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MORPHOLOGY OF NONSTOICHIOMETRIC Fe3O4 PARTICLES AS MAGNETIC RECORDING MEDIA
K. Haneda, H . Kojima
To cite this version:
K. Haneda, H . Kojima. MORPHOLOGY OF NONSTOICHIOMETRIC Fe3O4 PARTICLES AS
MAGNETIC RECORDING MEDIA. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-583-C2-
585. �10.1051/jphyscol:19792203�. �jpa-00218582�
JOURNAL DE PHYSIQUE Colloque C 2 , supplkment au n o 3, Tome 40, mars 1979, Page C2-583
MORPHOLOGY O F NONSTOICHIOMETRIC Fe304
PARTICLES AS
MAGNETIC RECORDINGMEDIA
K. Haneda and H. Koj ima
Research I n s t i t u t e for S c i e n t i f i c Measurements, Tohoku University, Sendai, Japan
R6sumB.- La morphologie des particules Fe304 non-stoechiomdtriques a dtd Btudide par effet ~Gssbauer et par des experiences cingtiques pour Blucider l'origine de l'augmentation du champ coercif en com- paraison avec Fe30t, pur ou y-FezOa. I1 est proposl que la particule FesOi, non-stoechiomktrique est en fait composde des cristallites finement divis6s de y-Fen03 et FesOt, pur.
Abstract.- The morphology of nonstoichiometric Fe304 particles has been examined through Miissbauer and kinetic experiments, in connection with the origin of its enhanced magnetic coercivity as compa- red with pure Fe304 or y-Fez03. It is suggested that a so-called non-stoichiometric Fe30t, particle is actually composed of finely divided crystallites of y-Fez03 and pure FesOr.
1. Introduction.- It is well known that when the oxidation from Fe304 to y-Fez03 is carried out at lower than normal-oxidation-temperatures, a series of compound of general formula (Fe 304) X(y-Fe~03) ,-x result. Those materials commonly recognized as solid solutions of Fe304 and y-FezO3, or
F ~ ~ + E F ~ ) +
1+(2~/3)F ~ : + ~ Q ~ ~ o ~ are particularly important as magne- tic recording media since they possesshigher coerci- vity than Fe304 or y-Fez03 11-31; it takes a maximum value at a certain composition. However the origin of that enhanced coercivity has not been established yet and remained puzzling. This paper reports the morphology of nonstoichiometric FesO4 particles in connection with the origin of the enchanced coerci- vity of the system.
2. Experiments and Results.- Our concern in this pa- per is confined to micron- or close-to-micron-sized regions of nonstoichiometric Fe304 particles. Figure 1 shows the Gssbauer spectra for commercially avai- lable nonstoichiometric Fe304 particles (%3100
1
in terms of sphere). Apparently in figure la, at 300 K the spectrum consists of at least two six-line patterns. One is associated with ferric ions. The other is attributed to the ensemble of ocathedral ferrous and ferric ions; they produce only one pat- tern because the electron hopping is much faster than the nuclear Larmor precession period. However the intensity ratio of the inner to outer sextuplet deviates well from 2, the value for stoichiometric magnetite. The character of the change in the absorp-tion profile between 300 K and 77 K spectrum indica- tes that the same Verwey transition occurs for oxi- dized samples as for stoichiometric magnetite between these two temperatures. Two alternative in- terpretations are possible for a 300 K spectrum.
-10 - 5 0 5 10
V E L O C I T Y c m m / s )
Fig. la
VELOCITY t m m / s ) Fig. Ib
Fig. 1 : M6ssbauer spectra for nonstoichiometric Fc30s particles (3100
1)
(a) at 300 K, and (b) at77 K.
First, the electron hopping occurs in a single phase solid solution of y-Fe203 and Fe304 / 4 , 5 / . Indeed Daniels et al.'s pair-localized model
141
is based on the nonstoichiometric magnetites being single phase. Second, so-called nonstoichiometric magneti- tes would in reality be a two phase mixture of r F e z 0 3 and Fe304 in which the spectrum for the y-Fez03 phase coincides with the A-site spectrum of Fe,O,, hence the electron hopping occurs only in Fe304 or almost pure Fe30r. A distinction between these two possibilities could be made if a kinetic study were made on the oxidation process of magneti- te in micron-sized region.For the kinetic experiments, the powders were heat-treated in air for various times, from 30 minu-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792203
c2-584 JOURNAL DE PHYSIQUE
tes to 43 hours, and at various temperatures, from 125 to 220'~. No hematite was observed by X-ray in any stage of the kinetic experiments. The technique was almost same as was used in reference 161 for ultrafine particles. The results are shown in figu- re 2.
