FR2664086A1 - IMPROVED PROCESS FOR THE OPTIMIZATION OF MAGNETIC PROPERTIES OF POWDER MAGNETIC MATERIALS AND PRODUCTS THUS OBTAINED. - Google Patents

Abstract

A method for improving the magnetic properties of a polyphase product having permanent-magnet-like properties at room temperature, acting as a precursor and having been processed by hydridizing crackling at a low pressure and a low temperature to obtain an intermediate hydride in powdered form. According to the method, the intermediate hydride in powdered form undergoes, after hydridizing, a first partially dehydridizing heat treatment in a vacuum and at a low temperature below its segregation temperature, and the non-segregated product thereby obtained undergoes a second, subsequent intense dehydridizing heat treatment in a vacuum and at a temperature of around 600 DEG C.

Description

IMPROVED PROCESS FOR THE OPTIMIZATION OF MAGNETIC PROPERTIES OF POWDER MAGNETIC MATERIALS AND
PRODUCTS THUS OBTAINED.

 The invention relates to an improved method for optimizing the magnetic properties of a material with permanent magnet properties, in order to obtain a product with high magnetic performance and in finely divided form. More specifically, it relates to a process capable of increasing the internal magnetic energy of such a material, of the rare earth / iron / boron alloy type, obtained after decrepitation by the hydriding-dehydriding procedure. Finally, it also relates to the products obtained by this process.

 The need for materials with high magnetic properties, in powder form is growing every day, especially in the context of their use for the production of sintered or bonded magnets (injected or pressed).

 The production of bonded magnets is basically carried out by introducing a large amount of magnetic material in the most divided form possible, into a continuous organic matrix, generally made of a synthetic polymer. This step is carried out in the traditional way by means of a twin-screw, at the melting point of the polymer. In this way, in order to obtain high performance bonded magnets, it is sought to introduce into the matrix the greatest possible quantity of magnetic material. In the context of optimizing such magnets, the aim is to minimize the size of the " particles "constituting the magnetic material, while increasing the magnetic properties, and in particular the coercivity of said particles. In addition, it is important that the size distribution of these "particles" be as tight as possible, in particular in order to optimize the magnetic properties (coercivity, induction) of the bonded magnet.

 This property is particularly important in the context of the production of magnets linked to strong anisotropy. Indeed, on this small distribution, and on the effective size of the "particles" obtained, depend on the one hand, the dispersibility of the powders, namely, their ability to disperse homogeneously, for example in the matrix or resin coating, and on the other hand their orientability, namely, their ability to orient themselves under magnetic field, and more precisely to align their direction of easy magnetization with the direction of the applied magnetic field, and this by mechanical rotation.

 The interest of obtaining powders with high magnetic properties of small homogeneous particle size thus appears clearly. Bonded anisotropic magnets made from these powders are likely to have a much higher residual induction than those made to date with the powders available today, and this, from the same "absolute" quantity of magnetic material. One of the aims of the present invention is to propose a process capable of manufacturing such powders, having a high coercivity.

 With regard to obtaining magnetic materials in powder form, there has been described in European patent EP-A-0 173 588, a process called "decrepitation" comprising at least one hydriding-dehydriding cycle. a compound corresponding to the general formula R2M14B, where B denotes boron, R denotes an element belonging to the Rare Earth family or Yttrium, M denoting essentially iron, and which can be partially substituted by other metallic elements, making it possible to result in a product of small particle size, and endowed with interesting magnetic properties in terms of its magnetization, but generally having an insufficient level of coercivity.

 We have indeed been able to show that the magnetic properties of a material were inter alia linked to the definition of its microstructure (domain, grain, texture, etc.). In this way, we sought to obtain materials finely divided into particles of small sizes, typically ten micrometers and less, by a process easy to implement, inexpensive and in the alternative harmless for experimenters and manufacturers. This process consists in obtaining the spraying of the base material by hydriding. This process, called decrepitation, consists briefly of absorbing hydrogen by a metal alloy under certain conditions of pressure and temperature. This absorption, the chemical reaction of which binds atomic hydrogen at particular sites of the material, causes a release of heat, induces an increase in the volume of the alloy, consecutive to the expansion of the crystalline mesh, leading in fact to a dislocation of the solid material. This takes place at two levels, namely at an inter-granular level, that is to say between the entities of the same phase, and at an intra-granular level, corresponding to the "bursting" of the entities of the same phase. A powder is obtained in the agglomerated state, which it is possible to disperse by simple stirring.

