Hydrogen-induced modification of the electronic structure and magnetic states in Fe, Co, and Ni monohydrides

Abstract

Hydrogen-induced modification of electronic structures, magnetic states, and crystal structures of transition metal ($\text{TM}=\text{Fe}$, Co, and Ni) monohydrides was investigated using TM $K$-edge x-ray magnetic circular dichroism (XMCD), x-ray diffraction, and first-principles calculations. The TM hydrides undergo narrowing of TM $3d$ density of states (DOS) and significant shifts in the Fermi energy ${E}_{\text{F}}$ due to hydrogenation, which is responsible for the magnetic properties of the TM hydrides: ferromagnetic FeH, ferromagnetic CoH, and paramagnetic NiH. The reconstruction of the electronic structure is mainly attributed to the appearance of bonding and antibonding states together with hydrogen-induced volume expansion. We demonstrate that the characteristic XMCD profile of the TM hydrides near the absorption edge probed the reconstruction of the electronic structure above ${E}_{\text{F}}$. The pressure dependence of XMCD revealed that the ferromagnetic state of FeH is less stable than that of CoH under pressure. The different hydrogenation processes between CoH and NiH at room temperature are reported by means of x-ray diffraction.