Eur. Phys. J. B 40, 427–431 (2004) DOI: 10.1140/epjb/e2004-00274-x T HE EUROPEAN P HYSICAL JOURNAL B Dynamics of the first-order magnetostructural transition in Gd 5 (Si x Ge 1-x ) 4 F. Casanova 1 , A. Labarta 1, a , X. Batlle 1, b , E. Vives 2 , J. Marcos 2 , L. Ma˜ nosa 2 , and A. Planes 2 1 Departament de F´ ısica Fonamental, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Catalonia, Spain 2 Departament d’Estructura i Constituents de la Mat` eria, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Catalonia, Spain Received 30 January 2004 / Received in final form 1st June 2004 Published online 7 September 2004 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. The dynamics at a mesoscopic scale of the first-order magnetostructural transition in a Gd5(SixGe1-x)4 alloy with x =0.05 is studied. We examine the effect of inducing the transition either by T or H on the entropy change. In addition, we analyse the avalanches between metastable states during the transition. The athermal character of the transition is evidenced. It is also shown that cycling through the transition leads to a reproducible pattern with no characteristic size of the avalanches. PACS. 75.30.Sg Magnetocaloric effect, magnetic cooling – 75.30.Kz Magnetic phase boundaries – 64.70.Kb Solid-solid transitions 1 Introduction There is renewed interest in using the magnetocaloric ef- fect (MCE) as an alternative for refrigeration [1]. The MCE can be defined as the isothermal entropy change or the adiabatic temperature change arising from the ap- plication or removal of a magnetic field, H , in a magnetic system. The MCE may be maximised in the vicinity of a first-order phase transition, when the transformation is field-induced, resulting in a large contribution to the en- tropy change [1]. Such a giant MCE has been found in Gd 5 (Si x Ge 1-x ) 4 compounds [2,3], in MnAs-based mate- rials [4,5] and in La(Fe x Si 1-x ) 13 alloys [6,7]. In this pa- per we study a Gd 5 (Si x Ge 1-x ) 4 sample with x =0.05. For this composition the first-order magnetostructural phase transition occurs from a high-temperature antifer- romagnetic (AFM) Sm 5 Ge 4 -type orthorhombic phase to a low-temperature ferromagnetic (FM) Gd 5 Si 4 -type or- thorhombic phase, at a transition temperature of T t ≃ 45 K [3,8–10]. Differential scanning calorimeters (DSC) are particu- larly suited to the study of first-order structural phase transitions since they measure heat flow. Integration of the calibrated signal yields the latent heat, L, and the entropy change, ΔS , of the transition. Recently, we reported a new high-sensitivity DSC operating under magnetic field, H , [11] and showed that measurements sweeping T at a constant H yielded accurate values of ΔS at the first-order transition [12,13]. However, a direct evaluation of the a e-mail: amilcar@ffn.ub.es b e-mail: xavier@ffn.ub.es MCE should be done while sweeping H at constant T . Our DSC is designed to operate in this sweeping-H mode. In the present paper, calorimetric results obtained by sweep- ing H and T are compared. Besides, calorimetric curves sweeping H through the transition reveal the discontinu- ous character of the transition dynamics which can be de- scribed in terms of avalanche events, whose analysis is also performed. Avalanches are associated with the nucleation and growth of domains of the new phase that take place during the first-order phase transition. Avalanche dynam- ics in phase transitions has been associated with many first-order phase transitions in disordered systems with athermal character [14]. In particular, it has been found in solids undergoing martensitic transformations (acous- tic emission pulses) [15,16] and ferromagnets (Barkhausen noise) [17]. Recently, it has been suggested the martensitic nature of the irreversible AFM-to-FM transition occurring in Gd 5 Ge 4 at low T [18]. Accordingly, burstlike effects may be present during the transformation [18]. The paper is organised as follows. In Section 2, exper- imental details are presented. In Section 3, calorimetric results of the entropy change are discussed. In Section 4, the dynamics of the transition is analysed. Finally, in Sec- tion 5 we summarise and conclude. 2 Experimental Gd 5 (Si x Ge 1-x ) 4 sample with x =0.05 was synthesized by arc melting the pure elements in the desired stoichiome- try under a high-purity argon atmosphere. The elements