FR2651366A1 - Surface-doped iron-based magnetic material and method of obtaining it - Google Patents

Abstract

Iron-based magnetic material surface doped with a lanthanide, the said material being in the form of particles whose iron atoms are combined with nitrogen atoms or are partially substituted by cobalt atoms especially. In a preferred embodiment of the invention, the lanthanide is neodymium or samarium. A method for preparing the said particles from precursors by treatment of the said precursors by a solution containing lanthanide ions and then heat treatment in a gaseous reducing phase.

Description

 The present invention relates to novel magnetic iron-based materials, their preparation and their application as magnetic recording materials.

It relates more particularly to particles for magnetic recording having controlled magnetic and morphological properties.

 Since the development of the first magnetic Fe203-gamma tapes, the magnetic materials used for recording have evolved considerably, especially to achieve a higher recording density.

High density recording requires materials with the following characteristics:
- a high coercive field (between 600 and 1500 Oe) ~ this feature requires the use of magnetic single-domain particles of controlled size and morphology.

 a relatively high saturation magnetization (greater than 100 u.e.m / g); this characteristic is dependent on the composition and the structure of the material concerned.

 On the other hand magnetic particles for high density recording must be smaller than one micrometer, have a narrow size distribution, be homogeneous in shape, and be free of porosity and dendrites.

 Fe2O3-gamma or Fe304ne iron oxide particles make it possible to achieve coercive field values of between 350 and 400 Oe. This coercivity can be increased by superficial doping, however the magnetization of the particles remains low, of the order of 75 u.e.m / g.

 Recently, iron particles have been used for high-density recording which have the advantage of having a high coercivity (1200 to 1503 Oe) in the passivated state and good saturation magnetization generally between 100 and 150 μm / g.

However, the size of these iron-based particles, corresponding to the magnetic monodomain, is small. Obtaining high values of coercive field implies very small particles. But this state of division leads to very high specific surface values having the disadvantage of a chemical degradation facilitated in the presence of oxygen or water vapor.
To these problems, research has been carried out on the basis of Fe4N iron nitride, which has the advantage over iron of having more chemical stability.
For illustrative purposes, the European patent US 0.102.881 relates to a new ferromagnetic material consisting of an iron carbonitride, obtained by substituting nitrogen atoms of the Fe4N network by carbon atoms, which gives it a better stability compared to the original materials.

Another evolution has been made in the direction of a search for less acicular particles that can be used for high density recording. However, the party
scarcely acicular iron sacs lose their coercivity; we therefore sought to improve their properties and in particular to obtain a high coercivity using a doping layer.

The article by T.MIYAHARA and K. KAWAKAMI (IEEE
Transactions on Magnetics-Vol. 23 N05 September 1987, pa
2877-2879) describes the treatment of acicular goethite particles (FeOOH) with an aqueous solution of neodymium nitrate and boric acid.

The surface layer obtained corresponds to the formula
Nd Fe B14 and is an amorphous layer, that is to say uncrisp
It is not very compatible with the nucleus of the particles.

The resulting particles have a coercivity greater than 2000 Oe and are unsuitable for high density recording due to the amorphous nature of their surface magnetic layer.

 The object of the present invention is to obtain a material, which can be put into the form of optionally non-acicular particles of more isotropic morphology, having an increase in the coercive field while maintaining saturation magnetization values adapted to one another. high density recording, and while increasing the resistance to chemical degradation at the particle surface, i.e. by increasing passivation.

 The subject of the present invention is therefore an iron-based magnetic material, characterized in that it consists of a ferromagnetic core comprising iron atoms, substituted or unsubstituted, said core being coated with a surface doping layer consisting of in particular atoms of said nucleus associated with atoms of one or more elements of the family of lanthanides having a magnetocrystalline anisotropy.

 The core magnetic material may be reduced or partially oxidized pure iron, or partially metal-substituted iron, such as cobalt, or a metalloid, such as nitrogen, or a defined composition phase, such as Fe4N or a mixed phase of defined composition, such as iron carbonitride corresponding to the formula Fe4N (Ct, in which E is less than or equal to 0.3, or such that iron boronitride corresponding to the formula Fe4N # 1 - # B, where s is less than or equal to 0.3, or a mixed nitride of the formula Fa (4 # x) MxN wherein M is a metal such as tin.

