Electronic structure and exchange interactions in $R$Ni$_{4}$Co ($R$ = Eu,Yb) compounds

The electronic structure and the exchange interactions in EuNi$_4$Co and YbNi$_4$Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + $U$ method); the variants of substitution of cobalt ion for nickel in the 3$d$ lattice in both types of crystallographic positions 2$c$ and 3$g$ are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3$g$ type. A Co ion in $R$Ni$_{4}$Co ($R$ = Eu,Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3$d$ metal sublattice.