Large negative magnetoresistance in the antiferromagnet ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. A high perpendicular magnetic field eventually suppresses the insulating behavior and allows ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ to reenter a metallic state. This effect is seemingly unrelated to any field-induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multiorbital electronic structure of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$.

Figure 1

(a) Crystal structure and G-type antiferromagnetic order of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$. (b) Band structure and density of states of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ with and without spin-orbit coupling.

Figure 2

(a) Temperature dependence of ${\rho }_a$ at $0\mathrm{T}$ plotted as a function of $T^{-1/4}$. The dashed line shows the fitting using a variable range hopping rule for the data at low temperatures. (b) Temperature dependencies of the in-plane electrical resistivity (${\rho }_a$) of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ measured under different strengths of perpendicular magnetic fields ($H_a$). The electric current $\mathbf{j}$ was applied along the $a$ axis (inset). (c) Longitudinal MR ($\theta =0^{\circ }$) observed at various $T$'s. (d) No hysteresis was observed in scanning $H_{ab}$ both in increasing and decreasing modes.

Figure 3

(a) The setting for the measurements of angle-resolved MR with current in plane in $\theta$ rotation of $H$. (b) The absolute values of MR as function of $\theta$ in polar plot. (c), (d) Same as (a) and (b) in $\varphi$ rotation of $H$. (e) The magnetic field dependence of MR for various $\theta \mathrm{s}$. (f) MR scaled by in-plane magnetic field ${\mu }_{0}Hcos\theta$.

Figure 4

(a) Temperature ($T$) and magnetic field ($H$) dependence of electrical resistivity (${\rho }_c$) of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ when the electric current $\mathbf{j}$ is parallel to the $\widehat{c}$ axis (out-of-plane geometry). (b) MR observed at various $T$'s. Negative MR becomes larger with a decrement in $T$. The positive MR is only observed at $T$ lower than $T^{*}$ under weak ${\mu }_{0}H$.

Figure 5

(a) The setting for the measurements of angle-resolved MR with current out of plane in $\theta$ rotation of $H$. (b) The absolute values of MR as function of $\theta$ in polar plot. (c), (d) Same as (a) and (b) in $\varphi$ rotation of $H$. (e) The magnetic field dependencies of MR for various $\theta$. (f) MR scaled by in-plane magnetic field ${\mu }_{0}Hcos\theta$.

Figure 6

$H_c$ dependencies of (a) ${\rho }_{yx}$ (current in plane) and (b) ${\rho }_{xz}$ (current out of plane) measured at various $T$'s under $\mathbf{H}\parallel \widehat{c}$ and $\widehat{a}$, respectively.

Figure 7

Temperature dependence of (a) Hall coefficient calculated from the slope near the zero-field region of Hall resistance. (b) Hall carrier density, (c) mobility, and (d) effective conductivity calculated from the fitting of Hall resistivity utilizing a single carrier model at $T>20\mathrm{K}$ and a two-carrier mode at $T\le 20\mathrm{K}$, respectively. We note that the reliability of the data points at $T\le 20\mathrm{K}$ can be questionable due to the validity of the two-carrier-type model in the context of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ (see text).

Figure 8

(a) Temperature dependence of specific heat ($C_P$) of BMBi. Analyses were made by employing the Einstein and the Debye models. Inset is the $C_p/T$ vs $T^2$ plot (red circle) and its theoretical fitting (black line) using the equation of $\gamma T+\beta T^3$. (b) Anisotropic temperature dependence of magnetic susceptibility ($\chi$) evaluated at ${\mu }_{0}H=0.1\mathrm{T}$ applied along the $\widehat{a}$ axis and the $\widehat{c}$ axis. (c) Magnetic field dependence of magnetic moment ($M$) per Mn ion.

Figure 9

The crystalline x-ray diffraction spectrum along the $\widehat{c}$ axis. Inset is a photo of single crystals grown in alumina crucible.