Journal of Magnetism and Magnetic Materials 226}230 (2001) 1561}1563
Mo K ssbauer and Random anisotropy studies on amorphous ribbons
Antony Ajan , Shiva Prasad *, S.N. Shringi , R. Krishnan , H. Lassri
Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India
Laboratoire de Magne&tisme et d+Optique C.N.R.S. Versailles, 45 Avenue des Etats Unis, 78035 Versailles Cedex, France Laboratoire de Mate&riaux et de Microe&le&ctronique, Universite& Hassan II, Casablanca, Morocco
Abstract
We have studied the MoKssbauer and magnetization of melt spun amorphous Fe
\VAl VEr
B
alloys with 0)x)15. MoKssbauer studies were carried at room temperature as well as at 80 K. The average hyper"ne"eld values are found to decrease with the Al concentration. Magnetization studies were carried out under magnetic"elds up to 140 kOe. The results were analyzed using random magnetic anisotropy model. 2001 Elsevier Science B.V. All rights reserved.
Keywords:Amorphous magnetic materials; Random anisotropy; MoKssbauer spectroscopy
Amorphous alloys based on rare-earth (RE) and transition metals (TM) and metalloids (B), such as TM}RE and TM}RE}B o!er the possibility to study various aspects of amorphous magnetism such as ran- dom anisotropy nature of magnetic interactions. In con- trary to the crystalline materials, these aspects can be investigated as a function of the rare-earth metal concen- tration in a continuous manner. RE atoms with spin orbit coupling are known to give rise to large random anisotropy in amorphous alloys [1]. Several important magnetic parameters such as local random anisotropy constant, exchange"eld, and ferromagnetic correlation length can be obtained by analyzing the approach to magnetic saturation on the basis of the model proposed by Chudnovsky et al. [2,3]. We are reporting studies on amorphous alloys Fe
\VAl VEr
B
with 0(x(15.
Amorphous ribbons under consideration were made by melt quenching technique in an inert atmosphere of Ar. Amorphization of the samples were checked by X-ray di!raction. The composition was determined by electron probe microanalysis. Room temperature and the 80 K MoKssbauer spectra were taken in standard transmission
*Corresponding author. Fax :#91-22-572-3480.
E-mail address:shivap@phy.iitb.ernet.in (S. Prasad).
geometry, using Co(Rh) source. The magnetization studies were carried out in the range of 4}290 K under applied"eld upto 140 kOe. The Curie temperatureT
was also determined using a vibrating sample magnetometer.
MoKssbauer spectra for all the samples taken at room temperature as well as at 80 K show a broad spectrum typical of amorphous alloys (Fig. 1). The quantitative analysis of the spectra were carried out by using the Window's program which assumes a probability distri- bution of hyper"ne"eldP(H). The parameters such as isomer shift (), line width and intensity ratio of the second and"fth line (b) were optimized to give the best value. The result obtained is shown in Table 1. The average hyper"ne"eld,H
, decreases with the Al addi- tion, but increases as the temperature is lowered. The H variation with Al is represented as a straight line (H"223.8}3.6x) at 80 K. At room temperature, this reduction is larger due to the e!ect of proximity ofT
and given by H
"205.8}5.1x. This result is similar to the one observed in the RE containing alloy like Fe\VY
VHo B
[4]. In the present case the decrease in H at low temperatures is attributed to the increase in the 3d}5s hybridization when the Fe concentration rela- tive to Er is decreased. This is also supported by the isomer shift () variation with Al concentration, where increases as the Al concentration increases.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 5 1 - 9
Fig. 1. MoKssbauer spectra obtained for Fe
\VAl VEr
B samples at temperatures of 300 K and 80 K.
Table 1
Hyper"ne"eld parameters of Fe\VAlVErBobtained from Window's method
X H b
($2) (kG)
($0.01) (mm s)
300 K 4 185 0.01 2.9 1.08
6 177 0.03 2.8 1.11
10 154 0.05 2.9 1.30
15 131 0.06 3.1 1.28
80 K 4 209 0.14 2.2 1.24
6 205 0.17 2.2 1.10
10 184 0.18 2.0 1.03
15 171 0.23 2.7 1.01
Table 2
Exchange"eld, random anisotropy and other parameters at 4 K from Chudnovsky's model [2}4]
x M T J$}$ J$}# A K* R
(emu/g) (K) (10\J) (10\J) (;10\erg/cm) (;10erg/cm) (As)
0 95 540 70 * 26.6 1.4 290 0.19
4 92 527 72.3 20 26.1 1.4 269 0.19
6 91 512 72.4 21 24.7 1.4 246 0.20
10 78 488 73.3 22 23.3 1.4 176 0.24
15 78 488 73.6 23 19.2 1.7 101 0.32
The magnetization M shows a monotonic decrease with increasing Al concentration. The net alloy moment in accordance with the antiferromagnetic interaction
can be written as"[(74!x)$ }6#]/100, where $ and# stand for Fe and Er moments, respectively.
We had found earlier that#"8 in similar amorph- ous alloys. Assuming these values, we "nd that $"1.9 at 4 K. TheT
for the various compositions is reported in Table 2.
The exchange parameters were obtained by using the
mean"eld approach and the obtained values are given in
Table 2. The details of the model and its application to amorphous alloys are described elsewhere [2}4]. It is seen thatJ
$}$ andJ
$}#increase when the Al concen- tration increases. A similar increase in J
0#}2+ has also been reported in intermetallic compounds and amorph- ous alloys. The 3d}5d interactions depend critically on 3d}5d hybridization according to Brooks et al. [5], there- fore the increase inJ
$}#could indicate an increase in the 3d}5d hybridization when the Fe concentration relative to Er is decreased.
The exchange constant,A can be obtained from the
mean"eld model proposed by Hasegawa [6] and from
the Curie temperature using the relation proposed by Heiman et al. [7]. The magnitude ofAdecreases gradual- ly with an increase of Al content at various temperatures.
This behavior of A is in agreement with the observed variation of the Curie temperature. The low-temperature magnetization studies indicated aH\dependence of M for these compositions in approach to saturation.
Slight deviation from the linear dependence observed for the high-"eld region arises from the instability of the anti-ferromagnetic coupling, as mentioned in the earlier work [8]. Studies of magnetization in approach of satu- ration also gives other interesting parameters like R , which is the length over which the local magnetization axis show correlation, R
, which is the ferromagnetic correlation length (R
"R /). Here is a parameter used to identify weak or strong anisotropy present in the material. Table 2 shows the various parameters obtained from the data analysis using the models described. It is seen that for the alloy with x"0, the random local anisotropy constant K
* is 1.4;10erg/cm, which is about one-third of what was obtained for Fe}Er}B}Si alloys for a similar Er concentration. This indicates that 1562 A. Ajan et al./Journal of Magnetism and Magnetic Materials 226}230 (2001) 1561}1563
the contribution to the anisotropy from Er is smaller in the present alloy system. It is found that in our alloys (1, which corresponds to a ferromagnetic system with wandering axis. Experimental data show that R
de- creases with increasingx. This is expected because the number of magnetic Fe nearest-neighbor atoms are de- creased as the Al concentration increases.
Transition metal sublattice moment is reduced in the rare-earth containing system by suitable doping of Al, which is inferred from magnetic and MoKssbauer studies.
The average hyper"ne"eld decreases by 3.60 kG/% Al at 80 K and 5.1 kG/% Al at 300 K. Ferromagnetic correla- tion length obtained from the magnetization studies shows that it decreases strongly with decreasing Fe con- tent. This is expected, because the number of magnetic Fe atoms is decreased with Al substitution.
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