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SURFACE MAGNETIC STRUCTURE OF SMALL
γ-Fe2O3 PARTICLES
A. Morrish, K. Haneda, P. Schurer
To cite this version:
A. Morrish, K. Haneda, P. Schurer. SURFACE MAGNETIC STRUCTURE OF SMALL
JOURNAL D E PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C6-301
SURFACE MAGNETIC STRUCTURE OF SMALL r-FezOS PARTICLES
A. H. MORRISH, K. HANEDA (*) and P. J. SCHURER Department of Physics, University of Manitoba, Winnipeg, Canada R3T 2N2
RBsum6. - Du Fer, enrichi en isotope Fe57, a BtB incorpork en surface de grains de y-Fez03 de la taille moyenne d'un micron dans le but d'Btudier Ia structure magn6tique descations extrsmes. Les spectres Mossbauer, en champ magnktique externe longitudinal de 50 kOe, demontrent I'exis- tence d'une structure non-collin6aire dans l'intervalle compris entre 4.2 K et la tempQature ambiante. Dans cet intervalle, l'angle d'inclinaison des spins, moyennB sur le nombre de cations, diminue de moiti6. Quelques modkles possibles sont prBsentBs pour dkcrire cette structure magnB- tique en surface.
Abstract. - Iron enriched in the isotope Fe57 has been incorporated into the surface of micron- sized y-Fez03 particles in order to study the magnetic structure of the outermost cations. Mossbauer spectra in external longitudinal magnetic fields of 50 kOe establish that a non-collinear structure exists from 4.2 K to room temperature. Over this temperature range the canting angle, averaged over all cations, decreases by a factor of about two. The surface structure is discussed in terms of some possible models.
1. Introduction.
-
Gamma ferric oxide (y-Fe,03) is a spinel, and is usually assumed to have a collinear magnetic structure consisting of two sublattices. In this model, the magnetization of one sublattice, the ensemble of ferric ions in tetrahedral or A sites, is anti- parallel to that of the other sublattice, the ferric ions in octahedral or B sites. Not all the cation sites are occupied ; the distribution of ferric ions per formula unit is given by (Fe) (Fe,13CI,13)0, where the two brackets identify A- and B-site cations respectively, and17
refers to the vacancies [l, 21.More recently, Fe5' Mossbauer spectra of y-Fe,03 micropowders taken with a large magnetic field applied show that some non-collinearity exists at the tempera- ture of liquid helium 13, 41. This conelusion is based upon the presence of the second and fifth lines when the field is applied along the direction of the y-ray. Further, the measurements have shown that the areas of the 2 and 5 lines, A,,,, increase relative to the areas of the 1 and 6 lines, A,
,,,
as the particle size decreases 141. Two alternative 'interpretations are possible. First, the degree of non-collinearity increases as the volume decreases. Second, the non-collinearity is only large near the surface of the particle, and will appear to increase for smaller particles because more surface is exposed as the particle volume decreases for a given mass of sample. A distinction between these two possi- bilities could in principle be made if the cations at or near the surface of the particles, and only these cations, were highly enriched in the Mossbauer isotope, Fe57. Natural iron itself only has a Fes7 concentration of about 2.2%.
Earlier, van der Kraan achieved some(*) Present address : Tohoku University, Sendai, Japan,
success in enriching the surface layer of ultrafine hematite particles [5]. The present communication reports on the progress made in enriching the surface cations of y-Fe,O, particles and on the information obtained on the surface magnetic structure by the methods of Mossbauer spectroscopy.
2. Experimental.
-
The Mossbauer spectra were obtained with a conventional electromechanical trans- ducer operated at constant acceleration in conjunction with a multichannel analyzer. The source of the 14.4-keV y-radiation was 57Co in chromium. The spectrometer was calibrated using a Fe-metal foil and a thin hematite absorber ; for a total of 250 channels the system was linear to within half a channel.A
variable temperature cryostat produced sample tempe- ratures between 4.2 and 300 K. For measurements in an external magnetic field, a 50 kOe superconducting solenoid was used together with a variable temperature insert dewar. The data were analyzed with a least- squares computer program that assumed the line shapes were Lorentzian.
In the present experiment, the y-Fe,03 particles investigated were acicular in shape, with a major axis usually ranging from 6 000 to 10 000 A (0.6 to 1.0 pm) in length and minor axis or width of about 1 000
A.
