DE4107499A1 - New magnetic material based up coupled exchange of hard and soft materials - with alloy including rare earth materials and with remanence to saturation magnetisation of more than 0.6 - Google Patents

Abstract

A ferromagnetic material is based upon an exhange coupling mechanism in which a hard magnetic phase (1) and a soft magnetic phase (20 are combined. While hard magnetic materials have a high anisotropy and a relatively low magnetic saturation, the soft materials exhibit the opposite effect. By coupling the materials an optimum is achieved. The volume of the soft magentic material is selected to be greater than that of the hard magnetic material. After removing a magnetic field from the material the remnant field effect is such that MRev is greater than 2.5. The material is an alloy of REx.Fey.Bz.Vu.Siv, where REx is a rare erth material. USE/ADVANTAGE - High performance material for permanent magnets, broad band microwave absorbers, magnetic recording material.

Description

The invention relates to a new category of ferromagneti materials consisting of two phases with exchange coupling, the composite material being both high seed magnetization as well as a high coercive force can own as well as the production and application of these Materials.

DE 23 44 644 includes a magnetizable material Exchange anisotropy known, consisting of a ferri- or ferromagnetic with an antiferro coupled to it magnetic. This material is characterized by that below a given temperature that is less than is the Nel temperature of the antiferromagnetic which is used irreversible rotation of the magnetization in antiferromagne critical magnetic field and material also the magnetic field to generate any magnetization structure leading to irreversible rotation of the magnetization leads in the antiferromagnetic, are stronger than the strongest technically feasible recording magnetic field. Thereby all magnetization structures disappear in magnetic field those who are smaller or the same as those by the strongest Magnetic field generated after switching off the generating magnet Fields in whole or in part again, such that previously at Si fixed at a temperature above this temperature gnale either completely or partially by itself can be restored or restored. So that can a magnetic recording medium records ge be made against subsequent unnoticed changes are protected.  

The present invention relates to another principle of exchange coupling and to a corresponding one accordingly constructed ferromagnetic material, namely a kind of exchange spring mechanism. Be at this two ferromagnetic phases, one magnetically hard and a magnetically soft, combined with each other. Magnetic hard materials generally have a high anisotropy with a relatively low saturation magnet sation, while magnetically soft materials are very large saturation magnetization and a very small aniso exhibit tropics. By combining the two above Phases through the exchange coupling succeed the benefits to get both phases at the same time. An exchange spring Ma gnet is characterized in that a coupling the spins between the magnetically hard and the magnetic soft phase is present, which is done either directly or through a coherence broker who checks the coherence mediated between the hard and soft magnetic phases.

In order to obtain the desired high total magnetization, the volume fraction of the hard magnetic phase is lower to choose as the proportion of the soft magnetic phase. There however, the necessary requirement for the existence an exchange spring magnet the exchange coupling between which is magnetically hard and which is magnetically soft phase, this depends on the distances between the hard magnetic Inclusions and their dimension itself. It has shown that for optimal implementation of the present Invention the distances between the hard magnetic one conclusions should be of the same order of magnitude as the dimension of the inclusions themselves.

To understand the spatial distribution of the hartmagneti inclusions in the soft magnetic matrix based on a model presentation in which the hard magnetic inclusions like in a face-centered cubic  Grid on the corners and center of a cube to sit. Since hard magnetic materials are generally a strong crystal anisotropy due to a corresponding crystal create structure (e.g. hexagonal or tetragonal) and additionally a magnetic (and therefore also a crystallo graphical) coherence between the magnetically hard and soft phase must be present, the crystal structure structures should be compatible with each other. Accordingly, e.g. Legs isotropic orientation of the hard magnetic inclusions in the soft magnetic matrix not be magnetically coherent. Particularly good properties can be achieved with a kubi obtained crystal structure of the soft magnetic matrix. In this case, due to the cubic symmetry Elimination crystallized in several directions be animal. So the C axes of the hard magnetic Inclusions parallel to the three (equivalent) crystal orientate directions. For statistical reasons, it is Orientation of the hard magnetic inclusions is then the same Share on the excellent directions of the soft magnet distribution, for example one third each on [100], [010] and [001]. In this way it forms the configuration mentioned above, and each of the particles has no magnetic preferred axis. From these considerations immediately follows a possible isotropic ratio of retentive for saturation magnetization greater than 0.5, preferably more than 0.6. In contrast, with a conventional Magnet, in which means from the prior art known methods the anisotropy axes in the material iso are distributed tropically, just a ratio of remanence to achieve saturation magnetization of 0.5.

