A role of the 3$d$ electron subsystem in the evolution of band structure and magnetic and optical properties of ErNi$_{5-x}$Co$_{x}$ compounds ($x$ = 0 – 4)

The evolution of the electronic structure and optical and magnetic properties of ErNi$_{5-x}$Co$_{x}$ compounds ($x$ = 0 – 4), occurring upon substitution of nickel by cobalt atoms, has been investigated. Spin-polarization calculations of the band spectrum of these intermetallic compounds have been performed within the local spin density approximation with a correction for strong electron correlations in the 4$f$ shell of a rare-earth ion (method LSDA+U). The values of magnetic moments for erbium, nickel, and cobalt ions (located at different crystallographic sites) are obtained. The exchange-interaction parameters for the 3$d$ sublattice of transition metals are determined. The spectral properties of the compounds have been investigated in the wavelength range of 0.22–15 $\mu$m by optical ellipsometry. The experimental frequency dependences of the optical conductivity in the interband absorption region are compared with the corresponding characteristics calculated based on the density of electronic states.