Noncollinear magnetic structure and anisotropic magnetoelastic coupling in cobalt pyrovanadate Co2V2O7

Co2V2O7 was recently reported to exhibit remarkable magnetic-field-induced magnetization plateaus and ferroelectricity [R. Chen et al., Phys. Rev. B 98, 184404 (2018)], but its magnetic ground state remains ambiguous. Magnetometry measurements and time-of-flight neutron powder diffraction (NPD) have been employed to study the structural and magnetic properties of Co2V2O7, which includes two nonequivalent Co sites. Upon cooling below the Néel temperature TN=6.0(2) K, we observe magnetic Bragg peaks at 2 K in NPD, which indicates the formation of long-range magnetic order of Co2+ moments. After symmetry analysis and magnetic structure refinement, we demonstrate that Co2V2O7 possesses a complicated noncollinear magnetic ground state with Co moments mainly located in the b-c plane and forming a noncollinear spin-chain-like structure along the c-axis. The ab initio calculations demonstrate that the noncollinear magnetic structure is more stable than various ferromagnetic states at low temperature. The noncollinear magnetic structure with a canted ↑↑↓↓ spin configuration is considered to be the origin of magnetoelectric coupling in Co2V2O7 because the inequivalent exchange striction induced by the spin-exchange interaction between the neighboring spins could be the driving force of ferroelectricity. It is also found that the deviation of lattice parameters a and b is opposite below TN, while the lattice parameter c and β stay almost constant below TN, evidencing the anisotropic magnetoelastic coupling in Co2V2O7.