 The surface layer consists of one or more elements of the family of lanthanides having a magnetocrystalline anisotropy, iron atoms with oxidation levels + 2 and / or + 3 and an anionic species likely to lead to a phase stable under the conditions of use.

 Elements of the family of lanthanides exhibiting magnetocrystalline anisotropy may in particular be neodymium and / or samarium. On the other hand, gadolinium does not meet the needs of the invention.

The surface layer can thus satisfy the following global formula:
Ln Fa X3 in which
Ln represents an element of the family of lanthanides,
X is any element capable of giving rise to an anionic species capable of leading to a stable phase under the conditions of use and in particular represents an atom of one or more elements taken from the group comprising S, O, N and P.

 The elements Ln and X are preferably chosen so as to ensure network compatibility, that is to say the arrangement of the atoms, between the core phase and the surface phase, for example cubic phases.

The degree of oxidation of the lanthanide (s) associated with the layer may be + 2 and / or + 3, in which case the surface layer has the following formula:
2+ 3+ 2+ 3+
Lnx.Ln (I # x) FeY.Fe Xz in which
x and y are independently less than or equal to 1
and z is a function of x, y and the valence of the element X.

In the case where X represents oxygen, z = 3- (X f Y)
2
Boron may also be used as an additional element, the latter being capable of either partially replacing the anionic species or of acting as a flux in the chemical process for constituting the surface phase.

 In addition, the effectiveness of the doping layer is obtained with a thickness of 5 to 50 Angstroms, preferably 20 to 30 Angstroms.

 In addition, the material may be in the form of a non-acicular or low acicular particle or in the form of a thin layer.

In the case of particulate products, the said parties
and length, diameter, or aspect ratio ratios in the range of 1: 1 to 10: 1.

 The products of the invention can therefore be particles having a length of 0.02 micron to 1 micron, with aspect ratios ranging from 1 to 10, having a coercivity of between 500 and 1500 Oe and a saturation magnetization between 100 and 150 μg.

 The invention furthermore relates to a method for the preparation of a material in the form of iron-based magnetic particles consisting of a ferromagnetic core, comprising iron atoms, substituted or unsubstituted, said core being coated with a surface doping layer consisting of atoms of said nucleus associated with atoms of one or more elements of the family of lanthanides having magneto-crystalline anisotropy, characterized in that the nucleus or precursor thereof is treated with a first solution containing ions of one or more lanthanides, in the presence of a second solution which can, by reaction with the first solution, give rise to a precursor phase of the doping layer adsorbing on the surface of the particles, in that the particles thus treated are dried and in that a reductive gas phase heat treatment is carried out.

 The reducing agent may be hydrogen or a mixture of hydrogen with nitrogen, ammonia or diborane (B2H6) and the treatment temperature is between 4000C and 4500C. The first particle treatment solution, containing lanthanide ions, may further contain boron ions, particularly in the form of boric acid.

 The reducing treatment is preferably carried out in a fluidized bed reactor and lasts 4 to 7 hours.

 It is also possible, prior to the addition of the lanthanide, to perform the substitution of iron atoms with manganese and / or tin atoms. This substitution can be performed by any method known in the state of the art and particularly by coprecipitation.

 According to a preferred embodiment of the invention, the lanthanide is neodymium or samarium.

 The present invention furthermore relates to a ferromagnetic material based on particles obtained by the process or one of the variants of this process previously described.

 The invention further relates to a magnetic recording element essentially comprising a non-magnetic support and a ferromagnetic material as previously described.

 The material according to the invention can be applied on strips or on any other support by means known to those skilled in the art.

 The invention will be further illustrated without being limited in any way by the examples hereinafter and by the appended figures.

 FIGS. 1 and 2 represent Nd-coated goethite particle photos respectively in transmission electron microscopy and in high magnification electron microscopy.

