3036 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 Influence of the Magnetic Anisotropy on the Magnetic Entropy Change of Memory Shape Alloy José Carlos Vieira Leitão , Daniel Leandro Rocco , João Sequeira Amaral , Mario de Souza Reis Junior , Vitor Brás Sequeira Amaral , Rodrigo Pacher Fernandes , Nuno V. Martins ,and Pedro B. Tavares CICECO and Physics Department, Universidade de Aveiro, Aveiro 3810-193, Portugal Technische Universität Hamburg-Harburg, Institut für Keramische Hochleistungswerkstoffe, Hamburg D-21073 Chemistry Department and CQ-VR, Universidade de Trás-os-Montes e Alto Douro, Vila Real 5001-911, Portugal The magnetocaloric effect (MCE) is a good chance to create a more efficient refrigeration technique, both in energy and environmental friendliness. On the search for materials with large MCE (mainly characterized by a great magnetic entropy variation) in a wide tem- perature range around room temperature, this work focuses on the widely studied Heusler alloy and the influence of bismuth alloying in the stoichiometry in an attempt to make the magnetic and structural transition temperatures, and , come closer, and therefore create a large MCE. In addition, we discuss the influence of alloying processes on the magnetic anisotropy. Our results have in fact increased and decreased , but Bi substitution in Ga site (from 0 up to 5%) has been insufficient to merge those two transitions. The maximum magnetic entropy change was found to be 3.8 J/kg.K for the pure sample (without Bi) and 2.2 J/kg.K for sample 4 (maximum Bi concentration). Index Terms—Heusler alloy, magnetocaloric effect, memory shape, . I. INTRODUCTION T HE Heusler alloy is widely studied due to its particular magnetic properties and applications. We can cite, for instance, the memory shape properties and a first-order structural transition from martensitic to austenitic phase [1], [2], both of which are ferromagnetic. Freestanding Ni-Mn-Ga thin films have also been used into the first prototypes of microactu- ators, for optical applications [3]. The studied alloy has a critical temperature of about 375 K [1], [4], [6]–[8], and undergoes a first-order structural transition between tetrag- onal martensitic and cubic austenitic phase at a temperature at about 200 K [1], [2], [4], [6], [8]. It has a saturation magnetiza- tion of 4.17 (per formula unit) below the structural transition temperature [4], [8], [9] and 3.90 above [8]. Accompa- nying the magnetic transition at and the structural transition at , this alloy presents large magnetocaloric effect (MCE), with a considerable magnetic entropy change. The MCE is in simple terms the increase in temperature of a magnetic material due to the application of a magnetic field, as it can be observed in an adiabatic process. It can also be un- derstood in an isothermal process as a heat exchange between the material and a thermal reservoir, also due to a magnetic field change. From the quantitative point of view, the MCE is mea- sured through the isothermal magnetic entropy change or adiabatic temperature change ; both quantities are de- rived from thermodynamic relationships and, to obtain those, the measurement of magnetization and specific heat as a func- tion of temperature and magnetic field are needed. It is straightforward the idea to produce a thermo-magnetic cycle based on the isothermal and/or adiabatic processes, using Digital Object Identifier 10.1109/TMAG.2008.2002794 therefore the Brayton and Ericsson cycles, respectively; and in- deed, this idea began in the late 1920s, when cooling via adi- abatic demagnetization was proposed by Debye [10] and Gi- auque [11]. The process was after demonstrated by Giauque and MacDougall, in 1933, by which they reached 250 mK [12]. In this particular work, we will only consider the isothermal process and the to characterize the MCE, using therefore the following equation (that results from one of Maxwell’s re- lations): (1) As is given as the derivative of the magnetization in order of temperature, it will be maximum around large jumps of mag- netization, such as those close to and ; also it may be described as a function of the area between magnetic isotherms [13]. Application of this equation for the calculation of the MCE for first-order transitions is a matter under discussion [14]. The structural transition at has an entropy change of 5 J/kg.K [8], while the magnetic transition at has 1.29 J/kg.K [9]. However, this alloy has a high compositional sensitivity, and both transitions have been coupled into a single one, either by changing stoichiometry, or by substituting some elements in the alloy, resulting in a giant magnetocaloric effect (GMCE) [5], with a magnetic entropy change of about 20 J/kg.K [15]. In typical systems, the dominant entropy change is associated with the first-order phase transition [16], but by merging the two transitions, or at least bringing them into close proximity, the coincidence of a first-order magnetic transition and its attendant structural phase change with a second-order magnetic transition causes that both the magnetic and crystallographic sublattices are easily affected by the magnetic field when it is applied [17]. The MCE of materials undergoing coupled magneto-structural transformations arises from the added difference of the entropies 0018-9464/$25.00 © 2008 IEEE