FR2650115A1 - MATERIAL WITH A LAMUNAR SPINEL FERRITE STRUCTURE WITH OPTIMIZED AND STABILIZED MAGNETIC OR MAGNETO-OPTICAL PROPERTIES, THEIR PREPARATION AND THEIR APPLICATION - Google Patents

Abstract

Magnetic material with optimized and stabilized magnetic or magneto-optical properties, consisting of a ferrite with a lacunar spinel structure, based on iron sesquioxide, cobalt oxide and at least one other metal oxide, this material being obtained at using heat treatments intended to reveal a directional order. The invention is based on a method which makes it possible to systematically determine the optimum heat treatments. The material is in particular in the form of needle-like particles or thin layers. Application in particular to magnetic or magneto-optical recording systems.

Description

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  The subject of the present invention is materials with a structure of spheroidal ferrite ferrites, with optimized and stabilized magnetic properties, their preparation and their application in the field of

  magnetic or magneto-optical recording.

  It is known that currently, in the high-density magnetic recording material industry, ferromagnetic metal oxide powders are mainly used in the form of particles.

  acicular, mainly based on iron sesquioxide.

  The properties sought for these materials include an important coercive field, which varies little at the temperatures of use, a ratio magnetization remanent / magnetization to saturation (middle) high,

  and of course a weak magnetostriction.

  The research focuses more particularly on magnetic oxides of the type ferrites spinel lacunary (iron sesquioxide)

  substituted) optionally containing various dopants.

  Among the many substituents and / or dopants that have been proposed, mention will be made especially of cobalt, chromium, zinc, copper, nickel, manganese, cadmium, alkali metals and / or alkaline earth metals, lead, silicon, phosphorus, boron, etc. It is known that iron sesquioxide Fe 2 O 3 has a structure 2 3

crystalline lacunar spinel.

  The acicular particles of iron sesquioxide can be

  prepared from various products according to known methods.

  The first method, which has been the subject of many variations, consists mainly of iron alpha-oxyhydroxide (or goethite) which can be obtained in the form of acicular crystalline particles by the action of the hydroxide of sodium on a ferrous sulphate solution in an oxidizing atmosphere. Colloidal particles of goethite are thus obtained, from which larger crystals can be grown. Dehydration of these crystals gives hematite crystals (alpha-Fe 2 O 3, non-magnetic). The reduction of hematite with hydrogen at temperatures above 300 ° C. leads to the production of magnetite particles Fe304 with a spinel structure. Finally, the oxidation of magnetite makes it possible to obtain particles of gamma-Fe 2 O 3, with conservation of the spinel structure, but with formation of vacancies in view of the oxidation of ferrous ions to ferric ions. In all these reactions, the

  acicular form of the particles is preserved.

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  A second method is to use as starting material iron gammaoxyhydroxide (lepidocrocite), obtained in the form of a precipitate by treatment of ferrous chloride with a base in an oxidizing atmosphere. Dehydration of lepidocrocite at moderate temperatures leads directly to the production of

  gamma-Fe203 that can be acicular under certain conditions.

  Particles of magnetite and / or gamma-Fe 2 O 3 modified by partial substitution of iron with various metals, or by addition to

  surface or volume of various doping agents, have already been described.

  The substituted magnetites have a spinel type structure, which can be partially incomplete in the case of the introduction of high valence metal cations. However, by convention, in the present application, the substituted magnetites will be

  called "non-lacunar spinel ferrites".

  The modified gamma-Fe 2 O 3 -based particles are compounds of the ferrite type, that is to say based on iron sesquioxide in which part of the iron ions and vacancies is replaced by other metal cations. These lacunar spinel ferrites are considered to be solid solutions of gamma-Fe 2 O 3 and ferrites which, in the case where

  the substituent M is divalent, are of the MOFe203 type.

  The integration of substituent ions in the crystal lattice has the effect of increasing the transformation temperature of the gammaFe203 crystalline system into alpha-Fe203 (non-magnetic). This

  temperature is close to 460 C for unsubstituted iron sesquioxide.

  Thus, the presence of the substituents makes it possible to optionally carry out heat treatments at higher temperatures, without compromising

magnetic properties.

  For example, the addition of chromium, cobalt, nickel, zinc salts, etc., to the starting ferrous sulphate, in the first process mentioned above, or the addition of various salts (eg alkaline earth metal salts, silicates, phosphates) to goethite particles has made it possible to obtain such modified magnetite and / or gamma-Fe 2 O 3 particles, the aim being to improve the magnetic properties, the texture, the dimensions and shape of

  particles, as well as the magnetic properties that result.

  The addition of various dopants to the magnetite obtained intermediately also makes it possible to obtain, after the oxidation step,

  particles of modified lacunar spinel ferrites.

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  The substituted magnetite and / or gamma-Fe 2 O 3 substituted particles (lacunar spinel ferrites) can also be obtained by decomposition in air of organic salt particles (for example acetates, oxalates) mixed, that is to say mixed salts of iron and metals used as substituents. The rest of the process is similar to that of the method using goethite. The oxides from the decomposition of the organic salt are reduced in order to convert the alpha-Fe 2 O 3 -type structure to a spinel structure (substituted magnetite), followed by oxidation to finally obtain

  substituted gama-Fe203 particles with lacunar spinel struture.

