Linear and non-linear AC susceptibilities in different magnetic phases of Fe-rich γ-FeNiCr alloys

Abstract

We have measured both real and imaginary parts of the low-field AC susceptibility and their DC field dependence in four different magnetic phases of Fe 80− X Ni X Cr 20 (14⩽ X⩽30) alloys within the same crystallographic phase. In the spin-glass (SG) phase ( X=19) the transition temperature ( T g) varies with frequency ( ν) and follows Fulcher's law of the form ν= ν 0 exp[− E a/( k( T g− T 0))] while the non-linear susceptibility ( χ 2) shows a peak at T g. In the re-entrant spin glasses (RSG) ( X=26 and 23), double transitions are clearly seen from the imaginary part of the linear susceptibility ( χ 0″). The peak near the upper transition vanishes in the presence of a small DC biasing field. We compare the lower transition of the RSG with that of an SG and the upper one with that of a ferromagnet (FM). Critical exponents, γ, β, and δ are obtained from χ 0 and its DC field dependence in the RSG ( X=26). They follow the scaling law obtained from the mean-field theory (MFT) although the values of the exponents are not in agreement with it. The non-linear susceptibility ( χ 1) shows a peak near T g in both the RSG's which is indicative of a long-range order. This is never observed in a pure SG, including X=19. The value of the critical exponent γ increases with the increase in Ni concentration ( X=23 to 30) as the system moves towards the FM phase ( X=30). In the antiferromagnet ( X=14), the transition temperature ( T N) shifts towards higher temperatures in the presence of small DC fields.