Frustration-driven spin freezing in the S=1/2 fcc perovskite Sr2MgReO6 -: art. no. 134410

The ordered perovskite Sr2MgReO6 of tetragonal symmetry [I4/m, a=5.5670(1) Angstrom, c=7.9318(2) Angstrom 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 (muSR) 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=1/2 spins providing a frustrated topology of edge-shared tetrahedra. The material exhibits a weak magnetic glassiness shown by a cusp at similar to50 K in the dc susceptibility, a weak but broad heat capacity anomaly, and a low-temperature muSR 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 similar to175 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 T(G)similar to14 K, and a large specific heat anomaly. In contrast, Sr2MgReO6 has a small anomaly, and only about 3% of the entropy is released at T(G)similar to50 K, which is comparable to other unconventional spin glasses such as the jarosite (H3O)Fe-3(SO4)(2)(OH)(6)(similar to6%). T(G)similar to50 K is unusually high for this class of materials.