  Several variants of this process have been described; see for example the

  French patents 2,180,575 and 2,587,990.

  Here again, other substituents or dopants may be

  added to the substituted magnetite before final oxidation.

  Among the various substituted oxides which have been described, it is those containing cobalt which have appeared the most interesting for magnetic recording. The cobalt-modified spinel ferrite particles may be prepared by co-precipitation of iron and cobalt salts (and salts of other optionally present cations) used as starting materials. In the final product, the cobalt ions are then

  distributed randomly in the spinel cell (volume doping).

  The particles obtained have an improved coercive field. However, for the volume-doped oxides thus obtained, the coercive field and the remanent magnetization vary significantly at temperatures slightly above ambient temperature, which makes their use difficult. For this reason, in the field of magnetic recording, surface-doped cobalt-doped particles are currently used, the disadvantages of which are lower. For example, lacunar spinel ferrites can be treated with cobalt ions in a basic medium at moderate temperatures. The coercive field is thus increased and the thermal dependence is less than in the case of particles doped in volume. However, cobalt has, even at the surface, the disadvantage of increasing the magnetostriction. In addition, cobalt ions brought by surface doping tend to migrate inward from the particles

with time.

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  The processes just described also allow the introduction of ferrous ions as substituents. It suffices for this, during the final oxidation step, to avoid the complete oxidation of the ferrous ions of the intermedian magnetite, either by performing the oxidation at a lower temperature or by limiting the oxygen consumption by setting

  the composition of the oxidizing atmosphere.

  The presence of ferrous ions in the final product is interesting because it is an inexpensive substituent with a positive magnetostrictive coefficient. Ions can notably be associated with

  cobalt whose magnetostriction coefficient is negative.

  In general, doping agents, a large number of which have been described, can be introduced either in volume, by adding the dopant to precursors of the lacunar spinel ferrite, or on the surface, by adding the dopant, or to the precursor magnetite. either ferrite

Lacunar spinel itself.

  In the field of particles for magnetic recording, current research includes the improvement of magnetic properties, the reduction of cobalt content and the improvement of particle texture and morphology, in particular by the development of various heat treatments; see for example French patent application No. 2,587,990. Until now, the various heat treatments accompanying the preparation of the particles were determined largely empirically. The improvements were

  partial, and sometimes divergent results.

  In the field of magneto-optical recording, thin layers of magnetic material, deposited on a substrate, are used. It is known that magneto-optical recording devices use the phenomenon of rotation of the plane of polarization of the light caused by the magnetic materials, either by transmission (Faraday effect) or by reflection (Kerr effect). The magnetic properties of lacunar spinel ferrites make them interesting materials, sub-form of thin layers, for magneto-optical recording. Here again, it is important to optimize the magnetic and / or optical properties and

to stabilize them.

  The subject of the present invention is novel materials with lacunar spinel ferrite structures, with improved magnetic and / or magnetooptical properties, which vary little at the temperatures of use, thanks to heat treatments that make it possible to optimize and stabilize

  systematically the magnetic and / or optical properties.

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  It has indeed been discovered that certain lacunar spinel ferrites generally exhibit, during appropriate thermal treatments, a directional order phenomenon, which is known to be related to the rearrangement of local electronic or cationic structures, under the influence of the temperature. and / or magnetic fields. Induced anisotropy appears in certain temperature zones and results in an increase of certain magnetic or optical parameters such as the coercive field, the remanent magnetization, the coercive field of the remanent magnetization, the middle part of the hysteresis cycle, the Faraday rotation or Kerr rotation. The study of the variation of one of these parameters, measured at ambient temperature, as a function of the sample treatment temperature, shows indeed a sometimes significant increase in the amplification of this parameter at certain temperatures o

appears a directional order.

  In what follows, the phenomenon observed and the method which is the basis of the invention are sometimes described with reference to the coercive field, but it should be understood that any of the parameters which have just been mentioned can be used as a criterion

  appearance of a directional order.

  The directional order, the theory of which was made by L. Neel, has been the subject of much work. It is not possible to determine in advance if a given material will present a directional phenomenon,

  nor at what temperatures will this directional order appear and disappear.

  The directional phenomenon had never been observed on monodomain acicular particles used in magnetic recording,

  nor on the thin oxide layers used in magneto-optics.

  When the directional order appears by heating, the field. coercive (or any other parameter selected from those mentioned above), increases with temperature, goes through a maximum,

  then decreases rapidly to a temperature greater than that of said maximum.

  This decrease corresponds to the fact that the directional order acquired is

  then destroyed by thermal agitation.

  During cooling, the temperature zone corresponding to the destruction of the directional order (during heating) becomes a zone of creation of the directional order, resulting in the growth of the magnetic or optical property studied, until at a maximum, but during the further cooling, the directional order is conserved and the measurement of the magnetic or optical property studied remains substantially constant and equal to that of the maximum corresponding to the

temperature zone considered.

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  The systematic study of heat treatments shows that sometimes several bands of directional order appear at various temperatures. The method of the invention allows the magnetic material to retain, during cooling, improvements in the magnetic or optical properties related to each of the directional orders that occur in the temperature range studied, up to room temperature. It is also possible, in order to reduce the duration of the heat treatments, to retain only the treatment temperature for which the gain observed for the magnetic or optical property studied is the most important.

important.

