Imprinting electrically switchable scalar spin chirality by anisotropic strain in a Kagome antiferromagnet

TL;DR

The paper demonstrates that anisotropic strain in Mn$_{3}$Sn thin films lowers the Kagome triangle symmetry and, together with an in-plane DM interaction, stabilizes a noncoplanar magnetic ground state with finite scalar spin chirality. This state produces a sizable Berry curvature and a large anomalous Hall effect in the Kagome plane at room temperature, previously absent in bulk Mn$_{3}$Sn. Furthermore, thermally assisted electrical switching enables multiple, non-volatile AHR memory states, controlled by bias fields and current pulses. The findings establish a route to imprint chiral spin textures in Kagome antiferromagnets via thin-film engineering, with potential implications for spintronics and neuromorphic computing.

Abstract

Topological chiral antiferromagnets, such as Mn$_{3}$Sn, are emerging as promising materials for next-generation spintronic devices due to their intrinsic transport properties linked to exotic magnetic configurations. Here, we demonstrate that anisotropic strain in Mn$_{3}$Sn thin films offers a novel approach to manipulate the magnetic ground state, unlocking new functionalities in this material. Anisotropic strain reduces the point group symmetry of the manganese (Mn) Kagome triangles from $C_{3v}$ to $C_{1}$, significantly altering the energy landscape of the magnetic states in Mn$_{3}$Sn. This symmetry reduction enables even a tiny in-plane Dzyaloshinskii-Moriya (DM) interaction to induce canting of the Mn spins out of the Kagome plane. The modified magnetic ground state introduces a finite scalar spin chirality and results in a significant Berry phase in momentum space. Consequently, a large anomalous Hall effect emerges in the Kagome plane at room temperature - an effect that is absent in the bulk material. Moreover, this two-fold degenerate magnetic state enables the creation of multiple-stable, non-volatile anomalous Hall resistance (AHR) memory states. These states are field-stable and can be controlled by thermal assisted current-induced magnetization switching requiring modest current densities and small bias fields, thereby offering a compelling new functionality in Mn$_{3}$Sn for spintronic applications.

Figures (5)

