Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Correlation between defects and magneto-structural properties in Ni-Mn-Sn metamagnetic shape memory alloys J. López-García a,b , I. Unzueta c,d , V. Sánchez-Alarcos b,e , V. Recarte b,e,* , J.I. Pérez-Landazábal b,e , J.A. Rodríguez-Velamazán a , J.A. García d,f , F. Plazaola f a Institut Laue-Langevin, 71 Avenue des Martyrs, 38000, Grenoble, France b Department of Physics, Universidad Pública de Navarra, Campus de Arrosadia, 31006, Pamplona, Spain c Department of Electricity and Electronics, University of the Basque Country UPV/EHU, 48940, Leioa, Spain d BCMaterials, University of the Basque Country UPV/EHU, 48940, Leioa, Spain e Institute for Advanced Materials (INAMAT), Universidad Pública de Navarra, Campus de Arrosadia, 31006, Pamplona, Spain f Department of Applied Physics II, University of the Basque Country UPV/EHU, 48940, Leioa, Spain ABSTRACT The effect of combined mechanical and thermal treatments in the magnetostructural properties was studied in Ni-Mn-Sn metamagnetic shape memory alloys, in which the extraordinary high stability of the L2 1 structure precludes the variation of atomic order by means of conventional thermal treatments. A Ni 50 Mn 35 Sn 15 alloy has been mechanically milled and then annealed at different temperatures in order to produce different micro- structural states. The evolution of both the internal stresses and the crystallite size upon annealing has been quantified and correlated to the evolution of the martensitic transformation features and the magnetic prop- erties. It is found that the relaxation processes brought by annealing leads to recovery of the martensitic transformation and the enhancement of the magnetism at both macroscopic and local level. In particular, the density of non-magnetic inclusions (defects) and their stress field decrease upon annealing, thus leading to an increase of the saturation magnetization and a decrease of the martensitic transformation temperature range, respectively, which results in a higher magnetocaloric effect. The obtained results confirm that, once the tran- sition temperature has been fixed by the composition, the modification of the microstructure through thermo- mechanical treatments appears as the best way to tune the functional properties of these alloys. 1. Introduction The thermoelastic martensitic transformation (MT) is a specific phase transition widely studied along the years. In metamagnetic shape memory alloys, it results in a large variation in the magnetization at the transformation temperature that makes possible the magnetic induction of the reverse MT, which is at the origin of interesting practical appli- cations in sensing and magnetic refrigeration [1–12]. In the case of Ni- Mn-based metamagnetic shape memory alloys, the MT is mainly driven by lattice dynamics and magnetism, being the electronic contribution very small [13–15]. The magnetism in these alloys arises from the coupling between the Mn atoms, in which the magnetic moment is mainly confined, and thus the magnetic exchange interactions strongly depend on the Mn-Mn distance. The control of the magnetostructural properties is crucial to en- hance the potential of these alloys for applications. The modification of the composition is the most used way to properly tune the MT features and the magnetic properties [16–18]. For a fixed composition, these properties may be modified from variations on the long-range atomic order, which have a direct influence on the magnetic-exchange cou- pling between Mn atoms and, as a result, on the free energy difference between austenite and martensite. In particular, the increase of atomic order degree stabilizes the austenite, which shows higher magnetic moment, thus increasing the transformation temperature [4]. In Ni-Mn- In and its quaternary systems, the long-range atomic order can be modified by means of standard thermal treatments (typically annealing and quenching), and shifts of the MT temperature up to more than 100 K can be achieved [19–21]. However, for Ni-Mn-Sn and Ni-Mn-Sb systems, the extraordinary high stability of the L2 1 structure precludes the variation of atomic order by means of conventional thermal treat- ments [22]. In these alloys, the modification of the microestructure appears as the only way to tune the functional properties. The microstructural parameters (grain size, vacancies, grain boundaries, dislocations, internal stresses, etc …) can be controlled https://doi.org/10.1016/j.intermet.2017.12.028 Received 15 September 2017; Received in revised form 20 November 2017; Accepted 30 December 2017 * Corresponding author. Department of Physics, Universidad Pública de Navarra, Campus de Arrosadia, 31006, Pamplona, Spain. E-mail address: recarte@unavarra.es (V. Recarte). Intermetallics 94 (2018) 133–137 0966-9795/ © 2018 Elsevier Ltd. All rights reserved. T