The most important characteristic of the exchange spring magnet is the extraordinarily high reversibility. This is understood here to mean that when a magnetic field is applied, which is less than the coercive field strength of the hard magnetic phase, the original remanence is almost reached again after removal of the field. This can apply even if the applied magnetic field is in the range of the coercive field strength of the hard magnetic phase. A simple one-dimensional model is shown in Fig. 1 ge. In the shaded areas there is the hard magnetic phase with a very large anisotropy constant, in the remaining areas there is the soft magnetic phase with a very small anisotropy constant and a high saturation magnetization. When an opposing field is applied, the magnetization in the soft magnetic areas is rotated in the field direction (arrow), while the magnetization in the hard magnetic areas remains in its original position. So there is no magnetic reversal in the traditional sense, rather energy is pressed into the spin chain (exchange energy) by the external field. After switching off the field, the energy stored in the spin chain is completely recovered, since the exchange energy tries to smooth the spin structure again. This means that an extremely high reversibility with partial magnetic reversal is to be expected. Only after reaching a sufficiently high field strength is the magnetization switched. The exchange spring magnets typically have a µ _rev of greater than 2.5, while conventional magnets have a µ _rev of 1-2. µ _rev is defined by the equation

ΔB = μo · μ _rev · ΔH.

A preferred system with exchange spring properties is an alloy of the composition RE _x Re _y B _z V _u Si _v , wherein
RE = one or more elements selected from rare Er den (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and / or Y, Zr and Hf,
and where
x not more than 8 atomic%
y not more than 85 atomic%
z not more than 25 atomic%
u not more than 6 atomic%
v not more than 2 atomic%
is.

The production of the aforementioned magnet according to the invention materials can be done as follows. A melt consisting from RE, Fe, V, Si and B, their stoichiometric ratio the composition of the final product is determined on a rotating metal roller sprayed, the resulting amorphous Baubles can be found in a grinder made from a stand the technology is known, if necessary to the desired Grain size to be ground. Then by tempering preferably in the range of 670-780 ° C for four to ten minutes the desired ratio of hard magnetic too soft magnetic phase.

Another method is mechanical alloying, in which the above together under an inert gas atmosphere for several days in a ball mill, for example Steel balls ground intensively and then annealed becomes.

Production example of a magnetic according to the invention Materials

A sample of the composition

Nd _3.8 Fe _73.3 V _3.9 B _18.0 Si _1.0

(In each case in atom%) is at a temperature of 1250 ° C brought. The liquid material is placed on a copper roller with a diameter of 15 cm, which rotates at 4300 rpm, sprayed. The resulting amorphous tinsel are annealed in fact  

Sample 1: 5 minutes at 708 ° C
Sample 2: 5 minutes at 725 ° C
Sample 3: 5 minutes at 775 ° C.

Figure 2 shows the measurement curve obtained with sample 2, namely hysteresis curve ( 1 ) and remanence curve ( 2 ). The samples have the magnetic characteristics listed in Table 1 below.

Table 1

Characteristic of the materials according to the invention is the high M _R / M _S ratio with high magnetization and the large ratio of remanent to normal coercive force, as can be seen from Table 1 and from Figure 2.

As materials with the described exchange coupling were also, for example, mixtures of the composition

- α-iron and Nd ₂ Fe ₁₄ B
- Fe ₂₃ B ₆ and Y ₂ Fe ₁₄ B

found.

