WO2010015769A1

WO2010015769A1 - Permanent magnet

Info

Publication number: WO2010015769A1
Application number: PCT/FR2009/051502
Authority: WO
Inventors: Dominique Givord; Nora Dempsey; Oliver Axel Gutfleisch; Alexey Dobrynin
Original assignee: Centre National De La Recherche Scientifique - Cnrs -
Priority date: 2008-08-08
Filing date: 2009-07-24
Publication date: 2010-02-11
Also published as: FR2934920A1

Abstract

Permanent magnet (10) comprising: an assembly of substantially single-crystal magnetic grains (12) of a first magnetic material A having a magnetization (M_A) at which at least one element of the 3d transition series provides the main contribution, and the magnetic anisotropy field of which, measured over at least 80% of the single-crystal grains (12), is greater than or equal to 80% of the magnetization (M_A) of the first magnetic material, a second magnetic material B covering at least 10% of the surface of each grain of the magnetic material A in which the second magnetic material B is such that its magnetization M_B is coupled, in a substantially antiparallel manner, to the magnetization M_A of the first magnetic material A.

Description

PERMANENT MAGNETS

The present invention relates to a permanent magnet having a strong coercive field and a method of manufacturing such a magnet.

The coercive field of a ferromagnetic material designates the magnetic field that it is necessary to apply to a material, having initially reached its saturation magnetization, to cancel the magnetization of the material. The coercive field is usually noted Hc.

The remanent magnetization of a magnetic material refers to the magnetization existing in the material in the absence of applied magnetic field. The remanent magnetization is usually denoted M. When the coercive field of a magnetic material is high, the material is called "hard". As an indication, the typical values of coercive field are between 10 ⁵ A / m and 2 10 ⁶ A / m. Such a material is then suitable as a material for the manufacture of permanent magnets, for example inside electric motors used in particular in the automobile or household appliances.

A permanent magnet is generally made of ferromagnetic or ferrimagnetic type materials. The most common permanent magnets are AlNiCo magnets, ferrite magnets and rare earth element magnets.

Among the magnets based on rare earth magnets neodymium-iron-boron, for example, Nd ₂ Fe _l4 B, and those of Sm (Co _b AIFE _w Zr _x Cu _y), usually designated by the term magnets type 2:17, are two main categories of permanent magnet.

NdFeB type magnets have significant values of remanent magnetization Mr and coercive field H _c , a moderate cost and ease of operation. manufacturing. The main weakness of the NdFeB type magnets is the moderate value of the Curie temperature of the order of 590 K. A significant decrease in the coercive field is observed at the Curie temperature approach. The value of the coercive field H _c at 100 ^° C. is for example equal to 50% of the value of said coercive field at ambient temperature, which makes it impossible to use such magnets above about 100 ^° C.

However, many applications require operation occasionally or frequently at temperatures above 15O ⁰ C.

For these, it is known from the state of the art that the replacement of a part of the neodymium by dysprosium in the R ₂ FeI ₄ B phase, where R is neodymium or dysprosium, leads to the formation of a pseudo-ternary compound, (Nd-Dy) ₂ Fei ₄ B. It makes it possible to substantially increase the value of the coercive field H _c because of the great anisotropy of this compound. This occurs at the expense of a significant loss of magnetization, as well as a substantial increase in the cost of the material because dysprosium is about 10 times more expensive than neodymium.

In Sm-Co magnets (SmCo ₅ or 2: 17 magnets) coercive fields higher than RFeB magnets are achieved. These magnets retain a significant coercitivity up to high temperature, sometimes up to 400 ⁰ C. But, they are much higher costs than NdFeB magnets and have a lower remanent magnetization. There is therefore a need for a permanent magnet that has a large coercive field and magnetization, in particular that could be used at high temperature without the disadvantages of the solutions of the state of the art, particularly in terms of cost. An object of the present invention is to provide a new permanent magnet reinforced coercive field and a method of manufacturing such a magnet.

The invention thus proposes a permanent magnet which comprises:

a set of substantially monocrystalline magnetic grains of a first magnetic material A having a magnetization M _A to which at least one element or combination of elements of the 3d transition series provides the main contribution, and whose field of magnetic anisotropy, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material, - a second magnetic material B covering at least 10% of the surface each grain of the magnetic material A where the second magnetic material B is such that its magnetization M _B couples substantially antiparallel to the magnetization M _A of the first magnetic material A, property retained when the permanent magnet is subjected to a field external magnet H substantially antiparallel to the magnetization M _A of the first material A.

When a permanent magnet according to the invention is placed in an applied magnetic field Happ, of opposite orientation to that of the magnetization M _A of the first magnetic material A, the magnetization of the second magnetic material B is oriented according to the field magnetic applied and tends to oppose the reversal of the magnetization of the first magnetic material A.

