy, a Nd-Al alloy, a Nd-Ga alloy, and a Nd-Pr alloy can be utilized.

A film with the structure illustrated schematically in FIG. 5 was formed by sputtering on a thermally-oxidized film (SiO 2 ) of a Si single crystal substrate. The conditions for film forming were as follows. In FIG. 5 “HM” stands for Nd 2 Fe 14 B layer (30 nm)+Nd layer (3 nm).

<Film Forming Conditions>

A) lower Ta layer: formed at room temperature

B′) Nd 2 Fe 14 B layer+Nd layer: film formation at 550° C.+annealing at 600° C. for 30min

C) Ta spacer layer+Fe 2 Colayer+Ta cap layer: film formation between 200 to 300° C.

wherein the Nd 2 Fe 14 B layer of B) is a hard magnetic phase, the Ta spacer layer of C) is an intercalated layer between a hard magnetic phase and a soft magnetic phase, and the Fe 2 Co layer of C) is a soft magnetic phase.

As illustrated in FIG. 5 , in the 1st cycle, the above A)+B′)+C) were conducted, then in the 2nd to 14th cycles B′)+C) were repeated, and in the 15th cycle B′)+film formation of Ta cap layer were conducted. In other words, 15 HM layers (=Nd 2 Fe 14 B layer+Nd layer) were stacked. In each HM layer, a Nd layer formed on a Nd2Fe14B layer diffused and infiltrated into a grain boundary phase of a Nd 2 Fe 14 B phase during annealing.

A TEM micrograph of a cross-sectional structure of the obtained nanocomposite magnet is shown in FIG. 6 .

<Evaluation of Magnetic Properties>

The magnetization curve of the nanocomposite magnet produced in the current Example is shown in FIG. 7 .

The directions of an applied magnetic field are vertical (plotted as filled circles in the Figure) and parallel (plotted as filled squares in the Figure) to the surface of a formed film.

Coercive force of 14.3 kOe, residual magnetization of 1.61 T, and maximum energy product of 62 MGOe were obtained in the vertical direction to the formed film surface. In particular, the value 1.61 T of residual magnetization exceeds a theoretical residual magnetization value of a single phase texture of Nd 2 Fe 14 B.

Comparative Example

As a Comparative Example, a conventional Nd 2 Fe 14 B based rare-earth nanocomposite magnet, in which a non-ferromagnetic phase according to the present invention was not intercalated between a hard magnetic phase and a soft magnetic phase, was produced.

A film with the structure illustrated schematically in FIG. 8 ( 1 ) was formed by sputtering on a thermally-oxidized film (SiO 2 ) of a Si single crystal substrate. The conditions for film forming were as follows. In FIG. 8 ( 1 ) “NFB” stands for Nd 2 Fe 14 B.

<Film Forming Conditions>

A) lower Ta layer: formed at room temperature

B) Nd 2 Fe 14 B layer: film formation at 550° C.+annealing at 600° C. for 30min

C) α-Fe layer+Ta cap layer: film formation between 200 to 300° C.

wherein the Nd 2 Fe 14 B layer of B) is a hard magnetic phase, and the α-Fe layer of C) is a soft magnetic phase.

A TEM micrograph of a cross-sectional structure of the obtained nanocomposite magnet is shown in FIG. 8 ( 2 ). There is not a non-ferromagnetic phase (Ta phase) intercalated between a Nd2Fe14B layer as a hard magnetic phase and an α-Fe layer as a soft magnetic phase. As remarked in FIG. 8 ( 2 ) as “No Fe”, an α-Fe layer as a soft magnetic phase has disappeared by diffusion at some region. At the region, a nanocomposite magnet structure is broken.

<Evaluation of Magnetic Properties>

The magnetization curve of the nanocomposite magnet produced in the current Comparative Example is shown in FIG. 9 .

The directions of an applied magnetic field is vertical to the formed film surface.

Coercive force of 6 kOe, residual magnetization of 0.7 T, and maximum energy product of 6 MGOe were obtained in the vertical direction to the formed film surface.

The magnetic properties obtained in the Comparative Example and Examples 1 to 3 are summarized in Table 1.

As obvious from Table 1, with respect to Nd 2 Fe 14 B based rare-earth nanocomposite magnets, in which combinations of components of a hard magnetic phase and a soft magnetic phase are equivalent, a texture according to the present invention including a non-ferromagnetic phase intercalated between the hard magnetic phase and the soft magnetic phase has improved significantly all of coercive force, residual magnetization, and maximum energy product, compared to a texture according to a conventional art not having a non-ferromagnetic phase intercalated between the hard magnetic phase and the soft magnetic phase.

Influences of the thickness of a non-ferromagnetic phase Ta and the thickness of a soft magnetic phase Fe 2 Co in a structure according to the present invention were examined. Further, for comparison, case without a Ta layer or a Fe 2 Co layer were also examined.

A film with the structure illustrated schematically in FIG. 10 was formed by sputtering on a thermally-oxidized film (SiO 2 ) of a Si single crystal substrate. The conditions for film forming were as follows. In FIG. 10 “NFB” stands for Nd 2 Fe 14 B.

<Film Forming Conditions>

A) lower Ta layer: formed at room temperature

B) Nd 2 Fe 14 B layer: film formation at 550° C.+annealing at 600° C. for 30 min

C′) Ta spacer layer+α-Fe layer+Ta cap layer: film formation between 200 to 300° C.

wherein the Nd 2 Fe 14 B layer of B) is a hard magnetic phase, the Ta spacer layer of C′) is an intercalated layer between a hard magnetic phase and a soft magnetic phase, and the α-Fe layer of C′) is a soft magnetic phase.

Thickness of Ta spacer layer: 0 nm to 8 nm

Thickness of Fe 2 Co layer: 0 nm to 26 nm

The thicknesses of a non-ferromagnetic phase Ta and a soft magnetic phase Fe 2 Co were measured by a transmission electron micrograph (TEM).

<Influence of Ta Spacer Layer>

Change of residual magnetization Br, when the thickness of a Ta spacer layer as a non-ferromagnetic phase intercalated between a hard magnetic phase and a soft magnetic phase is changed, is shown in FIG. 11 ( 1 ). With increase of the thickness of the non-ferromagnetic phase, the volume fraction of a region generating magnetism decreases, and therefore residual magnetization decreases monotonically. To generate practical residual magnetization, it is appropriate to select the thickness of the Ta spacer layer as a non-ferromagnetic phase at 5 nm or less.

Change of maximum energy product, when the thickness of a Fe 2 Co layer as a soft magnetic phase is changed, is shown in FIG. 11 ( 2 ). As seen in the Figure, when the thickness of a soft magnetic phase exceeds 20 nm, the maximum energy product decreases sharply. Presumably, this is because magnetization reversal occurred more easily due to existence of a soft magnetic phase beyond exchange interaction length, which made coercive force and residual magnetization decrease.

Therefore the thickness of a Fe 2 Co layer as a soft magnetic phase is preferably 20 nm or less.