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

Abstract

Single-crystal x-ray diffraction refinements indicate $\mathrm{SrS}{\mathrm{n}}_{2}\mathrm{F}{\mathrm{e}}_{4}{\mathrm{O}}_{11}$ crystallizes in the hexagonal $R$-type ferrite structure with noncentrosymmetric space group $P{6}_{3}mc$ and lattice parameters $a=5.9541(2)\phantom{\rule{0.16em}{0ex}}\AA{}, c=13.5761(5)\phantom{\rule{0.16em}{0ex}}\AA{}, Z=2$ ($R(F)=0.034$). 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 $\mathrm{SrS}{\mathrm{n}}_{2}\mathrm{F}{\mathrm{e}}_{4}{\mathrm{O}}_{11}$ single crystals undergo a ferro- or ferri-magnetic transition below a temperature ${T}_{C}=630\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with very low coercive fields ${\ensuremath{\mu}}_{\mathrm{o}}{H}_{c\ensuremath{\perp}}=0.27\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ and ${\ensuremath{\mu}}_{\mathrm{o}}{H}_{c\ensuremath{\parallel}}=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ at 300 K, for applied field perpendicular and parallel to the $\mathbit{c}$ axis, respectively. The value for ${T}_{C}$ 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 ${T}_{f}=35.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which results from crystallographic disorder of magnetic $\mathrm{F}{\mathrm{e}}^{3+}$ and nonmagnetic $\mathrm{S}{\mathrm{n}}^{4+}$ 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.