PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING

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PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING

W. Steck

To cite this version:

W. Steck. PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING.

Journal de Physique Colloques, 1985, 46 (C6), pp.C6-33-C6-40. �10.1051/jphyscol:1985605�. �jpa-

00224843�

PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING

W. Steck

BASF AG, 0-6700 Ludwigshafen, F.R.G.

Resume - L1auteur presente une selection de travaux dans le domaine des pigments magnetiques d'oxyde de fer, de fer metal et de Cr02.

Plusieurs methodes de synthese chimique par voie humides de particules iso- metriques FegO4 des compos6s Fe(OH)2 et Fe(3+) - non dopees ou dopees en volume avec du cobalt - sont comparees entre elles sur le plan de llinfluence de la composition et des dimensions g6ometriques du pr&curseur sur la taille des particules et sur les proprietes magnetiques des produits. L1oxydation de particules de magnetite dop'ees en volume avec du cobalt permet d'obtenir des pigments d comportement rnagnetique multiaxial.

La structure des couches de passivation des pigments metalliques est analy- see d llaide de differentes techniques. On mesure des epaisseurs de 4 - 5 nm, dont 0,5 d 1 nm pour les revetements superficiels d base de composes phos- phates destines d prevenir le frittage des particules. L1auteur compare plusieurs methodes de reduction pour la preparation des pigments m6talliques anisometriques.

Certains points de d6tail sont dGvelopp&s, concernant le mechanisme rGaction- nel de la synthsse hydrothemique des pigments aciculaires Cr02 en presence dlantimoine. Sur le plan qualitatif, les pigments de Cr02 presentent le m&me comportement qui dlautres materiaux dejd decrits en ce qui concerne la rela- tion de dependance entre la force coercitive et la taille des particules. Pour le Cr02 dop& avec du Sb et du Fe, la force coercitive maximale de 60 kA/m est obtenue avec une taille des cristallites (220) de 23 nm. Les pigments de Cr02 chimiquement stabilises sont proteges par des couches ayant seulement 0,7 d 1,3 nrn dl&paisseur. Pour l'emploi dans les materiaux d'enregistrement magn6- tique, le pigment metallique et le pigment Cr02 peuvent etre tous deux con- sider&~ comme ayant une structure noyau-couche superficielle.

Abstract - Selected contributions in the fields of iron oxide, metal and Cr02 magnetic pigments are presented.

Several wet chemical synthesis of undoped and cobalt bulk doped isometric FegO4-particles from Fe(OH)2 and iron(I1I)compounds are compared with regard to the influence of composition and geometrical dimensions of the ferric coreactant on the particle size and the magnetic properties of the product.

Oxidation of cobalt bulk doped magnetite particles leads to pigments with rnultiaxial magnetic behaviour.

The structure of passivation layers on metal pigments is analyzed by using different techniques. Thicknesses of 4 - 5 nm are measured 0.5 to 1 nm of which are due to surface coatings with phosphate compounds which are used to prevent sintering of the particles. Several reduction methods for the pre- paration of elongated metal pigment are compared.

Details of the reaction mechanism of the hydrothermal synthesis of needle shaped Cr02 in the presence of antimony are discussed. Cr02 pigments quali- tatively show the same behaviour with regard to the dependence of coercivity on particle size as described earlier for other materials. Maximum coercivity of 60 kA/m for Sb-Fe-doped CrO2 is obtained with crystallite sizes (220) of 23 nm. Chemically stabilized Cr02 pigments are protected by layers of 0.7 to 1.3 nm thickness only. For use in magnetic recording media both, metal pigment

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985605

JOURNAL DE PHYSIQUE

and Cr02 can thus be described by a core-shell-structure.

1. Introduction

It is well known that the properties of magnetic pigments depend strongly on the shape and volume of the individual particles. Consequently, chemistry has to provide suitable preparation methods by which geometrically well defined particles with narrow distibutions of shape and size are obtained. Ideally any magnetic powder therefore should be prepared by a direct synthesis as it is the case with the one-stage hydrothermal process of Cr02. Unfortunately however, there are no such processes for the production of acicular magnetic iron oxides or iron metal pigments respectively, which is due to the fact that theses substances crystallize in the cubic system. These materials therefore are made by the well known multi-stage processes mostly starting from acicular FeOOH which subsequently is pseudomorphously converted into magnetic pigments.

