THz ESR Study of Peculiar Co Pyrochlore System GeCo2O4 Using Pulsed High Magnetic Field

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THz electron spin resonance (ESR) measurements of the spinel compound GeCo2O4, which consists of the Co2+ pyrochlore structure, were performed. The temperature dependence measurements revealed new antiferromagnetic phases, AF1 and AF2, below TN. Moreover, a wide magnetic field range of ESR study using a single crystal at 1.8K 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 5.0T, 8.6T, and 11.0T for B//[111] and 5.1T, 7.7T, 11.2T, and 13.0T for B//[110] at 1.8K. Although it became difficult to observe ESR above 86K, the g-values of Co2+ 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 1.8K. Detailed frequency-field diagrams of the ESR modes at 1.8K suggested the existence of spin–lattice coupling energy of 250GHz (= 1.03meV = 12K) at the critical field resonances. The spin gap mode in the frequency-field diagram at 1.8K showed excitation energies of E[111] = 1004GHz (= 4.15meV = 48K) and E[110] = 1044GHz (= 4.32meV = 50K) for [111] and [110], respectively. These results will be discussed in connection with the di-tetramer model suggested previously.

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