High Field Magnetism of the Spinel Oxide CoAl2O4

We report the results of magnetization and electron spin resonance (ESR) measurements on sintered samples of the spinel oxide CoAl2O4 in magnetic fields up to 53 T. In CoAl2O4, Co2+ (S=3/2) ions make a diamond lattice, indicating naively that no geometrical frustration takes place. This spinel oxide, however, is thought to be in the vicinity of a quantum melting point of the antiferromagnetically ordered state because of no sharp peak in the temperature dependence of specific heat. Magnetization curve at 4.2 K is almost linear in magnetic fields and a broad ESR signal was observed on a straight resonance line with g=2.26, implying an antiferromagnetic long range order or a well developed antiferromagnetic short range order with very small anisotropy.