Figures 3a) and} b) represent spectra
EDAX respectively of the center and the periphery of particles coated with Nd, carried out using a probe of 7.4 angstroms. The characteristic peaks of Iron and Nd are denoted ND and FE.

 FIG. 4 represents a diagram obtained from an E.D.A.X image of a Nd-coated Fe4N particle. The light zone (A) representing the core of the particle and the zone (B) the Nd-rich surface layer.

 FIG. 5 represents an AUGER spectrum produced on the surface of Nd-coated Fe4N particles. On the abscissa is the kinetic energy and on the ordinate the number of strokes.

 Figures 6 and 7 show Auger profiles of Nd-coated Fe4N particles. The etching time is indicated on the abscissa and the atomic concentration of the various elements on the y-axis.

 Figure 8 shows the Mössbauer spectrum of 15lEu made on the surface layer of the magnetic particles coated EuFe03. The percentage of absorption in ordinate is measured according to the isomeric displacement in abscissa.

 FIG. 9 represents the influence of the variation of the neodymium level, on the abscissa, on the coercivity (He) and the saturation magnetization (6 s), on the ordinate, measured on particles coated with Nd.

EXAMPLE 1
Preparation of the goethite precursor FeOOH
In a 10 l beaker, 1 kg of Fe (NO 3) 3 .9H 2 O nitrate of iron is dissolved in 5 liters of pure water by means of magnetic stirring of the medium. The pH of the resulting solution is about 1.16. Over a period of one hour the pH was raised to 1.7 by addition of 50% NaOH solution. The volume is then increased to 9 1. The medium is gradually heated and heated to 800C for 4 hours. This period corresponds to nucleation: the number of nuclei (nucleation sites) increases as a function of time and the size of the subsequently obtained goethite particles is all the smaller (0.1 Xm).

The goethite particles are precipitated by increasing the pH to 11.8 (this value should not be exceeded beyond which iron oxide is obtained
Fe 2 O 3 - i) by addition of 50% sodium hydroxide solution. The reaction is complete after another 4 hours stirring at 800C. The color turns from blood red to yellow ocher color of goethite.

 After stopping the stirring and heating, the medium is allowed to stand overnight. The strongly basic solution is siphoned and the ocher precipitate is washed several times with pure water to bring the medium to a neutral pH.

The goethite is then filtered and dried.

EXAMPLE 2
Preparation of the goethite coated with Nd and B.

 About 59 g of goethite prepared in Example 1 are dispersed in 100 cm 3 of an aqueous solution of neodymium nitrate [Nd (NO 3) 3 914 20] and boric acid and the mixture is stirred through a magnetic stirrer. The concentration of the solution in Nd and in B is calculated with respect to the initial amount of goethite.

Neodymium is then precipitated under one of two forms
* Nd (OH) 3 (hydroxide) by adding an excess of ammonia to the suspension solution of goethite particles.

 * Nd2 (C204) 3 (oxalate) by adding an excess of an oxalic acid solution.

 The precipitate is then separated by filtration and allowed to dry without additional washing to preserve the boric acid.

EXAMPLE 3
Preparation of nitrided iron particles, borured and substituted with neodymium atoms

Goethite particles are prepared according to the method of Example 2 by treatment with an ammonia solution
The particles thus treated are then dried and nitrided in a fluidized bed reactor in a gaseous mixture
NH3 H2 at a temperature between 350 and 5000C for 4 hours.

EXAMPLE 4
Preparation of nitride particles, borurated and substituted with tin and neodymium atoms.

 Goethite substituted with tin atoms is prepared from iron nitrate and tin chloride.

 The goethite thus substituted is then treated as in Example 3 with a solution of neodymium nitrate, and is then nitrided.

EXAMPLE 5
Physicochemical Characterization of the Particles Obtained in Example 3

 a) X-ray crystallographic analysis.

The X-ray diffractogram of the magnetic particles obtained in Example 3 has a single isotype phase of
Fe4N.