 It has been found that, although the products obtained have a particle size entirely in relation to the desired objective, on the other hand, the magnetic properties obtained had to be optimized for certain particular applications, in particular for permanent magnets, or for bonded magnets. Indeed, the powders obtained certainly have a particle size close to ten micrometers, or even lower, on the other hand the internal magnetic energy (HB) max developed by them remains moderate, and in any case insufficient to be used as such in the production of high capacity permanent-polymer magnets.

 The object of the invention consists, starting from materials having qualities of permanent magnets - either intrinsically or potentially (example amorphous product) - to obtain powders having the same magnetic properties as their precursors by applying heat treatments corresponding to particular conditions. The invention also aims to obtain powders of small homogeneous particle size, endowed with these magnetic properties. After decrepitation obtained by hydriding, then partial dehydriding carried out under primary vacuum and at a temperature below the demixing temperature of the hydride obtained, temperature typically in the region of 520 "C, possibly followed by thermal steps, a second heat treatment is applied at a temperature in the region of 600" C, that is to say a temperature above the desorption temperature of the hydrides of the main phase of the material.

 It has been found that this thermal post-treatment makes it possible to obtain dehydriding of all of the constituent phases of the base alloy. Indeed, as we know, whatever the method of obtaining the latter, we must go through a step of melting the base material in order to obtain an alloy in massive form. This fusion not being congruent, there exists between the preponderant entities, constitutive of the "magnetic" phase proper, one or more secondary phases with eutectic behavior, richer in rare earth elements. In fact, the subsequent heat treatments aim to dehydrate this or these secondary phases. Finally, by a third heat treatment, the aim is to reshape the envelope with a concentration rich in rare earth elements.

 By "primary vacuum" is meant in the sense of the invention a vacuum preferably less than 10-2 to 10-4 millimeters of mercury (or about 1 to 10-2 Pa). This primary vacuum is intended to allow the evacuation of hydrogen gas as it is formed. The duration of the thermal dehydriding treatments is also linked to the restoration of the initial primary vacuum.

 The duration of the dehydriding treatment depends on the base material used. It is followed by cooling at constant speed, speed also depending on the starting material.

 In other applications, this first post-treatment can be followed by a thermal plateau, then by subsequent thermal treatments, the purpose of which is similar to the first.

The second advanced dehydriding treatment, as well as that of high temperature annealing (third treatment) cannot be carried out directly on bonded magnets, given the low melting temperature of their matrix. In this way, in order to increase the magnetic properties of the bound magnets, it was important to increase the magnetic performance of the powders obtained by decrepitation, according to the process described in the document.
EP-A-0 173 588 already cited.

In the publication TAKESHITA et al (N "18P0216 of the Oth International Workshop on Rare-Earth Magnets and
Their Applications, Kyoto, Japan, May 16-19, 1989), it has certainly been shown that by submitting an ingot of an alloy
Nd-Fe-B at a stream of hydrogen gas at a temperature between 750 and 900 "C for a period of between one and three hours, followed by a phase of about one hour under vacuum, at the same temperature, and finally from quenching to room temperature, powders with relatively good magnetic properties could be obtained, on the other hand, such a process leads to the demixing of the main phase of the basic multiphase material, and can only lead to the production of magnetically isotropic products, whatever the precursor.

 Indeed, only powders with anisotropic magnetic characteristics make it possible to obtain, by orientation under a magnetic field, sintered or bonded materials, the persistence of which increases by effect of the orientation in a parallel direction of the particles. On the contrary, the use of magnetically isotropic powders does not make it possible to increase the magnetization (the persistence) of sintered or bonded materials produced from them, when they are oriented under a magnetic field.

 To date, we do not know how to prepare fine and homogeneous powders of rare earth / iron / boron alloys, having a very strong anisotropy and high magnetic properties, allowing for example an increase of more than 60 to 80% of their magnetization. by orientation under field, and adapted to the realization of magnets linked to very strong magnetization and coercivity.

 Various methods have however been proposed for preparing powders having a certain degree of anisotropy. One of them, described for example in document EP-A-0 302 947, consists in mechanically grinding a solid anisotropic alloy which has previously undergone an appropriate mechanical treatment. If the products obtained certainly have a certain inherent magnetic anisotropy to the starting product, and not or only slightly altered by the treatment which it undergoes, on the other hand, the coercivity which they develop after treatment is relatively low, the latter being greatly altered by the particle size reduction treatments, in particular mechanical. I1 thus emerges that the mechanical grinding proves to be particularly ill-suited to obtaining homogeneous fine powders, taking into account the considerable degradations of the magnetic properties of the precursor which it generates.

