Ferromagnetism in EuH2

Magnetic measurements have been made on a Eu-H sample of composition represented by the formula Eu${\mathrm{H}}_{1.86}$ over a temperature range extending from 4.2° to 300°K. The magnetization-temperature data for the sample, which for simplicity is termed Eu${\mathrm{H}}_2$, indicate a Curie point at 24°K. Field dependency studies in the liquid helium range reveal saturation effects and remanence, confirming that Eu${\mathrm{H}}_2$ has become ferromagnetic at these temperatures. The observed saturation moment at 4.2°K is $\sim 6.0{\mu }_B$ (Bohr magnetons) per Eu atom. For $T>\mathrm{}30\mathrm{}°$K the susceptibility shows a Curie-Weiss dependence on temperature and the effective moment is ${8.4}_{\mathrm{\mu }\mathit{B}}$ per Eu atom. Eu${\mathrm{H}}_2$, the saturated hydride of Eu, is unlike the highest (i.e., saturated) hydrides of the other lanthanide metals which have been studied—Gd${\mathrm{H}}_3$, Tb${\mathrm{H}}_3$ and Ho${\mathrm{H}}_3$—in that it alone exhibits magnetic ordering at low temperatures. As the hydrogen-saturated metals lack conduction electrons, the coupling between the localized $4f$ electrons is expected to be very weak and magnetic ordering is expected to occur only at very low temperatures if at all. Thus the behavior of the several trihydrides is in keeping with expectation whereas that of Eu${\mathrm{H}}_2$ is not. The coupling mechanism in this case is not well understood; it may be due to direct overlap of the $4f$ orbitals.