  The method which is at the basis of the invention makes it possible both to search for and obtain new lacunar spinel ferrites having optimized magnetic or magneto-optical properties and a good one.

  stability of said properties at use temperatures.

  This method, which can be called "thermal spectroscopy of induced anisotropy", consists in performing series of annealing at increasing temperatures, each annealing being followed by cooling at room temperature, at which the magnetic parameter is measured. or chosen optics of the material, after which a new annealing is carried out at the

  next temperature, and so on.

  It is thus possible to establish a variation curve of said magnetic or optical parameter with the annealing temperatures. We observe the influence of thermal treatments, and the temperature zones characteristic of the creation and the destruction of the directional order. These temperature zones comprise a characteristic temperature corresponding to a maximum of the curve of variation of the

  magnetic or optical property studied.

  It has furthermore been found that by slowly cooling the sample studied, from the annealing temperature, the directional direction, even if destroyed, is again acquired during the cooling during the passage in the temperature zone corresponding to said maximum and is then retained by the sample during cooling to a

ambient temperature.

  It has also been observed that, in the case of the presence of several zones of creation of a directional order, the gains in magnetic or optical properties accumulate during slow cooling from the highest temperature zone of creation of an order

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  directional to the ambient temperature, the passage of each of the zones of creation of a directional order. The process that has just been described therefore makes it possible to determine for sure which are the best

  thermal treatments to be applied to a given sample.

  In addition, if a directional order appears at a temperature close to the ambient temperature, for example at a temperature between room temperature and 80.degree. C. or between room temperature and less than 100.degree. C., the method which has just been indicated allows to reject the study composition that would be difficult to use in practice, since a moderate heating would likely lead first to an increase, then a decrease, properties such as for example the coercive field. It is obvious that a material having such a variation of the magnetic (or optical) properties is not suitable for use in magnetic or magneto-optical recording, since normal or accidental heating, which can not be avoided,

  lead to a deterioration of the recording.

  It is also possible, after measuring the magnetic properties at room temperature, to continue the slow cooling down to temperatures below the usual ambient temperature, which may be encountered during the storage or use of 'recording. By carrying out additional magnetic and / or optical measurements at said temperature below room temperature, it is possible to determine whether a directional order is likely to occur between the usual room temperature and said temperature below room temperature, and eliminate the magnetic materials for which such a directional order appears. It is indeed obvious that the use of such a material at said lower temperature, or the storage, at said temperature, of a recording using said material, will lead to a deterioration of the qualities of the recording when returning to the room temperature or at

  a temperature of use higher than the ambient temperature.

  Another advantage of the method which is at the basis of the present invention is that the increase in the magnetic properties associated with the acquisition of a directional order is not accompanied by a significant increase in the magnetostriction or a thermal drift

  important magnetic properties.

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  For a material of given composition, the method of the invention comprises firstly a preliminary step of determining the variation of the coercive field (or of any other parameter chosen as indicated above), measured at ambient temperature, after series of annealed, as indicated above, from ambient temperature to the gamma transformation temperature ----> alpha, ie the transformation temperature of the spinel structure (gamma-Fe 2 O 3) into alpha structure -Fe203. The temperature ranges of the anneals chosen for this preliminary study can be, for example, of the order

from 10 to 50 ° C.

  Once these anneals have been carried out, we therefore know the temperatures and the optimal durations of the heat treatments leading to the best magnetic performances for the sample considered. At the same time, it is known whether there is a directional band in the vicinity of the ambient temperature, in which case it can be concluded that the studied material will not be usable in practice for magnetic recording, given the instability magnetic properties at

  neighborhood of the ambient temperature.

  After this preliminary study has been carried out, once and for all, it becomes possible to use the information obtained to determine the heat treatment which will make it possible to prepare the studied sample conferring on it the maximum magnetic properties, by proceeding directly to a treatment up to the highest temperature for which the acquisition of a directional order has been observed, corresponding for example to a maximum of the curve of variation of the coercive field (or up to the temperature of the maximum corresponding to the gain of most important coercive field) followed by slow cooling to room temperature. It is possible to replace the slow cooling to ambient temperature by a series of rapid cooling from a temperature corresponding substantially to a maximum coercive field to a temperature corresponding substantially to the maximum coercive field neighbor, and so on, and especially it is advantageous to carry out bearings at temperatures substantially corresponding to coercive field strengths, or, more exactly, at slightly lower temperatures (for example, lower than 5 to 15 C) at the temperatures for which an acquired directional order begins to be destroyed. Similarly, after the plateau at the lowest temperature for

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  which a maximum coercive field has been observed, or more exactly at said slightly lower temperature, it is possible, although not necessary, to rapidly cool the sample to the temperature

  ambient, without loss of acquired magnetic or optical properties.

  When the starting material is a non-oxidizable lacunar spinel at the highest annealing temperature (determined as indicated above), it is possible to anneal in air or in an inert atmosphere. When the starting lacunar spinel contains an oxidizable metal at the highest annealing temperature, it is then advisable to anneal under an inert atmosphere, for example under an atmosphere

  nitrogen, if it is desired to avoid such oxidation.

  It is also possible to use as starting material a spinel of the substituted magnetite type, by carrying out the treatment.

  conventional oxidation at the temperature suitable for the sample studied.

