Cluster-glass dynamics of the Griff phase in Tb-5-x Lax Si2 Ge2LaxSi2Ge2

Abstract

The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound Tb5−xLaxSi2Ge2 (x = 0.075) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature ∼140 K, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by τ0 = 10−13s and dynamic exponents zν ∼ 6 and β ∼ 0.4, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth Tb3+ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the Tb3+ atoms