Phase Competitions behind the Giant Magnetic Entropy Variation: Gd5Si2Ge2 and Tb5Si2Ge2 Case Studies

Abstract

Magnetic materials with strong spin-lattice coupling are a powerful set of candidates for multifunctional applications because of their multiferroic, magnetocaloric (MCE), magnetostrictive and magnetoresistive effects. In these materials there is a strong competition between two states (where a state comprises an atomic and an associated magnetic structure) that leads to the occurrence of phase transitions under subtle variations of external parameters, such as temperature, magnetic field and hydrostatic pressure. In this review a general method combining detailed magnetic measurements/analysis and first OPEN ACCESSEntropy 2014, 16 3814 principles calculations with the purpose of estimating the phase transition temperature is presented with the help of two examples (Gd 5 Si 2 Ge 2 and Tb 5 Si 2 Ge 2 ). It is demonstrated that such method is an important tool for a deeper understanding of the (de)coupled nature of each phase transition in the materials belonging to the R 5 (Si,Ge) 4 family and most possibly can be applied to other systems. The exotic Griffiths-like phase in the framework of the R 5 (Si x Ge 1-x ) 4 compounds is reviewed and its generalization as a requisite for strong phase competitions systems that present large magneto-responsive properties is proposed.

Suppression of magnetostructural transition on GdSiGe thin film after thermal cyclings

“…It is consensual that the magnetic materials undergoing first-order phase transitions are the ones exhibiting larger MCE, which is a key-parameter for their application in refrigeration systems. The Gd5(SixGe1-x)4, La(FexSi1-x)13, MnP1-xAsx, compounds belong to the group of first-order magnetic materials that exhibit a giant magnetocaloric effect (GMCE) 5,7,[12][13][14][15][16] . However one drawback of these compounds is that their physical properties change when subjected to magnetic/thermal cycling 16,17 .…”

Annealing influence on the magnetostructural transition in Gd5Si1.3Ge2.7 thin films

“…As previously reported, materials presenting coupled magnetic and structural transitions exhibit a much higher MCE because of the extremely large dM/dT value at the transition (which is a critical parameter for the enhancement of the magnetic entropy change) and also because there is an extra entropy change associated with the lattice transformation, -∆S lattice [16] . Therefore the reduction of the -∆Sm max value with annealing temperature was expected since the MST was suppressed.…”

Effects of the temperature dependence of the bulk modulus on magnetic exchange-entropy

“…In this case, the entropy change would be proportional to the volume difference realized during the phase change. The role of volume in giant MCE materials is reported by Pires et al [24]. Differently, the bulk modulus change, which is the macroscopic response of the atomic-bound dynamics in the solids, is the subject of the present study.…”