A material according to the invention with exchange spring action with the composition of the above example was compared with a conventional magnets based on NdFeB chen. Table 2 shows the comparison of the magnetic Key figures:  

Table 2

The measurement was done with a vibration magnetometer a maximum field strength of ± 1592 kA / m and in space temperature.

Furthermore, the corrosion resistance with both materials measured by the materials 1 hour respectively For 60 hours in a climate of 80 ° C and 80% relative Humidity were stored. The following resulted Changes Δ (%) in the magnetic properties, namely Coercive force and saturation magnetization.

For the exchange spring magnets according to the invention found the following preferred applications, the inventions is not limited to this.

A) For permanent magnets

Permanent magnets based on AlNiCo, rare earth cobalt, NdFeB or hard ferrite are used as permanent magnetic excitation, for example for electric motors and generators. While the AlNiCo alloy is characterized by a high magnetization and a small coercive field strength, in the case of hard ferrite there is the reverse case of a high coercive field strength with a small magnetization. In the case of rare earth compounds, the magnetization with a sometimes very large coercive field strength is much larger than with ferrite, but remains significantly smaller than with soft magnetic compounds or AlNiCo. The ability of a magnetic material to store energy is given directly by the maximum energy product, ie (BH) max. Under ideal conditions, the maximum energy product is given by µ _o M _s ^2/4 , it is assumed that the coercive field strength is sufficiently large. A further increase in the coercive field strength beyond the value 0.5 M _s has no influence on the maximum energy product, the maximum energy product thus only depends on the saturation magnetization.

In order to be able to magnetically store the greatest possible energy, is therefore a large magnetization with a sufficiently large one Coercive force of the magnetic material is desirable. These two requirements are met by the invention Material fulfilled, some problems are avoided:

• 1. Because the exchange spring magnet is relatively rare Earth is built in than in a conventional NdFeB or SmCo magnets, is the material according to the invention cheaper.
• 2. By a suitable choice of a soft and a hard magnet the properties of the composite can be set separately. So can Example by the proportion of the hard phase the coercive field strength or the panel strength through the appropriate choice of soft phase however, the saturation magnetization is set.
• 3. A suitable choice of the two components makes it possible to achieve a magnet with an extraordinarily high energy product. Due to the high magnetization, the maximum possible energy product µ _o M _s ² : 1.7 is significantly larger than that of a pure NdFeB magnet. Due to the extremely high reversibility, the dynamic energy product, which in the case of conventional magnets is significantly smaller than the static one, is considerably enlarged. In practical operation, for example a synchronous machine occur, due to the different magnetic lengths when the rotor rotates, various strong demagnetizations of the permanent magnet occur. In a conventional magnet, starting from the most demagnetized operating state of the magnet, the operating state is on a sub-loop, as shown in FIG. 3a. It can be seen that the demagnetization is largely irreversible. In contrast, the demagnetization in the case of the novel exchange spring magnet ( FIG. 3b) is almost completely reversible, which immediately follows that a much higher dynamic energy product (BH) can be achieved.
• 4. With regard to the magnetization of a permanent magnet in a magnetic circuit, the problem of irreversible demagnetization occurs with a conventional magnet. This means that the magnet partially demagnetizes after magnetization by removing it from the magnetization device (cf. FIG. 3a, point 1), which automatically results in an operating point on a sub-loop. When installing in z. B. an electrical machine is therefore not fully developed. This disadvantage of a conventional magnet can only be achieved by magnetization in e.g. B. encountered the electrical machine, but this requires large energies. The replacement spring magnet according to the invention does not have this disadvantage. It can therefore be used in the magnetized state without irreversible loss. Due to the high magnetization, less magnetic volume is required to achieve a desired magnetic flux.
• 5. Due to the operation of the exchange spring magnet for magnetic hardness not the size of the individual Grains relevant as in a permanent magnet, but from finally the microstructure of the soft and the hard magnetic phase. This eliminates the need a permanent magnet made of sufficiently small grains build up to the desired magnetic properties to obtain. It follows, among other things, that corrosive Influences that affect the high-quality, rare earth-based manufactured magnets are extremely strong, far be reduced. In addition, the corrosion susceptible hard magnetic phase due to a less strong one corrosion-sensitive soft magnetic matrix is protected.
• 6. Due to the intrinsic isotropic properties of the he material according to the invention is directional Stability against foreign fields. This plays into especially with rotating machines that naturally ro ting magnetic fields and thus magnetic cross-loading stungen of the permanent magnet mean a large role. With conventional magnets, external fields in Transverse irreversible demagnetization tion, while with the replacement spring magnet this after parts do not arise.
• 7. Since all hard magnetic materials first a certain one Grain size in the range from less than 1 µm up to larger than 10 µm to achieve the hard magnetic Properties require these materials first pulverized, or produced directly as a powder and then compacted using a sintering process. With these processes it is difficult to find the mechanical Accurate dimensions and complicated parts to create. Made of plastics (e.g. polyethylene or  Polypropylene) are much easier to complicate Win molded parts. For this reason, they become hard magnets embedded particles in the plastic mass, to transfer the benefits to permanent magnets. Man takes the resulting smaller magnetization in purchase, of course one becomes a filler Use material with the highest possible magnetization, thus the entire magnetization of the plastic its filling does not become too small. The fiction moderate material is special for this application suitable because it has a high magnetization and especially has a high isotropic remanence. While conventional hard magnetic materials within the Aligned plastic with a magnetic field must be in order to obtain optimal properties This process is not necessary when using an exchange feather magnets. Since an alignment process usually does not go through is a great advantage of the replacement spring Magnets in front.