Some idea of the range in particle sizes and shapes is given by the electron-microscope photograph shown in figure 1. Such particles are commonly used as the active ingredients of magnetic recording tapes.The last stage in the process to produce the acicular gamma-ferric oxide particles is an oxidation from magnetite (Fe304) to y-Fe,03, during which the size and shape of the particle changes but little [6]. The
C6-302 A. H. ~MORRISH. K. HANEDA AND P. J. SCHURER
FIG. 1. - Electron photomicrographs of acicular magnetite particles coated with Fe$704.
enrichment of the surface cations in the isotope Fes7 was begun when the particles were in the form of magnetite made with natural iron. These magnetite particles were coated with magnetite with 93
%
Fes7 enrichment by a method developed by D. L. Sanes and J. Roden of the 3M Co., St. Paul, U. S. A. Figure 1 is actually for coated magnetite particles. Examination of this photograph reveals the presence of some ultra- fine particles in the 100W
size range. Even for particles without coating, a background of a few ultrafine particles is normal. However, the possibility definitely exists that a percentage of the enriched magnetite appears as independent particles and not as a coating on the acicular particles.In the thermal treatment of the coated magnetite particles, there are at least four competing factors to be considered. One : the enriched iron must be incorporat- ed into the lattice of the acicular particle. Two : the diffusion of the enriched iron throughout the body of the acicular particle must not occur to any appreciable extent. Three : complete conversion from Fe30, to y-Fe2O3 must take place. Four : the conversion of y-Fe2O3 to a-Fe203 (hematite) must be prevented. Unfortunately, there appear to be no data available on the diffusion constants pertaining to the Fe304- y-Fe203 system. Although the conversion of magnetite to y-Fe20, has been studied by several investiga- tors [7, S], it is only recently that a study of the trans- formation kinetics for ultrafine particles has been made in our laboratory. These results were presented at the second International Conference on Ferrites held in Paris, September, 1976. Some information on activa-
tion temperatures for the gamma to alpha transforma- tion, which depends on the particle size, has been published [g, 101.
As a consequence, the conversion of the coated sample was carried out largely by trial and error, and monitored by X-ray diffraction and Mossbauer spec- troscopy. X-rays detected the presence only of Fe304 for both the uncoated and coated magnetite samples. The Mossbauer spectrum of the coated magnetite sample, taken at room temperature, possessed a central absorption with an area of 0.35 relative to that of the magnetite pattern ; no central absorption was observed for the uncoated sample (not shown). After heating at 270 OC for 30 minutes, the relative area of the unsplit to split patterns decreased slightly to 0.32 (Fig. 26) ; the
I I
0 5 10
VELOCITY (MM/S)
FIG. 2.
-
Mossbauer spectra at room temperature of acicular particles after various treatments : a) Magnetite with a Fe57 enriched magnetite coating ; b) After heating at 270 "C for 30 min. The split spectrum is identified with y-Fe20s ; c) Afterheating the sample of figure 2a at 320 OC for 30 min.
split pattern corresponded to y-Fe203. However, a coated sample heated at 320 O C for 30 minutes had a central absorption with a relative absorption of only 8
%.
X-ray superlattice lines indicated that only y-Fe,03 was present after either of these two heat treatments ; some cl-Fe203 was detected on heating at a temperature higher than 350 OC.SURFACE MAGNETIC STRUCTURE OF SMALL y-Fez03 PARTICLES C6-303
FIG. 3.
-
Mossbauer spectra in an external longitudinal magnetic field of 50 kOe : a) for uncoated y-Fez03 acicular particles at 4.2 K and for the surface enriched sample of figure 2cb) 300 K, c) 180 K, d ) 97 K, and e) 4.2 K.
line broadening occurs after any of the heat treatments. Further, the Mossbauer lines do broaden by about 10
%,
enriched iron ions incorporated into the surface lattice may experience a wider distribution in hyperfine fields, and hence lead to the line broadening observed. Upon cooling to liquid nitrogen temperature, the central absorption of figure 2c disappears. One concei- vable origin for the unsplit spectrum at room tempera- ture would be fast electronic relaxation of the cations lying in the outermost surface layers of the acicular particles [5] ; however this possibility would appear to be remote. Instead, it seems that the unsplit spectrum is to be associated with ultrafine y-Fe,O, particles, acting independently of the large acicular particles. It is unclear whether or not these ultrafine particles are attached to the surface of the acicular particles. Howe- ver, in experiments with smaller particles, a residual central absorption was always detected. It may be that these ultrdne particles originate from the enriched material either that does not adhere to the acicularparticles in the original coating, or that flakes off during the heat treatment.
Mossbauer spectra in a magnetic field of 50 kOe applied parallel to the direction of the y-ray at T = 300, 180, 97, and 4.2 K are shown in figures 3b, c, d, and e, respectively for the sample of figure 2c. For compari- son, the in-field spectrum at 4.2 K for a y-Fe,O, sample made from the original uncoated magnetite powder is also shown.
