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Submitted on 1 Jan 1988
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A STUDY OF SPIN PINNING FOR FINE IRON
PARTICLES
H.-X. Lu, J. Wu, Y.-W. Du, X.-K. Gao, T.-X. Wang
To cite this version:
H.-X. Lu, J. Wu, Y.-W. Du, X.-K. Gao, T.-X. Wang.
A STUDY OF SPIN PINNING FOR
JOURNAL DE PHYSIQUE
Colloque C8, Supplt5ment au no 12, Tome 49, d6cembre 1988
A STUDY OF SPIN PINNING FOR FINE IRON PARTICLES H.-X. Lu, J. Wu, Y.-W. Du, X.-K. Gao and T.-X. Wang
The Laboratory of Solid State ~ i c r o s t r u c t u r e s , Nanjing Uniuersity, China
Abstract. - The a ( T ) of fine iron particles in the field of 4 T and 2 T are measured in the temperature rising and dropping process. It is found that the u (T) increase anomalously as the temperature and are irreversible. It can be
explained by spin pinning in the surface oxide layer of fine iron particles.
1. I n t r o d u c t i o n
The magnetic properties of fine iron particles are an interesting research area. Compared with bulk iron, the specific magnetization, a
,
of fine iron particles is lower obviously. This is mainly due t o the oxide layers of the particles. Tasaki [l] thought the oxide layers to be non-magnetic, but Corner [2] dealt the magnetic moment of a particle as the sum of that of the iron core and the oxide layer. In fact, the oxide layers are composed of y-Fez03 and FesO4 instead of a-Fez03 [3]. We don't also find any superparamagnetic pat- tern in Mossbauer spectrum. The measurements show that calculation of the magnetic moment of fine iron particles is not simple. According t o the Mossbauer research on fine iron particles, a large non-collinearity in the spin structure of the oxide layer appears [4].In this paper, we report some new experimental re- sults about spin pinning in surface oxide layers of fine iron particles.
2. S a m p l e preparation
Fine iron particles are prepared by gas evaporation in NI atmosphere [5]. The average diameter of the par- ticles is 150
A
at the pressure of 80 Pa. X-ray diffrac- tion shows that the main component is a-Fe, and the lines of y-Fez03 or Fez04 are also found, which corre- spond t o the result of Mossbauer spectrum. ESCA andTEM dark field image prove that oxide is on the sur- face of the particles. The observation by means of high resolution electronic microscopy shows that the surface layer consists of microcrystalline and amorphous oxide.
3. T h e a (T) c u r v e i n t e m p e r a t u r e rising and d r o p p i n g process
The fine iron particles with average diameter of 150 A are pressed into a 1 mm thick, 6 mm in diameter tablet. The magnetization of the sample versus tem- perature, a (T) , is measured by the induction method. Before the sample to be magnetized, the tempera- ture is reduced to 1.5 K. Then, a magnetic field of
certain magnitude is acted with the direction paral- lel t o the surface of the tablet and the a (T) curve is measured in the temperature rising process.
After the temperature reaches about 100 K, we re- duce it and we measure the a (T) curve in the tem- perature dropping process. The a (T) curves in the magnetic field of 4 T and 2 T are drawn in figure 1 and figure 2, respectively. The curve of temperature rising process in figure 1 may be divided into four re- gions. In the region A, from 1.4 t o 20 K, a (T) in- creases anomalously as the temperature rises. In the
T (K) Fig. 1. - u (T) Curve in the magnetic field of 4 T.
dropping temperature
Fig. 2. - o (T) Curve in the magnetic field of 2 T. region B, from 20 t o 60 K, a (T) does not change re- markably. In the region C, from 60 to 85 K, a (T) decreases as the temperature rises. In the region D, from 85 to 105 K, a (T) does not change remarkably.
C8 - 1838 JOURNAL DE PHYSIQUE
The a (T) curve is irreversible in the temperature ris-
ing process. The a (T) curve in temperature dropping
process has the tendency of monotonous increasing as the temperature drops. The a (T) curve in the tem-
perature rising process in figure 2 is similar to that in figure 1, but the corresponding region moves to higher temperature. However, the u (T) curve in temperature rising process is about reversible.
4. Discussion
a (T) of bulk a-Fe at low temperature obeys T~~~ law which can be satisfactorily explained by spin-wave theory. a (T) of spinel such as y-Fez03 and Fe301 also decreases monotonously when temperature rises, cor- responding to the calculation by use of NBel's molecu- lar field theory.
In the study of the magnetic properties of fine iron particles, we have found that the increase of a (T) as the temperature rises is obviously different from that of bulk iron [3]. By comparing with Hc-T curve, we propose that the spins of surface oxide layer and iron near the interface are more or less pinned.
Look at the a (T) curve in temperature rising porcess in figure 1. Actually, the magnetic moment of the microcrystals in the surface layer of fine iron particles in an applied field is acted by anisotropic field, dipole field applied by adjoining a-Fe core as well as the applied field. Thermal energy also effects the behavior of the moments. At low temperature the anisotropy energy is much larger than the other kinds of energy and causes the moments in the oxide micro- crystals to be pinned. Therefore, before the sample to be magnetized, the contribution of the oxide layer to the total moment of a particle is zero. As temper- ature rises, anisotropy constants decrease and on the other hand thermal energy increases. When thermal energy and Zeeman energy are large enough to over- come the anisotropy obstacles the moments of some ox- ide microcrystals turn to the direction of applied field, which leads to the anomalous increase of a (T) in re- gion A. Regions B and D correspond to the removing of spin pinning in the oxide microcrystals with larger anisotropy constant at higher temperature. These mi- crocrystals are fewer than those corresponding to re- gion A, so that as temperature rises the a (T) does not decrease obviously. According to the temperature ris-
ing process, we think that the anisotropy constant of microcrystals in the surface layer of fine iron particles is different, so that the pinning is removed at different temperature.
In the temperature dropping process, the magnetic moments of oxide microcrystals are pinned again. However, they are pinned in the direction of applied field of 4 T and contribute to the a (T)
.
This gives reason for the irreversible a (T) curve.The applied field in figure 2 is 2 T instead of 4 T in figure 1, so that it is obvious that region C corresponds to higher temperature and region D corresponds to the temperature beyond 100 K. Because the Zeeman energy is lower than that of figure 1, it must be in higher temperature to overcome anisotropy obstacles. Following is the explanation to the fact that a (T) is
reversible. The oxide microcrystaIs are also acted by dipole field. When the applied field is much larger than the dipole field, the situation is in figure 1. When the applied field is smaller or near the dipo!e field, in the temperature dropping process, the magnetic moments on some microcrystals return to the direction opposite to those of a-Fe core under influence of dipole field and are pinned again, which leads to the decrease of the a (T)
.
Acknowledgments
This work has been supported by the NSF grant in China.
[I] Tasaki, .4., Takao, M. and Tokunaga, H., Jpn J.
Appl. Phys. 13 (1974) 271.
[2] Corner, W. D. and Mundell, P. A., J. Magn. Magn. Mater. 20 (1980) 148.
[3] Du, Y. W., Wu, J., Lu, H. X., Wang, T. X., Qiu, Z. Q., Tang, H. and Walker, 3. C., J. Appl. Phys.
61 (1987) 3314.
[4] Haneda, K. and Morrish, A. H., Surf. Sci. 77
(1978) 584.
