Charge distribution and hyperfine interactions in the CuFeO$_2$ multiferroic according to $^{63,65}$Cu NMR data

For the first time, the CuFeO$_2$ single crystal has been studied by $^{63,65}$Cu nuclear magnetic resonance (NMR). The measurements have been carried out in the temperature range of $T= 100$–$350$ K in the magnetic field $H= 117$ kOe applied along different crystallographic directions. The components of the electric field gradient tensor and the hyperfine coupling constants are determined. It is shown that electrons of copper $4s$ and $3d$ orbitals are involved in the spin polarization transfer Fe$\to$Cu. The occupancies of these orbitals are estimated.