 Another approach, described in particular in document EP-A-0 304 054, on the contrary consists of starting from an isotropic powder of fine and uniform particle size, obtained for example by decrepitation with hydrogen at very high temperature (600 to 900 "C), then subjecting this powder to a treatment of the hot plastic deformation type (analogous to that carried out in the previous case at the level of the precursor) intended to induce in said powder a certain degree of anisotropy without however risking causing its Sintering. It is true that powders of small particle size, but whose magnetic properties, in particular the possible anisotropy of the precursor, are considerably reduced or even canceled, due to the separation of the magnetic phases constituting the basic magnetic structure, this separation being inherent in treatment under hydrogen at high temperature.

 On the contrary, in the context of the present invention, a mode of treatment has been targeted associated with a composition of suitable precursor making it possible to induce a maximum level of magnetic anisotropy at the level of this precursor. A size reduction technique was used, such as hydrogen decrepitation practiced under moderate temperature conditions, followed by an appropriate post-treatment of dehydrogenation, capable of fully preserving the very strong anisotropy of the precursor. implemented for this purpose.

 The starting product therefore plays a fundamental role both in terms of its composition and its isotropic or anisotropic nature, the latter being preserved through successive stages of decrepitation and post-treatments. In fact, this product is advantageously a rare earth / iron / boron alloy, the iron possibly being partially substituted by cobalt or by other transition elements (3d, 4d, 5d). Advantageously, and when it is desired to obtain optimum magnetic properties, part of these iron or cobalt elements can be substituted by other elements such as copper or aluminum.

 For the subsequent production of sintered or bonded oriented magnets, it is essential to start with a precursor with anisotropic magnetic properties, so as to obtain a highly anisotropic decrepit powder, the particles of which are capable of being oriented under the effect of 'an external magnetic field. In this case, this orientation will in fact contribute to considerably increasing the residual induction of the sintered or bonded magnet thus produced, while in the case of a product with isotropic magnetic properties, the orientation treatment under field magnetic would have no effect.

 A highly anisotropic precursor is obtained (in terms of its magnetic characteristics) if materials derived from "powder metallurgy", a technique described in more detail in document EP-A-0 101 552, are used, or if the 'We start from massive magnet falls. The precursors resulting from a hot-working treatment of the hammering or pressing type, described in document WO 87/07425, also exhibit these characteristics of strong magnetic anisotropy. I1 also exist such precursors with magnetic anisotropy, resulting from a specific hyper-tempering treatment.

 Solid or ribbon precursors having, on the contrary, isotropic magnetic properties are obtained within the framework of hot working process carried out by spinning, also described in document WO 87/07425, or in the hyper-quenching process on rollers, described in particular in document EP-A-0 108 474.

 The invention also relates to the product obtained.

 It is a product having good magnetic properties, typically an internal energy (HB) max greater than or equal to 80 kJ / m3 for isotropic powders and 240 kJ / m3 for anisotropic powders, with a small homogeneous particle size, typically close to ten micrometers, or less, and in any event less than fifteen (15) micrometers. Advantageously, these products have a remanent magnetization, typically at least 40 Am2 / kg for isotropic powders and 80 Am2 / kg for oriented anisotropic powders, and a high coercivity of at least 700 kA / m. In addition, when the precursor is produced by hot working, the grains of the products obtained have a characteristic facies in the form of broken crystallites, typical of the morphology resulting from this manufacturing process.

 It was certainly known to obtain materials having significant magnetic properties, such as for example an internal magnetic energy greater than or equal to 200 kJ / m3. This type of material is for example described in the publication SHIMODA et al (J. APPL. PHYS.

64 10) in which it has been proposed to produce permanent magnets based on a praseodymium / iron / boron / copper mixture and this with a low reduction rate, in particular less than 90%. This production process is carried out by hot pressing, at a temperature of approximately 1000 "C.

The magnets obtained certainly have high magnetic properties, however their size is small. In addition, given their production process, in particular by sheath rolling, we certainly obtain a refinement of the grains (however insufficient with such a reduction rate of less than 90%), and above all a heterogeneous particle size and microstructure. Finally, good magnetic properties can only be obtained by using praseodymium. Indeed, it is clearly indicated that by replacing praseodymium with neodymium, the magnetic properties tend to drop drastically. However, the drawback of using praseodymium lies in its high cost, given its low proportion in nature compared to neodymium.

