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Submitted on 1 Jan 1988
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Co FERRITE COATING EFFECT ON FINE IRON
PARTICLES
H.-X. Lu, X.-Y. Mao, Y.-W. Du, W. Yu, W.-F. Chen
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988
Co FERRITE COATING EFFECT ON FINE IRON PARTICLES
H.-X. Lu, X.-Y. Mao, Y.-W. Du, W. Yu and W.-F. Chen
The Laboratory of Solid State Microstructures, Nanjing Uniuersity, China
Abstract. - The surface of 260 iron particles has been coated with a Co ferrite layer by chemical coprecipitation method. The Co ferrite layer causes a decrease of specific magnetization and increase of hyperfine field of a-Fe core. The effect of the Co ferrite layer on Hc belongs to an interface effect.
1. Introduction
Acidular y-Fez03 particles are well-known recording media. After crystallizing Co ferrite on the surface of y-FezO3, the magnetic anisotropy of Co ferrite causes an increase of coercivity 11, 21. Fine iron particles, with unique magnetic properties, have a future as a magnetic recording material. We have previously re- porded some results for the magnetic properties of fine iron particles [3]. In addition to higher magnetization and higher Curie temperature, they have higher co- ercivity, which reaches 1000 Oe at 300 K when the particles size is about 150
A.
The fine iron particles present a chain-of-spheres-like arrangement in the ob- served range 150-
400A.
If the fine iron particles is coated with Co ferrite, what change will the magnetic properties appear?2. Sample preparation
Fine iron particles were prepared by the gas- evaporation method in NZ atmosphere [4]. The av- erage size of,the particles for 530 Pa N2 pressure is about 260
A.
Co ferrite was coated on the surface of fine iron particles by means of a chemical coprecipita tion method. The suspension of the fine iron particles was poured into a mixed solution of cobaltous sulfate and ferrous sulfate with Co2+ / ~ e ' + molar ratio of 1:2,the amount of which was calculated according to the required thickness of the coating layer. Care was taken to prevent oxidization. Then an aqueous solution of sodium hydroxide, with excess 2 % of stoichiometric composition, was added to the suspension with fierce stirring. The reaction was continued about 4 hrs at
60 OC. Finally, the resultant product was washed with water.
At the same time, a sample coated with magnetite was prepared for comparison.
3. Experimental results and discussion
In addition to the characteristic peaks of a-Fe, the X-ray diffraction pattern includes that of spinel, which is lower and scattered. It is shown by means of mea- surement of TEM and the nitrogen adsorption BET method that the average size of the particles increases with coating and the measured values are in agree- ment with the calculated values. TEM observation also showed that chain-of-spheres-like arrangement of primary fine particles is turned to scattered spheres. Nitrogen, sulphur and sodium elements were not found in ESCA and AES analyses. The surface of the sam- ples contain co2+ and ~ eions, with the content of ~ + co2' ions reducing gradually with depth and the ap- pearance of bivalent iron, and finally iron. This in- formation was obtained by etching the surface. It is illustrated that the Co ferrite sedimentates to the sur- face of fine iron particles.
The specific magnetization and hyperfine field of Mossbauer spectra is measured by a vibrating sample magnetometer ( H , = 10 kOe) and a conventional con- stant acceleration spectrometer, respectively, as shown in figure 1.
The Co ferrite layer causes a decrease of the specific magnetization. If the magnetization of the sample is simply the mean of the magnetization of both fine iron
Co Ferrite
\
The analytical results for the samples after coating Fig. 1. - n and Hhf a s a function of the thickness of the are as follows Co ferrite layer at room temperature.
C8 - 1840 JOURNAL DE PHYSIQUE
particle and Co ferrite, then a is given by:
where a1 = 177 emu/g and a2 = 80 emu/g are the specific magnetization of 260
k
fine iron particles and Co ferrite respectively, and a is mass ratio of them[(r
+
d)3 - r3] p2a =
r3 pl
where r- the average radius of the fine iron particle is about 130 k . pl, pz- the density of iron and Co ferrite are 7.88 and 5.29 g/cm3, respectively. The dashed line in figure 1 represents results given by equation ( 1 ) . A comparison with the calculated results shows that the experimental values are low obviously. Oxidation of the iron partides on suspension is one of the reasons for u decreasing. When the iron particles were sus- pended in water for 1 hr, we observed the decrease of magnetization about 10 %
.
However, when the thick- ness of coated layer exceed 5k,
both the calculated and experimented curves tend to parallel.The Mossbauer spectra consist of patterns of a-Fe core and its coating of Co ferrite. Hyperfine field of a-Fe core may be written as
I
where Hht, Hdem, Hdip are hyperfine field of bulk iron,
demagnetism field and dipole field, respectively. Be- fore Co ferrite coating chain-of-spheres-like arrange- ment of fine iron particles gives rise to increase of dipole field. After Co ferrite coating, dispersed distri- bution and increase of distance between the particles cause a decrease of dipole field. In contrast, the demag- netism field increases. Thus hyperfine field increases with increase of the thickness of Co ferrite layer.
Figure 2 shows coercivity as a function of the thick- ness of Co ferrite layer at room temperature and 103 K. It seems as if Co ferrite coating layer gives reason for
Hc reducing. Before oxide coating chain-of-spheres
model is a suitable description of their magnetic behav- ior. After oxide coating, however, chain-of-sphereslike arrangement disappeared. According to Nee1 model
Hc for single domain particles is 0.64K1 / M , 200 Oe with K 1 = 4.2 x lo5 erg/cm3 and M, = 177 emu/g of bulk iron at room temperature. In fact, Hc is about
300 Oe for the fine iron particles with 10
k
magnetite coating layer, which is larger than the calculated value. It stands to reason that K l value of fine iron particles is larger than bulk value of iron and that the surface of them is coated with microcrystalline magnetite layer. However, Hc for the fine iron particles with Coferrite coating layer is about 510 Oe, which is larger
Fig. 2. - H, as a function of the thickness of Co ferrite layer at room temperature and 103 K.
than that of magnetite coating. Obviously, the in- crease of coercivity is not due t o the oxidation of fine particles but Co ferrite coating layer. In figure 2 Hc
do not change with the thickness of Co ferrite coat- ing layer. In other words, Co ferrite coating from 7
to 47 wt% makes same contribution to
Hc.
This result is in contrast with Amemiya et al. who had observed that the coercivity of 3.-Fe203 particles increases lin- early with content of Cobalt [ I ] . Therefore, the effect of Co ferrite coating layer on the coercivity of fine iron particles belongs to an interface effect, the same as we previously showed at fine Zno.zFez.804 particles [5].Acknowledgments
This work has been supported by the NSF grant in China.
[ I ] Amemiya, M., Kishimoto, M. and Hayarna, F.,
IEEE Trans. Magn. MAG-16 (1980) 17. [2] Sumiya, K., Matsumoto, T., Watatani, S. and
Hayama, F., J. Phys. Chem. Solids 40 (1979) 1097.
[3] Du, Y . W., Wu, J., Lu, X. H., Wang, T. X., Qiu,
Z. Q., Tang, H. and Walker, J. C., J. Appl. Phys.
61 (1987) 3314.
[4] Lu, H. X., Wu, J., Du, Y. W., Gao, X. K. and Wang, T. X., Submitted to J. Nanjing Univ.
(Natural Sci.)
