Quantum phase transitions in single-crystalMn1−xFexSiandMn1−xCoxSi: Crystal growth, magnetization, ac susceptibility, and specific heat

Abstract

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of ${\text{Mn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}\text{Si}$ and ${\text{Mn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ for temperatures down to $\ensuremath{\sim}2\text{ }\text{K}$ and magnetic fields up to 9 T. The suppression of the helimagnetic transition temperature ${T}_{1}$ above a critical composition ${x}_{1}$, as seen in the magnetization, ac susceptibility, and specific heat, suggests the existence of a quantum phase transition at ${x}_{1}$. A Vollhardt invariance at a temperature ${T}_{2}>{T}_{1}$, which may be attributed to the Dzyaloshinsky-Moriya (DM) spin-orbit interactions, is also suppressed with increasing $x$ and vanishes above a concentration ${x}_{2}$, where ${x}_{2}>{x}_{1}$. When suppressing the effects of the DM interactions in an applied magnetic field, the magnetization for sufficiently large fields shares the signatures expected of an underlying putative ferromagnetic quantum critical point for a critical concentration ${x}_{c}$, where ${x}_{1}<{x}_{c}<{x}_{2}$. As a function of normalized concentration $x/{x}_{c}$, where ${x}_{c}^{\text{Co}}\ensuremath{\approx}0.084$ and ${x}_{c}^{\text{Fe}}\ensuremath{\approx}0.192$, the properties of ${\text{Mn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}\text{Si}$ and ${\text{Mn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ are essentially identical with ${x}_{1}/{x}_{c}\ensuremath{\approx}0.78$ and ${x}_{2}/{x}_{c}\ensuremath{\approx}1.17$. Taken together, our study identifies ${\text{Mn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}\text{Si}$ and ${\text{Mn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ as model systems in which the influence of DM interactions on ferromagnetic quantum criticality may be studied.