Fig. 2 : The oxidation of Fe3Ok to y-FezOp.(a) The fraction of residual magnetite, c, is shown as a function of time for various temperatures. (b) Arrhenius plots; m is the rate constant or the slope of a straight line in (a).
It became evident from the kinetic experiments, mo- nitored by mainly chemical analysis, and qualitati- vely MGssbauer measurements, that the oxidation process can be represented by a third-order rate reaction with the form
--
dc = mc3, where c and m ared t
the fraction of unconverted residual magnetite ob- tained from the Fez+ ion concentration and the rate constant, respectively. The oxidation of magnetite involves 2 steps; 1 : the adsorption and ionization of oxygen at the outer surface, 2 : the diffusion of cations (and possibly anions) and subsequent oxidation of the ferrous ions. The slower one of either step will be rate determing; if the former, a homogeneous solid solution would be expected to form with - dc = constant. The observed third-order
dt
rate reaction (Fig. 2a), however, reveals that the studied so-called nonstoichiometric magnetite ac- tually consists of a two-phase y-Fe203-Fe301, system with the rate determing of diffusion step. This observation is consistent with Topsbe et al.'s 171.
The rate constant can be expressed by the equation m(t) = s exp(-TA/T) where TA is the activation tem- perature and s is the frequency factor (fig. 2b).
The values found for TA and s are 9140 K and 2.94 x 10' h.-', respectively for examined close-to-micron- sized particles. A relatively small value of TA and fairly small value of s may imply that the oxidation easily to occur but once the oxidation commences it tends to terminate at only the vicinity of the fo- cus, in other words the oxidation of magnetite par- ticles in micron-sized region proceedssmall-area by small-area. In a previous paper by one of the author (K.H) and Morrish /6/, the particle size were
shown to play an important role in determing the ease of the transformation from FesOr to y-Fe203 in the ultrafine particle region. While a micron- sized particle is known to be composed of finely divided crystallites, the size of d i c h determined by the earlier treatments 18.91. It seems that in micron-sized particles, the size of these divided crystallites of a particle might play such a role.
Figure 3 shows the variation of magnetic pro- perties upon oxidation. The ratio of u,/u, has ta- ken well below 0.5, the value for the random assem- bly of single-domain crystallites. Thus it might mean a particle of the sample is composed of multi- domain crystallites. These multi-domain crystalli- tes might behave in a way to increase coercivity when the oxidation is taken place, approaching to a single-domain behavior through the impregnation of other phase in a crystallite.
Fig. 3 : The variation of magnetic properties upon oxrdation.
as
: saturation magnetization,u : resi- dual magnetization, and .H : coercivity.1 C
Thus the initial morphology of a particle might greatly influence upon the size-related magnetic property, such as coercivity, of micron-sized par- ticles 191. In view of this model, when a particle is composed of ideal single-domain crystallites at Fe301,, as an exceptional case, the coercivity would show a minimum upon gradual oxidation to y-FenOa. A successful example has been shown in table I. The aging effect /2/ also might be explained by a re- construction process of a 2 phased mixture in a particle with an extended period of time.
Table I : Coercivity for another micron-sized sam- p)e. x : the fraction of residual magne- tite.
R e f e r e n c e s
/ I / Namikawa, M., S a t o , M., Imaoka, Y . , and T o c h i h a r a , S. Denki Kagaku
2
(1963) 37 ( i n J a p a n e s e ) . / 2 / Imaoka, Y . , Hoshino, Y . , and S a t o u , M., P r o c . I n t .Conf. o n F e r r i t e s (Univ. o f Tokyo P r e s s ) 1971, p. 467.
131 B o r r e l l i , N.F., Chen. S.L., and Murphy, J . A . , IEEE T r a n s . Mag. (1972) 648.
/ 4 / D a n i e l s , J . M . , and Rosencwaig, A., J . Phys. Chem.
S o l i d s
30
(1969) 1561./ 5 / Romanov, V.P., and C h e c h e r s k i i , V.D., Sov. Phys.
S o l i d S t a t e
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(1970) 1474.161 Haneda, K., and M o r r i s h , A.H., J . Physique C o l l o q . 38 (1977) Cl-321.
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/ 7 / Topsbe, H . , Dumesic, J . A . , and Boudart, M., J.Physi- que Colloq.
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(1974) C6-411.181 Berkowitz, A.E., S h u e l e , W . J . , and F l a n d e r s , P . J . , J . Appl. Phys.
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