Magnetostructural transition, metamagnetism, and magnetic phase coexistence in Co10Ge3O16
Abstract
Co10Ge3O16 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 TN = 203 K. This rhombohedral-to-monoclinic transition involves a slight elongation of the CoO6 octahedra along the apical axis. Below TN, the application of a large magnetic field causes a reorientation of the Co2+ 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 HC = 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. HC decreases as temperature is lowered and the loops at positive and negative fields merge beneath T = 20 K. The antiferromagnetism is described by kM = (00 1/2) and below T = 20 K a small uncompensated component with kM = (000) spontaneously emerges. Despite the Curie-Weiss analysis and ionic radius indicating the Co2+ is in its high-spin state, the low-temperature M-H trends toward saturation at MS = 1.0 uB/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.