Hydrogen induced change from superconducting to magnetically ordered state in Th7Fe3

The Pauli paramagnet Th₇Fe₃ becomes a superconductor at temperatures below 1.86 K.¹ This compound readily hydrogenates to Th₇Fe₃H₃₀, giving one of the densest hydrogen media known. Magnetization measurements on the hydride show that it is magnetically ordered. It has a Curie temperature above 300 K and a saturated moment measured at 4.2 K of 1.4 μ_B/Fe. The differing behavior in the host metal and the hydride is ascribed, in the latter, to competition between H and Fe for the electrons provided by Th. In the host metal the Fe d-band is filled by electron transfer and the Fe moment is quenched. In the hydride some of the electrons provided by Th are absorbed by H, leaving vacancies in the Fe d-band. These, in turn, give rise to the magnetic order. The present instance in which a superconductor is converted into a ferromagnet appears to be unique as regards the effects of hydrogenation on the properties of metallic systems.