 An analysis of the diffraction line widths obtained by the SHERRER method does not make it possible to demonstrate a significant variation in the size of the magnetic grains with respect to the nitrides prepared without prior treatment of the precursors with the solution of neodymium nitrate - and 'boric acid. This indicates that the surface treatment performed plays no role in the control of grain size during magnetic precursor grain processing.

 Characterization by electron microscopy and X-ray fluorescence "in situ".

 All of this work was done using a Philips C.M. 30-300 kV microscope.

The picture of Figure 1 shows the goethite particles, coated with neodymium hydroxide. In particular, we can observe the rows of parallel reticular planes
leles to the growth axis of the particles. Clichés of magnetic particles obtained after nitriding show that the general form of precursor crystallites is preserved, namely an acicular form. In addition, these clichés show a subdivision of particles into magnetic grains similar to that noted during the nitriding treatments carried out on precursors not encobed with neodymium hydroxide.

 It is important to underline that these particles, strongly magnetic, gather in singles then in loops thus making it possible to close the flux lines and to minimize the total magnetic energy.

 A high-magnification (550000 X) image of an isolated magnetic grain (Figure 2) shows that the core structure of the magnetic grain is different from that of the surface. The thickness of the surface layer can then be estimated at about 30 Angstroms.

An "in situ" X-ray fluorescence analysis is then performed on this grain using a nanometer of 7.4 nm in diameter in order to precisely locate the various constituent elements of the pigment. This analysis is carried out in two places of the grain: in the center then on the periphery on the surface layer. The two fluorescence spectra
X obtained are shown in Figure 3 and indicate that the NdîFe ratio is greater in the surface layer than in the core of the material.

X-ray fluorescence mappings of the Nd and Fa elements were also performed on a set of
ticles. Reconstituted digital images for each of these elements are shown schematically in Figure 4.

 In general, neodymium mapping is identical in appearance to that of iron. This is explained by a relatively homogeneous distribution of the neodymium-containing surface layer on the iron nitride grains.

c) Characterization by spectroscopy AUGER
The AUGER electron spectrum produced on the magnetic pigments is shown in FIG. 5. It shows the characteristic peaks of the elements Nd, B, Fa as well as oxygen. This last element indicates an oxidation of the surface of the particles. It should also be noted that nitrogen is not detectable on this spectrum, nitriding appearing to affect only the core of the material.

 In order to analyze the composition and the distribution of the various analytes in the presence and particularly to characterize the surface oxide layer, successive ionic etching of the particles is carried out. The concentration profiles obtained on two samples are shown in FIGS. 6 and 7.

 For a zero stripping time, that is to say with an untracked surface, the ratios of the concentrations measured for iron, neodymium and oxygen lead - to measurement uncertainties - to a neighboring Nd / Fa atomic ratio. In addition, the atomic ratio Nd / O is close to 3.

For increasing pickling times, an increase in nitrogen and iron concentrations is observed until a stoichiometry close to Fe4N is obtained. Rates of Nd, B and O after a slight decrease appear to be
ter constant for large stripping times. These last results are not significant since the Auger analysis of very fine particles actually concerns a set of randomly oriented particles, which always implies a contribution from the surface.

 d) Magnetic characterization.

 A hysteresis cycle is performed on the particles.

It is characterized by a magnetization close to 116 uem / g; a ratio of middle size equal to 0.50 and a coercive field of 735 Oe. This latter value represents an increase of about 450 Oe compared with iron nitrides or carbonitrides which have not been previously treated with neodymium nitrate.

EXAMPLE 6
Identification of the layer on the surface of the magnetic particles obtained in Example 3.

a) Mossbauer spectroscopic characterization of the surface
Since neodymium is not a resonant element in Mossbauer spectroscopy, magnetic particles passivated by an oxide layer containing europium have been prepared according to the process used for neodymium and described in Example 2.

 The Mössbauer spectrum of Eu produced on these pigments (FIG 8) consists of a peak located at 0.55 mm / s (reference: EuF3 at T = 273 K). This value is characteristic of the europium element at the degree of oxidation + III; in addition, this line has an enlargement with respect to the Eu line as observed in compounds such as EuF3.

 This spectrum is close to that of the Eu in octahedral site.

