Frustration-driven spin freezing in the S=12 fcc perovskite Sr2MgReO6

The ordered perovskite Sr2MgReO6 of tetragonal symmetry [I4/m, a=5.5670(1) Å, c=7.9318(2) Å at T=295 K] has been synthesized and characterized by x-ray and neutron diffraction, thermal gravimetric analysis dc susceptibility, heat capacity, and muon spin relaxation (μSR) experiments. The B site cations Re6+ and Mg2+ appear to be ordered due the large difference in formal charge. The Re6+ magnetic ions form a distorted fcc lattice of S=12 spins providing a frustrated topology of edge-shared tetrahedra. The material exhibits a weak magnetic glassiness shown by a cusp at ∼50 K in the dc susceptibility, a weak but broad heat capacity anomaly, and a low-temperature μSR line shape characteristic of a spin-glass state. A broad and strongly field- dependent maximum in the dc susceptibility suggests that magnetic correlations persist to ∼175 K, accompanied by a divergence in the field-cooled and zero-field-cooled susceptibility. The anisotropic nature of the superexchange pathways due to the tetragonal distortion is thought to disrupt the ideal frustrated environment and lead to weaker glassiness than Sr2CaReO6, which has TG∼14 K, and a large specific heat anomaly. In contrast, Sr2MgReO6 has a small anomaly, and only about 3% of the entropy is released at TG∼50 K, which is comparable to other unconventional spin glasses such as the jarosite (H3O)Fe3(SO4)2(OH)6(∼6%). TG∼50 K is unusually high for this class of materials.