 Also known are products endowed with magnetic properties, having a homogeneous and small particle size, the limit of which may be less than 0.1 Wu.

These products are for example obtained by means of the process described in the document already cited EP-A-0 173 588. As already said, the decrepitation of the products by hydriding dehydrating certainly allows to obtain products of small particle size but with a significant loss of magnetic properties.

It was then thought to apply mechanical treatments in order to reduce the particle size on products obtained by quenching and having good magnetic properties. These products are for example in the form of pre-oriented platelets, resulting in a certain anisotropy. However, it turns out in use that the application of mechanical grinding also alters the magnetic qualities, in particular because of mechanical shocks. Consequently, to date there are no powdery products having simultaneously a small homogeneous particle size, of the order of ten micrometers, or even less, and high magnetic properties, in particular for highly anisotropic powders, which develop a remanent induction in BrOrientee oriented form such as the ratio
Br0 nothing - BrnOn oriented BrO * ierltee is greater than or equal to 80 t.

 The starting material is a material which in the solid state already has high magnetic properties. The process according to the invention aims, following a decrepitation having reduced its magnetic properties, to restore them to result in magnetic properties, in particular in coercivity, and residual induction, close to those of the starting crude product.

 According to the process, the starting product is an isotropic or anisotropic polyphase alloy depending on the destination of the final product, of terrerare / iron / boron composition. In known manner, iron can be substituted by cobalt, in particular with a view to increasing the Curie point of the final product or by other 3d transition metals, such as copper, or 4d and 5d. In addition, iron can also be partially substituted by other metallic elements such as aluminum, and this cumulatively with the transition elements. One can advantageously substitute part of the rare earth atoms with others, such as dysprosium, according to the qualities of the magnetic properties required.

 This alloy is, as already said, in polyphase form, respectively a magnetic phase with high anisotropy, corresponding to the general formula R> -M14-B, and one or more other phases with a majority concentration of rare earth elements, consecutive to the mode basic material.

 I1 will not be described in detail how to obtain these starting materials. As already said, they can be obtained according to the so-called "rapid quenching" technique, mechanical hot grinding or wrought, or even by the powder metallurgy technique, that is to say sintered powders previously oriented under magnetic field.

 Once this basic material has been obtained, it is firstly hydrided by absorption of hydrogen under pressure (1 to 5 MPa) for example in an autoclave made of special steel, and generally at room temperature.

However, in some cases, thermal activation is necessary. Anyway, one or a few thermal cycles during the hydrogenation phase ensure better chemical and particle size homogeneity of the material. This hydriding leads to the fragmentation of the material, which thus becomes very easily dispersible. The revelation of the pulverulent form of the material can be obtained by simple mechanical stirring, or by simple grinding.

The hydrogenated pulverulent material undergoes, according to the process of the invention, three stages of treatment
During a first phase, partial dehydriding is carried out, which relates to the main hydrated phase R2-Nl4-B-Hx (where x is between 1 and 5), the latter transforming into R2-M14-B.

 Indeed, the hydrides formed being of the metastable type, the dehydriding must be carried out under primary vacuum at a temperature lower than their demixing temperature, failing which, the formation of rare earth hydrides, iron and an ill-defined iron-boron phase, the magnetic properties of the material then being definitively and prohibitively altered.

 The temperature of this partial dehydriding, which can start under primary vacuum around 150 "C, and which increases around 300" C, must not exceed 520 "C, demixing temperature of hydrides R2-M14-B-H.

 During a second phase of treatment, which takes place at a temperature of the order of 600 "C, complete dehydriding of the decrepit material can be carried out, in particular at the level of the eutectic phase rich in rare earths , which constitutes the film envelope of the magnetic domains This second phase is also carried out under primary vacuum.

 Finally, the dehydrated powder thus obtained can be subjected, in a third phase, to an annealing treatment between 450 and 1000 "C, aimed at completely restoring the magnetic properties, in particular the coercivity.

 It can be seen that by treating the materials under this temperature scale, any risk of sintering of the residual material is avoided, a result which would go against the desired morphology.

 The treatment can advantageously be supplemented with an in-situ passivation by introducing argon under normal pressure, before bringing the product back to its normal temperature.