3. Results and discussion.
-
If the enriched iron had diffused throughout the individua.1 particles, no change in the ratio A,.,/A,.,
would have occurred at any given temperature. The large increase actually observed (compare Fig. 3e with Fig. 3a) is convincing proof that the Fes7 enhancement was largely confined to the surface of the acicular particles. In addition, the results establish that the non-collinearity occurs primarily at the surface cations.It is known that the absorption associated with A-site ions should be fitted with a minimum of two over- lapping six-line patterns [4]. Nevertheless, for simpli- city the data were fitted with a total of only two six-line patterns, one for A-site cations, and the other for B-site cations. These two fits and their sum are shown as full curves in figures 3c, d, and e. The canting angle, 8, averaged over all the cations in a particle, is given by
8 = sin-'
The average canting angles for both the natural and the enriched samples are listed in table I for several tempe- ratures. For the larger angles, the probable error is 2 or 3 degrees. For the angles between 8 and 140, the upper bound is only 2-4 degrees larger, but the lower bound is much smaller. Where the resolution permits, the value of 8 for the A and B sites individually are also tabulated ; the errors are then about f 4 degrees. In view of the difference between the natural and enriched samples, a more realistic model is obtained by assuming a particle consists of two parts, an interior in which the canting is small, if not zero, and an outer shell in which the average canting angle is large. From the data shown in figure 2, and as discussed in section 2, the enriched material constitutes
A. H. MORRISH. K. HANEDA AND P. J. SCHURER
Parameters deduced from Mossbauer spectra of acicular y-Fe,03 particles in a longitudinal external magnetic field of 50 kOe
Temperature Sample - (K) - Natural 300 4.2 Surface-enriched 300 180 97 4.2
sites in the interior. Subtraction from the observed spectrum then yields the absorption of the surface cations. The average canting angles for the outer shell are calculated to be 580 for A-site cations and 470 for B-site cations. If the canting angle in the interioris assumed to be zero, a lower limit, then the canting angles deduced for the outer shell are 610 and 520 for A- and B-site cations, respectively.
Support for this two region model is given by consi- dering the linewidths observed for the 1 and 6 lines
for the A and B sites,
r,
and TB, and listed in table I. At 4.2 K for example, the A- and B-site linewidths of the enriched sample are 0.17 and 0.12 mm/s wider, respectively than those of the natural sample. In an applied field of 50 kOe, the effective hyperfine fields in the two regions of the particle will differ, because the canting is different, by about 18 kOe at both sites ; this difference corresponds to a peak displacement of about 0.29 mm/s. Since only 26%
of the absorption is ascribed to the enriched -material, the observed linewidth increase is in accord with the model within the experimental errors. Further support for the model is obtained by considering the location of the 2 and 5 lines for each site ; these lines have as their origin essentially only the surface cations. From the peak positions of the 2 and 5 lines, the corresponding hyperfine fields can be calculated. These hyperfine fields differ by about 16 kOe from the hyperfine fields deduc- ed from the peak positions of the 1 and 6 lines, and which are primarily determined by the cations in the interior. This 16 kOe difference it consistent with the canting angles. Finally, from table I is should be noted that I', and TB decrease with increasing temperature. This result is expected from the model because the canting angle also decreases when the temperature is raised.The canting angles of 580 and 470 obtained for the A- and B-sites for the outer shell seem large. However, the errors are also large ; no significance should be made of the difference between these two values, at least at this time. The ratio A,,,/A,,, averaged 2.5 for the different spectra, and indicated that the absorbers were not ideal. However, no thickness correction was made in the analysis.
Average canting angle (deg) A site B site Both sites
- - 8 13 14 16 20 19 27 26 26 32 28 30 When the temperature increases from 4.2 to 300 K,
8 decreases by a factor of about two. In comparison, the magnetization, and hence also the molecular field, decreases by 10-12
%
over the same temperature range [l]. At this time it is unclear what all the factors are that influence the temperature dependence of the surface spin structure.A more sophisticated model could consider a distri- bution in canting angles, at least in the outermost shell. The development of such a model may be feasible if more detailed data on the line shapes are obtained. The present investigation has been made with external magnetic fields of 50 kOe. Although at 4.2 K there is evidence that little change in the canting angle occurs when the field is increased to 90 kOe [4], it seems possible that the external field affects the canting at higher temperatures. Thus, future experiments in diffe- rent applied fields may be useful. Experiments on smaller particles of y-Fe203 with surface enrichment are in progress in our laboratory. The results are consistent generally with those for the larger acicular particles. A detailed report is planned sometime in the future.
4. Conclusions.
-
Iron enriched in the isotope Fes7 has been successfully introduced into the lattice at the surface of micron-sized particles of y-Fe20,. Moss- bauer spectra have unambiguously established that iron cations in both A and B sites at the surface have a non-collinear magnetic structure. The enhancement in the surface effects provided by the technique permits the change in surface canting to be detected from 4.2 K at least up to room temperature. The application of this method to the study of other ferrimagnetic micro- powders containing iron is a challenging and poten- tially rewarding undertaking for the future.SURFACE MAGNETIC STRUCTURE OF SMALL y-Fez03 PARTICLES
References
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2752. 1049.
[2] ARMSTRONG, R. J., MORRISH, A. H. and SAWATZKY, G. A., L71 AHARONI, A., FREI, E- H- and SCHEBER, J - 9 Phys. Ckm.
Phys. Lett. 23 (1966) 414. Solids 23 (1962) 545.
[S] IMAOKA, Y., HOSHINO, Y. and SATOU, M., Proc. Int. Con$ on [3] COEY, J. M, D. and KHALAFALLA, D., Phys. Stat. Sol. a 11 Ferrites (Univ. of Tokyo Press) 1971, p. 467.
(1972) 229. [g] KACHI, S., MOMIYAMA and SIUMIZU, S., J. Phys. Soc. Japan [4] MORRISH, A. H. and CLARK, P. E., Proc. Znt. Conf. Mug. 18 (1963) 106.
