Spin glasses in amorphous and crystalline RE alloys

Abstract

Abstract. -Recent experimental results on spin glasses in crystalline and amorphous RE systems are reviewed. Main emphasis of this paper will be on the bulk magnetic properties of metallic spin glasses containing Gd. The effects of Gd in the dilute limit (x < 1 at. %) are compared with those observed in 3d impurity systems. The case of amorphous alloys is discussed. Finally, we present a cluster description of magnetic properties in concentrated (1 < x < 32 at. %) amorphous 'L& ao _ x G& li £M 20 spin glasses.1. Introduction. -Regardless of the (short-or long-range) forces which are responsible for the spinglass phenomena [1], both dilute (~ 1 at. %, typically) and concentrated spin glasses are phenomenologically characterized by an ordering temperature below which short-range magnetic order [2] and field cooling effects are observed [3][4]. This ordering or freezing temperature T M is usually defined by a sharp cusp in zero-field ac susceptibility [5]. Considerable effort has developed among theoreticians to explain this cusp in terms of a phase transition [6][7]. Meanwhile, experimentalists have accumulated evidences for the complex nature of the spin-glass ordering process : namely, both the shape and the position of T M depend on the strength of a dc applied field and on the frequency of an ac field [8][9][10][11]. Moreover, specific heat measurements exhibit no singularity at T M ; instead, a broad maximum occurs at a temperature T which varies as a function of applied field [12]. In addition, recent neutron scattering experiments were interpreted as indicating a non-unique freezing temperature depending on the momentum of incident neutrons [13]. Finally, the remanence below T M is time dependent [4]. All these findings do not favor the phase transition picture for the spin-glass freezing. In contrast, they can be interpreted rather easily within a phenomenological model of superparamagnetic clouds [3][4] inspired from Neel's studies on fine grains [14]. Such an approach is very appealing for an experimentalist by providing him with a tool for a quantitative analysis of his data. In particular, the scaling behaviour observed in the dilute limit for T M and for its frequency dependence [9], for the magnetization [15-16] above

Quatrième partie. — Les propriétés magnétiques moyennes des métaux et alliages amorphes

“…basse température, la symétrie locale autour des ions de terres rares légères. Les résultats obtenus pour le Ce [54], le Pr et le Nd [55] dilués dans le LagoAu2o montrent des écarts à la symétrie uniaxiale, qui doivent être attribués à des termes non axiaux.…”