Enhanced magnetocaloric effects in R 3(Ga 1− xFe x) 5O 12 (R=Gd, Dy, Ho; 0< x

Abstract

A series of R 3(Ga 1− x Fe x ) 5O 12 (R=Gd, Dy, Ho; 0< x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd 3(Ga 0.5Fe 0.5) 5O 12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473 K for 15 h, rather than at 1173 K for 15 h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (Δ S m) with respect to the basic paramagnetic garnet (i.e., x=0). In addition, with variation in “ x”, the optimal Δ S m was measured for the x=0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal Δ S m values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd 3(Ga 0.5Fe 0.5) 5O 12, despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.