Magnetic properties and instability phenomena in doped Euo

A. Mauger , C. Godart , M. Escorne , J.C. Achard , J.P. Desfours

The influence of free carriers concentrations on magnetic properties of EuO single crystals has been studied. Increasing electronic concentrations from quasi stoichiometric EuO have been achieved by departure from stoichiometry (Eu rich EuO) and by doping (2 % Gd doped EuO). The magnetization of bulk samples, characterized by transport measurements, has been measured with a vibrating sample magnetometer. For low doping, magnetization curves do not depart from the Brillouin law, with a Curie temperature Tc = 69.4 K. On the contrary, for heavily Gd doped sample, Tc increases up to 143 K and the reduced magnetization is smaller than that expected from the Brillouin law. We propose a model taking into account both the superexchange interaction which explains the magnetic behaviour of insulating EuO and an indirect exchange due to the conduction electrons in semiconducting and metallic samples. The main feature is that, contrary to usual models we take into account the carrier statistics, i.e. splitting of spin subbands and spread out of the Fermi level. There is a good quantitative agreement between theory and experiment for heavy doping (n > 1.5 x 1020 cm -3). For a low doping (n < 1.5 x 1020 cm-3 ), the model predicts a first order magnetic transition. This model, considering only magnetic interactions, shows that it becomes energetically favourable for the electron system to create bound magnetic polarons at T > 40 K. Then a metal insulator transition occurring near this temperature suppresses the indirect exchange mechanism and the correlated first order magnetic transition.

Mots clés: europium compounds; ferromagnetic properties of substances; gadolinium; magnetic semiconductors; magnetisation; superexchange interactions

Domaines: Articles anciens

A. Mauger, C. Godart, M. Escorne, J.C. Achard, J.P. Desfours. Magnetic properties and instability phenomena in doped Euo. Journal de Physique, 1978, 39 (10), pp.1125-1133. ⟨10.1051/jphys:0197800390100112500⟩. ⟨jpa-00208853⟩