Interplay of spin magnetism, orbital magnetism, and atomic structure in layered van der Waals ferromagnet VI3

Abstract

Recently discovered ferromagnetism of the layered van der Waals material ${\mathrm{VI}}_{3}$ attracts much research attention. Despite substantial progress, in the following important aspects no consensus has been reached: (i) a possible deviation of the easy axis from the normal to the ${\mathrm{VI}}_{3}$ layers, (ii) a possible inequivalence of the V atoms, (iii) the value of the V magnetic moments. The theoretical works differ in the conclusions on the conduction nature of the system, the value, and the role of the V orbital moments. To the best of our knowledge there are no theoretical works addressing issues (i) and (ii) and only one work dealing with the reduced value of the V moment. By combining the symmetry arguments with density functional theory (DFT) and $\mathrm{DFT}+U$ calculations we have shown that the antidimerization distortion of the crystal structure reported by Son et al. [Phys. Rev. B 99, 041402(R) (2019)] must lead to the deviation of the easy axis from the normal to the ${\mathrm{VI}}_{3}$ layers in close correlation with the experimental results. The antidimerization accompanied by breaking the inversion symmetry leads to the inequivalence of the V atoms. Our $\mathrm{DFT}+U$ calculations result in large value $\ensuremath{\sim}0.8\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$ of the V orbital moments of the V atoms leading to reduced total V moment in agreement with a number of experimental results and with the physical picture suggested by Yang et al. [Phys. Rev. B 101, 100402(R) (2020)]. We obtained large intra-atomic noncollinearity of the V spin and orbital moments revealing strong competition between effects coursed by the onsite electron correlation, spin-orbit coupling, and interatomic hybridization since pure intra-atomic effects lead to collinear spin and orbital moments. Our calculations confirm the experimental results of strong magnetoelastic coupling revealing itself in the strong dependence of the magnetic properties on the distortion of the atomic structure.