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 Co _3−x Rh _x O _4 in the range of 0 ≤ x ≤ 2.0, which is an isostructural S = 3/2 system for the well-known frustrated antiferromagnet CoAl _2 O _4 . In the Co _3−x Rh _x O _4 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 Co _2 RhO _4 and CoRh _2 O _4 . 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 CoAl _2 O _4 and Co _3 O _4 , 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 .

Keywords

• magnetic transition
• pressure effects
• diamond lattice
• antiferromagnetism
• magnetic frustration