Magnetotransport and electronic band structure of EuNi$_2$As$_2$ antiferromagnet
We investigated the magnetotransport properties of single-crystals of tetragonal van der Waals compound EuNi$_2$As$_2$, that orders antiferromagnetically below 14.6 K in an incommensurate helical structure. Metamagnetic transitions are revealed by the magnetization measured in the magnetic field applied transverse to the axis of the helix, and are clearly reflected in the magnetoresistance. Overall, the magnetoresistance is small, but shows complex changes with the temperature, the strength, and the angle of the applied magnetic field. In magnetically ordered state, magnetoresistance shows prominent anomalies related to the metamagnetic transitions. For temperatures above the N\'eel point the negative magnetoresistance can be modeled very well with de Gennes-Friedel mechanism of the spin-disorder-scattering reduction. Hall resistivity data indicate hole-dominated multi-band conductivity in antiferromagnetic state and single-band one above the N\'eel temperature, with carrier concentrations of the order of 10$^{22}$cm$^{-3}$. This metallic character of the compound seems to obscure the plausible topological contribution to the Hall resistivity. Our \textit{ab-initio} calculations of electronic band structure showed that the electronic structure changes very strongly upon magnetic ordering, but the density of states at the Fermi level differs by a factor smaller than two, in agreement with experimental Hall resistivity data. Meaningful changes in the density of states, magnetic moments, and screening length of Eu-4$f$ orbitals are discussed in terms of the effects of Hubbard corrections.