Magnetic transitions and thermomagnetic properties of GdCu 6 M.K. Chattopadhyay, P. Arora, P. Mondal, S.B. Roy à Magnetic and Superconducting Materials Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India article info Article history: Received 9 November 2009 Received in revised form 11 May 2010 Available online 1 June 2010 Keywords: Antiferromagnetic transition Specific heat Magnetic entropy Magnetocaloric effect Magnetic regenerator material abstract We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu 6 . These results clearly reveal the evidences of multiple magnetic transitions in GdCu 6 . In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low ð o10 kOeÞ applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic–antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu 6 is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu 6 as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature. & 2010 Elsevier B.V. All rights reserved. 1. Introduction In cryogenic engineering a large specific heat is the most important property of the materials used as heat generators [1]. As a result, magnetic materials having large specific heat associated with a magnetic transition in the temperature region 2–20 K are of considerable interest as magnetic regenerator materials for applications in low temperature cryocoolers [2,3]. These materials can help to overcome the lack of efficiency of the conventional regenerator materials like Pb below 20 K, which is one of the main obstacles for the cryocoolers to reach low temperatures. Finding a new potential regenerator material, however, still remains a matter of intuition and trial and error [2,3]. There exist certain guidelines for the desirable magnetic properties, which include high spin unquenched by crystal field- interactions and large resultant magnetic moment, so that the magnetic entropy and also the change in magnetic entropy by applied magnetic field are large [2]. The Gd-based compounds are promising here because of relatively high spin ðJ ¼ 7 2 Þ and lack of crystal-field quenching in Gd [2]. There exist now considerable amount of studies of magnetic properties and specific heat in various Gd-based dielectric [2] and intermetallic compounds [4]. Continuing the search of such Gd-based potential magnetic regenerator materials, we have studied the magnetic properties and specific heat of the intermetallic compound GdCu 6 in detail. With large concentration of Cu in the material, GdCu 6 would also be attractive from economy points of view. This compound forms in the orthorhombic CeCu 6 structure and orders antiferromagne- tically around 16 K [5]. This magnetic transition is clearly reflected in magnetization, resistivity and heat capacity measurements [5]. Further in the temperature region well below the ordering temperature there is an indication of a field induced metamag- netic transition [5]. However, since this early work [5] on a single crystal sample of GdCu 6 , to the best of our knowledge no further study on the magnetic and thermal properties of GdCu 6 have appeared in the literature. From large scale application points of view it is more likely that polycrystalline samples of GdCu 6 rather than single crystals will be of considerable interest. In this respect it is worth exploring the magnetic and thermal properties of polycrystalline samples of GdCu 6 . We shall present here a detailed study of field-temperature dependence of magnetization and specific heat in a polycrystalline sample of GdCu 6 . While confirming some of the results of the earlier study on the single crystal sample of GdCu 6 [5], the present study has revealed newer and interesting aspects of the magnetic and thermomagnetic properties of GdCu 6 . Application of magnetic field causes distinct changes in the magnetic response in the magnetically ordered as well as the paramagnetic regime. These changes are reflected both in the magnetization and specific heat measurements. The experimentally measured specific heat in and around the temperature region of antiferromagnetic transition is consider- ably higher than that of Pb in the same temperature regime [3] and comparable to various Gd-based compounds [4] and other potential magnetic regenerator materials like HoCu 2 and ErNi 0.9- Co 0.1 [3]. Furthermore we have estimated the isothermal magnetic field induced change in entropy (i.e. magnetocaloric Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.05.049 à Corresponding author. E-mail address: sbroy@rrcat.gov.in (S.B. Roy). Journal of Magnetism and Magnetic Materials 322 (2010) 3142–3147