Magnetocaloric effect in La 0.67 Sr 0.33 MnO 3 manganite above room temperature A. Rostamnejadi a,b,n , M. Venkatesan a , P. Kameli b , H. Salamati b , J.M.D. Coey a a CRANN and School of Physics, Trinity College, Dublin 2, Ireland b Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 15 February 2011 Received in revised form 18 March 2011 Available online 5 April 2011 Keywords: Manganite Magnetocaloric effect Magnetic entropy Magnetic refrigeration Phase transition abstract The La 0.67 Sr 0.33 MnO 3 composition prepared by sol–gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic–ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Dm0H ¼5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic–ferromagnetic transition. The obtained results show that La 0.67 Sr 0.33 MnO 3 could be considered as a potential candidate for magnetic refrigeration applications above room temperature. & 2011 Elsevier B.V. All rights reserved. 1. Introduction The entropy of a magnetic material changes when it is subjected to an applied magnetic field. Under adiabatic condi- tions, the lattice and magnetic parts of the total entropy change by the same amount but with opposite sign [1–3]. In this process there is a temperature change, which is known as the magneto- caloric effect (MCE) [1–3]. Magnetic refrigeration (MR) is based on MCE and has potential as a future cooling technology. Near room temperature (RT) MR is a promising alternative to conventional gas compression refrigeration due to its unique advantages such as high efficiency and minimal environmental impact [1–4]. While refrigeration involves about 15% of the worldwide energy consumption, it has been estimated that MR has the potential to reduce the energy consumption by 20–30% over the conventional vapor compression technology [6]. Therefore, there is an ongoing quest to find suitable materials that have a large enough magnetic entropy change at moderate magnetic fields near room tempera- ture [1–6]. Since the discovery of the colossal magnetoresistance effect, the physical properties of doped perovskite manganites have been studied extensively in view of their complex physics and potential applications [7–9]. Manganites are good candidate materials for MR near RT [1–3,5], where Gadolinium (Gd) is a leading material [1–5]. Although the change of magnetic entropy near RT in some manganites is much higher than in Gd [3] their specific heat is larger so their adiabatic temperature change is relatively smaller. This problem can be eased at higher temperatures, because the adiabatic temperature change is proportional to temperature [1–3]. La 1 x Sr x MnO 3 is one of the most attractive manganite families because it has the highest Curie temperature of 370 K at optimum doping x ¼ 0.33 [7–9]. In this work, we use dc magne- tization measurements to study the MCE in a La 0.67 Sr 0.33 MnO 3 (LSMO) powder sample. Around the Curie temperature, a large magnetic entropy change has been observed and the changes of magnetic entropy and adiabatic temperature at moderate mag- netic fields are quite significant. The results suggest that LSMO has potential for magnetic refrigeration above RT. 2. Experiments The powder samples of LSMO were prepared by the sol–gel method [10]. Stoichiometric amounts of La(NO 3 ) 3  6H 2 O, Mn(NO 3 ) 2  4H 2 O and Sr(NO 3 ) were dissolved in water and mixed with ethylene glycol and citric acid (molar ratio of 1:4:3 for cations: citric acid: ethylene glycol). The gel was dried and calcined at 500 1C for 5 h. The resultant powder was then annealed at 1000 1C for 5 h. Structural characterization of the sample was investigated by X-Ray diffraction (XRD using Philips X’Pert PRO X-ray diffractometer equipped with a Cu–K a X-ray source (l ¼ 1.5406 ˚ A). Magnetization measurements were per- formed as a function of field and temperature using a Quantum Design MPMS XL SQUID magnetometer. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.03.036 n Corresponding author at: Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran. Tel.: þ98 311 3912375; fax: þ98 311 3912376. E-mail address: ali@ph.iut.ac.ir (A. Rostamnejadi). Journal of Magnetism and Magnetic Materials 323 (2011) 2214–2218