# Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of Fe$_{1-x}$Co$_x$Si

## TL;DR

This study investigates the structural and dynamical properties of the helimagnetic transition in Fe$_{1-x}$Co$_x$Si, revealing a gradual transition with complex dynamics that challenge the universality of helimagnetic transitions in chiral magnets.

## Contribution

It provides the first detailed neutron scattering analysis of Fe$_{1-x}$Co$_x$Si, showing its transition differs from MnSi and questioning the universality of helimagnetic transitions.

## Key findings

- Transition in Fe$_{1-x}$Co$_x$Si is gradual with coexistence of long- and short-range order.
- Dynamics involve long relaxation times with stretched exponential behavior.
- Results challenge the universality of helimagnetic transition in chiral magnets.

## Abstract

We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of the helimagnetic transition in Fe$_{1-x}$Co$_x$Si with $x$ = 0.30. In contrast to the sharp transition observed in the archetype chiral magnet MnSi, the transition in Fe$_{1-x}$Co$_x$Si is gradual and long-range helimagnetic ordering coexists with short-range correlations over a wide temperature range. The dynamics are more complex than in MnSi and involve long relaxation times with a stretched exponential relaxation which persists even under magnetic field. These results in conjunction with an analysis of the hierarchy of the relevant length scales show that the helimagnetic transition in Fe$_{1-x}$Co$_x$Si differs substantially from the transition in MnSi and question the validity of a universal approach to the helimagnetic transition in chiral magnets.