Frustration-driven spin freezing in the S=½ fcc perovskite Sr2MgReO6

The ordered perovskite Sr 2 MgReO 6 of tetragonal symmetry [14/m, a=5.5670(1) A, c=7.9318(2) A 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 Re 6 - and Mg 2 + appear to be ordered due the large difference in formal charge. The Re 6 + magnetic ions form a distorted fcc lattice of S= 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 Sr 2 CaReO 6 , which has T G ∼14 K, and a large specific heat anomaly. In contrast, Sr 2 MgReO 6 has a small anomaly, and only about 3% of the entropy is released at T G ∼50 K. which is comparable to other unconventional spin glasses such as the jarosite (H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 (∼6%). T G ∼50 K is unusually high for this class of materials.