Studies of Curie-Point Increases in EuO

The ferromagnetic transition temperature ${\mathrm{(T}}_{\mathrm{C}})$ of $\mathrm{EuO}$ has been increased from 69° to 135°K by selectively doping to increase the electrical conductivity. Three different chemical systems have been studied: the $\mathrm{EuO}‐\mathrm{Eu}$ , the $\mathrm{EuO}‐\mathrm{Eu}‐{\mathrm{RE}}_2{\mathrm{O}}_3$ , and the $\mathrm{EuO}‐\mathrm{RES}$ systems (RE is a rare earth other than europium). Compositions in the latter system were multiphase because the solubility of the rare‐earth sulfides in $\mathrm{EuO}$ is very small and it was necessary to add about 10% to cause significant Curie‐temperature increase. Single crystals have been grown from ternary compositions in the $\mathrm{EuO}‐\mathrm{Eu}‐{\mathrm{RE}}_2{\mathrm{O}}_3$ system with magnetizations at 77°K as high as 170 G cm³/g and ferromagnetic Curie temperatures ${\mathrm{(T}}_{\mathrm{C}})$ close to the paramagnetic θ values, a condition not observed in the other doped europium chalcogenides. The effect of the Eu metal on the formation of these crystals and its role in “forcing” the trivalent rare‐earth oxide into the rock‐salt lattice is discussed. From Kerr measurements and the optical absorption data it is shown that the magnitude of the rotation of the doped material at 80°K is equal to that of pure $\mathrm{EuO}$ at 18°K. This makes this material attractive for use in magneto‐optical devices operating at nitrogen temperature.