  If the highest temperature of occurrence of a directional order resulting in, for example, a maximum coercive field (for the final product) is greater than the oxidation temperature, then additional heating should be performed up to said highest temperature, and then cool the sample as indicated above. If the highest temperature of appearance of a directional order is lower. at the oxidation temperature, it is sufficient to cool the sample, after oxidation, as previously indicated, from its preparation temperature to room temperature. Once the oxidation has been carried out, it is generally possible to carry out the subsequent heat treatments in the air (if oxidation

  additional is no longer to be feared) or in an inert atmosphere.

  It is therefore important to distinguish between the preliminary process (thermal spectroscopy induced anisotropy) and the annealing process that will be used in practice to prepare the material

  final magnet, which will be described later.

  The subject of the present invention is therefore a magnetic material constituted by a ferrite with a lacunar spinel structure, based on iron sesquioxide, cobalt oxide and at least one other metal oxide, with possibly one or more doping agents, characterized in that said ferrite can be obtained from a lacunar spinel ferrite containing said oxides, by a process consisting of carrying out, optionally in an applied magnetic field, successive annealing at increasing temperatures ranging from room temperature to a temperature below the gamma -> alpha transformation temperature, for the material under consideration, said annealing temperatures being separated by intervals of 10 to 50 C, observing a step of at least 0.5 hour at each annealing temperature chosen, after each annealing the composition is cooled to room temperature, after which After each annealing followed by cooling, at least one magnetic or optical property selected from the coercive field, the remanent magnetization, the coercive field of the remanent magnetization, the middle part of the hysteresis cycle, the rotation, is measured at ambient temperature. Faraday and the rotation Kerr, that one thus establishes the curve of variation of said property with the annealing temperature, that one locates the characteristic temperature or annealing corresponding to a maximum of said property thus measured, that the the materials for which at least a maximum of said magnetic or optical property, as measured, are selected for an annealing temperature of greater than 80 ° C., but no maximum between ambient temperature and C, that said material is subjected to a final annealing temperature greater than the temperature corresponding to the largest maximum of said property, and which is cooled slowly ledi t material since

  said upper temperature up to room temperature.

  In the definition just given of magnetic materials, the method by which said materials can be obtained is to some extent arbitrary, given the explanations that have been given previously. In particular, it is possible to choose the final annealing temperature not with respect to a temperature corresponding to the largest maximum of the magnetic or optical property chosen, but with respect to the highest temperature for which a maximum of said property (when the variation curve of the magnetic or optical property chosen, established as indicated above, comprises for example two maxima, corresponding to coercive field gains of x and y Oe, respectively, the most important maximum is of course that corresponding to the gain x, if x is greater than y, and at gain y if y is greater than x). Instead of performing the final annealing to a temperature higher than said temperature, it can be carried out to a slightly lower temperature, especially at a temperature below 5 to 15 C, for example, at the temperature for which observe a

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  beginning of destruction of the directional order acquired in the temperature zone considered. Of course, it is appropriate in this case to carry out a step at the final annealing temperature chosen during the time necessary for the acquisition of the directional order. Slow cooling can also be replaced by steps at the characteristic temperatures that have been identified (or more accurately at slightly lower temperatures, as explained above). The various anneals can be carried out in the air if oxidation is not feared in the temperature range studied, or in an inert atmosphere or

  Controlled oxygen content in other cases.

  The magnetic materials of the invention may especially be in the form of acicular particles having for example dimensions of 0.5 to 11 μm and a needle ratio of 1.5 to 10 or in the form of thin layers, deposited on a substrate, having for example a

thickness from 10 to 1000nm.

  In the case where the material of the invention is a thin layer, the determination of the characteristic temperatures can be made, not on the thin layer, but on acicular particles having the same chemical composition. As the thin films do not have their own uniaxial anisotropy, unlike the acicular particles (which therefore have an internal field), in the case of thin films, treatments (preliminary or preparatory) should be carried out under a magnetic field. applied, for example in the plane of the layer or in a perpendicular plane. The applied field must be substantially constant and may be for example between 100 and 5000 Oersted. For the actual preparation of the thin layers, it is convenient in practice to apply the magnetic field at the maximum annealing temperature and during cooling to room temperature, or simply during the temperature steps performed at the characteristic temperatures. For the preparation of materials in the form of acicular particles, a magnetic field can be applied, but it is also possible to operate without a field

  magnetic applied, considering the internal field indicated previously.

  In the process described above, which serves to define the magnetic materials of the invention: the temperature ranges of the anneals can be most often of the order of 20 C, provided that it is always possible to tighten subsequently some of these intervals to more accurately determine the characteristic temperature corresponding to a maximum of the selected magnetic or optical property, which is tantamount to finding, in practice, the temperature at which the acquired directional order begins

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  to be destroyed, because these two temperatures are very close to one another and can be confused in practice; in the present application, therefore, no distinction is made between the temperature corresponding to the maximum of the magnetic or optical property studied, during the acquisition of the directional direction of heating, and the (slightly higher) temperature of the beginning of the destruction of the directional order acquired, while continuing heating at said slightly higher temperature. It is this temperature which is called here "characteristic temperature"; the duration of the steps, in the preliminary process of searching for the remarkable temperatures, can, for each composition studied, be determined by simple routine experiments; it can be generally chosen between 0.5 and 2 hours; the temperature above 80 C which has been mentioned is preferably a temperature at least equal to 100 C, but lower than C; the maximum temperature of the various anneals studied, which, as already indicated above, is below the gamma transformation temperature.

  generally less than or equal to about 600 ° C.

