THz ESR Study of Peculiar Co Pyrochlore System GeCo 2 O 4 Using Pulsed High Magnetic Field

THz electron spin resonance (ESR) measurements of the spinel compound GeCo 2 O 4 , which consists of the Co 2+ pyrochlore structure, were performed. The temperature dependence measurements revealed new antiferromagnetic phases, AF 1 and AF 2 , below T N . Moreover, a wide magnetic field range of ESR study using a single crystal at 1.8 K also revealed various field-induced phases due to the competition between the spin–lattice coupling and the spin frustration. Critical field resonances were observed at 5.0 T, 8.6 T, and 11.0 T for B //[111] and 5.1 T, 7.7 T, 11.2 T, and 13.0 T for B //[110] at 1.8 K. Although it became difficult to observe ESR above 86 K, the g -values of Co 2+ ions were estimated to be g [111] = 3.34 and g [110] = 3.27 for [111] and [110], respectively, from the observed ESR mode above the saturation field at 1.8 K. Detailed frequency-field diagrams of the ESR modes at 1.8 K suggested the existence of spin–lattice coupling energy of 250 GHz (= 1.03 meV = 12 K) at the critical field resonances. The spin gap mode in the frequency-field diagram at 1.8 K showed excitation energies of E [111] = 1004 GHz (= 4.15 meV = 48 K) and E [110] = 1044 GHz (= 4.32 meV = 50 K) for [111] and [110], respectively. These results will be discussed in connection with the di-tetramer model suggested previously.