Remanent magnetization and thermal relaxation in a frustrated spin-glass: Eu_1-xGd_xS

The Eu1-xGdxS system evolves from ferromagnetism for small x to antiferromagnetism for x→1. We have studied the remanent magnetization and the thermal relaxation in the concentration range 40%≲x≲58% where a spin-glass state is observed (1). Both properties are found to decrease exponentially with T ln (t/τo) but with different coefficients (T is the temperature, t the measuring time and τo a characteristic relaxation time). This appears to be a general property of spin glasses at low temperature consistent with a description in terms of two levels systems.