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.