MAGNETISM IN Ni-Cu AND Co-Re SOLID SOLUTIONS

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MAGNETISM IN Ni-Cu AND Co-Re SOLID SOLUTIONS

S. Mishra, E. Hayes, R. Tustison, P. Beck

To cite this version:

S. Mishra, E. Hayes, R. Tustison, P. Beck. MAGNETISM IN Ni-Cu AND Co-Re SOLID SOLUTIONS.

Journal de Physique Colloques, 1974, 35 (C4), pp.C4-195-C4-197. �10.1051/jphyscol:1974434�. �jpa-

00215625�

JOURNAL

DE PHYSIQUE Colloque C4, supplkment au ^no 5 , tome 35, Mai 1974, page C4-195

MAGNETISM IN Ni-Cu AND Co-Re SOLID SOLUTIONS (*)

S. MISHRA, E. J. HAYES, R. TUSTISON and P. A. BECK Department of Metallurgy and Materials Research Laboratory

University of Illinois at Urbana-Champaign Urbana, Illinois 61801, U. S. A.

Rbsum6. - On a corrigk les vaieurs assignees aux moments magnktiques des atomes de Ni dans les alliages Ni-Cu pour diverses configurations (en fonction du nombre des plus proches et des seconds voisins de Ni). Ces corrections sont basks sur des informations nouvelles concernant les paramttres d'ordre a courte distance et I'aimantation a saturation d'alliages Ni-Cu trts purs prks de la concentration critique.

Les moments atomiques assign& au Co dans les alliages Re-Co hexagonaux ferromagnetiques et superparamagnktiques sont donnes en fonction de la configuration des atomes voisins les plus proches.

Abstract. - An improved version of the moment assignments to Ni will be discussed for various nearest and second-nearest neighbour configurations in Ni-Cu alloys, using revised short range order parameters and new saturation magnetization data for ferromagnetic and superparamagnetic high purity Cu-Ni alloys near the critical composition.

Using magnetization vs. field data at various temperatures for ferromagnetic and superparama- gnetic HCP solid solutions of Co and Re, the moment assignments to Co will be discussed as a function of the nearest neighbour atomic environment.

Recent work with a virtually iron-free superpara- magnetic Cu,.,Nio., alloy [I] resulted in the analysis of the magnetization measured as a function of field in the temperature range from 1.5 K to room tem- perature in terms of a field- and temperature-inde- pendent susceptibility and two Brillouin functions corresponding to average magnetic cluster moments of pl = 5.3 pB and p2 = 23.2 pB. From this analysis, the average permanent moment per alloy atom was calculated : ji = 0.0097 pB. Similar work with an iron-free paramagnetic alloy Cuo.,Nio., gave the value = 0.00036 ^pB. These average moment values, together with those derived from saturation magne- tization measurements for three virtually iron-free ferromagnetic alloys with 42, 45 and 48 percent Ni, as well as the average moment values for the more Ni-rich ferromagnetic alloys [2] were used to revise the moments assigned to Ni atoms with various local atomic environments, characterized by the number n of nearest and the number m of second-nearest neighbour Ni atoms. In arriving at these moment assignments, use was made of short-range order parameter values determined by neutron scattering [3, 41. The revised moment assignments, which differ relatively little from the values previously publish-

ed [5] but give an improved fit to the experimental average moment values, particularly at alloy composi- tions near the equiatomic, will be published in detail elsewhere [6].

The results of work so far completed on Co-Re alloys show the existence of a continuous solid solution phase field all the way from the low temperature modification of Co t o Re. The study ofmagnetism in these HCP alloys indicates the occurrence of ferromagnetism in alloys with 60 %, or more, cobalt.

The saturation magnetization drops steeply with increasing Re content, well in excess of simple dilution.

The Curie temperature of the 65 % Co alloy is slightly above 4.2 K, while the alloy with 60 % Co shows remanence only at. 1.65 K. In the absence of short- range order information, a preliminary moment assignment scheme was worked out for Co atoms with various numbers n of Co nearest neighbours, assuming randomness. According to this moment assignment, a Co atom must have eight or more Co nearest neighbours in order to have any permanent moment. The moment increases with the number of nearest cobalt neighbours from p = 0.09 pB for eight to 1.65 pB for eleven Co nearest neighbours.

For the superparamagnetic alloys with 60 % and

(*)

Work supported by grants from the U. S. Atomic Energy 50 % Co the-magnetization us. field isotherms in the Commission through the Materials Research Laboratory of the temperature range between about 2 and 10 K could University of Illinois and from the National Science Foundation. be analysed in terms of two Brillouin functions and a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974434

C4-196 S. MISHRA, E. J. HAYES, R. TUSTISON AND P. A. BECK

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constant susceptibility. However, between 10 K and 300 K the magnetic susceptibility increases far in excess of the values extrapolated from the low tempera- ture data ; it reaches a value at 300 K of three times the extrapolated value for the 60 % Co alloy and five times the extrapolated value for the alloy with 50 % Co.

It seems probable that the excess temperature- and field-dependent susceptibility, which reaches a maxi- E,/U

mum at around 150 to 200 K and disappears at near 10 K, is associated with single Co-moments, which FIG. 3. have high spin fluctuation temperatures, dependent D(w)

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MAGNETISM IN Ni-Cu AND Co-Re SOLID SOLUTIONS C4-197 on the local atomic environment. It is estimated that moments is near 0.1 of the total and that roughly in the 50 % Co alloy the fraction of the Co atoms 90 % of the Co atoms have no moment even at high participating in the magnetic clusters is somewhat temperatures. The study of the magnetic properties less than 0.01, the fraction of Co atoms with isolated of these alloys is continuing.

References

[I1 MISHRA, S. and BECK, P. A., Int.

of Magnetism 4 (1973) [4] CABLE, J. W., WOLLAN, E. 0. and CHILD, H. R., Phys. Rev.

277. Lett. 22 (1969) 1256.

121 AHERN, S. A., MARTIN, M. J. C. and SUCKSMITH, W. A.,

Proc. R. Soc. 248A (1958) 145. [5] ROBBINS, C. G., CLAUS, H. and BECK, P. A., Phys. Rev.

[3] MOZER, B., KEATING, D. T. and Moss, S. C., Phys. Rev. 175 Lett. 22 (1969) 1307.

(1968) 868. [6] MISHRA, S., to be published in Int. J. of Magnetism (1974).