THz ESR study of Spinel Compound GeCo2O4

We performed terahertz ESR measurements of spin frustrated spinel compound GeCo2O4 using pulsed magnetic fields of up to 40 T. A very broad EPR absorption line was observed at 86 K. The g-values at 86 K were estimated to be g = 5.26 +/- 0.07, 5.16 +/- 0.12 and 4.98 +/- 0.07 for B//[111], [100] and [110], respectively. High-field ESR measurements revealed complicated phase structures and a field-induced magnetic phase below 3 K. Critical fields of magnetic phases at 1.8 K for B//[111] were observed at 1.8 T, 5.0 T, 8.6 T, 11.0 T and 12.9 T for B//[111]. An energy gap of 300 GHz (= 14.4 K = 1.24 meV), which was related to the lattice deformation, was observed for B//[111]. The zero field gap of omega(4) mode, which is considered to be a singlet-triplet excitation of the di-tetramer, was estimated to be 1120 GHz.