Magnetostructural transition, metamagnetism, and magnetic phase coexistence in Co 10 Ge 3 O 16
Co${}_{10}$Ge${}_{3}$O${}_{16}$ crystallizes in an intergrowth structure featuring alternating layers of spinel and rock salt. Variable-temperature powder synchrotron x-ray and neutron diffraction, magnetometry, and heat capacity experiments reveal a magnetostructural transition at ${T}_{\mathrm{N}}=203$ K. This rhombohedral-to-monoclinic transition involves a slight elongation of the CoO${}_{6}$ octahedra along the apical axis. Below ${T}_{\mathrm{N}}$, the application of a large magnetic field causes a reorientation of the Co${}^{2+}$ Ising spins. This metamagnetic transition is first order as evidenced by a latent heat observed in temperature-dependent measurements. This transition is initially seen at $T=180$ K as a broad upturn in the $M$--$H$ near ${H}_{\mathrm{C}}=3.9$ T. The upturn sharpens into a kink at $T=120$ K and a ``butterfly'' shape emerges, with the transition causing hysteresis at high fields while linear and reversible behavior persists at low fields. ${H}_{\mathrm{C}}$ decreases as temperature is lowered and the loops at positive and negative fields merge beneath $T=20$ K. The antiferromagnetism is described by ${k}_{\mathrm{M}}=(00\frac{1}{2})$ and below $T=20$ K a small uncompensated component with ${k}_{\mathrm{M}}=(000)$ spontaneously emerges. Despite the Curie-Weiss analysis and ionic radius indicating the Co${}^{2+}$ is in its high-spin state, the low-temperature $M$--$H$ trends toward saturation at ${M}_{\mathrm{S}}=1.0$ ${\ensuremath{\mu}}_{\mathrm{B}}$/Co. We conclude that the field-induced state is a ferrimagnet, rather than a $S=1/2$ ferromagnet. The unusual $H$--$T$ phase diagram is discussed with reference to other metamagnets and Co(II) systems.