The inventors have demonstrated that the threshold of 10% of the surface of each grain of the magnetic material A covered by the second magnetic material B makes it possible to increase the value of the field coercive significantly.

By this mechanism, the coupling between the first and the second magnetic material, said super-ferrimagnetic coupling, is a source of increase in coercivity and a source of improvement in the performance of permanent magnet systems.

A permanent magnet according to the invention may further comprise one or more of the following optional features, considered individually or according to all the possible combinations:

the element of the transition series 3d is chosen from the list consisting of iron, cobalt, nickel, manganese and chromium;

the second magnetic material B covers at least the surfaces substantially perpendicular to the direction of the magnetization M _A of each magnetic grain;

the second magnetic material B covers at least 50% of the surface of each magnetic grain; the second magnetic material B is in the form of at least one layer on the surface of each magnetic grain with a thickness greater than or equal to 1 nm and / or less than or equal to 5 μm;

the second magnetic material B is arranged in such a way that, when it covers said magnetic grains, said second magnetic material B has a magnetization M _B substantially antiparallel to the magnetization M _{A of} said magnetic grains; the first magnetic material A is chosen from the list comprising FePt, CoPt and the compounds of type RM ₅ or R ₂ M ₁₇ (type 2: 17), or R ₂ M ₁₇ N ₈ , or R ₂ M ₁₄ B with R a rare earth element and M an element of the 3d transition series, or a hard magnetic compound such as MnBi or MnAl, or a hexagonal ferrite with Sr or Ba base;

the second magnetic material B is chosen from the list comprising rare earth-based compounds or alloys, compounds or alloys based on a member of the 3d transition series, compounds of the R-Fe or R-Co type; with R a lanthanide.

the second magnetic material B is chosen from the list comprising:

the compounds and alloys of rare earths (R), R being predominantly Gd, Dy or Tb or their combination,

compounds and alloys of RM type where R is defined as above and M is predominantly Mn, Fe, Co, Ni or their combination, ferromagnetic or ferrimagnetic compounds based on Mn, for example MnAl, MnBi or an alloy Heusler such as Cu2MnAl, or ferromagnetic or ferrimagnetic compounds based on Cr, for example CrO2.

The invention also relates to a method for manufacturing hard magnetic grains, comprising the following steps:

a set of substantially monocrystalline magnetic grains is provided composed of a first magnetic material A having a magnetization M _A to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field , measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material,

at least 10% of the surface of magnetic grains is covered with a second magnetic material B, the second magnetic material B being such that its magnetization MB couples substantially antiparallel to the magnetization M _A of the first magnetic material A.

The invention also relates to a method of manufacturing a permanent magnet comprising the following steps:

- Powders containing a first magnetic material A and a second magnetic material B are mixed, densifying said mixture of the first and second magnetic materials A and B; where the first magnetic material has a magnetization M _A to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material.

For example, the manufacturing method according to the invention may comprise a sintering step in the liquid phase at a sintering temperature greater than or equal to the melting temperature of the second magnetic material B and less than the melting temperature of the first magnetic material A The manufacturing method according to the invention may also comprise a compaction step by any suitable method, at room temperature or at high temperature.

The invention also relates to a magnet motor comprising at least one permanent magnet according to the invention.

The invention also relates to an electrical generator comprising at least one permanent magnet according to the invention. The invention also relates to any system magnets, for example magnets for medical imaging, traveling wave tubes, or magnetic bearings comprising at least one permanent magnet according to the invention. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings in which:

- Figure 1 is a schematic representation of a section of a permanent magnet according to a first embodiment of the invention;

- Figure 2 is a schematic representation of a section of a permanent magnet according to a second embodiment of the invention; and - Figure 3 shows hysteresis cycles of permanent magnets.

For the sake of clarity, the different elements shown in the figures are not necessarily to scale. For the purposes of the invention, the term "magnetic grain" means a piece of material of substantially monocrystalline structure having ferromagnetic or ferrimagnetic properties.

In the context of the invention, X-Y is the alloy of elements X and Y, XY is the compound consisting of elements X and Y.

The invention relates to a permanent magnet comprising first and second magnetic materials. The first and second materials are such that the interfacial exchange coupling between the two materials promotes the antiparallelism of the magnetizations of said first and second magnetic materials.

According to a first embodiment shown in FIG. 1, the permanent magnet 10 comprises a set of magnetic grains 12 of a first material magnetic magnet A having a magnetization M _A and a second magnetic material B covering the surfaces 14 of the magnetic grains 12 substantially perpendicular to the direction of the magnetization M _A of the magnetic grains 12. The first magnetic material A has a magnetization M _A at which minus one element or combination of elements of the 3d transition series provides the main contribution, and whose magnetic anisotropy field, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material.

