Crystal structure, magnetocaloric effect and magnetovolume anomalies in nanostructured Pr_2Fe_(17)

摘要

Using high-energy ball milling, nanostructured Pr_2Fe_(17) powders can be obtained from their arc-melted bulk alloys. High-resolution X-ray and neutron powder diffraction experiments reveal that the Th_2Zn_(17)-type rhombohedral crystal structure is maintained, after milling for 10 h, with almost unchanged values for both crystalline lattice parameters (DELTA a; DELTA c < 0.05 percent) and vanishing mechanically induced microstrain (<0.1 percent). Although the mean crystalline size decreases down to 20 + - 3 nm, magnetovolume anomalies observed in pure Pr_2Fe_(17) are still present with a significant volume decrease on heating from 5 up to 320 K. After the milling, a significant increase in the magnetic anisotropy, due to the drastic reduction in crystalline size, is observed, while the value of the magnetic moment seems to be increased slightly (5 percent). In addition, the magnetocaloric effect of bulk and nanostructured Pr_2Fe_(17) has been investigated. The magnetic entropy change, |DELTA S_M|, decreases from 6.3 to 4.5 J kg~(-1) K~(-1) under an applied magnetic field mu_0 H=5T after the milling process. However, the width of the |DELTA S_M|(T) curve is substantially enlarged and hence the refrigerant capacity is enhanced. These findings make the iron-based nanostructured Pr_2Fe_(17) powders interesting for applications in magnetic refrigeration at around room temperature.