Magnetic studies of (Gd_1−xM_x)₅Si₄ alloys (M=La or Y)

In 1967, Holtzberg, Gambino, and McGuire [J. Phys. Chem. Solids 28, 2283 (1967)] observed that intermetallic Gd₅Si₄ exhibits T_C = 336 K. This is remarkable in that the T_C of Gd (∼291 K) is increased by chemical union with a nontransition metal, which is rare, if not unique. In the present study, magnetization‐temperature behavior of Gd₅Si₄ and (Gd_1−xM_x)₅Si₄ systems with M=La or Y were studied and compared with the Brillouin function (BF) to see if molecular‐field theory is obeyed. Demagnetization near T_C is much sharper for the alloys than expected from the BF. As examples, at T/T_C = 0.9, the BF for Gd₅Si₄ gives M/M₀ = 0.38, whereas the observed values range from 0.99 [for (Gd_0.8Y_0.2)₅Si₄] to 0.5 [for (Gd_0.6La_0.4)₅Si₄]. The factors responsible for these strong deviations from the BF and the high‐T_C values of these alloys are as yet unclear. X‐ray patterns for yttrium‐doped samples with x=0.1–0.5 and La with x=0.05 and 0.1 appear identical to the pattern for Gd₅Si₄, indicating no structural change. For La substitution of x=0.2 and above, there is a change from orthorhombic to tetragonal structure. All of the compounds show a decrease in Curie temperature and magnetic moment as Gd is decreased, as expected because of the replacement of Gd by a nonmagnetic species.