Evolution at the nanoscale of magnetic clustering of the Griffiths-like phase in Tb₄.₉₂₅La₀.₀₇₅Si₂Ge₂

Abstract

Griffiths-like phases (GP) are connected to disordered magnetic nanostructures. Here, we focus on the Giant Magnetocaloric compound Tb₅−xLaxSi₂Ge₂ with x = 0.075 where a re-entrant cluster-glass state (CGS) emerges at a characteristic freezing temperature TF ∼ 140 K within the GP (110–180 K), i.e. above the Curie temperature (TC). This unconventional magnetic state has been studied via temperature-dependent DC magnetization (5–300 K), time-dependent macroscopic AC susceptibility (80–200 K), including ageing and memory experiments, and magnetic small-angle neutron scattering (SANS), above TC (110–250 K). This approach allows to reveal the microscopic structure of the GP at the nanoscale in this system. AC susceptibility and DC magnetization confirm the presence of interacting short-range (< 2 nm) ferromagnetic (FM) clusters in the GP. The Langevin-like field dependence of the isothermal magnetization provides a quantitative assessment of the temperature dependence of the cluster size. Memory effects and ageing phenomena within GP are indicative of magnetically-frustrated states. Our results reveal that the dynamics is affected by the progressive coupling among CGS and GP towards the FM state. SANS correlation lengths between 1-5 nm above TF are determined from the calculated magnetic correlation function C(r), which is computed from the magnetic SANS intensity. A phenomenological model based on the formation of FM clusters with intercluster (FM) interactions within a PM matrix is proposed to explain the unusual re-entrant glassy behaviour in the PM state. These findings serve as another experimental reference for the global understanding of disordered magnetic compounds.