ARTICLE Copyright © 2020 by American Scientific Publishers All rights reserved. Printed in the United States of America Science of Advanced Materials Vol. 12, pp. 391–397, 2020 www.aspbs.com/sam Tailoring the Magnetic Properties and Magnetocaloric Effect in Double Perovskites Sr 2 FeMo 1−x Nb x O 6 Imad Hussain 1 , S. N. Khan 2 , Tentu Nageswara Rao 1 , Riyaz Uddin 1 , Jong Woo Kim 3 , and Bon Heun Koo 1, * 1 School of Materials Science and Engineering, Changwon National University, Changwon, Gyeongnam, 51140, Republic of Korea 2 Department of Physics, Abdul Wali Khan University Mardan, 23200, Pakistan 3 Powder & Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam, 51508, Republic of Korea ABSTRACT The crystal structure, magnetic and magnetocaloric properties of the Sr 2 FeMo 1−x Nb x O 6 (0 ≤ x ≤ 0.3) samples prepared by solid state reaction method were investigated using X-ray diffraction (XRD) and magnetic mea- surements. The room temperature XRD profiles obtained for all the samples revealed the formation of the double perovskite tetragonal structure with I4/mmm symmetry. Maximum values of spontaneous magnetization (17.6 emu/g at 150 K) and Curie temperature, T C (380 K) were observed in the Sr 2 FeMo 0.9 Nb 0.1 O 6 sample indicating that low Niobium (Nb) substitution (x = 0.1) at the Mo site in the host material resulted in higher magnetization and T C . Lower values of magnetization and T C were recorded in the samples with higher Nb concentration (x = 0.2, 0.3) that was attributed to the decrease in orbital hybridization and increase in anti- site disorder resulting from heavy doping. A second order of the magnetic phase transition in each sample was confirmed by the magnetization measurements and Arrott plots. The maximum magnetic entropy change and relative cooling power (RCP) were enhanced in lowest Nb doped sample (x = 0.1) suggesting that this compound can be used in magnetic refrigeration technology. KEYWORDS: Double Perovskite, Magnetization, Magnetocaloric Effect, Magnetic Refrigeration, Relative Cooling Power. 1. INTRODUCTION For the last few decades, refrigeration technology has attracted enormous attention from the scientific research point of view. Most cooling devices are currently based on the conventional vapor compression refrigeration systems. These systems use hazardous gases such as chlorofluoro- carbons (CFC), hydrofluorocarbons (HFC) and hydrochlo- rofluorocarbons (HCFC) that cause the depletion of the Ozone layer, climate change and other irrecoverable dam- ages to the environment. 1–3 In order to overcome the unde- sirable effects of the hazardous gases, it is essential to establish a different refrigeration technology that is cheap, energy efficient and most importantly “friendly” to the environment. Magnetic refrigeration presents an attractive refrigeration technology that carries the potential to replace the conventional vapor compression refrigeration. 4 5 ∗ Author to whom correspondence should be addressed. Email: bhkoo@changwon.ac.kr Received: 31 January 2019 Accepted: 9 April 2019 Magnetocaloric effect (MCE) refers to an intrinsic prop- erty of a magnetic material to heat up when an external magnetic field is applied and cool down when the field is removed. The phenomenon of MCE makes magnetic materials interesting for both basic research and possible technological applications. The idea in hand is to attain high cooling efficiency in these materials while manipu- lating the external magnetic field. Most magnetic mate- rials, however, yield high cooling efficiency only under high applied magnetic field (H ≥ 5 T). 6 This value of the applied magnetic field cannot be easily achieved or main- tained in household refrigeration appliances. For practical applications though it is highly desired to obtain magnetocaloric materials that can produce high cooling efficiency under easily attainable applied mag- netic field values and over a larger range of tempera- ture. Gadolinium (Gd) and some alloys containing Gd such as Gd 5 (Si 2 Ge 2  have been considered as the most effective MCE materials near room temperature 7 so far. Fe 2 P, La(Fe, Si) 8 9 13 and few other materials 10–15 were Sci. Adv. Mater. 2020, Vol. 12, No. 3 1947-2935/2020/12/391/007 doi:10.1166/sam.2020.3648 391