The square-lattice spiral magnet Ba_2CuGe_2O_7 in an in-plane magnetic field

TL;DR

This study investigates the magnetic structure of Ba_2CuGe_2O_7 under in-plane magnetic fields, revealing field-dependent spiral propagation vector rotation and reorientation, explained by a free energy model including exchange, Dzyaloshinskii-Moriya, Zeeman, and anisotropy effects.

Contribution

It provides a detailed neutron diffraction analysis of Ba_2CuGe_2O_7's magnetic behavior in in-plane fields and develops a theoretical model to describe the observed phenomena.

Findings

Propagation vector rotates with field orientation.

Spin rotation plane reorients with magnetic field.

Model accurately describes experimental behavior.

Abstract

The magnetic structure of Ba_2CuGe_2O_7 is investigated by neutron diffraction in magnetic fields applied along several directions in the $(a,b)$ plane of the crystal. In relatively weak fields, $H\lesssim 0.5$~T, the propagation vector of the spin-spiral rotates to form a finite angle with the field direction. This angle depends on the orientation of $H$ itself. The rotation of the propagation vector is accompanied by a re-orientation of the plane of spin rotation in the spiral. The observed behaviour is well described by a continuous-limit form of a free energy functional that includes exchange and Dzyaloshinskii-Moriya interactions, as well as the Zeeman energy and an empirical anisotropy term.