Magnetocaloric properties and exchange bias effect in Al for Sn substituted Ni 48 Mn 39.5 Sn 12.5 Heusler alloy ribbons Pawel Czaja a,n , Wojciech Maziarz a , Janusz Przewoźnik b , Czeslaw Kapusta b , Lukasz Hawelek c , Artur Chrobak d , Piotr Drzymala a , Magdalena Fitta e , Aleksandra Kolano-Burian c a Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str., 30-059 Krakow, Poland b AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Department of Solid State Physics, Al. Mickiewicza 30, 30-059 Krakow, Poland c Institute of Non Ferrous Metals, 5 Sowinskiego Str., Gliwice 44-100, Poland d A. Chelkowski Institute of Physics, University of Silesia, 4 Uniwersytecka Str., Katowice 40-007, Poland e The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences,152 Radzikowskiego Str., 31-342 Krakow, Poland article info Article history: Received 21 October 2013 Received in revised form 9 January 2014 Available online 2 February 2014 abstract The influence of Al substitution for Sn on magnetocaloric properties and exchange bias behavior in Ni 48 Mn 39.5 Sn 12.5x Al x (x ¼0, 1, 2, 3) melt spun ribbons have been investigated. All the studied ribbons undergo a martensitic and reverse transformation. At low temperature martensite region, below 100 K, the alloys exhibit exchange bias effect, which appears to enhance with the increase of Al concentration. The loop shift difference (ΔH E ) of up to 7960 A m 1 is recorded between the ribbon containing no Al and the ribbon with x ¼3. The presence of exchange bias behavior in these samples is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions. The magnetic entropy change and refrigerant capacity are evaluated for the ribbons studied around both the structural and magnetic transformations under the applied magnetic field induction of 2 T. The maximum entropy change around the magnetic transition and around the structural transition is reported for the Ni 48 Mn 39.5 Sn 12.5 ribbon, and the entropy values amount to 1.8 and 7.8 Jkg 1 K 1 , respectively. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Ni–Mn–Sn Heusler alloys have received attention due to their magnetoelastic (ME), magneto resistance (MR), exchange bias (EB) and magnetocaloric effect (MCE) properties [1–4]. They owe it to the coupling between magnetism and structure, which upon the appli- cation of a magnetic field yields increased entropy change of structural and magnetic contribution (magnetic entropy changes ΔS M of up to 10.4 J kg 1 K 1 at 1 T have been reported for this system) [5]. This relates to the fact that the magnetic field can induce in these materials the reverse martensitic transformation (RMT) from the martensite to the parent austenite phase, interchanging other- wise in result of cooling or heating [6]. This gives the structural contribution ΔS S associated with the latent heat involved in the transition and is the basis of the inverse MCE (ΔS M 40) [7]. The structural transition is of first order nature and features a thermal hysteresis, which may introduce hysteretic losses associated with the reversibility of the MT upon temperature change as well as upon the application of a magnetic field, and may therefore be detrimental to the cooling efficiency [8–11]. The magnetic contribution ΔS M on the other hand originates from the different magnetic exchange interac- tions in the austenite and martensite phase resulting in an abrupt change of magnetization upon the transition [12]. In addition increasing temperature brings about a ferromagnetic–paramagnetic (FM–PM) transformation, occurring at the respective Curie tempera- ture (T C ) and being of second order nature. The latter gives rise to the conventional MCE (ΔS M o0). The combination of the conventional and the inverse MCE has been proposed for refrigeration systems in which the latter would act as a heat sink absorbing heat released on isothermal magnetization prior to the adiabatic cooling step [13]. The magnetic and structural transformation temperatures in these alloys vary widely with composition. This than allows for temperature tuning. It can be accomplished by changing the valence electron concentration given by the electron to atom ratio, e/a, and calculated as the weighted sum of the 3d and 4s electrons of Ni and Mn and 5s, 5p electrons of Sn [14]. It is realized through the composition modification [15,16]. Alternatively it may be done by introducing a doping element or by altering the degree of atomic order [17–19]. Recently Chen has shown that substitution of Sn with Al reduces the unit cell volume, due to the smaller radii Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2014.01.069 n Corresponding author. Tel.: þ48 122952815; fax: þ48 2952804. E-mail addresses: p.czaja@imim.pl, pawelczaja@poczta.fm (P. Czaja). Journal of Magnetism and Magnetic Materials 358-359 (2014) 142–148