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

Griffiths-like phases (GP) are connected to disordered magnetic nanostructures. Here, we focus on the Giant Magnetocaloric compound Tb 5 − x La x Si 2 Ge 2 with x = 0.075 where a re-entrant cluster-glass state (CGS) emerges at a characteristic freezing temperature T F ∼ 140 K within the GP (110–180 K), i.e. above the Curie temperature (T C ). 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 T C (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 T F 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. • Novel insights are revealed into the re-entrant Cluster-Spin Glass state within the Griffiths-like phase. • Microscopic structure of GP is investigated at the nanoscale. • AC and DC measurements reveal interacting short-range FM clusters < 2 nm. • Memory effects and ageing indicate magnetically frustrated states in GP. • SANS correlation lengths between 1–5 nm above the Cluster-Spin Glass are observed.