Comparison of magnetic properties of the monocrystalline and microcrystalline Gd7Pd3 compound
|Monika Oboz , Ewa Talik , Antoni Winiarski , Grzegorz Ziółkowski|
University of Silesia, Institute of Physics, Uniwersytecka 4, Katowice 40-007, Poland
Materials based on gadolinium are prospective for applications in magnetic refrigeration cycles. Recently, magnetic materials with relatively high Curie temperatures (TC) are researched as potential candidates for magnetic refrigeration if they show large enough magnetocaloric effect (MCE). Several intermetallics based on rare earth and transition metals show interesting magnetocaloric properties [1-4]. However, their ordering temperatures are very often below room temperature. As a part of current research concerning the development of new magnets for magnetic refrigeration devices we characterized the monocrystalline and microcrystalline Gd7Pd3 compound. A single crystal of Gd7Pd3 of Th7Fe3-typewas grown by the Czochralski method from a levitating melt while microcrystalline Gd7Pd3 sample was obtained by rapid cooling of the melt (Figure). The samples were characterized by the following methods: X-ray diffraction, SQUID magnetometry, X-ray photoelectron spectroscopy and scanning electron microscopy (SEM/EDX). The SEM images showed grains size of the microcrystalline Gd7Pd3 sample. Magnetic and magnetocaloric properties were compared. The exchange interactions between Gd ions have a dominating mechanism being responsible for the formation of a magnetic ordering. It is realized by the RKKY interactions via the conduction electrons. From ac and dc magnetic susceptibility a transition to the ferromagnetic state has been found below TC = 334 K for mono- and microcrystalline Gd7Pd3. It seems that microcrystalline Gd7Pd3 exhibits similar behaviour to a axis of monocrystalline Gd7Pd3 while Gd7Pd3 in powder form shows similar behaviour as c axis of monocrystalline Gd7Pd3. For both compounds the saturation magnetic moment per Gd ion is enhanced in relation to the theoretical value. Such behavior can be well ascribed to the contribution of itinerant ferromagnetism induced in the d-bands by the f–d exchange interactions. The value of the magnetocaloric effect is the highest for monocrystalline Gd7Pd3 and then decreases for microcrystalline and powder form of Gd7Pd3. The calculated entropy changes ΔSm for the examined compounds amount to -6.7 J/Kkg, -5.3 J/Kkg and -2.8 J/Kkg for Gd7Pd3 single crystal and -5.4 J/Kkg, -4.2 J/Kkg and -2.07 J/Kkg for microcrystalline Gd7Pd3 at 7, 5 and 2 T respectively.
Fig. (a) Monocrystalline and (b) microcrystalline Gd7Pd3 compound.
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Presentation: Poster at 15th Summer School on Crystal Growth - ISSCG-15, by Monika Oboz
See On-line Journal of 15th Summer School on Crystal Growth - ISSCG-15
Submitted: 2013-04-08 10:14 Revised: 2013-04-08 10:20