The physical chemistry and rheology of the magnetic media manufacturing pro- cess is largely governed by surface properties of the magnetic pigments. Also the magnetic and chemical stability are often influenced by the chemical com- position of surface layers on the particles. Thus, metal pigments usually are passivated by controlled surface oxidation to reduce their pyrophoric charac- ter. Cr02 on the other hand shows oxidizing properties and can be stabilized by means of reducing agents to form a protecting surface layer.

The present paper intends to contribute to the knowledge about how to prepare magnetic particles of different, well defined shapes which can be used in magnetic recording media. Furthermore, results on the geometric dimensions of

surface oxide films on passivated iron powders and on chemically stabilized Cr02 pigments will be presented.

2. Preparation and properties o f isometric magnetic iron oxides

Although most of the presently used magnetic pigments are of acicular shape there is still research going on with isometric oxide particles which were first used in magnetic tapes about 50 years ago 1 J. One reason for the continued interest in these materials are potential applications like the so-called isotropic magnetic recording mode C2J. Some selected wet chemical single-stage syntheses for magnetite and bulk-doped cobaltmodified magnetite, respectively, starting from Fe(OH)2 and Fe(II1)-oxides or -hydroxides are compared in table 1.

All syntheses depend in a similar way on -the reaction conditions like pH, temperature, reaction time, concentration, kind of ions present, dissolution rate and solubility of the iron compounds. The different particle sizes (as derived from specific surface areas SSA in m2/g) and thus the magnetic pro- perties of Fe304 - ferrimagnetic or superparamagnetic - are primarily led back to different nucleation rates which in turn are determined by the solubility and the dissolution rate of the iron( I I I) -compounds at the given reaction con- ditions. Therefore all methods presented can be described as reactions of ferric hydroxo-complexes with ferrous hydroxo species like rFeOHJ+ or L-HFe022-, being dominated by different nucleation rates.

The higher solubilities of Fe(OH)2 and Fe(OH)3 lead to high nucleation rates.

Thus very small superparamagnetic particles are obtained, which are well sui- ted for the preparation of magnetic fluids. Accordingly the lower solubili- ties of FeOOH or 4,-Fez03 lead to coarser ferrimagnetic particles, which can be used for magnetic printing inks or in magnetic recording media.

Additionally, it is well known that the dissolution rate (which controls the

nucleation rate of the product) of small particles is bigger than the disso-

lution rate of large particles of the same composition. Therefore one would

by the results with 8-FeOOH t3.7.

Typical reaction conditions and characteristic properties of Fej04

Table I - Preparation of FejO4 from Fe(OH)2 and ferric coreactants:reaction conditions and powder properties.

From transmission electron micrographs it can be seen that method 3 with A-FeOOH and $-FeOOH yields narrow particle size distributions. The same is true for the methods 1 and 2. Contrary to this method 3 with 6-FeOOH and 4 with &-Fez03 lead to much broader distributions and less homogenous particle shapes.

In order to prepare bulk-doped C0~Feg_~O4-particles with higher coercive force (Hc) the cobalt-doping is preferably done during the synthesis from Fe(OH12 and a-FeOOH, p-FeOOH or k-Fe203, respectively , ! - 4 -7. After oxida- tion in the dry state to C0~Fe3-~04.5-0.5~ with O < x l 1 very often a further increase in Hc is observed. With decreasing Fe(I1)-content of the particles due to oxidation the reduced saturation remanence mR increases (figure 1).

The same results have been obtained in tapes prepared with such particles.

These findings can be explained according to Kamiya [5J who assumed that

the induced uniaxial anisotropy in the magnetite system becomes lower with

decreasing iron( I I ) -content thus leading to more pronounced multiaxial be-

haviour in Fe(I1)-poorer systems.

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Relative remanence mR versus amount of Fe(ll) in volume doped

isometric iron oxide

Fig. 1 - Relation between reduced saturation remanence mR and Fe( I I )-content of cobalt bulk doped isometric iron oxide.

