Quantum phase transitions in spiral magnets without an inversion center

It is shown that the $T$–$x$ magnetic phase diagram of ${\rm Mn}_{1-x}{\rm Fe}_x{\rm Si}$ substitutional solid solutions exhibits two quantum critical points: $x^*\sim 0.11$, which corresponds to the disappearance of long-range spiral magnetic order, and $x_{\rm c}\sim 0.24$, which marks the suppression of the phase with short-range magnetic order. It is established that the microscopic reason for the observed complicated quantum critical behavior is the evolution of the Fermi surface structure driven by an increase in iron concentration.