Cristallite size tuning on magnetocaloric effect of ball milled HoAl2

Abstract

The magnetocaloric effect (MCE) is the basis for alternative, environment-friendly new refrigeration technologies. It is an intrinsic property of all magnetic materials, occurring as a consequence of the coupling among external applied field and the magnetic sub-lattice. [1] In this study, we report on the synthesis, microstructural and magnetic characterization of mechanically milled HoAl2 compound. The samples were investigated by using x-ray diffraction, scanning electron microscopy (SEM) and magnetization measurements. HoAl2 polycrystalline sample were submitted to a milling process using high energy ball mill in order to produce metallic nanoparticles. The bulk sample show single phase with cubic MgCu2- type structure. The lattice parameter was not significant changed by milling process up to 20 h of milling time. On the other hand, the crystallite size is strongly reduced reaching 28 nm for 20 h sample. Magnetic measurement show ferromagnetic phase transition around 32 K for bulk and all milled samples. A contribution related with spin reorientation is also observed below 20K, which becomes more pronounced as milling time increase. The magnetization of saturation reduces from 8.4 (bulk sample) to 6.2 μB/u.f. (20 h sample), due to the weakening of long-range magnetic ordering caused by size effects. The mechanical milling reduces the maximum values of magnetic entropy change (−DSM max) from 30 J/kgK (bulk sample) to 15 J/kgK (20 h milled sample). A significant broadening in the −DSM peak profile is observed as the milling time increase, which enhance in the full width half maximum (δTFWHM) of entropy change peak. High values of δTFWHM are associated with a large operation temperature range of magnetic refrigerator prototypes making the milled HoAl2 interesting for magnetic refrigeration purposes.