THE CORRELATION OF THE MAGNETIC-PROPERTIES AND THE MAGNETOCALORIC EFFECT IN (GD1-XERX) NIAL ALLOYS

A study of the magnetic properties of several (Gd1-xErx)NiAl alloys (w here x = 0, 0.30, 0.40, 0.46, 0.50, 0.55, 0.60, 0.80, and 1.00) was un dertaken using both ac and dc magnetic and heat capacity measurements in an attempt to understand the table-like magnetocaloric effect previ ously observed in (Gd0.54Er0.46)NiAl. Results indicate the presence of both antiferromagnetic and ferromagnetic ordering processes in all al loys containing Gd. For ErNiAl, a metamagnetic transition from an anti ferromagnetic ground state was observed. Within each alloy, several ma gnetic transitions occur over a temperature range from 10 K [in (Gd0.2 0Er0.80)NiAl] up to 35 K (in GdNiAl), with all but the lowest temperat ure transition shifting to higher temperatures with increasing Gd cont ent. The change in magnetic entropy (Delta S-mag) induced by a change in field is observed to peak around the Neel temperature for ErNiAl wh ile gradually broadening and shifting toward the Curie temperature as the Gd content is increased. For Gd-rich alloys, a significant contrib ution to Delta S-mag is observed at both the low and high temperature transitions, resulting in a rounded, skewed caret-like temperature pro file of the magnetocaloric effect. Factors, which are believed to cont ribute to this effect, include the presence and temperature spacing of multiple zero-field transitions, which most likely result from compet ing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. This leads to broad peaks in the magnetic heat capacity that span several transition temperatures, providing for a substantia l Delta S-mag over an extended temperature range. This characteristic is desired for application to magnetic refrigeration, where certain th ermodynamic cycles (e. g., Ericsson cycle) require specific temperatur e profiles of the magnetocaloric effect in refrigerant materials (e. g ., a constant change in magnetic entropy as a function of temperature within the region of cooling). In general, the best materials are thos e which supply the maximum amount of cooling over the widest temperatu re range. (C) 1998 American Institute of Physics. [S0021-8979(98)03122 -3].