The impact of fourth-order exchange interactions on the critical temperatures of Eu_xSr_1-xS and Eu_xSr_1-xTe

Abstract

The Curie phase line T C (x) of the diamagnetically diluted ferromagnets Eu x Sr 1−x S and the Néel phase line T N (x) of the diamagnetically diluted antiferromagnets Eu x Sr 1−x Te are analysed empirically on the basis of a meanfield approximation. Experimental data for the transition temperatures are collected from neutron scattering, magnetic specific heat and susceptibility measurements. Neither phase line, T C (x) or T N (x), is proportional to the concentration x of the magnetic atoms. It is shown that the deviations from linearity correlate with the fourth-order interaction sum in the paramagnetic phase. We therefore also ascribe the nonlinear x dependence of both phase lines to fourth-order interactions. Furthermore, our zero field neutron scattering measurements show that even for the strongly diluted Eu x Sr 1−x S samples the order parameters exhibit a T 2 Bloch law at low temperatures instead of a T 3/2 law. The T 2 law is a characteristic signature of fourth-order interactions in 3D materials with half-integral spin. It is also observed that the critical exponent β of the order parameter changes from β = 0.5 for x = 1 towards β = 1 at about x 2/3. This confirms that a new universality class is reached for a random distribution of magnetic moments. However, all phase transitions are first order and the critical power law applies only for the continuous part of the rise in the order parameter.