3. Preparation and properties o f needle shaped iron metal pigment

Preparing geometrically well defined aciular iron particles which have approximately preserved the shape of the nonmagnetic precursor FeOOH is still a challenge to chemistry. On going from FeOOH to -Fe a complete rebuilding of the crystal strucure occurs accompanied by an enormous change in densities from about 4.0 g/cm3 to 7.8 g/cm3 for FeOOH to L - F e , respectively. One way to handle the problem is to absorb a shape-stabilizing surface coating on the

Fs00H prior to reduction.

Effect of shape - stabilizing coating during pseudomorphic conversion of a- FeOOH into iron metal pigment

//>a-~eoOH

I

-

^SSA

Reducrion conditions 8 hours, 310°C, hydrogem 30 1

(S.TP3

Fig. 2 - Comparison of the effectiveness of shape-stabilizing surface coa-

tings for the preparation of elongated metal pigments from r-FeOOH.

carboxylic acids 163 leads to powders with the best magnetic properties (process A). Phosphate itself is very effective in preventing sintering, too.

However the rate of reduction of only phosphate-treated FeOOH drops to a level which is by far too low for industrial processes (cf M~lfvalues). By adding carboxylic acids to the surface coating the reduction rate is strongly en- hanced without loosing the anti-sintering and shape preserving effect of the phosphate. Highly anisometric particle shapes are obtained by using decompos- able organic compounds with high reduction potential together with hydrogen C7.7. It is possible to carry out the reduction as either a one-stage ( 0 ) or two-stage (C) process, as shown in figure 3 together with process A. Table 2 shows that especially process C yields particles which lead to an improved noise level in tapes. The effect is thought to be due to the higher specific surf ace.

One or two - stage synthesis of metal pigment:

presence of decomposable organic compounds

, e b

I

metal pigment

improved wise level in tape

metal pigment Fe304 2 7 z

first stage second stage

Fig. 3 - Synthesis of metal pigments from FeOOH in the presence of decompos- able organic compounds.

Preparation of iron metal pigments in the presence of decomposable organic compounds

a1 After paswation ir a seeam of N2 and ai

b> R e f e r e m $ r e F f m t s d na- ram spans( n wa p e .Ec w

Table 2 - Preparation of iron metal pigments in the presence of decomposable

organic compounds: Comparison of powder and tape properties.

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After stabilizing iron metal particles prepared according to process A by a controlled surface oxidation at below 60 OC the passivated particles consist of a metal core coated with a shell of iron oxides and phosphates. It was ex- perimentally proved by the little known 12-method l 7 , 8 ] that the particle structure corresponds to the assumed metal core-oxide shell-model. After selectively dissolving the iron metal core in I2/MeOH according to L 7 2 the thickness of the remaining empty oxide-phosphate shell was found by electron microscopy to be in the order of 4 to 5 nm.

Since the iodine/methanol method is not sensitive to wether the shell of the pigments consists of iron oxides or other substances it should be possible to further analyze the structure of the shell itself by comparing I2/MeOH results with results from magnetization measurements. The latter give the thickness of the iron oxide shell only, neglecting the shape-preserving coating which is present in both the pyrophoric and passivated metal pigment. For evaluation of magnetic data a cylindrical shape of the particles was assumed and the mean particle diameter was taken from electron micrographs. Supposing a shell com- posed of magnetite this estimation gives a shell thickness d in the order of 4 nm. On the other hand an assumed shell composed of the non magnetic &-Fez03 would yield a d-value of about 3 nm. Comparing both methods it is most prob- able that the oxidized layer contains Fe304 and/or f-Fe2Og which is in agree- ment with published data r10-7. Additionally it may be estimated that the thickness of the adsorbed phosphate layer is in the order of 0.5 to 1.0 nm.

4. Preparation and properties of needle-shaped Cr02

The direct, hydrothermal synthesis of Cr02 allows the preparation of perfectly crystallized elongated particles the size and axial ratio of which can exact- ly be controlled by doping with elements like Sb and Te. Addition of Fe in- creases the magneto-crystalline anisotropy and thus the coercive force of Cr02 particles. The chemical synthesis of Cr02 is represented by the equation

but little has been published on the reaction mechanism and the occurence of intermediates during the hydrothermal process. Own investigations have shown that in the absence of any doping agents crystalline Cr02 is for the first time observed at 260 OC. At about 300 OC as an intermediate phase Cr205 occurs, whilst above 325 OC only Cr02 can be found as crystallized solid.

