Anomalous magnetic susceptibility behavior of some Yb compounds: Thermally excited interconfiguration crossover

The magnetic susceptibility of the intermetallic compounds YbCu, YbAg, YbAu, and YbZn has been measured in the temperature range 4.2-1000 K. The susceptibility of all the compounds is constant at low temperature and is not highly exchange enhanced. In the case of YbCu and YbZn, the susceptibility increases as the temperature is raised, while for YbAg and YbAu it remains constant until the crystallographic FeB→CsCl phase transition at which there is a sharp jump. The susceptibility of YbAg stays independent of temperature after the phase transition, while that of YbAu increases with temperature. The phase transition in YbAg is reversible, whereas that of YbAu is not. The temperature behavior of the susceptibility of all the compounds can be successfully explained according to the recent ionic model of interconfiguration fluctuation. The susceptibility increase with temperature is due to thermal excitation of the nonmagnetic ground-state ${\mathrm{Yb}}^{2+}$ ion into the excited magnetic ${\mathrm{Yb}}^{3+}$ configuration. The applicability of the resonant-scattering theory based on the virtual bound state can not, however, be unequivocally ruled out for the case of YbCu and YbAu (CsCl phase).