Chiral criticality in doped Mn$_{1-y}$Fe$_y$Si compounds

TL;DR

This study investigates the critical spin fluctuations in doped Mn$_{1-y}$Fe$_y$Si compounds, revealing a sequence of crossovers from paramagnetic to chiral helimagnetic phases through detailed susceptibility and scattering measurements.

Contribution

It identifies and characterizes the temperature-driven crossovers to chiral magnetic states in doped Mn$_{1-y}$Fe$_y$Si, highlighting the role of Dzyaloshinskii-Moriya interactions.

Findings

Identification of two inflection points in ac-susceptibility indicating phase crossovers.

Observation of the transition from paramagnetic to partially chiral and highly chiral fluctuating states.

Correlation length analysis showing crossover conditions at specific wavevector ratios.

Abstract

The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous, yet well distinguishable crossovers: (i) from paramagnetic to partially chiral, (ii) from partially chiral to highly chiral fluctuating state. The crossover points are identified on the basis of combined analysis of the temperature dependence of ac-susceptibility and polarized SANS data. The whole transition is marked by two inflection point of the temperature dependence of ac-susceptibility: the upper one corresponds to the crossover to partially chiral state at $T^*$, where the inverse correlation length $\kappa \approx 2 k$, the lower one corresponds to the transition to the spin helix structure. The intermediate crossover to the highly chiral phase is observed at the inflection point $T_k$ of the first derivative of ac-susceptibility, where $\kappa \approx k$. The temperature crossovers to the highly chiral fluctuating state is associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.