Magnetism in Eu-based amorphous alloys from local and bulk investigations

Abstract

The magnetic properties of Eu2+ (4f7 electronic configuration) based amorphous alloys Eu1-xXx, where X=Mg, Zn, Cd and Al and x approximately=0.3, have been investigated by studying local moments from 151Eu Mossbauer spectroscopy and bulk magnetisation in low AC and high DC fields. The divalent 4f7 state of Eu atoms implies vanishing interactions with crystal fields. The magnetic properties are then assumed to be depicted by localised 4f7 moments interacting via RKKY interactions, in view of the pure s (Mg, Zn, Cd) or sp (Al) character of the valence electron shells of the components. Amorphous Eu1-xAlx (x=0.225, 0.30 and 0.40) are concentrated spin glasses (mictomagnets). Amorphous Eu0.70Zn0.30 (TC=133K) and Eu0.75Cd0.25 (TC=134K) reveal all the properties of soft ferromagnets in the presence of some distribution of the exchange interaction, resulting from the disorder. Amorphous Eu0.70Mg0.30 exhibits a double magnetic transition, i.e. from paramagnetic to ferromagnetic, at TC=131K and from ferromagnetic to spin glass at TSG approximately=52K. The latter re-entrant spin-glass transition is consistently characterised both from AC susceptibility and from the anomalous temperature dependence of the hyperfine field distribution. Amorphous Eu0.70Mg0.30 is concluded as representing an example of a cluster spin glass (with intercluster frustration) whereas the amorphous Eu1-xAlx alloys would present short-range frustration.