Physical properties of $Cu_xMn_{1-x}Cr_2S_4$ solid solutions

Magnetic and electrical properties of solid solutions of the $Cu_xMn_{1-x}Cr_2S_4$ system have been studied for $x$ = 0; 0.1; 0.2. It has been found that the magnetic moment of these compounds measured at 4.2 K increases with $x$ and corresponds to a valence distribution $\vec{Cu_x^{2+}}\vec{Mn_{1-x}^{2+}}Cr_2^{3+}S_4^{2-}$. The introduction of copper in the ferrimagnetic semiconductor $MnCr_2S_4$ leads to a significant increase in the Curie temperature (above 300 K) and retention of the semiconductor type conductivity. Thus, we have discovered new nondegenerate semiconductors with magnetic ordering temperatures above the room temperature, which is an important factor for their practical application. It is suggested that the growth of $Т_с$ is due to the existence of a strong s-d exchange interaction in compositions with $x$ = 0.1 and 0.2. The magnitude of this interaction is estimated at $AS\approx 0,4$ eV.