The trivalent europium diamagnetic (fundamental term
tal 7Fo) is normally not sensitive to the magnetic field.

Nevertheless, in some compounds an enlargement, or even a break-up of the line, has been demonstrated by the fact of
the presence of a field transferred to the Eu ion site.

This one appears during a mixture of the state
of Eu III with certain states during exchange interactions between europium and a neighboring atom, especially iron.

b) Crystal structure and magnetic properties of
NdFeO3
The compound NdFeO3 belongs to the family of rare earth orthoferrites. The crystalline structure of this phase is of the distorted (orthorhombic) perovskite type.

The parameter of the pseudo-cubic mesh has, = 3,876
Calculated from the orthorhombic mesh, is close to that of Fe4N (3,797 A).

The magnetic structure of NdFeO3 was determined by neutron diffraction according to the method of KOEHLER, WOLLAN and WILKINSON: (Phys Rev, 118, 58-70, (1960)) the antiferromagnetic order of the iron atoms is of type G 35 and the Néel temperature of 760 K. In addition a ferromagnetic contribution was observed
All of these results suggest that the surface-formed layer is a poorly crystallized layer of perovskite NdFeO3.

EXAMPLE 7
Physical characterization of the particles prepared in Example 4.

 The characteristic magnetic properties of the particles thus obtained can be deduced from the hysteresis cycle.

By combining the control of the size of the magnetic grains by partial substitution of the iron by Sn (or Mn) within the Fe4N type structure network to the action of a layer containing Nd, Fa, B and O and entralnant an increase
In the case of surface magnetocrystal anisotropy, it has been possible to achieve values of the coercive field of the order of 900 Oe. Such unexpected values in the case of Fe4N-type nitrides or carbonitrides are in fact suitable for magnetic recording applications.
It should also be emphasized that the values of the saturation magnetization observed are in the samples of 80 to 130 uem / g, values which are also perfectly appropriate for such an application.
EXAMPLE 8
Effect of neodymium percentage on the magnetic properties of particles.

 The particles were prepared as described in Example 2, varying the proportion of neodymium incorporated in the particles. Figure 9 shows the evolution of coercivity and saturation magnetization (ordinate) as a function of the percentage of neodymium incorporated in the particles (abscissa), with a percentage of boron set at 15%. This figure shows that the optimal proportions for both the coercivity and the saturation magnetization of the particles lie in a range of neodymium content between 2 and 3%. For a neodymium content of 2.5%, a coercive field strength of 740 Oe and a saturation magnetization of 165 uem / g are obtained.

 In addition, a measurement of the thermal coefficient of the variation of the coercive field with the temperature led to low values of the order of 0.2 Oe / OC.

EXAMPLE 9
Comparison of the effect of neodymium and lanthanum on the magnetic properties of particles.

 The nitrided or non-nitrous iron particles are obtained from the goethite particles according to the process described in Example 2, using neodymium oxalate or lanthanum oxalate for the iron particles, and neodymium nitrate. or lanthanum nitrate for the goethite particles and not adding boric acid to the reaction mixture. The results are shown in Tables 1 and 11 and show that in all cases neodymium has a higher effect both with regard to the coercive field of saturation magnetization compared to lanthanum under the same conditions. In addition, much higher values are obtained with respect to the coercive field and the saturation magnetization when using goethite particles with respect to the use of iron particles as a precursor.

EXAMPLE 10
Preparation of Nd-doped carbonitrided Fe203 particles

 Ferric oxide crystallites (Fe 2 O 3 - o () of submicron size (<1 μm) are dispersed in an aqueous solution of neodymium nitrate (typically at 10% -at / amount of iron) .The medium is homogenized by means of After stirring, a solution of oxalic acid is added to the slurry, resulting in absorption of the Nd3 + ions as a precipitate coating the iron oxide particles.

