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NMR STUDY OF 59Co HYPERFINE FIELD
DISTRIBUTIONS IN AMORPHOUS Co-M ALLOYS
(M = Si, B)
M. Misawa, Y. Tanaka, H. Nagai, A. Tujimura
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dkembre 1988
NMR STUDY OF
5 9HYPERFINE FIELD DISTRLBUTIONS IN
~ ~AMORPHOUS
Co-M ALLOYS
(M
=Si, B)
M. Misawa (I), Y. Tanaka (2), H. Nagai (2) and A. Tujimura (2)
(I) Department of electronics, Nagano Precision Industry Research Institute, Okaya, 394, Japan (2) Department of Physics, Faculty of Science, Shinshu University, Matumoto, 390, Japan
Abstract.
-
The mean hyperfine field and the magnetic moment at 77 K per Co atom in amorphous Co-M (M = Si, B) sputtered films decrease with M concentration. The analysis by using a rigid band model shows that the charge transfer per M atom is larger than the number of p electrons of M atom.Amorphous films can be obtained by sputtering or coevaporation in a wider range of composition than liquid-quenched ribbons, because of higher quenching rate in thin films. The magnetic properties of amor- phous Col-, (Si,B), ribbons for 0.2
<
z5 0.25 have
been studied by several workers 11-31, but it has lit- tle been discussed for other composition. Therefore we studied magnetic properties of amorphous Co-M (M = B, Si) alloys over a wide range of concentration in this paper.The Bms were prepared by rf magnetron sputtering in an argon atmosphere and a multi target system. The multi targets were pure Co disk of 4 inches in diameter and square plates of Si and B approximately
5 rnm in a side. The concentration of these films was determined by the number of plates. The samples were deposited on the water-cooled substrates which were glasses and polymide films. The thicknesses of film wer&.about 1 ,um. The compositions of film were found from electron microscope analysis by referring to bulk Co, SI and B. The amorphous state was confirmed by X-ray diffraction. The prepared samples were Co-Si (25
5 Si
<
36 at %), Co-B (205 B
5
30 at %) and C075 ( S ~ I - ~ B ~ ) ~ ~ (0.2<
Y<
0.8).The saturation magnetization was measured a t 77 K by a vibrating-sample magnetrometer in a dc field up to 8 kOe.
he
magnetic moment p of Co atom was calculated from this value. The hyperfine field Hhf of 5 9 ~ o nuclei were measured at 77 K by the zero-field spin echo NMR method.The magnetic moment of Co atom in amorphous Col-,M, (M =, B, Si) as shown in figure 1 decreases with increasing concentration of M atom. The effective electron transfer can be found from the reduction of the magnetic moment of Co atom. If it is assumed that n electrons are transfered from M atom to rn holes of a Co atom; the mean magnetic moment of Co atom,
( p )
,
is expressed as follows [4]:(p) = m - n x / ( l - x ) . (1) In equation (I), by using m = 1.6 [4] we obtained
n = 2.5 for Si, 1.5 for B respectively. These values
Fig. 1. - Concentration dependence of the magnetic m e ment per Co atom in amorphous Col-,M= (M = Si, B) at 77 K.
are larger than the number of p electrons which are equal to 2 for Si and 1 for B. In the case of amorphous Co75 (Si, B)25 films, the magnetic moment of Co atom increases significantly upon substitution of B for Si. This means a smaller transfer from B than Si to Co 3d band. For B atom in amorphous Co-M alloys the observed data are in agreement with those of liquid- quenched ribbons [I-31.
The Co NMR lineshape of amorphous Co-Mn film is nearly a Gaussian distribution. Since the magnetic interaction neems to be short-ranged, the distribu- tion in exchange interaction may be large. Then the quadrupole broadening would be negligible. The mean
Nhf at 5 9 ~ o nuclei in amorphous Col-,B, as shown in figure 2 decreases with increasing B concentration. The corresponding hyperfine coupling constant is cal- culated to be 110 kOe/pB. The reduction value A H of Co hyperfine field per B atom in nearest neighbours of a Co atom is calculated to be 31 kOe/pB by using the relation of the mean hyperfine field (Hhf) of CO atom as follows 131:
C8 - 1374 JOURNAL DE PHYSIQUE
u
0 0.1 0.2 0.3 0.4 0.5X A I - X I
C O N C E N T R A T I O N
Fig. 2. - Mean hyperfine field at 77 K us. x/ (1
-
x) in amorphous Col-,B, , C075 (Si, B)25 and fcc-CO.where, H:* is the hyperfine field at 5 9 ~ o nuclei with- out B neighbour, ( n ~ ) is the mean B concentration of a Co atom. By using the above AH value, the observed hyperfine field distributions can be fitted by a superposition of Gaussian distributions correspond- ing to Co atoms with different numbers of B near- est neighbours assuming random distributions. For
C075 (SI,
B)25
films,(I&)
increase upon substitution of B for Si.In conclusion, the magnetic properties of amorphous Co-B films are similar to those of the liquid-quenched samples.
On the other hand, the magnetic properties of Co-Si and C075 (Si,
B)25
films are different from those of Co-B films. The magnetic moment cb Co atom decreases more rapidly by increasing Si content than B content. It is difficult to be explained by the simple charge transfer model the fact that the charge transfer of Si atom is larger than the number of p electrons.
(11 van der Borst, J. and den Eiroeder, F. J. A., J. Appl. Phys. 48 (1977) 1724.
121 Durand,
J.,
Lemius, B., Hasegawa, R., Aliaga- Guerra, D. and Panissod, F'., J. Magn. Magn.Mater. 15-18 (1980) 1373.
[3] Boehner,
W.,
Lutgemeier, El. and Zinn, W., J.Magn. Magn. Mater. 62 (1986) 152.
[4] Yamauchi,
