Spin-flip-driven giant magnetotransport in A-type antiferromagnet $\mathrm{NaCr}{\mathrm{Te}}_2$

The value of angle-dependent magnetoresistance (MR) synergistically and simultaneously depends on the magnitudes of magnetoresistance (MR) and magnetocrystalline anisotropy energy (MAE). In a magnetic material, the concurrence of gigantic angle-dependent MR and MR signals is rather difficult due to weak spin-lattice coupling and small MAE. Here we report the considerable magnetotransport effect in layered A-type antiferromagnetic (AFM) $\mathrm{NaCr}{\mathrm{Te}}_2$ by realigning the spin configurations. Above 3 (8) T, the antiparallel spins of adjacent layers experience a spin-flip transition to a parallel alignment along the $c$ axis ($ab$ plane). Theoretical calculations reveal that the energy band gap narrows from 0.39 to 0.11 eV, accompanying a transition from semiconductor (high-$R$ state) and half semiconductor (low-$R$ state), respectively. Thus, a gigantic negative MR ratio of −90% is obtained at 10 K. More importantly, the decrement of $R$ along $H\parallel c$ is far quicker than that of $H\parallel ab$ because the MAE of the Ising-like ferromagnetic (FM) state is $1017\mu \mathrm{eV}/{\mathrm{Cr}}^{3+}$ lower than that of XY-like FM. The distinct trends result in the angle-dependent MR ratio of 732% at 10 K. These findings unravel the intrinsic origin of magnetoresistance in $\mathrm{NaCr}{\mathrm{Te}}_2$ and will stimulate us to explore the $H$-sensitive transport property in more AFM materials.

Figure 1

Spin-flipped transition of A-type antiferromagnet $\mathrm{NaCr}{\mathrm{Te}}_2$. (a) Spin-flip transition in an A-type layered antiferromagnet under $H$. By applying large enough $H$ along the easy axis and the hard axis, two kinds of FM states can be obtained. (b) Relative total energy of A-type AFM, Ising-like type FM and XY-like type FM. (c) Crystal structure of $\mathrm{NaCr}{\mathrm{Te}}_2$. Blue ball is Cr, green ball Te, and orange ball Na. Interlayer spacing is 7.44 Å. Right panel shows a single Cr layer. $J_1$, $J_2$, and $J_3$ represent the nearest neighbor, the next-nearest neighbor, and interlayer magnetic exchange constants, respectively. (d) Temperature dependence of magnetic susceptibility (χ) of $\mathrm{NaCr}{\mathrm{Te}}_2$ under $H\parallel c$ and $H\parallel ab$. [(e),(f)] Isothermal $M-H$ curves of $\mathrm{NaCr}{\mathrm{Te}}_2$ under $H\parallel c$ and $H\parallel ab$.

Figure 2

Electrical transport and magnetoresistance (MR) of $\mathrm{NaCr}{\mathrm{Te}}_2$ single crystal. [(a),(b)] $T$-dependent electrical resistivity ${\rho }_{ab}$ and ${\rho }_c$ under $H\parallel c$. With increasing $H$, the jump due to PM to AFM transition is gradually suppressed. [(c),(d)] Isothermal $\mathrm{M}{\mathrm{R}}_{ab}$ ($J\parallel ab$) of $\mathrm{NaCr}{\mathrm{Te}}_2$ under $H\parallel c$ and $H\parallel ab$. [(e),(f)] Magnetic phase diagrams in $\mathrm{NaCr}{\mathrm{Te}}_2$ under $H\parallel c$ and $H\parallel ab$. The data are taken from the onset $H$ ($H_{\mathrm{sf}1}$, green) of $R$ drop and onset $H$ ($H_{\mathrm{sf}1}^{\prime }$, blue) of low-$R$ state. $H$-induced FM transition temperature $T_{\mathrm{c}}^{\prime }$ (orange) is determined from the derivative of the $M-T$ curves at high $H$.

Figure 3

Anisotropic magnetoresistances of $\mathrm{NaCr}{\mathrm{Te}}_2$ single crystal. (a) $R_{ab}$ of $\mathrm{NaCr}{\mathrm{Te}}_2$ single crystal (sample 1) under various $H$ at 90 and 10 K. (b) $R_c$ of $\mathrm{NaCr}{\mathrm{Te}}_2$ single crystal (sample #) under various $H$ at 90 and 10 K. The data at selected $H$ are presented for clarity. Inset arrows show schematic spin alignments under given $H$ and angle. [(c),(d)] Giant angle-dependent MR ratios are observed in $R_{ab}$ and $R_c$. Insets are the schematic geometry of the relative orientation between current ($J$), magnetic field ($H$), and crystal axis of the slab sample. All measurements were carried out by rotating the angle θ from 0 ° to 360 ° under a specific $H$.

Figure 4

Electronic structure and origin of giant angle-dependent MR. [(a),(b)] Band structure and PDOS of the A-type AFM and Ising-like FM $\mathrm{NaCr}{\mathrm{Te}}_2$ calculated by HSE06+$U$+SOC. [(c),(d)] Schematic electronic structure of A-type AFM semiconductor and FM half semiconductor under different $H$. (e) MR under $H\parallel c$ and $H\parallel ab$ at 10 K measured from 0 to 14 T. (f) MAE per ${\mathrm{Cr}}^{3+}$ of FM $\mathrm{NaCr}{\mathrm{Te}}_2$. In the inset, $\phi =0^{\circ }$ and 90 ° represent the Ising-like type and XY-like type FM configurations, respectively.