 After filtration, the particles are dried in an oven at 700C. The dried particles are then nitrided under a reducing atmosphere in a fluidized bed reactor (experimental conditions: 1 <NH3 / H2 <2 T = 350-4500 ° C., 1 h at 1300 ° C. and 6 h at 45 ° C.). The decomposition of oxalate into tralne the formation of carbon monoxide which at the temperature of the experiment is in equilibrium with carbon dioxide and elemental carbon. The nitriding atmosphere is thus coupled with a fuel atmosphere resulting in the formation of a final product which is iron carbonitride.

 The magnetic powder thus obtained is stored under a non-oxidizing solvent such as cyclohexane (previously dried under sodium metal). The particles thus prepared have the following characteristics: X-ray diffraction: Fe4N phase (Fe4 (N, C), magnetic measurements: Hc = 700 Oe, = s = 140 ug, r r / s = 0.3.

EXAMPLE 11
Comparison of the doping of various lanthanide iron particles.

a) Preparation of Fe-amorphous precursor
The preparation technique derives from that described by Oppegard et al. (J. Phys Phys., S32, 1961).

 100 cm3 of a molar solution of NaBH4 or KBH4 are added to 100 cm3 of a molar solution of stirring iron sulphate solution for homogenization. The addition must be done regularly over a period of 3 minutes using a burette, under a hood. A weakly magnetic black precipitate is obtained. It is washed several times with distilled water by decantation by keeping the powder in the beaker with the help of a magnet. The powder is then dried under a stream of air. A quantity of 3 g of a very fine black powder is obtained.

 It is also possible to substitute for the iron sulphate solution a solution of iron sulphate and sulphate of one of the following elements: Co, Ni, Cu, Sn, Ru and Ir.

b) Preparation of particles coated with Nd and / or with Sm
About 1 g of the iron powder prepared in a) is dispersed in an aqueous solution of neodymium nitrate (10% by weight based on the initial amount of iron). Mechanical agitation ensures the homogeneity of the medium.

A solution of oxalic acid is then added to the suspension. This results in an absorption of Nd3 + ions in the form of a neodymium oxalate precipitate coating the iron particles. After filtration, the particles are dried under a stream of air.

 The precursor powders are subjected to a heat treatment under hydrogen flow (grade of purity U) at 550-6000C for 4 hours. The reaction is carried out in a silica tube placed in a horizontal furnace suitable for gas flow rates. The reaction is carried out simultaneously on the coated product and on an untreated control surface.

 After reaction, the product is collected in cyclohexane and the physicochemical characterizations are carried out.

 X-ray diffraction: Fe-06 type phase plus characteristic peaks of an iron oxide phase with neodymium.

 Magnetic measurements: Hc = 750 Oe, r # = 110 uem / g (Fe without surface treatment - control: Hc-400 Oe, 6> 100 ug / g).

Electron microscopy:
The samples were characterized by transmission electron microscopy: no difference in size or morphology with respect to the precursor particles was observed.

 A similar treatment with a samarium salt or an equimolar mixture of Sm and Nd gives identical results.

 c) Particles coated with Gd.

 The precursor iron particles obtained in a) were subjected to a treatment similar to treatment b). Only neodymium salt has been replaced by a gadolinium salt.

The characteristics of the final particles are as follows:
X-ray diffraction: Fe-Otplus type phase of the characteristic lines of a mixed iron oxide.

 Magnetic measurements: Hc = 400 Oe, 6s = 110 ug. (Fe without surface treatment - control -: Hc-380 oye, rs 100 uem / g).

 The results in b) and in c) clearly show that Nd and / or Sm doping makes it possible to obtain particles having a high coercive field (Hc) while Gd doping does not allow it.

 In this example only the obtaining of a surface-doped pigment Fa has been sought because of the additional passivation provided by nitriding or carbonitriding. Thus, the magnetic role of the doping layer is at the origin of any improvement in the magnetic characteristics required for the recording, in particular the coercive field Hc.

 The increase of the coercive field observed for doping layers based on Nd, Sm or Nd + Sm (systematic increase of 50% of Hc) is interpreted through the examination of the electronic configuration of rare earth ions: Xe 4f (n) with n = 3 for Nd and n = 5 for Sm, ie an underlayer 4f (which can contain 2x7 electrons in total) incomplete and anisotropically occupied. On the other hand Gd3 + which has a sub-layer 4f half filled with 7 electrons has an isotropic electronic configuration: Xe 4f (7).