  The spine ferrites which give rise to the directional phenomenon described above are especially those containing, in addition to iron sesquioxide and cobalt oxide, at least one divalent metal oxide selected from zinc, magnesium, cadmium or iron and / or at least one oxide of a metal with several valences. The metal having several valences is for example chosen from manganese, molybdenum, copper, vanadium, chromium and rare earths (yttrium and lanthanides). Oxides other than iron sesquioxide may be present (especially in the case of acicular particles) or in the

mass, either on the surface.

  The materials of the invention may further contain one or more usual doping agents, in volume or surface. The doping agents are in particular (in the form of oxides), alkali or alkaline earth metals, silicon, phosphorus, boron, etc. The oxides constituting the magnetic materials of the invention can be considered as ionic compounds made up

  by 02- anions and metal cations.

  The proportions of metals other than iron (III) may vary in proportions compatible with the structure of lacunar spinel ferrites. The maximum allowable proportions can be

  determined by routine experiments.

  The proportions of cobalt can vary for example from 1 to

  Z in moles, relative to the total number of moles of metal cations.

(doping agents excepted).

  The materials of the invention are especially those

  contain, in moles, from 1 to 5Z of cobalt.

  Generally, the proportions of all the divalent metals mentioned can vary from 1 to 30 mol%, relative to the number

  total moles of metal cations (doping agents excepted).

  The proportions of all the metals with several valences mentioned above may vary from 1 to 30 mol%, relative to

  total number of moles of metal cations (doping agents excepted).

  In addition, the sum of the molar percentages of metals

  divalent and multivalent and cobalt is not more than 33%.

  The rare earths are preferably Eu, Gd, Tb, Dy, Sn, Nd,

Ho, Pr or Ce.

  The proportions of doping agents can vary from 0 to 5% by weight of doping elements, relative to the total weight of the oxide composition. In other words, to express this proportion, we take into account

  the weight of the elements, not the weight of the corresponding oxides.

  Doping agents may be especially those which have been

mentioned above.

  The magnetic materials of the invention are especially those whose chemical composition (optional doping agents excepted) has the following formula: MM 'Co 2+ Fe 3 + O 2- (I) wxy 3 -xyxy + t in which M represents at least one cation of a divalent metal selected from zinc, magnesium, cadmium and iron, M 'represents at least one cation of a metal with a plurality of valencies selected from manganese, molybdenum, copper, vanadium, chromium and rare earths,

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  w represents the number of moles of said one or more divalent metal cations, x represents the number of moles of said one or more metal cations, 2+ y represents the number of moles of Co2 cations, t is a number representing the number of moles of moles of anions 02- in excess of 4, this number t being greater than or equal to 0.01 and less than or equal to 1, it being understood that the sum w + x + y is less than or equal to 1.

  preferably, t is less than or equal to 0.5.

  The number of 02- anions in excess of 4 is of course due to the fact that formula I does not show the cationic gaps. The numbers w, x and y are notably such that the proportions

  of M, M 'and Co are those which have been indicated previously.

  In formula I, the Fe 2 + cations are arranged with the divalent cations and not with the multivalent metal cations because the

  Fe3 + ions are shown separately.

  It should be noted that the novelty of the magnetic materials of the invention does not necessarily reside in their chemical composition (in fact some of these compositions are not new from the point of view of their crude chemical composition), but in the existence induced anisotropy by the acquired directional order, which results in the fact that, for example, the coercive field (or another magnetic or optical property chosen as indicated above) does not increase by heating, but remains substantially stable until the temperature corresponding to the first maximum of coercive field, then decreases, and so on to the transition to the temperature corresponding to

each maximum of coercive field.

  The invention also relates to a preparation process

  magnetic materials defined above.

  This process is mainly characterized by the fact that a lacunar or non-lacunar spinel ferrite is used as starting material, which is carried out at least the operation of bringing the starting material to an acquisition temperature of a lower directional command, of about 5 to 15 C, at a predetermined temperature, that said material is maintained at said lower temperature for a time sufficient to allow acquisition of the corresponding directional order, which is cooled material thus obtained up to ambient temperature, it being understood that said predetermined temperature is the temperature corresponding to a maximum of the variation curve

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  of a magnetic or optical property chosen from the coercive field, the remanent magnetization, the coercive field of the remanent magnetization, the middle part of the hysteresis cycle, the Faraday rotation and the Kerr rotation, said variation curve being established according to the method which has been described previously, that said maximum is the largest maximum of said variation curve, and it being understood that when the starting product is a non-lacunar spinel ferrite, said starting material is heated beforehand in an oxidizing atmosphere at a temperature sufficient for the

  transform into lacunar spinel ferrite having the desired composition.

  According to a particular embodiment, the method of the invention may further be characterized in that said predetermined temperature is the highest temperature corresponding to a maximum of said magnetic or optical property and that the material is cooled to a maximum at ambient temperature by observing steps at temperatures below about 5 to 15 C at each of the temperatures corresponding to

  a maximum of the magnetic or optical property studied.

