Magnetic properties and magnetocaloric effect in Heusler-type glass-coated NiMnGa microwires A. Zhukov a,b,⇑ , V. Rodionova a,c,d , M. Ilyn e , A.M. Aliev f , R. Varga g , S. Michalik g , A. Aronin h , G. Abrosimova h , A. Kiselev h , M. Ipatov a , V. Zhukova a a Dpto. Fisica de Materiales, Fac. Quimicas, UPV/EHU, 20009 San Sebastian, Spain b IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain c Faculty of Physics, Moscow State University, Leninskie Gory, 119991 Moscow, Russia d Immanuel Kant Baltic Federal University, 236041 Kaliningrad, Russia e Center of Physics of Materials, CSIC-UPV/EHU, P. Manuel de Lardizabal, 5, 20018 San Sebastian, Spain f Amirkhanov Institute of Physics of Daghestan Scientific Center, RAS, 367003 Makhachkala, Russia g Inst. Phys., Fac. Sci., UPJS, Park Angelinum 9, Kosice, Slovakia h Insitute of Solid State Physics, Chernogolovka, 142432 Moscow, Russia article info Article history: Received 18 February 2013 Received in revised form 10 April 2013 Accepted 12 April 2013 Available online 19 April 2013 Keywords: Heusler alloys Magnetic shape memory effect MCE effect Thin microwires abstract We studied magnetic properties, structure and magnetocaloric effect (MCE) in thin NiMnGa glass-coated microwires. After annealing studied microwires exhibit ferromagnetic behavior at room temperature. We directly measured the temperature change, DT, originated from the MCE in annealed NiMnGa glass- coated microwires and after glass removal. After glass removal DT increases from 0.06–0.08 K to 0.22 K. Observed temperature dependence of magnetization and MCE should be associated with both magnetic (paramagnetic–ferromagnetic) and structural (austenite–martensite) phase transitions. Consid- erable magnetization increase in vicinity of Curie temperature can be also attributed to the Hopkinson effect. We evaluated the possibility to prepare the magnetocaloric materials for magnetic refrigerators in the form of glass-covered microwires and showed that glass-coated microwires might be interesting for the MCE applications owing to high ratio of surface to volume. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Studies of magnetic shape-memory alloys (MSMAs) has at- tracted growing attention within the last few years owing to a number of functional properties, such as: significant magnetic- field-induced strain (MFIS), originating from the coupling between magnetic and structural ordering; the magnetocaloric effect, MCE; and giant magnetoresistance, GMR [1–5]. It is evident that the di- rect and inverse magnetic shape-memory effects are useful for applications in actuators, sensors and for energy harvesting [1–3]. In fact previously the MFIS effect has also been observed in ferroelectrics in the early 1990s (rare-earth molibdates) [6]. It is commonly accepted that the MFIS or shape memory effect is related to a martensitic transformation when the samples may change their shape between two states during the heating and the cooling [1–4,7]. Therefore the martensitic transformation exhibited by the MSMAs might be induced by stress. Although the largest MFIS values (up to 10%) have been re- ported for Ni–Mn–Ga single crystals, the MFIS in polycrystalline Ni–Mn–Ga initially exhibiting quite small MFIS (<0.01%) can be en- hanced to 2–9%. In polycrystalline Ni–Mn–Ga MFIS depends on their structure (grain size, size distribution) [7,8]. Lower MFIS has been attributed to the sample’s structure consisting of small grains. In this case grain boundaries effectively suppress twin boundary motion [2,3]. Consequently larger MFIS (up to 9%) has been observed in polycrystalline samples with high level of poros- ity, with reduced incompatibilities between grains [8]. The miniaturization of MSMA devices based on small-size MSMA particles, wires, ribbons, films, bi- and multilayers, pillars is quite important for future technological applications [1]. Therefore fabri- cation of the Heusler-type thin wires from brittle NiMnGa alloy might be interesting from the point of view of application of MSMAs. It is worth mentioning, that studies of glass-coated microwires produced by the Taylor–Ulitovski technique allowing the fabrica- tion of a few km long metallic microwires coated by glass with metallic nucleus diameters ranging from 1 to 30 lm and the thick- ness of the insulating glass coating between 0.5 and 20 lm at- tracted certain interest in the field of applied magnetism. This method allows a high enough quenching rate to be achieved, 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.083 ⇑ Corresponding author at: Dpto. Fisica de Materiales, Fac. Quimicas, UPV/EHU, 20018 San Sebastian, Spain. Tel.: +34 943018611; fax: +34 943017130. E-mail address: arkadi.joukov@ehu.es (A. Zhukov). Journal of Alloys and Compounds 575 (2013) 73–79 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom