From antiferromagnetic insulator to ferromagnetic metal: Effects of hydrogen substitution in LaMnAsO

Manganese oxyarsenide hydrides LaMnAsO_1-xH_x (x=0-0.73) were synthesized by a solid-state reaction under a high pressure of 2 GPa. The hydride ions H^- in O^2- sites induced an insulator-to-metal conversion and suppressed the antiferromagnetic ordering of Mn spins. Instead, ferromagnetism with negative magnetoresistance was observed. The Curie temperature and saturated moment per Mn increased with x up to 264 K and 1.5μ_B, respectively, at x=0.73. These results indicate that H^- donate conduction electrons to the MnAs layer, mediating a direct ferromagnetic interaction between Mn atoms.