ESR of Yb3+ ions in a single crystal of the fluctuating-valence compound YbB12

The temperature and angular dependences of electron spin resonance (ESR) spectra of Yb3+ ions in a single crystal of fluctuating-valence compound YbB12 were studied. The existence of Yb−Yb ion pairs was observed in a cubic-symmetry crystal. The ions forming the pairs are coupled by the isotropic exchange but interact also with the other pairs by the dipole and exchange coupling. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of the Kondo dielectric. A strong temperature dependence of the amplitude of the ESR signals is found at 1.6–4.2 K and interpreted as a result of the capture of electrons by Yb3+ ions from electron traps with a binding energy of 18 K. ESR spectra of Yb3+ single ions in the Γ6 state were observed also. The decrease of temperature from 4.2 to 1.6 K indicates a tendency to the ferromagnetic ordering of Yb−Yb pairs.