B) For magnetic recording media example 1

With the described magnetic materials according to Sample 1-3 becomes a magnetic recording medium made by the amorphous tinsel in an inert ground gas atmosphere to a desired particle size will. The pigments that are created in this way remain intrinsic properties fully preserved. Subsequently the pigments consist of a mixture of binders from a vinyl chloride-vinyl acetate copolymer and one Polyester-polyurethane, which in an organic Lö be dissolved, dispersed and without alignment treatment in a magnetic field on a polyester film applied and dried.  

On the magnetic recording medium thus obtained signals in longitudinal track, oblique lane or vertical recording can be applied; he is equally suitable for analog or digital signals. Such a material is particularly preferred for diskettes suitable for the media. By increasing the maximum achievable Remanence compared to conventional magnetic pigments a significantly higher read signal is obtained. Furthermore is the dispersing behavior of the pigments in the binder solution much cheaper than conventional magnetic Pigments that are magnetic immediately after manufacture are neutral because, as stated above, the preference axes of the magnetically hard phase isotropic in each particle are distributed.

Example 2

A melt with the composition Ce ₂ Fe ₁₄ B is processed further as in Example 1. A magnetic recording medium is built up with the pigments obtained. Due to the low Curie point of Ce (Tc = 435 ° K), the material obtained is ideally suited as a slave band for thermo magnetic duplication.

Examples 3 to 4

A melt of the composition as in Example 1 or 2 is by vacuum evaporation on a metallic Layer support applied with a thickness of 0.3 microns. By Annealing for 1-10 minutes at 650-750 ° C can be a magnetic medium for high density longitudinal or create vertical recording.

Example 5

The procedure is similar to that in Examples 3 to 4, however the composition according to Example 1 or 2 by sputtering  applied to a metallic substrate. Also this becomes a medium for longitudinal high-density sealing or vertical recording.

The additional advantages achieved by the invention for magnetic recording media can be as follows sum up.