According to an embodiment of the invention, the at least one element of the 3d transition series having a dominant contribution to the magnetization of the first magnetic material A is chosen from the list consisting of iron, cobalt, nickel, manganese and chromium.

In addition, the first magnetic material A may have ferromagnetic or ferrimagnetic hard properties. The first magnetic material A may consist of a high-anisotropy type compound RM ₅ , in which R denotes a rare earth element or a combination of rare earth elements, for example Sm, Pr or Y, and M denotes an element of the 3d transition series for example Co.

The first magnetic material A may also be an uniaxial anisotropic compound of the RM type in which R denotes a rare earth element or a combination of rare earth elements and M denotes an element from the series of 3d, Fe transitions and optionally including additional elements such as Zr, Cu.

The first magnetic material A can also be a compound of the type R ₂ M ₁₄ B in which R is chosen from Nd and / or Pr and M is predominantly Fe or Co. The first magnetic material A can also be a compound of FePt or Co or Pt type.

According to the invention, the second magnetic material B covers at least 10% of the surface of each magnetic grain 12 and its magnetization M _B is substantially antiparallel to the magnetization M _A of the first magnetic material when the permanent magnet according to the The embodiment shown in FIG. 1 is subjected to an external magnetic field H substantially antiparallel to the magnetization M _A of the first magnetic material 1.

According to one embodiment of the invention, the second magnetic material B represents less than 20%, for example less than 10% of the total volume of the permanent magnet according to the invention. The second magnetic material B may be a rare earth metal of the second series, for example Gd, Tb or Dy or an alloy based on such elements. The material B may also be composed of an alloy of such a rare earth metal of the second series and of a non-magnetic element such as Al, Zn to constitute a ferromagnetic material.

The second magnetic material B may consist of a ferromagnetic or ferrimagnetic alloy R-M, where R contains one or more elements of the second part of the lanthanide series and M contains Fe and / or Co.

The second magnetic material B may be a ferromagnetic or ferrimagnetic compound or alloy based on elements of the series of 3d transitions, for example Mn or Cr. The second magnetic material B may for example be chosen from the following compounds:

- MnBi of hexagonal structure,

- MnAl of LlO structure,

- Heusler alloys, for example Cu ₂ MnAl - CrO ₂ . According to an embodiment shown in FIG. 2, the permanent magnet 10 comprises a set of substantially monocrystalline magnetic grains 12 of a first magnetic material A having a magnetization M _A and a second magnetic material B covering substantially the entire surface of the grains 12 and being arranged so that the magnetization M _B of the second magnetic material B is substantially antiparallel to the magnetization M _A of the first magnetic material A when the permanent magnet 10 is subjected to an external magnetic field H substantially antiparallel to the magnetization M _A of the first magnetic material A.

Advantageously, such an arrangement of the second magnetic material B makes it possible to increase the value of the coercive field of the permanent magnet 10.

The first and second magnetic materials A and B may have the same composition as in the first embodiment. A permanent magnet according to the invention may comprise hard magnetic grains prepared according to a manufacturing method comprising the following steps:

the set of substantially monocrystalline magnetic grains 12 is composed of a first magnetic material A having a magnetization M _A to which at least one element of the 3d transition series provides the main contribution, and whose anisotropy field magnetic measurement, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material,

a second magnetic material B is provided, for example in the form of a powder,

the sintering in the liquid phase is carried out at a sintering temperature greater than or equal to melting temperature of the second magnetic material B and less than the melting temperature of the first magnetic material A.

A permanent magnet according to the invention can be obtained by a method comprising the following steps:

- Powders are mixed comprising a first magnetic material A and a second magnetic material B,

densifying said mixture of materials A and B by any suitable method, wherein the first magnetic material has a magnetization M _A to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field , measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material.

The method according to the invention may also comprise a step of densifying the mixture A-B at low temperatures.

A magnet according to the invention can be obtained by coating the magnetic grains 12, then a suitable compaction process, for example under the effect of a pulsed magnetic field. Advantageously, such a compaction method allows temperature-based densification, preserving the interfacial exchange coupling between the first magnetic material A and the second magnetic material B according to the so-called super-ferrimagnetic magnetization configuration. A manufacturing method according to the invention makes it possible advantageously to obtain a permanent magnet with a strong coercive field.

A manufacturing method according to the invention may also comprise a step where the grains A of a powder of a layer of B are covered, for example according to a structure of the heart / shell type.

A permanent magnet according to the invention can also be obtained by means of a process comprising the deposition of successive layers of magnetic materials A and B, for example by a technique such as cathodic sputtering, pulsed laser deposition or any other suitable method. .