 Thus an anisotropic electronic configuration of the rare earth is at the origin of a magnetocrystalline anisotropy resulting in the increase of the coercive field of the elementary magnetic particle.

 In the case where the electronic configuration is isotropic (example Gd (3+)) no improvement of the magnetic characteristics is observed.

d) Iron particles substituted at the core and doped at the surface
According to the methodology described in a) precursors
Fe-Co (Fa0.7 Cl0.3) were prepared and subjected to the treatments mentioned in b). The magnetic powders obtained had improved characteristics compared with iron powders coated with rare earths alone (Hc = 800
Oe, rs-130 uem / g.

 The increased anisotropy of cobalt with respect to that of iron added to the anisotropy provided by the surface doping layer. The two combined effects bring about an increase in the coercive field.

 Any other element having a magnetocrystalline anisotropy greater than that of iron and forming a stable alloy with it is suitable in this example.

TABLE I
Effect of partial substitution of iron atoms by
lanthanide atoms in iron particles

<tb><SEP> field <SEP> magnetization
<tb><SEP> coercive <SEP> to <SEP> saturation
<tb> Precursor <SEP> Conditions <SEP> Hc <SEP> (Oe) <SEP> (uem / g)
<tb><SEP> experiment. <September>
<Tb>

Particle <SEP> current <SEP> H2 <SEP> 200 <SEP> 180
<tb> of <SEP> goethite <SEP> T <SEP> = <SEP> 560 C
<tb> Particles
<tb> of <SEP> goethite <SEP> same <SEP> 620 <SEP> 100
<tb> + <SEP> 10% <SEP> at. <SEP> Nd <SEP> treatment
<tb> Particles <SEP> same <SEP> 450 <SEP> 95
<tb> of <SEP> goethite <SEP> treatment
<tb> + <SEP> 10% <SEP><SEP> and. <SEP><SEP> The
<Tb>
TABLE II
Effect of partial substitution of iron atoms by
lanthanide atoms in particles of
nitrile iron

<tb><SEP> field <SEP> magnetization
<tb><SEP> coercive <SEP> to <SEP> saturation
<tb><SEP> Precursor <SEP> Conditions <SEP> Hc <SEP> (Oe) <SEP> (uem / g)
<tb><SEP> Experimental
<tb><SEP> Goethite <SEP><SEP> Current <SEP> NH3 / H2 <SEP> 460 <SEP> 211
<tb><SEP> 1 <SEP><NH3 / H2 <SEP><SEP><SEP> 2
<tb><SEP> T <SEP> = <SEP> 4200C
<tb><SEP> Goethite <SEP>
<tb><SEP> + <SEP> 2.5% <SEP> and. <SEP><SEP> same <SEP> | <SEP> 740 <SEP> 164
<tb> I <SEP> Nd <SEP><SEP> treatment
<tb><SEP> + 15% <SEP> At. <SEP> B
<tb><SEP> Goethite
<tb><SEP> + 2.5% <SEP> At.The <SEP> same <SEP> 663 <SEP> 110
<tb><SEP> +15, ó <SEP><SEP>at; B <SEP> treatment
<Tb>

Claims (29)

