Magnetic ground state, field-induced transitions, electronic structure, and optical band gap of the frustrated antiferromagnet GeCo 2 O 4

摘要: Systematic studies of magnetic ordering, magnetic-field-induced transitions, electronic structure, and optical properties the frustrated spinel ${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$ (GCO) are reported. Our results reveal that GCO orders antiferromagnetically (AFM) at ${T}_{\mathrm{N}}=20.4$ K but with significant short-range ferromagnetic (FM) order up to $T$ $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{N}}$. The paramagnetic susceptibility ($\ensuremath{\chi}$) fits modified Curie--Weiss law, $\ensuremath{\chi}={\ensuremath{\chi}}_{\mathrm{o}}$ + $C$/($T\ensuremath{-}$ $\ensuremath{\theta}$), $\ensuremath{\theta}=+51$ for 100 $T800$ K. fit high-temperature-series expansion $\ensuremath{\chi}(T$) yields J${}_{1}$/${\mathrm{k}}_{\mathrm{B}}=14.7$ as dominant FM exchange coupling pyrochlore lattice ${\mathrm{Co}}^{2+}$ spins consisting alternate planes Kagom\'e (KGM) Triangular (TRI) lying perpendicular [111] direction. From analysis M-H plots 2 published results, three critical fields identified: ${H}_{d}\ensuremath{\sim}11$ kOe due AFM domains, ${H}_{C1}\ensuremath{\approx}$ 44 related spin-flips ordering TRI spins, ${H}_{C2}\ensuremath{\approx}97$ KGM spins. For $H$ g ${H}_{C2}$, is a forced ferromagnet some canting Magnetic field dependence ${T}_{\mathrm{N}}$ follows relation ${T}_{\mathrm{N}}(H)={T}_{\mathrm{N}}$(0)--${D}_{1}{H}^{2}$ valid antiferromagnets ${D}_{1}=6.63\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ K/${\mathrm{Oe}}^{2}$. This magnitude T${}_{\mathrm{N}}$(H) along temperature H${}_{d}$, H${}_{C1}$, H${}_{C2}$ used construct H-T phase diagram. magnitudes Curie constant ($C$) saturation magnetization it shown ions in have ground state effective spin $S$ = 1/2. High resolution x-ray photoelectron spectra 2p 3d orbitals Co Ge confirm divalent tetravalent states Ge, respectively, GCO. energy band gap (${E}_{g}=3.28$ eV) evaluated using DFT+U calculations good agreement experimental (${E}_{g}=3.16$ obtained from diffuse reflectance spectroscopy.