Effect of a non-magnetic atom in double-exchange ferromagnets

Using the exact diagonalization technique for a finite-size cluster, we have studied the effect of a non-magnetic atom on the magnetism of the double-exchange ferromagnet constituted of the Hubbard model and localized spin system. The non-magnetic atom raises the critical correlation energy U_c for the full spin polarization, but spreads a partial spin polarization region below U_c. This is caused by the effect that a double occupation of electrons, realized at the non-magnetic atom, reduces the total spin but stabilizes the ferromagnetic phase with the help of kinetic-energy gain of the conduction electrons. Based on the present results, we discuss the ferromagnetism of some Mn or Fe alloys containing non-magnetic elements such as B, C, N, Al, Si, etc.