Effect of hydrogen in solid solution upon the magnetic properties of monocrystalline erbium

The authors compare the magnetic properties of a single crystal of erbium before and after charging with hydrogen to a concentration x=0.035 at.(H)/(Er). The decrease with x of the Neel temperatures T N (along the c axis) and T H (within the basal plane) can be explained by: (i) a change in the electronic structure of the metal, i.e. a decrease of the number of conduction electrons (or of the Fermi level density of states); (ii) a modification of the interaction between itinerant and localised spins. Interesting features are observed for the low temperature transitions towards ferromagnetism. Along the c axis and in zero field there is an increase of the transition temperature T c with x; for T>T C , the same transition is observed in a finite critical field the value of which is smaller for x not=0 than for x=0. Within the basal plane, the transition towards ferromagnetism is observed only in a finite field; contrary to what is observed along the c axis, higher fields are needed to induce the transition for the x not=0 case. The interplay between magnetostriction and hydrogen-induced dilatation is most probably at the origin of these results; anisotropies in the hydrogen distribution may also be invoked.