Magnetic and Magnetocaloric Properties of High-Energy Ball-Milled Nanocrystalline CeMn 2 Ge 2 Compound
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INTRODUCTION
INTERMETALLIC RMn2X2 (R = rare-earth, X = Si or Ge) compounds have been comprehensively investigated owing to their exciting physical and magnetic properties.[1–7] CeMn2Ge2 exhibits unusual and attractive properties which crystallize in body-centered tetragonal (space group I4/mmm) with Ce, Mn, and Ge atoms located at 2a(0,0,0), 4d(0,½,¼), and 4e(0,0,z), respectively.[8] The CeMn2Ge2 compound exposes magnetic moments solely at Mn sites even at the lowest temperatures.[9] CeMn2Ge2 bulk compound has been studied by multiple methods,[10–15] due to its unexpected magnetic form; nevertheless, there are inadequate data in the literature about nanoformed CeMn2Ge2 regarding its magnetic or magnetocaloric properties. Decreasing mean crystalline size down to the nanometer scale may suggest both modification of the magnetic properties as far as those of corresponding bulk compositions are concerned and occurrence of new physical properties. High-energy ball milling (HEBM) is a commonly used technique for alloying and mechanical processing of materials.[16–19] In the present study, we focus on the effect of particle size reduction of the intermetallic CeMn2Ge2 compound obtained by the HEBM technique that has been presented and discussed by their magnetic and magnetocaloric properties.
MELIKE KAYA, Ph.D. Student, and ILKER DINCER and YALCIN ELERMAN, Professors, are with the Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler, Ankara, Turkey. Contact e-mail: [email protected] SELCUK AKTURK, Professor, is with the Department of Physics, Faculty of Science, Mug˘la Sıtkı Koc¸man University, 48000 Ko¨tekli, Turkey. Manuscript submitted January 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
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EXPERIMENTAL
The bulk CeMn2Ge2 was produced on a water-cooled arc-melting furnace under argon atmosphere. The purity of the elements was 99.9 pct Ce, 99.98 pct Mn, and 99.98 pct Ge. To compensate the mass loss of Mn, additional 2 pct Mn was added to the stoichiometric amount. To reach homogeneity, the polycrystalline bulk was remelted a few times. The bulk CeMn2Ge2 compound was grinded then ball-milled using planetary milling system (PULVERISETTE 7) for 5 and 10 hours in the hardened steel vial under ambient atmosphere. Before the milling process, bulk compound was ground using an agate mortar. The rotation speed kept constant at 500 rpm and the weight ratio of sample was around 12:1. To prevent cold welding and overheating (plastic deformation or agglomeration), milling system was stopped every half an hour then continue with the same process. At room temperature, to characterize the crystal structures, X-ray diffraction data were taken by XRD, RIGAKU D-MAX 2200 diffractometer equipped with Mo Ka radiation for bulk compound and nanopowders. The size and shape of the nanopowders were inspected by SEM (JEOL JSM 7600F at 20 kV) and TEM (JEOL JEM 2100F at 200kV). To find out the elemental composition of the nanopowders, JEOL JSM 7600F equipped with an Oxford EDS was used. For TEM
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