Curie temperatures of amorphous RFe2 alloys

Curie temperatures of the series of amorphous rare earth-iron alloys RFe2 (where R = Gd, Tb, Dy, Ho, Er, and Y) have been determined from Belov-Goryaga plots of the magnetization isotherms. The Curie temperature of GdFe2 is 5000K, and drops sharply (e.g. ErFe2 T/sub c/ = 1050K) as one proceeds to the right in the above series (decreasing R spin). The observed Curie temperatures exhibit a smooth variation with the DeGennes factor of the rare earth ion, with the zero spin limit YFe2 exhibiting no long-range order. This is in marked contrast to the analogous crystalline Laves phase compounds for which YFe2 has a 5350K Curie temperature and which show a much weaker dependence on the rare earth spin. These results imply a more significant effect of the structural disorder on the direct Fe--Fe exchange versus the RKKY Fe--R and R--R couplings. A molecular field model describing these interactions has been used to calculate the expected Curie temperature for both the amorphous and crystalline series. Overall agreement with the observed T/sub c/'s was less satisfactory for the amorphous than for the crystalline materials