Optical and magnetic characterisation of rare earth hydride films

It is well known that rare earth metals like yttrium or europium show a metal insulator transition, if they are exposed to a hydrogen atmosphere. Yttrium changes from a reflective metal to a very weakly transparent metal (YH2) and finally to a transparent insulator (YH3). Europium also undergoes a metal to insulator transition however, in addition a transition from antiferromagnetism to ferromagnetism has been reported to occur in powder samples. Due to the reactivity of the pure rare earth metals it is difficult to grow stable films. We have grown in-situ hydrogenated films of yttrium and europium by pulsed laser deposition in a hydrogen atmosphere. By variation of the hydrogen pressure thereby, we succeeded to grow layers with different hydrogen concentrations. The EuHx films have been covered in-situ with a gold layer to protect against oxidation and deterioration. After removal from the vacuum chamber the optical and magnetic properties of the films have been determined by spectroscopic ellipsometry and SQUID magnetometry, respectively. The former measurements demonstrate the formation of the optical gap, while the latter indicate an increasing moment with increasing hydrogen content. For the highest hydrogen concentrations a semiconductor with a gap of 1.9 eV and a ferromagnetic transition at 17 K is obtained