Coexistence of ferromagnetism and spin glass freezing in the site-disordered kagome ferrite SrSn₂Fe₄O₁₁

Single-crystal x-ray diffraction refinements indicate SrSn₂Fe₄O₁₁ crystallizes in the hexagonal R-type ferrite structure with non-centrosymmetric space group P6₃mc and lattice parameters a = 5.9541(2) Å, c = 13.5761(5) Å, Z = 2 (R(F) = 0.034). Octahedrally coordinated sites are randomly occupied by Sn and Fe; whereas tetrahedrally coordinated sites are exclusively occupied by Fe, whose displacement from ideal trigonal-bipyramidal coordination causes the loss of inversion symmetry. DC magnetization data indicate SrSn₂Fe₄O₁₁ single crystals undergo ferro- or ferri-magnetic order below a transition temperature T_C = 630 K with very low coercive fields Hc⊥ = 0.27 Oe and H_c// = 1.5 Oe at 300 K, for applied fields perpendicular and parallel to the c-axis, respectively. The value for T_C is exceptionally high, and the coercive fields exceptionally low, among the known R-type ferrites. Enhanced coercivity and thermomagnetic hysteresis suggest the onset of short-range, spin glass order occurs below T_f = 35 K. Optical measurements indicate a band gap of 0.8 eV, consistent with wide-gap semiconducting behavior and a previously established empirical correlation between the semiconducting gap and T_C for R-type ferrites based upon Ru.