Insulating spin-glass systemEuxSr1−xS

Abstract

A systematic experimental study of the magnetic properties in insulating ${\mathrm{Eu}}_x{\mathrm{Sr}}_{1-x}\mathrm{S}$ is reported, giving evidence of spin-glass behavior for concentrations $0.13<~x<~0.5$ due to short-ranged exchange interactions with opposite sign. The magnetic properties below the spin-glass transition temperature $T_f$ are explored by low-field magnetization, ac susceptibility, and neutron-diffraction measurements. The magnetization and remanent magnetization are found to depend on the thermomagnetic history. An unusually pronounced maximum is observed in the thermoremanent magnetization as function of the previously applied field. The saturation values of the remanent magnetizations remain only a few percent of the saturation magnetizations, even for the high Eu concentrations. Their temperature, field, and time dependence are studied. Comparison is made with metallic spin-glass samples. A detailed experimental study of the susceptibility at the spin-glass transition $T_f$ in (Eu,Sr)S is presented exhibiting a strong sensitivity of $\chi \left(T_f\right)$ and $T_f$ to small external magnetic fields and different measuring frequencies. In an attempt to analyze the data, the distinction between the spin-glass phenomenon and superparamagnetism is stressed. Near the percolation threshold of the exchange interactions, $x_p=0.13\mathrm{}$, a transition from spin-glass behavior to "pure" superparamagnetism is clearly observed in ${\mathrm{Eu}}_x{\mathrm{Sr}}_{1-x}\mathrm{S}$.