Abstract
The magnetisation of polycrystalline Er and alpha ErHx solid solutions (0<or=x<or=0.03) was measured between 4.2 and 200K in applied fields 0.2<or=H<or=55 kOe. The authors have observed a drop in the paramagnetic Curie temperature, theta p, from 40K in Er to 31.5K in ErHx (with no significant x dependence); a decrease of the antiferromagnetic and the helical transition temperatures, TN and TH, with increasing x; an increase in the Curie temperature, TC, and in another critical point, TCl, with increasing x; and a strong magnetic hardening of the solid solutions, with the appearance of an x-dependent critical field, Hc=18, 16 and 13 kOe for x=0.01, 0.02 and 0.03. The decrease in TN and TH is attributed to a weakening RKKY exchange interaction due to a pumping of conduction electrons by hydrogen. The other results and, in particular, the striking similarity between the present data and those reported by other workers using Er single crystals measured in fields parallel to the basal plane are explained through an anisotropic magnetoelastic interaction of hydrogen atoms with the crystal field, leading to an opening-up of the top angle of the ferromagnetic cone structure thus turning the total moment towards the basal plane.