Anomalous spin relaxation and quantum criticality in Mn$_{1-x}$Fe$_{x}$Si solid solutions

We report results of the high frequency ($60$ GHz) electron spin resonance (ESR) study of the quantum critical metallic system Mn$_{1- x}$Fe$_{x}$Si. The ESR is observed for the first time in the concentration range $0 < x < 0.24$ at temperatures up to $50$ K. The application of the original experimental technique allowed carrying out line shape analysis and finding full set of spectroscopic parameters, including oscillating magnetization, line width and $g$ factor. The strongest effect of iron doping consists in influence on the ESR line width and spin relaxation is marked by both violation of the classical Korringa-type relaxation and scaling behavior. Additionally, the non-Fermi-liquid effects in the temperature dependence of the ESR line width, which may be quantitatively described in the theory of Wölfle and Abrahams, are observed at quantum critical points $x^{*} \sim 0.11$ and $x_{c} \sim 0.24$.