• 1) Due to the high reversibility with partial entma gnetisation results in a significantly higher read signal. If the magnetic head is reading the medium magnetically short-circuits, the remanence almost rises to hers Original value. Conventional media point through the described recoil mechanism a drop in Lesesi gnals from normally 4 to 5 dB, at which he medium according to the invention is inherently avoided.
• 2) Since there is no preferred magnetic direction, get one always isotropic since the alignment process is omitted Remanence. This is particularly cheap for disk media and hard disk media, which have an unequal orientation is desired because otherwise the signal level fluctuates. In this Case allows the use of an exchange spring magnet an increase in the reading signal compared to conventional ones Media. Attention is also drawn to the cheap ones In contrast, dispersing properties of the isotropic pigments to conventional fine-particle magnetic pigments, for example, highly coercive acicular pigments. These always consist of magnetic dispersion the problem of re-agglomeration of the pigments as a result the mutual attraction, which by recipe and / or Procedural measures must be resolved.
• 3) Since the hysteresis curve is largely reversible, can by using a current in the reading head without legs the read signal is affected by the read information be enlarged. The reading current in the reading head only has to  be such that field strengths are generated which smaller than the switching field strength of the hard magnetic Phase. With a medium according to the prior art if an additional read current is used, the rema nent magnetization changed.
• 4) When the coercive field strength is reached, the invention appropriate record carriers also without external fields be magnetized. In contrast to conventional media the stored information is not lost, because it remains stored in the replacement spring. The maxi Male recording density is accordingly in the Invention according to the magnetic recording medium not by the Demagnetization determined.
• 5) There are also special advantages for metallic thin parts layers, in which by vapor deposition or sputtering binder-free layers are generated. At konventio light layers have a thickness of 0.2 - 0.3 µm and must have a sufficiently fine microstructure. This is typically columnar, so that the columns ideally exist as self-sufficient magnetic districts. Due to thermal stability problems (Superpara magnetism) the particles in thin film media cannot be made arbitrarily small. Completely different on the other hand, a thin film reacts with exchange the magnet is built up. With this lies, as already mentioned, not a one-domain behavior in traditional Senses before. The storage takes place in principle in the hard magnetic inclusions, the soft magnetic matrix smoothes the magnetization. Because the hard magnetic Inclusions are very small, despite the smoothing effect very dense magnetic storage possible. Therefore a thin film according to the invention behaves like an exchange coupled traditional ferromagnetic Layer that has an extremely fine microstructure.  
• Because also the demagnetization is not storable limited, there is therefore no need, possible As thin as possible media for high-density longitudinal spoke to use. In the case of a smaller writing depth than the layer thickness that occurs with high writing density, acts on the part of the magnetic layer not described due to the reversible permeability as a magnetic back Enough. The magnetic inference is also through characterized the high reversibility which means that the medium according to the invention is limited Writing depth will not completely demagnetize, because a magnetic inference from the non-exposure tized part of the magnetic layer can be accomplished can. In the case of vertical recording, it does not work described part automatically similar to a soft magnet lower layer of a conventionally constructed table magnetic recording media.
• 6) For microwave absorbers
  A ferro- or ferrimagnetic material absorbs microwaves due to the ferromagnetic remanence. In the case of single domain particles, this absorption depends on the strength of the ansiotropic field strength. For applications in the technically interesting area (e.g. shielding a microwave oven), anisoptropy field strengths of around 80 kA / m must be available. One possibility of realizing such an absorption is accordingly the production of magnetic single-domain particles with an anisotropy field strength of approximately 80 kA / m. In order to achieve a broad band effect, however, a broad distribution of the anisotropy field strengths is required according to the prior art.
• However, there is also a resonance in multi-domain particles Domain walls possible. These resonance frequencies are at far lower frequencies, typically in MHz  Area. Since only in an exchange spring magnet On the one hand there are fragments of domain walls the resonance frequencies are significantly higher than with complete ones Domain walls, but on the other hand there are domain wall pieces of very different sizes. The reason for this is to see that the angle between the easy axes in the hard magnetic areas due to the magnetization can be bridged in the soft magnetic phase. Since the Positions of the hard magnetic preferred axes differ to each other, domain wall pieces are created under of different sizes, the different natural frequencies to have. In this way, a broadband microwave get absorber. The magnetic material can be embedded in a plastic mass.

