Solid State Communications, Vo1.54,No.4, pp.313-316, 1985. Printed in Great Britain. 0038-1098/85 $3.00 + .OO Pergamon Press Ltd. Low ~MPERATIRE M&XBAUER STUDY OF AMORPHOUS Fe-Ni-Cr-MO-Si-B Shiva Prasadl , Girish Chandra', G. Rajaram', V. Srinivas', S. N. Shringil and R. Krishnan 3 1. Physics Department, Indian Institute of Technology, Powai, Bombay-400076, India 2. Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay-400005, India 3. Laboratoire de Magnetisme, C.N.R.S., 1, Place Aristide Briand, 92195, Meudon - Bellevue, France. (Received on 29-5-84 & in revised form on 24-9-84 & 23-l-85 by A.R. VERMA) Chromium containing amorphous alloys of composition Fe40-x/2Ni40-x/2CrxMo2si10B8~ x = 0,1,3,4,5 have been studied by Mossbauer effect at 4.4 K. The results indicate that the large reductions in the saturation magnetisation observed in these alloys on addition of Cr may be largely due to the reduction of the Fe moment in the neighbourhood of Cr. 1. Introduction The addition of Cr to transition metal- metalloid glasses is considered important be- cause it improves their soft magnetic proper- tiesl. However, it produces large decrease in the saturation magnetisation and in Curie temperature. It also increases the high field susceptibility. One such studied alloy is Fe32Ni36Crl4Pl2B8 (METGLAS 28263 Of Allied Chemical Corp.), whose various properties are well documented in the literature2r3. MGssbauer studies have been carried out on this alloy in order to obtain the distribution of hyperfine fields, P(H). There are controversies regard- ing the actual shape of P(H) for this alloy. However, these studies have indicated that the number of Fe atoms with low hyperfine fields is comparatively larger in this Cr containing alloy4-8. There are some studies reported on alloys containing varying amounts of Cr9-15. One finds from these studies that the average magnetic moment at 4.2 K as well as the hyper- fine field at Fe nucleus decreases rapidly with Cr concentration. It has been indicated by some of these authors9rl3 that a model with constant Fe and Ni moments and with Cr moment of -4pB fits the experimental data well (nega- tive sign implies antiferromagnetic coupling between Cr and Fe atoms). On the other hand, other authors assume Cr moment to be zeroll,l4 which is sometimes supported by Mijssbauer and magnetisation datall. We undertook the M&ssbauer study of Cr containing alloy in the dilute range of Cr concentration in view of the controversy over the clustering of Cr atoms. Our room temperature study of Fe40_x/2Ni40_x/2CrxMo2Si10B8 with x=0,1,3,4,515 indicated that the fraction of Fe atoms with low hyperfine fields increased with the addition of Cr. The present paper reports the behaviour of saturation hyperfine fields obtained from Mijssabuer spectra at 4.4 K, which are compared with the magnetic properties such as magnetisation obtained by Krishnan et a113. 2. Experimental Procedure The samples were melt spun alloys prepared by Vacuum Schmelze, Hanau. These were ribbons about 30 ym thick and 3 to 10 mm wide. The Mijssbauer measurements were made with the absorber at 4.4 ?:0.1 K in the transmission geometry with Co57(Rh) source. A natural iron absorber was used to calibrate the source velocity. 3. Analysis The spectra, as for most amorphous alloys, consist of highly overlapped six line patterns due to distribution of hyperfine fields. The spectra were analysed using Window's method16, in order to derive the probability distribution, P(H), of hyperfine fields. There is controversy about the reliability of this and other methods of determining P(H) especially when applied to hyperfine field distributions that extend to low fields5-8. However, using Keller's con- clusionsl7, we expect the value of the average hyperfine field obtained in the analysis to be insensitive to the shape of P(H) curve. A qua- drupole splitting term has not been explicitly taken into account in our analysis. At large hyperfine fields, one may use modified line shapes for the basic spectral8 or a broadened Lorentzian line, as is done in our analysis. It is possible that Fe atoms with low magnetic hyperfine fields but with finite quadrupole splitting may contribute to the peaks at low hyperfine fields %50 kG. We, however, observe systematic changes in the shape of P(H) as a function of Cr concentration indicating an increase in the number of Fe atoms with fields in this range (see results and discussions). It should be pointed out here that, according to Dubois and Le Caerl9,when the magnetic and qua- drupole terms are comparable, the usual treat- ment of the latter as a first order perturbation of the former is invalid and the problem is less tractable. 313