 The purpose of the final heat treatments (thermal bearings) is to optimize the cohesion of the granular material at the level of the elementary particles, namely the phase of the R2-M14-B type and of its eutectic intergranular envelope. The different parameters of these heat treatments, respectively temperature, duration, cycle, are a function of the composition of the base material and their metallurgical synthesis process.

 We choose different types of starting materials (in the sense of the metallurgy of their preparation), depending on whether we want to favor the magnetization properties or coercivity properties. Likewise, and as already said, the choice of starting material also depends on the desired anisotropy of the final product. This choice is involved both in its composition and in its synthesis process.

 There is shown in Figure 1, a block diagram of the different steps involved in the production of a bonded magnet. In addition to the steps mentioned above, the powders obtained after the various heat treatments are dispersed before being coated in a resin, then oriented in the field.

 In order to illustrate the process according to the invention, various embodiments of products with the resulting magnetic properties have been described below.

Example 1: Phase a
An isotropic compound corresponding to the following chemical formula Ndl4Dylt5Nbot7Fe76tsB, obtained by hyper-quenching at the speed of 50 m / s, is used as precursor material. Sintered in massive form, such a material is known to have good magnetic properties, such as
residual induction: 85 Am2 / kg
Coercive field: 1,250 kA / m
This material undergoes a decrepitation treatment by hydriding, and the desorption is carried out by a heat treatment beyond 1800C. This treatment, aimed at desorbing the hydrogen from the main phase, is carried out at the speed of 300 "C / hour. I1 constitutes the so-called dehydriding phase, carried out under primary vacuum. I1 is followed by a thermal plateau for 1 hour at 520 "C and finally cooling at the speed of 1500C / hour.

 For this finely divided isotropic material, a residual induction of 42 Am2 / kg is obtained, but a very reduced coercive field of 120 kA / m, which renders this material unusable for shaping in the state of a bonded magnet.

Example 1: Phase b
The same treatment is repeated as phase a, from the same material and then the latter is subjected to a second heating phase at 600 "C, temperature obtained at the speed of 300" C / hour. This treatment is followed by heating to 640 "C, temperature obtained at the speed of 50 C / hour, the thermal plateau at 640" C being maintained for 30 minutes. This phase is followed by rapid cooling to 600 "C, at a speed of 1000" C / hour, followed by a temperature drop of 1500C / hour.

The results obtained are respectively
a residual induction of 56 Am2 / kg on a
non-oriented compaction sample 0.4
and a coercive field of 1,100 kA / m.

 It can be noted that the influence of the second treatment on the material makes it possible to restore the initial magnetic properties in a way.

Example 2: Phase a
We start with an isotropic precursor material corresponding to the formula Ndl6Fe77AlB6 obtained by hot working by spinning, with a working rate of 12. This material has the following magnetic characteristics in a solid state
Residual induction: 75 Am2 / kg
Coercive force: 950 kA / m
This material is decrepit and then heat treated, in the same manner as that described in Example 1 phase a. The residual induction of the non-oriented 0.4 compaction sample is 43 Am2 / kg, the coercive field being only 320 kA / m.

Example 2: Phase b
The same precursor material having undergone the treatment of Example 1 (phase a), then undergoes heating at 600 "C, temperature obtained at the speed of 300" C / hour.

 I1 is then treated according to the same process as that indicated in example 1 phase b.

 The residual induction measured on a non-oriented 0.4 compaction sample is 43 Am2 / kg, and the coercive field of 880 kA / m. As in the previous case, the isotropic magnetic characteristics of the solid material are therefore largely restored.

Example 3
We start from an anisotropic precursor material corresponding to the crude formula Nd15DyFe78B6 obtained by spinning and forging by hot working (working rate: 10). The initial magnetic characteristics of the material in massive form are respectively
. remanent induction: 75 Am2 / kg
. Coercive force: 1,430 kA / m
The material is decrepit by hydriding, then heated to 520 "C under primary vacuum, temperature obtained at the speed of 300" C / hour. It undergoes a thermal plateau lasting one hour at this temperature and is then heated to 600 "C, a temperature obtained at the speed of 300" C / hour. It is then heated to 680 "C, obtained at the speed of 100" C / hour. It then undergoes a thermal plateau for 20 minutes at 680 "C, then is rapidly cooled -up to 600" C at the speed of 600 "C / hour, followed by a temperature drop to 150 CC / hour.

 The sample, in the form of a non-oriented anisotropic powder of compaction 0.4, exhibits a residual induction of 40 Am2 / kg for a coercive field of 1,200 kA / m.

