Record increase in the Curie temperature up to room values in the noncentrosymmetric magnet Mn$_{1-x}$Rh$_x$Si

The magnetic properties of Mn$_{1-x}$Rh$_x$Si solid solutions with a noncentrosymmetric structure B20 synthesized at a pressure of $8$ GPa and temperatures of $1500$–$1770$ K have been studied in detail in a wide temperature range of $2$–$300$ K in magnetic fields up to $9$ T. An anomalous increase in the Curie temperature $T_C$ of compounds with $0.15\leq x\leq0.8$ by a factor of $8.4$–$11.5$ compared to the pure helical magnet MnSi has been found. It has been established that the Curie temperature $T_C$ increases with the rhodium content to $T_C(x=0.15)=(244\pm4)\,$K, $T_C(x=0.4)=(299\pm5)\,$K, and $T_C(x=0.8)=(334\pm6)\,$K. Uniquely high magnetic transition temperatures up to room values occur in the disordered ferromagnetic Griffiths phase and may be due to the spin-fluctuation mechanism of enhancement of the magnetic interaction.