  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 T_N = 203 K. This rhombohedral-to-monoclinic
transition involves a slight elongation of the CoO6 octahedra along the apical
axis. Below T_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_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_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_M = (00
1/2) and below T = 20 K a small uncompensated component with k_M = (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 M_S = 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.