  It is understood that all the annealing temperatures used in the process of the invention are lower than the gamma transformation temperature ----> alpha, for the composition under consideration. This temperature is easily determined by the degradation of the magnetic properties, either by the appearance of diffraction lines corresponding to

alpha-Fe203.

  When the starting product is a lacunar spinel ferrite, the heating rate up to the temperature of approximately 5 ° C. to 15 ° C. at said predetermined temperature is in particular

  -200 C / hour, for example 150 C / hour.

  The cooling rate between the lower temperature of about 5 to 15 C at the temperature corresponding to said largest maximum and the ambient temperature can be slow (for example 2-10 C / hour) or fast (for example 150-1000 ° C). /hour). Likewise, the cooling rate between two temperatures corresponding to two consecutive maximum zones, furthermore such a temperature and the ambient temperature, can be slow, or can be fast when it is decided to observe steps at each of the zones. temperature corresponding to a maximum. A rapid cooling rate can in this case be for example of the order of 100 to 500 C / hour. Of course, the bearings are carried out at a lower temperature, for example from 5 to 15 C, at

temperature of each maximum.

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  The duration of the steps can be determined by simple routine experiments, by determining the time at which the magnetic or optical property chosen does not increase with the extension of the duration of the plateau. This time is generally between 1 and 20 hours, most often between 2 and 10 hours. When the starting product is a non-lacunar spinel ferrite (substituted magnetite), the preliminary oxidation step to obtain a lacunar spinel ferrite having the desired composition is carried out by heating to the appropriate temperature, this heating being generally carried out at a speed 60 - 200 C / hour, followed by a plateau. at the chosen oxidation temperature, of a duration of between 0.5 and 10 hours. The oxidation temperature, until the desired product is obtained, is determined by routine experiments. It is generally between 100 and 600 C. This preliminary oxidation step is of course carried out in an oxidizing atmosphere. The composition of the oxidizing atmosphere and / or the oxidation temperature can be adjusted when it is desired to only partially oxidize certain metals, for example in the case where it is desired to retain ferrous ions. In the latter case, if the implementation of the annealing treatments according to the invention involves heating to a temperature above the chosen oxidation temperature, this heating and subsequent cooling are performed in neutral atmosphere. In other cases, the heat treatments that are the subject of the process of the invention can generally be carried out either in air or in a neutral atmosphere. In certain cases where the heat treatment, carried out at a high temperature, higher than the oxidation temperature at which the lacunar spinel ferrite composition was obtained, could lead to unwanted additional oxidation, it should also be carried out

  this annealing, made at said elevated temperature, under a neutral atmosphere.

  When the magnetic material of the invention is in the form of acicular particles, the starting particles (substituted magnetites or lacunar spinel ferrites) can be prepared according to the known methods, which have been recalled in the beginning of

the description above.

  When the magnetic material of the invention is in the form of a thin film deposited on a substrate, it can be obtained in the following manner. A composition having substantially the composition of the substituted magnetite or of the desired lacunar spinel ferrite, which constitutes the starting material in the composition, is deposited on the substrate.

  heat treatment process subject of the invention.

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  The starting material may be deposited on the substrate, for example by a sputtering process, using as a target a compacted composition of spheroidal lacunar ferrite particles obtainable for example by one of the methods described above and having the desired chemical composition. It is thus possible to deposit on the substrate a composition having substantially the composition of the substituted magnetite precursor or the

  desired lacunar spinel ferrite.

  It is then possible to subject the thin film deposited on the substrate to the reduction operations that have already been described above.

  in the description of the preparation of the particulate compositions, for

  transforming the deposited thin film into a substituted magnetite having a non-lacunated spinel ferrite structure, and then to an oxidation treatment, as already described above for the particulate compositions, for converting the substituted magneetite into ferrite with a lacunar spinel structure. It is then possible to subject the thin layer deposited on the substrate, thus obtained, the heat treatment process

  of the invention which has been described previously.

  The substrate is for example a glass, a metal, a thermoplastic polymer, a ceramic, etc. As has already been pointed out, the preliminary method for determining the characteristic temperatures may, in the case of thin films, be carried out on particles of spinel ferrites

  lacunaires of the same composition as the thin layer studied.

  The conditions for implementing the sputter deposition method can be determined according to the methods

  usual by routine experiments.

  The materials of the invention can be used, in view of their magnetic and / or optical properties of interest, in the production of materials for magnetic recording or

  magneto-optical. This use is also part of the invention.

  The following examples illustrate the invention without however

limit it.

-18- 2650115

EXAMPLE 1

  Ferrite spinel based on Fe, Co, Mn -

  Influence of the oxidation temperature The starting particles consist of a substituted magnetite of formula: Mn0.44 C 0.18 Fe2.38 4 They were obtained as follows: - Precipitation of an oxalic precursor Mn 0 , 15Co 0.06Fe0.79C204.2 H2

in an alcoholic environment.

  - Thermal decomposition of the precursor at slow heating rate (10 C / h) up to 300 C then rapid heating (200 C / h) up to 600 C, followed by a step of 0.5

hour.

