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57Fe HYPERFINE FIELD DISTRIBUTION IN Co AND Mn SUBSTITUTED INVAR ALLOYS
C. Bansal, Girish Chandra
To cite this version:
C. Bansal, Girish Chandra. 57Fe HYPERFINE FIELD DISTRIBUTION IN Co AND Mn SUB- STITUTED INVAR ALLOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-202-C2-203.
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JOURNAL DE PHYSIQUE
Colloque Cl, supplément au n° 3, Tome 40, mars 1979, page C2-202
5 7F e HYPERFINE FIELD DISTRIBUTION IN Co AND Mn SUBSTITUTED INVAR ALLOYS
C. Bansal and Girish Chandra
Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay-4000053 India
Résumé.- Les spectres d'absorption Mossbauer des alliages Feo, esC^i-i-x^x^ &>35 (° £ x *$ 0»19) et Feo, 65(Nix-xCox)o, 35 ( 0 ^ x >< 0,08) ont été obtenus à 80 K et à plus haute' température. Le champ hyperfin moyen dépend fortement de la concentration en Mn mais il est pratiquement indépendant de la concentration en Co, comme le laissaient attendre les mesures d'aimantation. A partir de ces spectres nous avons calculé la distribution de champ magnétique hyperfin, indépendemment des modè- les.
Abstract.- Mossbauer absorption spectra of alloys Fe0. esCNij-^jMnx) o. 35 with 0 .£ x .< 0.19 and Feo.65(Ni,_xCox)o.35 with 0V< x ^ 0.08 have been obtained at 80 K and above. Model independent distributions of magnetic hyperfine fields have been calculated from the spectra. The mean hyper- fine field is strongly dependent on Mn concentration but practically independent of Co concentra- tion as expected from magnetization data.
1. Introduction.- The invar alloys are of particular interest due to number of anomalous properties /]/.
They show large magnetovolume effects and unusually sharp drop in the magnetic moment. The magnetization of invar alloys, such as Feo.65Nio.35 is very sensi- tive to small changes in composition. Magnetization behaviour of Fe0.6s(Nii-xMnx)0.35 and Fe0.65
(Nij_xCox)0.35 are found to be considerably diffe- rent for the two alloys 111. The 57Fe Mossbauer spectroscopic studies have been carried out on same samples (kindly sent to us by Dr. L.R. Edwards) and behaviour of magnetic hyperfine field by substitu- ting either Co or Mn for Ni in Feo.65Nio.35 are re- ported here.
2. Experimental procedure.- The alloys Feo.6 5 (Ni1_xMnx)0.3 5 with x = 0, 0.09, 0.16 and 0.19 and Feo.esCNij-xCo^o.ss with x = 0, 0.02, 0.04, 0.06 and 0.08, as obtained, were spark cut into thin discs and then repeatedly annealed and rolled into foils of thickness of about 35 pm. The samples were disordered by quenching from 800°C into liquid ni- trogen. Mossbauer absorption spectra at 80 K and 300 K were recorded against a source of 2 mCi of
57Co in copper matrix.
3. Results and discussion.- The 80 K spectrum of Feo.65Nio.35 alloy shows broadened six-line pattern due to distribution of hyperfine field. The line- widths and intensities of the lines are asymmetric in agreement with others /3/. With Co substitution there is very little change in the spectra except in the case of 8% Co alloy which shows a larger
hyperfine field. Shapes of spectra are very sensiti- ve to Mn concentration in the Mn substituted alloys.
The x = 0.09 Mn alloy shows a broad single line at 300 K which splits into broad six-lines at 80 K. The x = 0.16, 0.19 Mn alloys show sharp single lines at 300 K, but former changes to a composite spectrum of broadened six-lines and a central single line at 80 K and the latter alloy shows only a broadened single line at 80 K.
Model independent hyperfine field distributions have been obtained from the 80 K Mossbauer spectra using Window's method / 4 / . The linewidths and inten- sities of the component spectra were taken from a standard Fe absorption spectrum. Six terms were re- tained in the series expansion of the hyperfine field. The average hyperfine field were also evalua- ted from the field distributions. The field distri- butions at 80 K of Co and Mn substituted alloys are shown in figures 1 and 2 respectively. The x = 0 and 0.06 Co alloy shows a maximum in the distribu- tion at about 330 kOe and x = 0.08 Co alloy shows the maximum at about 345 kOe. There is an fee to bec transition for the latter alloy at 160 K which has higher saturation magnetization. In the case of Mn substituted alloys the field distribution (Fig. 2) show a considerable dependence on Mn concentration.
The field distributions show two broad maxima which shift to lower value with increase in Mn concentra- tion.
The average hyperfine field at 80 K as a func- tion of Co and Mn concentration is shown in figure 3. This behaviour is similar to the magnetization data 111. Qualitatively, the difference in behaviour of Mn and Co substitution can be explained on the Present address : School of Physics, University of
Hyderabad, Hyderabad-500001, India.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979271
basis of molecular field approximation 151.
HYPERFINE FIELD (kG)
Fig. 1 : The 5 7 ~ e magnetic hyperf ine field distri- butions in Feo.6s(Nil-,Co,)o.ss alloys at 80 K for x = 0, 0.06 and 0.08.
The hyperfine field at an Fe nucleus is dependent on the average exchange interaction at the Fe atom due to nearest neighbour atoms. Since Mn-Mn exchange interaction is negative, with the increase in Mn concentration the relative number of Mn atoms in a given nn configuration increases which reduces the average exchange interaction leading to reduction in hyperfine field.
H ( kilogauss
Fig. 2 : The "Fe magnetic hyperfine field distribu- tions in Feo.65(Nil-,MnX)o.35 alloys at 80 K for x = 0, 0.09, 0.16 and 0.19.
-
Acknowledgment.- Authors are grateful to Dr. L.R.
Edwards of Sandia Laboratories, New Mexico, for sup- plying the alloy samples.
References
/I/ Nakamura, Y., IEEE Trans. Magnetics, Mag-12 (1976) 278.
/2/ Edwards, L.R. and Bartel, L.C., Phys. Rev.
(1974) 2044.
131 Window, B., J . Phys.
3
(1974) 329./ 4 / Window, B., J. Phys. $ (1974) 401.
/ 5 / Coey, J.M.D. and Sawatzky, G.A., Phys. Status S o l i d i s (1971) 673.
Fig. 3 : The average 5 7 ~ e magnetic hyperfine field at 8 0 K as a function of Co and Mn concentration.
