Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field

TL;DR

This study investigates the magnetic fluctuations and phase transition in MnSi under magnetic fields, revealing that precursor phenomena are not the driving force behind the first order transition, which persists up to 0.4 T.

Contribution

The paper provides experimental evidence that precursor chiral fluctuations do not cause the first order transition in MnSi under magnetic fields, challenging previous theoretical scenarios.

Findings

First order transition persists up to 0.4 T

Precursor fluctuations weaken and align with the field above 0.2 T

No tricritical point observed in the phase diagram

Abstract

The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius $2\pi/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above $T_C$, both of general relevance to chiral magnetism, remain an open question.