Fluctuation-induced first-order phase transition in Dzyaloshinskii-Moriya helimagnets

TL;DR

This paper demonstrates that in MnSi, a Dzyaloshinskii-Moriya helimagnet, the phase transition from paramagnetic to helimagnetic state is a fluctuation-induced first-order transition, confirmed through neutron scattering and thermodynamics, aligning with Brazovskii's theory.

Contribution

The study provides experimental evidence for fluctuation-induced first-order phase transition in a DM helimagnet, clarifying long-standing puzzles about MnSi's phase transition and extending the understanding to similar materials.

Findings

Confirmation of fluctuation-induced first-order transition in MnSi

Agreement between experimental data and Brazovskii's theoretical model

Implications for other DM helimagnets with weak anisotropies

Abstract

Two centuries of research on phase transitions have repeatedly highlighted the importance of critical fluctuations that abound in the vicinity of a critical point. They are at the origin of scaling laws obeyed by thermodynamic observables close to second-order phase transitions resulting in the concept of universality classes, that is of paramount importance for the study of organizational principles of matter. Strikingly, in case such soft fluctuations are too abundant they may alter the nature of the phase transition profoundly; the system might evade the critical state altogether by undergoing a discontinuous first-order transition into the ordered phase. Fluctuation-induced first-order transitions have been discussed broadly and are germane for superconductors, liquid crystals, or phase transitions in the early universe, but clear experimental confirmations remain scarce. Our results from neutron scattering and thermodynamics on the model Dzyaloshinskii-Moriya (DM) helimagnet (HM) MnSi show that such a fluctuation-induced first-order transition is realized between its paramagnetic and HM state with remarkable agreement between experiment and a theory put forward by Brazovskii. While our study clarifies the nature of the HM phase transition in MnSi that has puzzled scientists for several decades, more importantly, our conclusions entirely based on symmetry arguments are also relevant for other DM-HMs with only weak cubic magnetic anisotropies. This is in particular noteworthy in light of a wide range of recent discoveries that show that DM helimagnetism is at the heart of problems such as topological magnetic order, multiferroics, and spintronics.