Claims (14)

1. Magnetizable material, which consists of two phases, which are coupled to one another by exchange coupling, characterized in that the ferromagnetic material is composed of a hard magnetic and a soft magnetic phase with exchange coupling of the spins of both phases, the volume fraction of the soft magnetic phase being greater than is the proportion of the hard magnetic phase and, when a magnetic field is applied, which is approximately equal to the coercive field strength of the hard magnetic phase, after removal of the field the original remanence is practically reached again, where µ _rev <2.5. 2. Magnetizable material according to claim 1, characterized records that the soft magnetic phase is a cubic Structure and that the orientation of the hard magnetic phase statistically in relation to the main directions the soft magnetic phase is distributed and the magnetic material has an isotropic ratio of remanence preferred to saturation magnetization greater than 0.5 wise greater than 0.6. 3. Magnetizable material according to claims 1 to 2, characterized by an alloy of RE _x Fe _y B _z V _u Si _v wherein
RE = one or more elements selected from rare earths (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and / or Y, Zr and Hf and where x is no longer than 8 atomic%
y not more than 85 atomic%
z not more than 25 atomic%
u not more than 6 atomic%
v not more than 2 atomic%
is. 4. Process for the production of magnetizable material according to claim 3, characterized in that a melt consisting of RE, Fe, B, V and Si, their stoichiometric The ratio of the final product true, in an inert gas atmosphere on a rotating one Metal roller is sprayed and that the resulting amor phen flitter by tempering in the range of 670-780 ° C to the desired ratio of hard magnetic to soft magnetic phase. 5. Process for the production of magnetizable material according to claim 3, characterized in that a together set from RE, Fe, B, V and Si, their stoichiometric Ratio determines the composition of the end product, in an inert gas atmosphere in a meal device intensively ground for a long time (mechanical Alloying) and then by tempering in the range of 670-780 ° C to the desired ratio of hard magne table is brought to soft magnetic phase. 6. Method of making a magnetic record tion material according to claims 4 or 5, characterized ge indicates that the composition in a fine disper yaw device finely ground to the desired grain size, then by tempering in the range of 670-780 ° C to the desired ratio of hard magnetic to Soft magnetic phase is brought and that the ent standing magnetizable pigments together with poly merem binder on a substrate by a casting device without subsequent magnetic alignment of the pigments are applied. 7. Method for producing a magnetic record nung carrier with a material according to claims 1 to 3, characterized in that an alloy, be consisting of RE, Fe, B, V and Si, their stoichiometric  Ratio determines the composition of the end product, applied to a substrate by evaporation in vacuo is brought, whereupon the desired by tempering the layer Ratio of hard to soft magnetic phase set becomes. 8. Method for producing a magnetic record nung carrier with a material according to claims 1 to 3, characterized in that a target consisting from RE, Fe, B, V and Si, its stoichiometric ratio the composition of the final product determined by sput tern is applied to a substrate, whereupon by annealing the layer the desired ratio of hard to soft magnetic phase is set. 9. A magnetic recording medium according to claims 1 to 8, characterized in that the magnetic Auf a hard disk, a floppy disk or a is a tape-shaped recording medium. 10. A magnetic recording medium according to claim 9, characterized in that RE = Ce and that the Auf Drawing medium as a thermomagnetic duplication measure material is used. 11. Process for producing a permanent magnet, because characterized in that a material according to the claims Chen 1 to 5 is used. 12. Process for producing a permanent magnet, because characterized in that a composition according to Claims 1 to 5 embedded in a plastic mass and is shed. 13. Electric generator or electric motor, characterized net that the permanent magnetic excitation by a Magnet according to claims 11 or 12 happens.   14. absorber for microwaves, characterized in that the absorber has a composition according to the claims 1 to 3 and 12.