Abstract

The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound ${\mathrm{Tb}}_{5\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ ($x=0.075$) 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 $\ensuremath{\sim}140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by ${\ensuremath{\tau}}_{0}={10}^{\ensuremath{-}13}s$ and dynamic exponents $\mathrm{z}\ensuremath{\nu}\ensuremath{\sim}6$ and $\ensuremath{\beta}\ensuremath{\sim}0.4$, 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 ${\mathrm{Tb}}^{3+}$ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the ${\mathrm{Tb}}^{3+}$ atoms.

Highlights

  • The R5(SixGe1−x )4 (R = rare earth) compounds have been a subject of intensive research since the discovery of the giant magnetocaloric effect (GMCE) near the magnetostructural transition in Gd5Si2Ge2 compound [1,2]

  • The static and dynamic susceptibility data presented in this paper provide clear evidence for the existence of magnetic relaxation resembling the standard cluster-glass state below a characteristic freezing temperature within the Griffiths phase in the magnetocaloric compound Tb5−xLaxSi2Ge2 (x = 0.075)

  • Nonlinear susceptibility response reveals that ferromagneticlike correlations are built up at 155 K, which become frozen at a lower temperature 140 K, in analogy with a reentrant spin glass behavior

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Summary

INTRODUCTION

The crystallographic structure and the magnetic properties in R5(SixGe1−x ) compounds are closely related since both are controlled by the number of the interlayer T -T bonds connecting the slabs [6] Both theoretical and experimental work have been reported supporting the picture that the intralayer magnetic structure is essentially ferromagnetic (FM), whereas the sign of Jinter depends on the distance between the slabs and tends to be either FM or antiferromagnetic (AFM), depending on the number of interslab pairs that are covalently bonded [8,9]. Such a behavior was explained by the preferential Lu occupation of R1 sites, which interfere with exchange interactions of the R1-T3-T3-R1 chains Inspired by these results, Belo and coworkers extended the study to Tb5Si2Ge2, in order to understand the role of nonmagnetic atoms (La) on parent compounds exhibiting spontaneous magnetostructural transitions and on how it could affect the magnetic exchange mechanism [18]. Complex clustered systems and directly probe the magnetic order prevalent within the clusters [22]

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