Temperature Variation of the Magnetization Curve of Superparamagnetic Alloys

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Abstract
Thermodynamical considerations suggest that in precipitation alloys there will, in general, be a gradual transition of composition between precipitate and matrix over at least a few atom layers. Since the Curie temperature of solutions is a continuous function of composition, this implies that the magnetically effective volume of ferromagnetic precipitates in a nonferromagnetic matrix depends on temperature. This interface effect may be rather large, especially for small particles. It is shown that characteristic deviations of the magnetic behavior of superparamagnetic alloys from that predicted by current theory of superparamagnetism, which assumes temperature-independent particle volumes, can be explained quantitatively by the interface effect. In particular, a corresponding analysis of initial susceptibility data taken on 2 Fe-24 Ni-74 Cu as aged at 873°K yields for the interface thickness Δρ of spherical precipitates, in the size range between 19 and 45 Å in radius, the value Δρ≃12.3 Å. It is suggested that the analysis of magnetic data, as outlined in this paper, may be utilized to determine concentration profiles of precipitates in suitable alloy systems.