History dependence of the magnetic properties of single-crystal Fe$_{1-x}$Co$_{x}$Si

TL;DR

This study investigates the magnetic phase diagrams of Fe$_{1-x}$Co$_{x}$Si single crystals, revealing history-dependent skyrmion phases and slow dynamics, highlighting its potential for topological magnetic research.

Contribution

It provides detailed magnetic phase diagrams for Fe$_{1-x}$Co$_{x}$Si, showing history dependence and metastability of skyrmion phases, which was not previously characterized.

Findings

Skyrmion lattice phase exists just below the helimagnetic transition.

Phase diagrams depend on cooling history and magnetic field.

Metastable skyrmion states persist at low temperatures.

Abstract

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x \leq 0.50$. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to lowest temperatures. Taken together with the large variation of the transition temperatures, transition fields, and the helix wavelength as a function of composition, this hysteresis identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.