We have succeeded in preparing RCo4Si compounds (R = Y, Nd, Pr, Sm, Gd, Tb, Dy, Ho and Er). All samples are single phase with the CaCu5 structure as revealed by X-ray analysis. Si substitution drastically reduces the Curie temperature of the host RCo5 compounds. In particular, we have observed a spontaneous magnetization of 2.34 μB/f.u. at 5 K for SmCo4Si, which is significantly lower than the value of 3.0 μB/f.u. found for YCo4Si. Also the temperature dependence of the spontaneous magnetization of SmCo4Si shows ferrimagnetic characteristics. These results suggest an antiparallel coupling of the 3d and 4f moments in SmCo4Si, which has so far not been observed in light-rare-earth-transition-metal compounds. The Co magnetocrystalline anisotropy and the crystalline electric field interaction of the compounds have been studied. The results suggest the distinctive role of Si substitution in modifying the magnetic properties of the host RCo5 compounds.
