Materials based on gadolinium are prospective for applications in magnetic refrigeration cycles. Recently, magnetic materials with relatively high Curie temperatures (T_C) 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 T_C = 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 ΔS_m 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.

 References:

[1] V. K. Pecharsky, K. A. Gschneidner, Jr., Int. J. Refrig. 29, (2006) 1239.

[2] K. A. Gschneidner, Jr., V. K. Pecharsk, A. O. Tsokol, Rep. Prog. Phys. 68, (2005) 1479.

[3] D. T. Cam Thanh, E. Brück, O. Tegus, J. C. P. Klaasse, K. H. J. Buschow, J. Magn. Magn. Mater. 310, (2007) e1012.

[4] E. Brück E, J. Phys. D Appl. Phys. 38, (2005) R381.

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