• Figure 1: Anisotropic strain in Mn$_{3}$Sn/Ta heterostructures. (a) Three-dimensional crystal structure of Mn$_{3}$Sn, which consists of bilayer (AB) stacking of Kagome planes. Top view of the pristine Kagome plane and Mn Kagome triangles with $C_{3v}$ point group symmetry. (b) (left) Top view of the Mn Kagome triangle with anisotropic strain with a reduced symmetry ($C_{1}$) and experimentally measured lattice parameters. Side view of the relaxed structure of Ta(110)/Mn$_{3}$Sn(0001) heterostructure considered in the density functional theory (DFT) calculations (middle) and a top view of the highlighted rectangular region (right), revealing a reduction of the $C_{3v}$ symmetry of the Kagome triangles to $C_{1}$ as well as lifting of the inversion symmetry of the pristine Mn$_{3}$Sn crystal structure. (c) Out-of-plane $\theta$-2$\theta$ XRD scan of Mn$_{3}$Sn/Ta heterostructures. Inset shows the pole figure plot of Mn$_{3}$Sn {$20\bar{2}1$} family of diffraction peaks showing a six-fold symmetry, establishing the epitaxial nature of Mn$_{3}$Sn. (d) RSM plots of ($0002$) and ($20\bar{2}1$) Bragg peaks. The in-plane lattice parameters are estimated by measuring the $q$ vectors of the {$20\bar{2}1$} family of Bragg peakssuppl.
• Figure 2: Anomalous Hall effect in Mn$_{3}$Sn/Ta heterostructures. Anomalous Hall conductivity at 300K of (a) Ta(11nm)/ Mn$_{3}$Sn(90nm)/ AlO$_{x}$(8nm) and (b) Ta(11nm)/ Mn$_{3}$Sn(90nm)/ Ta(11nm)/ AlO$_{x}$(8nm) thin film heterostructures synthesized on c-plane sapphire substrates. The arrows indicate the direction of magnetic field sweeps, which is applied perpendicular to the Kagome plane (along [$0001$]) and the current is applied along [$\bar{1}100$]. Insets show corresponding Hall conductivity as a function of magnetic field. Magnetization as a function of magnetic field at 300K for (c) Ta(11nm)/ Mn$_{3}$Sn(90nm)/ AlO$_{x}$(8nm) and (d) Ta(11nm)/ Mn$_{3}$Sn(90nm)/ Ta(11nm)/ AlO$_{x}$(8nm) thin film heterostructures with the magnetic field applied perpendicular to the Kagome plane. A linear diamagnetic background has been subtracted in both the cases.
• Figure 3: Non-coplanar spin structure in Mn$_{3}$Sn/Ta heterostructures. (a) Phase diagram of the magnetic ground state as a function of anisotropic strain and in-plane DM interaction. Scalar spin chirality($\chi$) of the magnetic ground state as a function of $\lambda$ and $D_{\parallel}/J$ is shown on the left (all parameters defined in the text). Phase diagram as a function of $D_{\parallel}/J$ for two specific $\lambda$ values viz. $\lambda$ = 0, and $\lambda$ = +39 are shown on the right. (b) Illustration of non-coplanar magnetic ground states (E-noncolpanar$_{2}$) having an out-of-plane magnetic moment of $M_{z}$$\approx$ 9.3 $m\mu B$/Mn corresponding to $\lambda$ = +39 and $\lambda$ = -39, as described in the text. Band structure obtained with (c) coplanar inverse triangular spin structure (E-coplanar$_{6}$, $\lambda$ = 0) and (d) non-coplanar (E-noncoplanar$_{2}$) spin configurations corresponding to $\lambda$ = +39 and $\lambda$ = -39, as shown in (b). The bottom panels show the spin-configurations and the color bar represents the Berry curvature $\Omega_{xy}$. (e) Calculated anomalous Hall conductivities (AHC)as a function of binding energy for $\lambda$ = 0, +39, and -39. Note that AHC is non-zero for $\lambda$$\neq$ 0, and is identically zero otherwise.
• Figure 4: Thermal assisted switching of anomalous Hall effect in Mn$_{3}$Sn. Switching of anomalous Hall resistance (a) under zero magnetic field and (b) under a finite magnetic field of $+0.2$ T in Ta(11nm)/Mn$_{3}$Sn(90nm)/Ta(11nm)/AlO$_{x}$(8nm) heterostructures. The initial and final experimentally measured Hall resistance (R$_{xy}$) before and after the application of a current pulse are shown in the middle with (a) a cyan and a green square (b) a yellow and a violet circle, respectively. The evolution of the Hall resistance with magnetic field is shown in red as a reference. The corresponding domain configurations before and after the application of electric pulse are shown schematically on the left and right, respectively. $"Up"$ and $"down"$ domains are shown schematically with triangles with a green and violet dots, respectively. Under a zero magnetic field, post demagnetization due to the current pulse, domains with both $"up"$ and $"down"$ magnetization nucleate in equal proportions, resulting in zero anomalous Hall resistance, shown in (a). In contrast, when a finite cooling field of $+0.2$T is applied during the switching process, all domains align in the $"up"$ direction, leading to a significant anomalous Hall resistance, shown in (b). Switching behavior as a function of (c) pulse amplitude with the pulse width fixed at 12ms and (d) pulse width with the pulse amplitude fixed at 100mA. The transient temperature during the application of pulse is estimated by measuring the longitudinal resistance (R$_{xx}$) suppl. R$_{xx}$ is measured directly by an NI-DAQ device for low voltages, shown in green, and through a divider circuit for higher voltages, shown in blue.
• Figure 5: Multiple-stable anomalous Hall resistance memory states in Mn$_{3}$Sn. (a) Anomalous Hall resistance ($R_{xy}$) states after application of electrical pulse under different bias fields in Ta(11nm)/Mn$_{3}$Sn(90nm)/Ta(11nm)/AlO$_{x}$(8nm) heterostructures. Results with positive and negative bias fields are shown in green and orange, respectively. The hysteresis curve of the AHE as a function of magnetic field is shown in blue for reference. (b) The change in $R_{xy}$ due to electrical switching under a bias field w.r.t the initial state. The initial state is obtained by applying an electric pulse with zero bias field that brings $R_{xy}$ to zero (see discussion in the text). The black dashed lines are linear fits. The red dashed line in (b) shows saturation behavior for fields above 1300 Oe. (c) Symmetrical switching behavior for positive and negative bias fields shown for different magnitudes of the bias field. The steps involved in obtaining a resistance loop are shown for a particular bias field, as described in the main text, in blue.(d) Evolution of the AHR memory states with magnetic field. The field range over which the AHR remains within $\pm 0.0021 \Omega$ of its value at zero field (see text) is shown with a black dot-dash line. The estimated stability field ranges as a function of bias fields is shown at the bottom. The dashed line in black corresponds to 1769 Oe, the minimum value obtained for the field range. In all cases, a single pulse of 100 mA and of duration 12 ms is applied for electrical switching.