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Croissance épitaxiale et propriétés magnétiques de films de Sm1-xGdxAl2 : un ferromagnétique sans moment

Abstract : The single-crystal growth of REAl2 (RE=Rare Earth) and slightly Gd-substituted Sm1-xGdxAl2 (SGA) thin films was performed for the first time by Molecular Beam Epitaxy. This SGA compound is the first experimental evidence of a ferromagnet without magnetisation. The growth direction of the films is [111] or [110], depending on the deposition temperature.Thermal strains in REAl2 epitaxial films, due to the high deposition temperature, lead to a magnetic anisotropy that is different from the one reported in the bulk compound. Moreover, the strains sign can be tuned by the choice of the substrate, which allows the control of the easy magnetisation direction. Thin films of SGA(111) grown onto a Al2O3 substrate present a perpendicular anisotropy, with very high coercive fields attributed to the presence of structural defects. The different temperature dependencies of antiparallely-coupled orbital and spin contributions to the moment give rise to an original thermal variation of the magnetisation that permit to consider the SGA compound as a self ferrimagnet. The substitution of Gd to Sm atoms in the films generates a magnetic compensation point, the temprature of which decreases when increasing the Gd content. At the compensation temperature, XMCD experiments show that, despite the zero magnetisation, the spin moments keep a long range ferromagnetic order. The polarisation sign is linked to the dominant contribution to the moment at the temperature where the film is previously magnetised. This spin polarisation at compensation is shown to induce an exchange anisotropy in coupled bilayers. This exchange field depends in sign an magnitude on the bilayer thermo-magnetic treatment.
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Aymeric Avisou. Croissance épitaxiale et propriétés magnétiques de films de Sm1-xGdxAl2 : un ferromagnétique sans moment. Autre [cond-mat.other]. Université Henri Poincaré - Nancy 1, 2006. Français. ⟨NNT : 2006NAN10141⟩. ⟨tel-01754304⟩



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