Evidence for biquadratic exchange interactions in GdAg1_xZnx

Abstract

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. We have re-examined the magnetic-phase diagram of GdAg 1−x Zn x , an intermetallic solid solution of a ferromagnet (GdZn) and an antiferromagnet (GdAg). Samples of the intermediate composition range 0.48<x<0.59 show new magnetic-ordering phenomena and all exhibit nearly the same phase diagram at the application of a magnetic field. These phase diagrams comprise two ferrimagnetic phases with ground-state magnetization values of m≃(1)/(3) and m≃(2)/(3) and one uncommon high-temperature phase which is nearly ferromagnetic but which has a finite susceptibility and therefore no spontaneous magnetization. All ordered phases seem to coexist at one multicritical point. It is argued that the observed variety of magnetic phases is due to biquadratic exchange interactions which remain as the only active ones in a situation where the ferromagnetic and antiferromagnetic bilinear interactions compensate each other. The easiest method to identify biquadratic interactions is the Curie−Weiss law of the cubic susceptibility χ 3 . However, in metallic systems the Curie−Weiss temperature regime of χ 3 is not nearly reached even at five times the ordering temperature, thus preventing the evaluation of t 3 which provides a measure for the strength of the biquadratic interaction in insulating systems. This shows that there are much stronger and strongly temperature-dependent individual biquadratic interaction processes of either sign than one might assume according to the macroscopically observed average interaction. Although the existence of the new magnetically ordered states is confirmed with magnetization, dilatometric and specific heat measurements, no corresponding neutron-diffraction intensities are observed in zero-magnetic field which points to strongly fluctuating or non-collinear magnetic structures. Since the outlined observations fall into a composition and temperature range in this random magnetic-bond system where magnetic non-equilibrium phenomena become visible, special precautions are necessary to obtain the low-temperature equilibrium magnetization curves.