Colossal low-field negative magnetoresistance in CaAl${}_2$Si${}_2$-type diluted magnetic semiconductors (Ba,K)(Cd,Mn)${}_2$As${}_2$

We report the magnetic and magnetotransport properties of the layered CaAl${}_2$Si${}_2$-type diluted magnetic semiconductor (Ba${}_{1-x}$K${}_x$)(Cd${}_{1-y}$Mn${}_y$)${}_2$As${}_2$ over a broad Mn (spin) substitution range of $0.05\le y\le 0.5$. K substitution introduces hole carriers, whereas Mn provides local moments, resulting in bulk ferromagnetism with Curie temperatures up to $\sim 17$ K. Intrinsic magnetic ordering is further supported by an anomalous Hall contribution and a specific-heat anomaly near $T_{\mathrm{C}}$. A key performance feature is a colossal negative magnetoresistance: for heavily Mn-doped compositions ($y\ge 0.3$), $\mathrm{M}\mathrm{R}=[\rho (H)-\rho (0)]\mathrm{/}\rho (0)$ reaches approximately $-100\mathrm{\%}$ at 2 K and nearly saturates at a relatively low magnetic field of $\sim 0.35\mathrm{T}$. The combination of soft ferromagnetism, strong spin-charge coupling, and low-field MR saturation highlights (Ba,K)(Cd,Mn)${}_2$As${}_2$ as a promising bulk platform for low-temperature magnetoresistive functionalities.