 1. Magnetic material based on iron, characterized in that it consists of a ferromagnetic core having substituted or unsubstituted iron atoms, said core being coated with a surface doping layer consisting in particular of atoms of said associated core to atoms of one or more members of the family of lanthanides having a magnetocrystalline anisotropy.  2. Material according to claim 1, characterized in that the core consists of reduced pure iron, partially oxidized iron, or iron partially substituted by a metal or a metalloid.  3. Material according to claim 2, characterized in that the iron is partially substituted by cobalt.  4.Material according to claim 2, characterized in that the iron is partially substituted by nitrogen.  5.Material according to claim 1, characterized in that the core consists of a defined composition phase or a mixed phase of defined composition.  6.Material according to claim 5, characterized in that the core consists of a defined composition phase of iron nitride Fe4N formula.  7. Material according to claim 5, characterized in that the core consists of a non-stoachiometric structure phase of iron carbonitride Fe4N formula (1 g) Ce in whichgest less than or equal to 0.3 or iron bromonitride of formula Fe4N (Li) B i, in which G0 less than or equal to 0.3, or of mixed nitride of formula Fe4 # xMxN, in which M represents a metal atom.  8. Material according to any one of claims 1 to 7, characterized in that the surface layer consists in particular of one or more elements of the family of lanthanides having a magnetocrystalline anisotropy, iron atoms with degrees of oxidation + 2 and / or + 3 and an anionic species likely to lead to a stable phase under the conditions of use.  9. Material according to claim 8, characterized in that the lanthanide (s) and the anionic species (s) form compatible networks.  10. Material according to any one of claims 1 to 9, characterized in that the lanthanide is neo dyme.  11. Material according to any one of claims I to 9, characterized in that the lanthanide is samarium.  12. Material according to any one of claims 1 to 11, characterized in that the surface layer has the following formula:  Ln FeX3  in which  Ln represents an element of the family of lanthanides,  X corresponds to any element capable of giving rise to an anionic species capable of concluding a stable phase under the conditions of use and in particular represents an atom of one or more elements taken from the group comprising S, O, N and P.  13. Material according to claim 11, characterized in that the degree of oxidation of the lanthanide associated with the surface layer is +2 or +3, in which case the formula of the surface layer is as follows:  2+ 3+ 2+ 3+  Lnx. Ln (1 # x) FayFe (1 # y) Xz in which  Ln and X are as previously defined.  and z is a function of x, y and the valence (s) of element X.  x and y are independently less than or equal to 1,  14. Material according to any one of claims 11 and 12, characterized in that X represents an oxygen atom, in which case z = 3 (x t y).  2  15. Material according to any one of claims I to 14, characterized in that the doping layer has a thickness of between 5 and 50 Angstroms approximately and preferably between 20 and 30 Angstroms approximately.  16. Material according to any one of claims 1 to 15, characterized in that it is in the form of particles.  17. Material according to claim 16, characterized in that the particles have a length: diameter ratio in a range from 1: 1 to 10: 1.  18. Material according to any one of claims 16 and 17, characterized in that the particles have a preferred length in a range from 0.02 to 1 micrometer.  19. Material according to any one of claims 1 to 15, characterized in that it is in the form of a thin layer.  20. Process for the preparation of a material in the form of iron-based magnetic particles consisting of a ferromagnetic core comprising substituted or unsubstituted iron atoms, said core being coated with a surface doping layer made up of atoms of said iron nucleus associated with atoms of one or more elements of the family of lanthanides having a magnetocrystalline anisotropy, characterized in that the nucleus or the precursor thereof is treated with a first solution containing in particular ions of one or more lanthanides in the presence of a second solution which, by reaction with the first solution, can give rise to a precursor phase of the doping layer adsorbing on the surface of the particles, in that the particles thus treated are dried and in that a reducing gas phase heat treatment.  21. The method of claim 20, characterized in that the particles are treated with a reducing gas at a temperature between 4000C and 4500C.  22. Process according to any one of claims 20 and 21, characterized in that the reducing gas is hydrogen, nitrogen, ammonia, or diborane (B2H6).  23. Process according to any one of claims 20 to 22, characterized in that the treatment with a reducing gas is carried out in a fluidized bed reactor.  24. Process according to any one of claims 20 to 23, characterized in that the first particle treatment solution contains boron ions, in particular in the form of boric acid.  25. Process according to any one of claims 20 to 24, characterized in that, prior to the addition of the lanthanide, the substitution of the iron atoms by manganese and / or tin atoms is carried out.  26. Method according to any one of claims 20 to 25, characterized in that the lanthanide is neodymium or samarium.  27. Particle obtained by a process according to any one of claims 20 to 26.  The particulate magnetic material of claim 27.  Magnetic recording material comprising a non-magnetic support and a ferromagnetic material according to any of claims 1 to 19 and 28.