Quantum phase transitions in single-crystal Mn1-xFexSi and Mn1-xCoxSi: Crystal growth, magnetization, ac susceptibility, and specific heat

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Mn1-xFexSi and Mn1-xCoxSi for temperatures down to similar to 2 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)(Co) approximate to 0.084 and x(c)(Fe) approximate to 0.192, the properties of Mn1-xFexSi and Mn1-xCoxSi are essentially identical with x(1)/x(c) approximate to 0.78 and x(2)/x(c) approximate to 1.17. Taken together, our study identifies Mn1-xFexSi and Mn1-xCoxSi as model systems in which the influence of DM interactions on ferromagnetic quantum criticality may be studied.