Chemical and physical pressure effects in the A-site spinel antiferromagnets CoM2O4 (M = Al, Co, and Rh)
Abstract
Magnetic phase transitions under high pressure are reported for the diamond lattice antiferromagnet Co3-xRhxO4 in the range of 0 <= x <= 2.0, which is an isostructural S = 3/2 system for the well-known frustrated antiferromagnet CoAl2O4. In the Co3-xRhxO4 system, magnetic and specific-heat measurements at ambient pressure revealed that a second-order antiferromagnetic transition occurred at the Neel temperature (T-N) which exhibits a nonmonotonic x-variation.
The physical pressure variations of T-N were determined by ac-calorimetry under hydrostatic pressures up to p = 2.6 GPa for Co2RhO4 and CoRh2O4. The rates of change of T-N with pressure (i.e., the pressure coefficients), 1.93 and 1.61 K GPa(-1), respectively, were comparable to those for CoAl2O4 and Co3O4, respectively.
The pressure coefficients of magnetic ordering temperature for these A-site spinel compounds were considerably larger than those for other spinel and iron-garnet compounds which follow the empirical '10/3 law'. Simple analysis of the chemical and physical pressure coefficients of T-N revealed that T-N depended on both the lattice volume and the oxygen positional parameter u.