  - Reduction of the decomposition product at 320 C for 1 h under H2 (10%) and N2 (90%), followed by treatment with

450 C under N2 (100%) during lh.

  The particles are heated in air, at a rate of C / hour, at various increasing temperatures, being maintained each time for 2 hours at the chosen temperature, and then cooled at a rate of 100 ° C. the ambient temperature at which the coercive field and the remanent magnetization measurements are

each time done.

  The results are shown in Figures 1 and 2 attached.

EXAMPLE 2

Influence of the duration of the bearings

  The starting material is the same as in Example 1.

  Two samples of this substituted magnetite are air-oxidized by raising them to 380 C (heating rate: 150 C / hour) for 2 hours for the first sample, and for 6 hours for

the other sample.

  The results obtained are as follows: Sample Coercive field Magnet. remanent (Mr) (Oe) (uem / g)

1 1300 41

2 1365 42

EXAMPLE 3

  Annealing of a Lacunar Spinel Ferrite Lacunar spinel ferrite particles were obtained by air oxidation of the starting material of Example 1, by direct heating, in air, at 350 ° C., at a plateau of 5 hours at

  this temperature, then rapid cooling to room temperature.

  heating rate: 150 C / hour cooling rate: 1000 C / hour The particles obtained have the following composition: MnO 0, 44 Co0.17 Fe2.39 4.44 Hc = 1.240 Oe Mr = 40.2 emu / g Mr / Ms = 0.73

(Ms = saturation magnetization).

  The obtained particles were subjected to series of air anneals similar to those described in Example 1, but at temperatures below the oxidation temperature (350 ° C.). The state

  oxidation of the product does not vary.

-20- 2650115

  The results of coercive field measurements and magnetization

  the remanent, are shown respectively in Figures 3 and 4 attached.

EXAMPLE 4

  The procedure is analogous to that described in Example 3, starting from lacunar spinel ferrite particles of composition: Mn0.33 C 0.08 Fe2.59 4.35 Hc = 494 Oe Mlr = 40.2 uem / g Mr / Ms = 0.65 These particles were obtained by oxidation of a substituted magnetite of composition: Mn 0.33 Co0.08 Fe2.59 4

  Oxidation temperature: 350 C (5 hour stage).

  Fast cooling at room temperature.

  The results obtained after the annealing carried out on these lacunar spinel ferrite particles under conditions similar to those of Example 3 are shown in Table I below: T annealed Hc (Oe) Mr (uem / g) Carrure

494 40.2 0.65

100 497 40.1 0.65

210 506 40.6 0.66

260,551 40.9 0.67

300,536 40.4 0.66

350 473 38.7 0.54

26-2650115

* -21- dd- l

EXAMPLE 5

  The procedure is analogous to that described in Example 3, starting from lacunar spinel ferrite particles of composition: inO, 07 C 0.12 Fe2.81 04.43 Bc 725 Oe Mr = 42.2 uem / g Mr / Ms = 0.67 These particles were obtained by oxidation of a substituted magnetite of composition: Mn Fe 0 0.07 CoO, 12 2.81 4

  Oxidation temperature: 350 C (5 hour stage).

  Fast cooling at room temperature.

  The results obtained after the annealing carried out on these lacunar spinel ferrite particles, under conditions similar to those of Example 3, are summarized in Table 2 below: T annealed Hc (Oe) Mr. (uem / g) Carrure d hc / dT (Oe /) *

725 42.2 0.67 - 6

110 735 41.8 0.66 - 6.3

210 751 41.9 0.66 - 6.5

260,795 42.4 0.68 - 6.8

_ _____

300, 770 41.4 0.66 - 6.5

340,750 42.1 0.66 - 6.5

  * measured between 20 and 60 ° C Conversion CGS units to SI 1 Oe = 103/4 't A / m 1 uem / g 1 AM 2 / kg -22-

EXAMPLE 6

  R8ole of cooling rate When lacunar spinel ferrite has a peak of

  coercive field, as defined in the description, at temperature

  less than its production temperature (by oxidation of a substituted magnetite), a slow cooling allows a large gain of field

  coercive, compared to rapid cooling.

  Thus, particles of lacunar spinel ferrites of composition (with the exception of the dopants): Co0.14 Zn0.04 Fe2.82 4.41 '0.02 BaO, 0.05 B203O obtained by heating in air at 310 C for 2 hours of the corresponding substituted magnetite, are cooled either at the speed of 500 C / hour, or at the rate of 5 C / hour. Coercive fields are

820 Oe and 910 Oe respectively.

EXAMPLE 7

  The same cooling experiments are carried out as in Example 6, with a lacunar spinel ferrite of formula Mo 0.04 C 0.12 Fe 2.84 4.49 obtained by oxidation at 300 ° C. for 2 hours of the corresponding substituted magnetite. . Fast cooling: Hc = 705 Oe; Mr = 40.1 uem / g Slow cooling: He = 744 Oe; Mr = 40.6 uem / g -23-

Claims (15)

