Interactions of hydrogen in solution with magnetic structures in rare earth metals

Abstract

The existence of a low temperature metastable α phase in the Tm-H and Er-H systems enables the interaction of hydrogen with magnetically ordered structures in these metals to be studied. The temperature dependences of the electrical conductivity and the magnetization were measured and, in combination with neutron diffraction data, the following results were obtained. The antiferromagnetic transition temperatures T N in α-TmH x and α- ErH x , the helicoidal transition temperature T H in α-ErH x , the paramagnetic Curie temperatures θ p and the effective magnetic moments μ p eff decrease with increasing x. This behaviour is explained in the framework of the Ruderman-Kittel-Kasuya-Yoshida model as the result of a diminution in the electronic density of states near the Fermi surface owing to the pumping of conduction electrons responsible for the exchange interaction by hydrogen (anion-like behaviour). The interaction with the ferromagnetically ordered structure is dominated by magnetoelastic effects due to crystal field anisotropy. Thus the addition of hydrogen to thulium produces a reduction in the Curie temperature T c and an increase in the saturation magnetic moment μ sat (0).This can be understood as a result of preferred magnetization in the c-axis-oriented antiphase structure of thulium (three spins up and four spins down) through crystal field distortion caused by the strain. In erbium, however, T C and T c' , (higher order harmonic) increase with increasing x and magnetic hardening of the specimens takes place, i.e. the polycrystalline ErH x specimens behave like single crystals of pure erbium oriented along the b axis. These results suggest that hydrogen interacts with the conical structure which exists in erbium below T C mainly by affecting the c axis component of the spins, thus transforming the sample into a magnetic monocrystal. The strong anisotropy of the magnetoelastic interaction of hydrogen suggests that the hydrogen atoms are in a uniaxially oriented configuration, probably in pairs along the c direction.