Abstract: The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound Tbs-x laz Si2 Ge2 (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 ?140K, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by ?0=10?13s and dynamic exponents z??6 and ??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.

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