Tunable decoupling of coexisting magnetic orders in Co$_{1/3}$TaS$_2$

Abstract

In multiferroics, new physical responses and functionalities emerge when symmetry-distinct order parameters couple. This conventionally occurs when lattice and magnetic degrees of freedom order independently in a material. Here, we report an all-magnetic analogue of multiferroic behavior in the antiferromagnet Co$_{1/3}$TaS$_2$, where topological scalar spin chirality and nematicity coexist on the same spin lattice. While the chiral spin texture generates an anomalous Hall effect (AHE), the nematic order breaks threefold rotational symmetry and dominates longitudinal transport. Crucially, in zero field these symmetry-distinct orders merely coexist yet magnetic fields induce strong coupling between them, thus realizing a new type of multiferroic bebhavior via tuning of the coupling itself instead of direct manipulation of secondary orders. In sub-domain sized devices with achiral geometry, we demonstrate that nonreciprocal transport serves as a symmetry-based probe of the global spin chirality, co-aligned with the strong topological AHE of the system. In Co$_{1/3}$TaS$_2$ the topological Hall state inherits a large resistance anomaly via chiral-nematic coupling, thus our results showcase how hybrid magnetic orders can achieve advanced functionalities by merging symmetry-forbidden material responses.

Figures (13)

• Figure 1: a The real-space and reciprocal-space sketch of chiral phse, nematic phase and their combination. The volume spanned by Si, Sj and Sk is propotional to the chirality. The time-reversal symmetry and the C$_{3z}$ rotational symmetry of the three phases are indicated. b SEM image of the device for transport measurement the $b'$ axis denotes the crystallographic $b$ axis rotated by $30^\circ$ clockwise in device plane. The current is applied along $a$ direction. Scale bar 10 $\mu$m. c Temperature dependence of longitudinal resistivity.
• Figure 2: a The phase diagram of Co$_{1/3}$TaS$_2$. The area with blue stripes is the new phase defined by our experiment, which is pure chiral phase or chiral+nematic phase. b Magnetoresistance at different temperatures. The orange circles denote the metamagnetic transition for nematicity, and the green squares denote the chirality reversal transitions.
• Figure 3: Longitudinal voltage and its second harmonics (eMChA) and the Hall voltage measured at a 3 K and b 20 K respectively. The 20 K eMChA is calibrated by subtracting a constant background induced by heating (see supplement Thermal effect on $V_\mathrm{2\omega}$).
• Figure 4: a 2D analogue of the spin texture and the chirality defined by it. b Net magnetization and scalar spin chirality with and without an external magnetic field. In the spin model for net magnetization, the arrows represent the in-plane component of the spins and the color in circles represents the out-of-plane component of the spins. The local out-of-plane magnetization $S^{z}_{\mathrm{local}}$ and the local scalar chirality $\chi_{\mathrm{local}}$ are indicated by the values and colors on each triangular plaquette, respectively. The parameters used in the simulation are $J_1 = 1$, $J_2 = 0.5$, $D = 0.05$, $B = 0.25$ and $K = 0.01$. For simulations with an external field, we choose $H = 0.3$, corresponding to a similar energy scale as in the experiment, estimated using data from Ref. Park2023
• Figure 5: a Calculated nematic order parameter $\phi$ with the field-induced coupling ($\kappa\neq 0$) to the chiral order parameter $\chi$. The nematic transitions are marked by empty orange circles and the chirality reversal transitions are marked by empty green squares. b Temperature dependence of the nematic transition field $H_\mathrm{nem}$ extracted from experiment (solid circles) and calculations (open circles).