Chromium spinel oxides in high magnetic fields

In this article, we review magnetic properties of the geometrically frustrated chromium spinel oxides AB₂O₄ (A=Hg, Cd, Mg, and Zn) in high magnetic fields. In these oxides, magnetic chromium ions occupy the B site, forming a pylochlore lattice composed of a three-dimensional arrangement of corner sharing octahedra. The nearest neighbor chromium spins are coupled by an antiferromagetic interaction with an isotropic Heisenberg nature, regarding these spinel oxides as ideal substances for highly frustrated antiferromagnets. Strong geometrical frustration and significant spin-lattice coupling inherent in the chromium spinel oxides result in unconventional magnetic phase transitions. In particular, the appearance of a magnetization plateau with one-half of the saturation magnetization in high magnetic fields has attracted much attention. In this plateau phase, a peculiar symmetry breaking into acollinear ferrimagnetic state with treeup and one-down spins on each tetrahedron is realized. Recent developments of high field experimental methods enable us to investigate the plateau phase in detail. Especially, we focus on the high-field/multi-frequency ESRwhich offers precise evaluations of exchange parameters. It is demonstrated that the spin-lattice coupling, which modulates the exchange parameters, is an essential ingredient to bring about the distinct magnetic phase transition in chromium spinel oxides.