 The original magnetic characteristics of the material are again restored.

Example 4
We start from an anisotropic precusor material corresponding to the crude formula Ndl6Cu2Fe76B6 obtained by forging by hot working, with a working rate of 10. The initial magnetic characteristics of this material in massive form are respectively
. Residual induction: 116 Am2 / kg
. Coercive field: 1.030 kA / m
This material is then treated as indicated in Example 3. The magnetic properties of the powder thus obtained, with its non-oriented nature, are 0.4.
. Residual induction: 53 Am2 / kg
Coercive field: 720 kA / m.

This powder is then heat treated at 1000 "C and then at 500" C and is then coated in a resin under magnetic field. The characteristics of the oriented powder thus introduced become
. Residual induction: 96 Am2 / kg
Coercive field: 720 kA / m.

 In this way, it emerges from the preceding examples that the products obtained of typical particle size, close to ten micrometers, have quite advantageous magnetic properties, as soon as an appropriate heat treatment is applied to them. We note in particular the very good values obtained for the coercivity, values which approach the values obtained for this quantity with the massive precursor products. It therefore appears clearly the fundamental role played by the precursor material, in particular when one wishes to favor coercivity.

 All of these materials have a large residual induction, which has been characterized on a non-oriented powder sample, with a relative compactness close to 0.4.

 The powders thus obtained, taking into account their small homogeneous particle size on the one hand, and their high magnetic properties on the other hand, enabled the production of anisotropic bonded magnets, for which the measured residual induction is increased by 30 to 40% compared to the anisotropic bonded magnets available today, and this for substantially the same charge of magnetic material.

Claims (9)

 1 / Process for optimizing the magnetic properties of a multiphase product endowed with permanent magnet properties at ambient temperature, acting as a precursor, having undergone a decrepitation treatment by hydriding under low pressure at low temperature in order to obtain an intermediate hydride in powder form, characterized  in that the pulverulent intermediate hydride undergoes, following hydriding, a first heat treatment under vacuum for partial dehydriding at a temperature below its demixing temperature,  - And, in that the non-demixed product then obtained undergoes a second thermal post-treatment under vacuum of dehydriding pushed to a temperature close to 6000C.  2 / A method for optimizing the magnetic properties of an isotropic multiphase product according to claim 1, characterized in that the precursor is an isotropic material obtained by the method chosen from the group consisting of hyper quenching such as quenching on a wheel and hot working (with a working rate of at least 10)  3 / A method for optimizing the magnetic properties of an anisotropic multiphase product according to claim 1, characterized in that the precursor is an anisotropic material obtained by orientation under a magnetic field and sintering of powders, or by forging a solid or derived material a fall in the production of magnets.  4 / Method according to one of claims 1 to 3, characterized in that the dehydrated product obtained undergoes a third thermal post-treatment under -neutral atmosphere or under vacuum, at a temperature between 450 and 1,000 "C, both post -treatments which can be separated or not from a thermal bearing.  5 / Method according to one of claims 1 to 4, characterized in that the precursor mainly responds to the quadratic phase R2-M14-B, whose demixing temperature is close to 520 "C, in which  - B denotes boron  - R denotes an element of the land family  rare or Yttrium,  - and M denotes iron, possibly partial  ment substituted by a transition element,  such as cobalt, and / or other elements  metallic, such as in particular aluminum and  the copper.  6 / Magnetic compounds obtained according to the method defined in claim 5, of homogeneous morphology of quadratic majority phase R2-M14-B, in which:  - B denotes boron;  - R denotes an element of the land family  rare or Yttrium,  - and M denotes iron, possibly partial  ment substituted by a transition element,  such as cobalt and / or other metallic elements, characterized in that they are in the form of a homogeneous particle size powder of average size less than or equal to fifteen micrometers, and in that they develop a coercive field at least 700 kA / m and a residual induction of at least 40 Am2 / kg.  7 / magnetic compounds according to claim 6, characterized in that they are magnetically isotropic  8 / New magnetic compounds according to claim 6, characterized in that they are magnetically anisotropic in that they develop a remanent induction in Broriente oriented form such that the ratio  Brorientée - Brnon oriented Brovi = toe is greater than or equal to 80 *.  9 / Magnetic compounds according to one of claims 6 to 8, characterized in that the grains which constitute them mainly contain crystallites of the R2-Ml4-B phase, the morphology of which is identical to that of the grains of the precursor.