Figure 4 summarizes the results also including the course of pressure (solid curve) and temperature (dashed curve) with reaction time for a selected example with no doping.

Hydrothermal synthesis of Cr02 (no doping)

reaction time (hl

Fig. 4 - Reaction mechanism of the hydrothermal synthesis of Cr02 (no doping).

seed crystals of CrSbOq of rutile type structure at 140 OC. Due to this the isostructural Cr02 is now detectable already at 225 "C, the intermediate Cr205 at about 250 OC. Finally above 275 "C only Cr02 can be found as crystalline reaction product.

Particle sizes of Cr02 pigments can be varied over a wide range by doping with suitable agents. Based on own experiments figure 5 shows the influence of the particle diameter of Sb- and Fe-doped Cr02 on the coercive force. The data follow the same type of curve which has been established for other materials before indicating the transition from multidomain via single domain to superparamagnetic behaviour [ 1 1 J . In this case the particle sizes of the Sb- and Fe-doped Cr02 have been determined by x-ray line broadening. Maximum coercivity in the order of 60 kA/m is reached with particles with a crystal-

lite size (dcr(220)) of about 23 nm. For the crystallite size measurements the (220)-reflex is taken, which lies perpendicular to the needle axis.

Relation between coercive force and mean cristallite size from experiments with Sb203/ Fe203- doped Cr02

Fig. 5 - Relation between coercivity and particle size of Sb- and Fe-doped Cr02.

Finally also Cr02 particles can have a core-shell-structure. This is due to the fact that Cr02 has oxidizing properties and thus for example can be treated with reducing agents in order to form a protecting surface layer prior to application in magnetic recording media. The protecting layer consists of nonmagnetic chromium oxides which makes the determination of layer thickness by means of magnetic measurements much more unambigous than it is the case with passivated metal pigment. Evaluating Mslj'data by using the same model described above leads to 0.7 nm for the thickness of the nonmagnetic shell.

Particle mean diameters were again taken from electron micrographs.

In case of Cr02 also x-ray line broadening can be used as a method to investi-

gate the protecting oxide layer's thickness. Cr02 particles for magnetic re-

cording media are single crystals and thus the crystallite size determined by

x-ray line broadening is identical with the mean particle size. Values of

0.55 to 1.3 nm for the layer thickness are obtained which is in good agreement

with the values based on the magnetic measurements. Although this chemically

generated shell on Cr02 is much thinner than the surface oxide layers formed

on metal pigments by controlled oxidation it provides a sufficient stability

for applications in audio, video and data recording media.

C6-40 JOURNAL DE PHYSIQUE

Acknowledgements

I wish to thank Dr. H. Jakusch for the magnetic measurements and the eva- luation of the surface layers on Cr02 and Dr. B. Meyer for his contributions to the reaction mechanism of the hydrothermal synthesis of Cr02. I am also indebted to Dr. K. D. Hoppe and Dr. E. Schwab for their calculations of the oxide layers on metal pigments from magnetic measurements.

Literature

[ I 1 B r i l l , i i . a n d S c h o e n e n m a n n , K . , D P 7 1 2 4 5 7 ( 1 9 3 5 ) . C 2 ] Lernke, J . , J . Appl. Phys. 5 3 , 2561 (1932).

[3_7 National Standard Co., US patent 4 115 106 (1976).

[4] Steck, W. et al., BASF, patent application filed.

L 6 i ' Steck, W., Schneehage, H., Jaeckeh, C., Huebner, W., Ohlinger,

M. and Loeser, W., BASF, DE-A 2 546 345 (1976).

7 Steck, W., Rudolf, P., Sarnecki, W., Loeser, W., Kovacs, J., and Jakusch, H., BASF, DE-A 3 228 669 (1982).

[ 8 J Vernon, F., Wormwell, F. and Nurse, T., J. chem. Soc. 1939, 621.

L 9 - 7 Feitknecht, W. and Durtschi, A., Helv. Chim. Acta 47, 174 (1964).

C l U J Van Diepen, A.M., Vledder, H.J. and Langereis, C., Appl. Phys. 15, 163 (1977).

1 Luborsky, F.E., J. Appl. Phys. 32, 172 S (1961).