Magnetostructural transition, metamagnetism, and magnetic phase coexistence in Co10Ge3O16

TL;DR

This study investigates the magnetostructural transition, metamagnetism, and phase coexistence in Co10Ge3O16, revealing complex magnetic behavior and phase transitions driven by temperature and magnetic field.

Contribution

It provides the first detailed analysis of the magnetostructural and metamagnetic transitions in Co10Ge3O16, highlighting its unique phase diagram and magnetic properties.

Findings

Identified a magnetostructural transition at 203 K involving lattice distortion.

Observed a first-order metamagnetic transition with hysteresis and butterfly-shaped phase diagram.

Determined the low-temperature magnetic state is ferrimagnetic, not ferromagnetic.

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 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.