  1. Magnetic material constituted by a ferrite with a lacunar spinel structure, based on iron sesquioxide, cobalt oxide and at least one other metal oxide, optionally with one or more doping agents, characterized in that said ferrite can be obtained from a lattice spinelite ferrite containing said oxides, by a process consisting of performing, optionally in an applied magnetic field, successive annealing at increasing temperatures ranging from room temperature to a lower temperature at the gamma transformation temperature>> α, for the material in question, said annealing temperatures being separated by intervals of 10 to C, observing a step of at least 0.5 hour at each chosen annealing temperature, after each annealing, the composition is cooled to room temperature, after each annealing followed by cooling, it is measured at room temperature. ambient temperature, at least one magnetic or optical property selected from the coercive field, the remanent magnetization, the coercive field of the remanent magnetization, the middle part of the hysteresis cycle, the Faraday rotation and the Kerr rotation, which is thus establishes the curve of variation of said property with the annealing temperature, which one finds the annealing temperature or temperatures corresponding to a maximum of said property thus measured, that one selects the materials for which one observes at least a maximum of said magnetic or optical property, thus measured, for an annealing temperature greater than 80 ° C, but no maximum between ambient temperature and 80 ° C, that said material is brought to a final annealing temperature greater than the temperature corresponding to the largest maximum of said property, and that said material is slowly cooled from said   higher temperature up to room temperature.   -2. Material according to claim 1, characterized by the fact that said annealing temperature greater than 80 ° C. is at least 1000 ° C., and that no maximum is observed between the temperature ambient and 100 C.   3. Material according to claim 1 or 2, characterized by   that said slow cooling rate varies between 2 and 10 C / hour. -24-   4. Material according to any one of the claims   characterized in that it contains, in addition to iron sesquioxide and a cobalt oxide, at least one divalent metal oxide selected from zinc, magnesium, cadmium or iron and / or at least one multi-valence metal oxide, it being understood that the sum of the molar proportions of the cobalt, the divalent metal and the polyvalent metal is less than or equal to 33%.   5. Material according to claim 4, characterized in that the proportion of all of said divalent metals may vary from 1 to 30 mol%, relative to the total number of moles of the metal cations. (doping agents excepted).   6. Material according to claim 4 or 5, characterized in that said metal with several valences is selected from manganese, molybdenum, copper, vanadium, chromium and rare earths (yttrium). and lanthanides).   7. Material according to any one of claims 4 to 6,   characterized in that the proportion of all of said multi-valence metals can vary from 1 to 30 mol%, based on the number of   total moles of metal cations (doping agents excepted).   8. Material according to any one of the claims   of the above, characterized by the fact that the proportion of cobalt may vary from 1 to 30 mol%, relative to the total number of moles of   metal cations (doping agents excepted).   9. Material according to any one of the claims   characterized in that its chemical composition (optional doping agents excepted) has the following formula: MM 'Co2 + F 3+ 02- (I) wxy 3 -xyxy + t in which M represents at least one cation of a divalent metal selected from zinc, magnesium, cadmium and iron, M 'represents at least one cation of a metal with a plurality of valencies selected from manganese, molybdenum, copper, vanadium, chromium and rare earth, w represents the number of moles of said one or more divalent metal cations, x represents the number of moles of said multi-valence metal cation or cations, 2+ y represents the number of moles of said cation cations. -25- 2650115   t is a number representing the number of moles of anions o2 in excess of 4, this number t being greater than or equal to 0.01 and less than or equal to 1,   it being understood that the sum v + x + y is less than or equal to 1.   10. Material according to any one of the claims   preceding, characterized by the fact that it is in the form of   acicular particles or a thin layer deposited on a substrate.   11. A process for preparing a material as defined in   any of the preceding claims characterized by the fact that   whether a lacunar or non-lacunated spinel ferrite is used as the starting material, at least the operation of bringing the starting material to an acquisition temperature of a lower directional order, from 5 to 15 C, at a predetermined temperature, that said material is maintained at said lower temperature for a time sufficient to allow the acquisition of the corresponding directional order, that the material thus obtained is cooled to the temperature ambient, it being understood that said predetermined temperature is the temperature corresponding to a maximum of the variation curve of a magnetic or optical property chosen from the coercive field, the remanent magnetization, the coercive field of the remanent magnetization, the middle part of the hysteresis cycle, Faraday rotation and Kerr rotation, said variation curve being established according to the method mentioned in the claim 1, that said maximum is the largest maximum of said variation curve, and it being understood that when the starting product is a non-lacunar spinel ferrite, said starting material is heated beforehand in an oxidizing atmosphere at a temperature sufficient to transform it. hollow ferrite spinel having the desired composition.   12. The method of claim 11, characterized in that said predetermined temperature is the highest temperature corresponding to a maximum of said magnetic or optical property and that the magnetic material is cooled to room temperature by observing bearings at temperatures below about 5 to 15 C at each temperature corresponding to a maximum of the property magnetic or optical studied. -26- 26 5 0 115   13. Process according to any one of Claims 11l and   12, characterized in that the cooling rate between the lower temperature of about 5 to 15 C at the temperature corresponding to said largest maximum and the ambient temperature, or between said highest temperature and the ambient temperature, is 210 C / hour.   14. Process according to any one of claims 11 and   12, characterized in that the cooling from said lower temperature to ambient temperature, or said cooling between two temperatures corresponding to two consecutive maximum zones or between such a temperature and the ambient temperature is rapid, and that one observe steps at each of the zones of   temperatures corresponding to a maximum.   15. Use of a material as defined in one of   any of claims 1 to 10 in the preparation of materials for   magnetic and / or magneto-optical recording.