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 Néel 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.