A manufacturing method according to the invention may also comprise an etching step. FIG. 3 represents hysteresis cycles illustrating the increase of the coercive field of the magnets according to the invention.

The hysteresis cycle 20 in FIG. 3 corresponds to that of a layer of a first NdFeB magnetic material of 100 nm in thickness. The coercive field corresponding to the hysteresis cycle 20 of FIG. 3 is equal to 0.01 Tesla.

The hysteresis cycle 22 shown in FIG. 3 corresponds to a 50 nm thick layer of GdFe ₂ , the coercive field of such a layer can be measured at 0.002 Tesla.

The hysteresis cycle 24 shown in FIG. 3 corresponds to that of a permanent magnet according to the invention. Said permanent magnet comprises a structure as shown in FIG. 1 with the first magnetic material having a columnar structure of NdFeB of approximately 100 nm in length and 30 nm in section and as second magnetic material two layers of GdFe ₂ of 50 nm in diameter. each thickness. Each column of NdFeB corresponds to a monocrystalline grain according to the invention. The GdFe layers ₂ cover the surfaces of the NdFeB columnar structures substantially perpendicular to the direction of permanent magnetization of said NdFeB columnar structures. The coercive field of said permanent magnet Ambient temperature is about 1 Tesla, about 100x greater than that of the NdFeB layer alone.

The invention is not limited to the embodiments described and will not be interpreted in a limiting manner, and encompasses any equivalent embodiment.

Claims

A permanent magnet (10) comprising:

a set of substantially monocrystalline magnetic grains (12) of a first material having a magnetization (M _A ) to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material,

a second magnetic material B covering at least 10% of the surface of each grain of the magnetic material A where the second magnetic material B is such that its magnetization M _B couples substantially antiparallel to the magnetization M _A of the first magnetic material AT.

2. Magnet according to claim 1, characterized in that the at least one element of the 3d transition series is selected from the list consisting of iron, cobalt, nickel, manganese and chromium.

3. Magnet according to one of claims 1 or 2, characterized in that the second magnetic material (B) covers at least the surfaces (14) substantially perpendicular to the direction of magnetization (M _A ) of each magnetic grain ( 12).

4. Magnet according to any one of the preceding claims, characterized in that the second magnetic material (B) covers at least 50% of the surface of each magnetic grain (12).

5. Magnet according to any one of preceding claim, characterized in that the second magnetic material (B) is in the form of at least one layer on the surface of each magnetic grain (12) having a thickness greater than or equal to 1 nm and / or less or equal to 5 μm.

6. Magnet according to any one of the preceding claims, characterized in that the second magnetic material (B) is arranged so that when it covers said magnetic grains (12) said second magnetic material (B) has a magnetization (M _B ) substantially antiparallel to the magnetization (M _A ) of said magnetic grains (12).

Magnet according to any one of the preceding claims, characterized in that: the first magnetic material (A) is chosen from the list comprising FePt, CoPt and the compounds of type RM ₅ or R ₂ M ₁₇ (type 2: 17 ), or R ₂ M ₁₇ Ns, or F ^ M ₁₄ B with R a rare earth element and M an element of the transition series 3d, or a hard magnetic compound such as MnBi or MnAl, or a hexagonal ferrite based Sr or Ba;

the second magnetic material (B) is chosen from the list comprising the compounds or alloys based on rare earth, the compounds or alloy based on an element of the 3d transition series, the compounds of the R-Fe or R-type Co with R a lanthanide.

8. A method of manufacturing hard magnetic grains, comprising the following steps: there is a set of substantially monocrystalline magnetic grains (12) composed of a first magnetic material (A) having a magnetization (M _A ) to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization ( M _A ) of the first magnetic material, at least 10% of the surface of magnetic grains (10) is covered with a second magnetic material (B), the second magnetic material (B) being such that its magnetization (M _B ) couples substantially antiparallel to the permanent magnetization (M _A ) of the first magnetic material (A).

9. A method of manufacturing a permanent magnet comprising the steps of: mixing powders comprising a first magnetic material A and a second magnetic material B, densifying said mixture of the first and second materials A and B, characterized in that the first magnetic material having a magnetization (M _A ) to which at least one element of the 3d transition series provides the main contribution, and whose magnetic anisotropy field, measured on at least 80% of the monocrystalline grains (12), is greater than or equal to 80% of the magnetization (M _A ) of the first magnetic material.

10. The method of claim 9 characterized in that it comprises a compaction step.

11. Magnet motor characterized in that it comprises at least one permanent magnet (10) according to any one of claims 1 to 7.

12. Electric generator characterized in that comprises at least one permanent magnet (10) according to any one of claims 1 to 7.

13. Magnet system characterized in that it comprises at least one permanent magnet (10) according to any one of claims 1 to 7.