Field induced phase transitions in the helimagnet Ba2CuGe2O7

TL;DR

This paper theoretically investigates field-induced phase transitions in the 2D helimagnet Ba2CuGe2O7, revealing an intermediate phase during incommensurate-to-commensurate transition and explaining experimental observations with a nonlinear sigma model.

Contribution

It introduces a nonlinear sigma model analysis of Ba2CuGe2O7, predicting an intermediate phase and accounting for experimental magnetic structure rotations and weak ferromagnetism.

Findings

Identification of an intermediate phase during phase transition

Explanation of magnetic structure rotations with broken U(1) symmetry

Prediction of a weak ferromagnetic component in Dzyaloshinskii-Moriya anisotropy

Abstract

We present a theoretical study of the two-dimensional spiral antiferromagnet Ba2CuGe2O7 in the presence of an external magnetic field. We employ a suitable nonlinear sigma model to calculate the T=0 phase diagram and the associated low-energy spin dynamics for arbitrary canted fields, in general agreement with experiment. In particular, when the field is applied parallel to the c axis, a previously anticipated Dzyaloshinskii-type incommensurate-to-commensurate phase transition is actually mediated by an intermediate phase, in agreement with our earlier theoretical prediction confirmed by the recent observation of the so-called double-k structure. The sudden pi/2 rotations of the magnetic structures observed in experiment are accounted for by a weakly broken U(1) symmetry of our model. Finally, our analysis suggests a nonzero weak-ferromagnetic component in the underlying Dzyaloshinskii-Moriya anisotropy, which is important for quantitative agreement with experiment.