Magnetic structure and Dzyaloshinskii-Moriya interaction in the S=1/2 helical-honeycomb antiferromagnet α-Cu2V2O7

Magnetic properties of the S = 1/2 antiferromagnet alpha-Cu2V2O7 have been studied using magnetization, quantum Monte Carlo (QMC) simulations, and neutron diffraction. Magnetic susceptibility shows a broad peak at similar to 50 K followed by an abrupt increase indicative of a phase transition to a magnetically ordered state at T-N = 33.4(1) K. Above T-N, a fit to the Curie-Weiss law gives a Curie-Weiss temperature of Theta = -73(1) K suggesting the dominant antiferromagnetic coupling. The result of the QMC calculations on the helical-honeycomb spin network with two antiferromagnetic exchange interactions J(1) and J(2) provides a better fit to the susceptibility than the previously proposed spin-chain model. Two sets of the coupling parameters J(1) : J(2) = 1 : 0.45 with J(1) = 5.79(1) meV and 0.65 : 1 with J(2) = 6.31(1) meV yield equally good fits down to similar to T-N. Below T-N, weak ferromagnetism due to spin canting is observed. The canting is caused by the Dzyaloshinskii-Moriya interaction with an estimated bc-plane component vertical bar D-p vertical bar similar or equal to 0.14J(1). Neutron diffraction reveals that the S = 1/2 Cu2+ spins antiferromagnetically align in the F d'd'2 magnetic space group. The ordered moment of 0.93(9) mu(B) is predominantly along the crystallographic a axis.