Frustration-driven spin freezing in theS=12fcc perovskiteSr2MgReO6

Abstract

The ordered perovskite ${\mathrm{Sr}}_{2}{\mathrm{MgReO}}_{6}$ of tetragonal symmetry $[I4/m,$ $a=5.5670(1)\AA{},$ $c=7.9318(2)\AA{}$ at $T=295\mathrm{K}]$ has been synthesized and characterized by x-ray and neutron diffraction, thermal gravimetric analysis dc susceptibility, heat capacity, and muon spin relaxation (\ensuremath{\mu}SR) experiments. The B site cations ${\mathrm{Re}}^{6+}$ and ${\mathrm{Mg}}^{2+}$ appear to be ordered due the large difference in formal charge. The ${\mathrm{Re}}^{6+}$ magnetic ions form a distorted fcc lattice of $S=\frac{1}{2}$ spins providing a frustrated topology of edge-shared tetrahedra. The material exhibits a weak magnetic glassiness shown by a cusp at \ensuremath{\sim}50 K in the dc susceptibility, a weak but broad heat capacity anomaly, and a low-temperature \ensuremath{\mu}SR 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 \ensuremath{\sim}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 ${\mathrm{Sr}}_{2}{\mathrm{CaReO}}_{6},$ which has ${T}_{G}\ensuremath{\sim}14\mathrm{K},$ and a large specific heat anomaly. In contrast, ${\mathrm{Sr}}_{2}{\mathrm{MgReO}}_{6}$ has a small anomaly, and only about 3% of the entropy is released at ${T}_{G}\ensuremath{\sim}50\mathrm{K},$ which is comparable to other unconventional spin glasses such as the jarosite $({\mathrm{H}}_{3}\mathrm{O}){\mathrm{Fe}}_{3}({\mathrm{SO}}_{4}{)}_{2}(\mathrm{OH}{)}_{6}(\ensuremath{\sim}6%).$ ${T}_{G}\ensuremath{\sim}50\mathrm{K}$ is unusually high for this class of materials.