Colossal low-field negative magnetoresistance in CaAl$_{2}$Si$_{2}$-type diluted magnetic semiconductors (Ba,K)(Cd,Mn)$_{2}$As$_{2}$

Abstract

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 ($y \ge 0.3$), $\mathrm{MR}=[ρ(H)-ρ(0)]/ρ(0)$ reaches approximately $-100\%$ 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.

Figures (4)

• Figure 1: Crystal structure and phase characterization of CaAl$_2$Si$_2$-type (Ba,K)(Cd,Mn)$_2$As$_2$.a Crystal structure of (Ba,K)(Cd,Mn)$_2$As$_2$ (space group $P\bar{3}m1$), composed of CdAs$_4$ tetrahedra and BaAs$_6$ octahedra and forming a layered Cd$_2$As$_2$ network. b Powder X-ray diffraction patterns of (Ba$_{1-x}$K$_x$)(Cd$_{0.9}$Mn$_{0.1}$)$_2$As$_2$ for $x$=0.01, 0.03, 0.04, 0.05, 0.07, and 0.10. All patterns can be indexed by the CaAl$_2$Si$_2$-type structure, indicating no structural transition across the measured K-doping range. c Lattice parameters refined from XRD as a function of composition. Both $a$ and $c$ expand with K substitution, whereas Mn substitution leads to a contraction (inset), supporting successful chemical doping.
• Figure 2: Ferromagnetism and Hall response in (Ba,K)(Cd,Mn)$_2$As$_2$ with decoupled charge and spin doping.a Temperature dependence of magnetization $M(T)$ measured under 500 Oe for the K-doping series (Ba$_{1-x}$K$_x$)(Cd$_{0.9}$Mn$_{0.1}$)$_2$As$_2$ with $x$=0.01--0.10, showing ferromagnetic ordering; inset: Curie--Weiss analysis in the paramagnetic regime. b Magnetic hysteresis loops $M(H)$ at 2 K for the Mn-doping series (Ba$_{0.96}$K$_{0.04}$)(Cd$_{1-y}$Mn$_y$)$_2$As$_2$ with $y$=0.05, 0.07, 0.10, and 0.20, demonstrating soft ferromagnetism with small coercive fields; inset: $T_{\mathrm{C}}$ evolution with Mn content. c Temperature dependence of magnetization for heavily Mn-doped compositions (representative high-$y$ samples), illustrating the reduction of $T_{\mathrm{C}}$ at higher Mn concentrations. d Hall resistivity $\rho_{xy}(H)$ of (Ba$_{0.96}$K$_{0.04}$)(Cd$_{0.9}$Mn$_{0.1}$)$_2$As$_2$ at 2 K, showing an anomalous Hall contribution at low fields (top-left inset); bottom-right inset: temperature dependence of hole concentration extracted from the normal Hall coefficient, indicating $p$-type carriers with $n_p \sim 10^{19}$ cm$^{-3}$.
• Figure 3: Specific-heat evidence for intrinsic magnetic ordering.a Zero-field specific heat $C(T)$ of (Ba$_{0.96}$K$_{0.04}$)(Cd$_{0.9}$Mn$_{0.1}$)$_2$As$_2$, showing an anomaly near $T_{\mathrm{C}}$ (arrow); inset: low-temperature fit using $C_{\mathrm{electron}}+C_{\mathrm{lattice}}=\gamma T+\beta_3T^3+\beta_5T^5$, yielding $\gamma$=14.94 mJ$\cdot$mol$^{-1}\cdot$K$^{-2}$. b Magnetic contribution $C_M(T)$ obtained after subtracting electronic and lattice terms, exhibiting a broad feature around $T_{\mathrm{C}}$; inset: susceptibility/magnetization data for the same composition, consistent with the transition temperature.
• Figure 4: Transport evolution and colossal low-field negative magnetoresistance in (Ba,K)(Cd,Mn)$_2$As$_2$.a Temperature dependence of resistivity $\rho(T)$ for the parent compound BaCd$_2$As$_2$ and representative K-doped sample(s); inset: Arrhenius plot and thermal-activation fit used to extract the activation energy. b Composition dependence of $\rho(T)$ for (Ba$_{1-x}$K$_x$)(Cd$_{0.9}$Mn$_{0.1}$)$_2$As$_2$ (varying $x$; open symbols) and (Ba$_{0.96}$K$_{0.04}$)(Cd$_{1-y}$Mn$_y$)$_2$As$_2$ (varying $y$; solid symbols), illustrating opposite effects of charge (K) doping and spin/disorder (Mn) doping on resistivity. c Magnetoresistance at 2 K for (Ba$_{0.96}$K$_{0.04}$)(Cd$_{1-y}$Mn$_y$)$_2$As$_2$ with increasing Mn content, defined as $\mathrm{MR}=[\rho(H)-\rho(0)]/\rho(0)$, highlighting the strong enhancement toward the heavily Mn-doped regime. d Field-dependent transport of the heavily Mn-doped composition (Ba$_{0.96}$K$_{0.04}$)(Cd$_{0.7}$Mn$_{0.3}$)$_2$As$_2$: $\rho(T)$ measured under various magnetic fields; inset: ${\rm MR}(H)$ at selected temperatures showing a colossal negative MR approaching $-100\%$ at 2 K with near-saturation at a low field of $\sim$0.35 T.