Coexistence of ferromagnetism and unconventional 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 noncentrosymmetric space group P63mc and lattice parameters a=5.9541(2)Å, c=13.5761(5)Å, Z=2 (R(F)=0.034). Octahedrally coordinated 2a [M(1) and M(1a)] and 6c sites [M(2)] have random, mixed occupation by Sn and Fe; whereas the tetrahedrally coordinated 2b sites [Fe(3) and Fe(3a)] are exclusively occupied by Fe, whose displacement from the ideal position with trigonal-bipyramidal coordination causes the loss of inversion symmetry. Our dc and ac magnetization data indicate SrSn2Fe4O11 single crystals undergo a ferro- or ferri-magnetic transition below a temperature TC=630K with very low coercive fields μoHc⊥=0.27Oe and μoHc∥=1.5Oe at 300 K, for applied field perpendicular and parallel to the c axis, respectively. The value for TC is exceptionally high, and the coercive fields exceptionally low, among the known R-type ferrites. Time-dependent dc magnetization and frequency-dependent ac magnetization data indicate the onset of short-range, spin-glass freezing below Tf=35.8K, which results from crystallographic disorder of magnetic Fe3+ and nonmagnetic Sn4+ ions on a frustrated Kagome sublattice. Anomalous ac susceptibility and thermomagnetic relaxation behavior in the short-range-ordered state differs from that of conventional spin glasses. Optical measurements in the ultraviolet to visible frequency range in a diffuse reflectance geometry indicate an overall optical band gap of 0.8 eV, consistent with observed semiconducting properties.