First Observation of a Magnetic-Exchange—Induced Valence Transition

A magnetic-exchange—induced valence transition has been observed for the first time for different compositions $x$ of ${\mathrm{Tm}}_{1-x}{\mathrm{Eu}}_x\mathrm{Se}$. This new effect is explained in terms of an exchange splitting $2\Delta E$ of the $5d$ conduction band which leads to a crossing of the lower $5d$ band edge with the ${\mathrm{Tm}}^2$ $4f^{13}$ state at a certain temperature. The polar magneto-optical Kerr effect of the ${\mathrm{Eu}}^{2+}$ $4f^7\to 4f^{6}5d$ transition is used to study $\Delta E$ as a function of temperature via the magnetic red shift. Volume changes of more than 1% have been observed which correspond to an exchange-induced valence change of the Tm ions by 0.15.