Magnetic structure of ground and field-induced ordered states of low-dimensionalα-CoV2O6: Experiment and theory

Abstract

In this work, we investigate the magnetic properties of the monoclinic $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$ by powder neutron diffraction measurements and ab initio calculations. An emphasis has been pointed towards the magnetic structure and the interaction between the Co ions leading to magnetic frustrations in this compound. Neutron diffraction experiments were carried out both in the ground state (zero magnetic field) and under applied external field of 2.5 and 5 T corresponding to the ferrimagnetic and ferromagnetic states, respectively. The antiferromagnetic ground state below 14 K corresponds to $k=(1,0,\frac{1}{2})$ magnetic propagation vector in $C1$ space group. The magnetic structure can be described by ferromagnetic interactions along the chains ($b$ axis) and antiferromagnetic coupling between the chains (along $a$ and $c$ axes). The ferrimagnetic structure implies a ninefold unit cell (3a, b, 3c) in which ferromagnetic chains follow an ``up-up-down'' sequence along the $a$ and $c$ axes. In the ferromagnetic state, the spin orientations remain unchanged while every chain lies ferromagnetically ordered. In all cases, the magnetic moments lie in the $ac$ plane, along the CoO${}_{6}$ octahedra axis, at an angle of 9.3${}^{\ensuremath{\circ}}$ with respect to the $c$ axis. The magnetic structure of $\ensuremath{\alpha}$-CoV${}_{2}$O${}_{6}$ resolved for all the ordered states is successfully related to a theoretical model. Ab initio calculations allowed us to (i) confirm the ground-state magnetic structure, (ii) calculate the interactions between the Co ions, (iii) explain the frustration leading to the stepped variation of the magnetization curves, (iv) calculate the orbital magnetic moment (1.5 ${\ensuremath{\mu}}_{B}$) on Co atoms, and (v) confirm the direction of the magnetic moments near the $c$ direction.