Formation of incommensurate long-range magnetic order in the Dzyaloshinskii-Moriya antiferromagnet Ba$_2$CuGe$_2$O$_7$ studied by neutron diffraction

TL;DR

This study uses neutron diffraction to investigate magnetic phase transitions in Ba$_2$CuGe$_2$O$_7$, revealing incommensurate antiferromagnetic and ferromagnetic orders and a ring-shaped distribution of magnetic intensity above the transition.

Contribution

It demonstrates the coexistence of incommensurate AFM and FM orders and suggests a Brazovskii-type soft fluctuation mechanism for the weak first-order transition.

Findings

Incommensurate AFM and FM Bragg peaks vanish at T_N

Magnetic intensity above T_N forms a ring in momentum space

Transition exhibits characteristics of a weak first-order transition

Abstract

Neutron diffraction on a triple-axis spectrometer and a small-angle neutron scattering instrument is used to study the magnetic phase transition in tetragonal Ba$_2$CuGe$_2$O$_7$ at zero magnetic field. In addition to the incommensurate cycloidal antiferromagnetic (AFM) long-range order, we establish that weak incommensurate ferromagnetism (FM) also arises below the transition temperature $T_N$ identified by sharp Bragg peaks close to the $\Gamma$ point. The intensities of both the incommensurate AFM and FM Bragg peaks vanish abruptly at $T_N$ indicative of a weak first-order transition. Above $T_N$, evidence is presented that the magnetic intensity within the tetragonal $(a,b)$ plane is distributed on a ring in momentum space whose radius is determined by the incommensurate wavevector of the cycloidal order. We speculate that the associated soft fluctuations are at the origin of the weak first-order transition in the spirit of a scenario proposed by Brazovskii.