Piezomagnetic transport in van der Waals noncoplanar Antiferromagnets

The piezomagnetic effect-strain-induced linear modulation of magnetization, arises in magnets with broken time-reversal symmetry (BTRS), offering a pathway to bidirectional strain-based control of magnetism, which is an essential straintronic and spintronic functionality in solids. Metallic antiferromagnets with BTRS provide an ideal platform to study this effect through transport measurements, yet experimental demonstrations are limited. Van der Waals (vdW) nanomagnets, with their mechanical flexibility, are particularly promising for realizing large piezomagnetic responses and effective transport control. Here we demonstrate piezomagnetic control of electronic transport in nano-devices of the vdW antiferromagnets CoNb$_3$S$_6$ and CoTa$_3$S$_6$, archetypal vdW metals with BTRS that exhibit a spontaneous Hall effect. Applying uniaxial strain linearly modulates both the antiferromagnetic transition temperature and coercive field, consistent with strain-driven tuning of exchange coupling, key signatures of the piezomagnetic effect. Moreover, spontaneous Hall effect is controllable via strain, evidencing piezomagnetic tuning of Berry curvature and its associated geometric transport. These findings establish piezomagnetism as a powerful route to manipulate antiferromagnetic transport, opening avenues for straintronic and spintronic applications in vdW magnetic systems.

💡 Research Summary

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This work investigates the piezomagnetic effect— the linear coupling between strain and magnetization— in van der Waals (vdW) non‑coplanar metallic antiferromagnets, specifically CoNb₃S₆ and CoTa₃S₆. Both compounds crystallize in a non‑centrosymmetric P6₃22 structure with Co ions intercalated between NbS₂ (or TaS₂) layers. Below their Néel temperatures (≈26 K for CoNb₃S₆, ≈37 K and ≈26 K for CoTa₃S₆) the Co spins adopt an all‑in‑all‑out (AIAO) configuration, which cancels the net magnetization but yields a finite scalar spin chirality χᵢⱼₖ = Sᵢ·(Sⱼ × Sₖ). This chirality breaks time‑reversal symmetry (BTRS) and generates a Berry curvature in momentum space, giving rise to a spontaneous Hall effect even at zero external magnetic field.

The authors exfoliate thin flakes (≈90–100 nm) onto flexible polymer substrates equipped with bottom electrodes, and apply uniaxial strain by bending the substrate to radii that produce tensile or compressive strains ranging from –2 % to +2 %. Electrical transport is measured in a four‑probe geometry while varying temperature (2–300 K), magnetic field (±9 T along the c‑axis), and strain.

Key experimental observations for CoNb₃S₆ are:

1. Linear strain tuning of magnetic order – Both the Néel temperature (T_N) and the coercive field (H_c) shift linearly with applied strain. Tensile strain enhances the antiferromagnetic exchange, raising T_N and reducing H_c, whereas compressive strain has the opposite effect. This behavior is interpreted through a strain‑dependent exchange constant J(ε), where mean‑field theory predicts T_N ∝ J/k_B, and through a strain‑modified magnetic anisotropy energy that controls H_c.

2. Piezomagnetic generation of out‑of‑plane magnetization – The magnetic point group (32’) permits a piezomagnetic tensor component Λ_zxx = Λ_zyy, leading to an out‑of‑plane magnetization M_z = Λ_zxx ε_xx + Λ_zyy ε_yy under in‑plane strain. The strain‑induced imbalance of exchange interactions creates a net moment, which is directly linked to the observed linear shifts of T_N and H_c.

3. Strain control of Berry curvature and spontaneous Hall conductivity – The zero‑field Hall conductivity σ_SHE_xy, originating from the Berry curvature of the non‑coplanar spin texture, varies linearly with strain. At 5 K, σ_SHE_xy(ε) follows a straight line, and a scaling analysis σ_SHE_xy ∝ σ_xx^1.8 places the system in the “bad‑metal” intrinsic regime, indicating that the Hall response is governed by Berry curvature rather than extrinsic scattering. Normal Hall coefficient R_o also changes with strain (compressive strain reduces carrier density, tensile strain increases it), but the dominant contribution to σ_SHE_xy modulation is the strain‑induced change of Berry curvature rather than a simple shift of the Fermi level.

4. Universality and material dependence – Similar measurements on CoTa₃S₆ reveal analogous linear strain dependencies of T_N1, H_c, and Hall angle, confirming the generality of the piezomagnetic mechanism. However, the sign of the strain‑induced slope is opposite to that of CoNb₃S₆, implying that the piezomagnetic tensor Λ has opposite sign in the two compounds. Moreover, CoTa₃S₆ exhibits two magnetic transitions; between T_N1 and T_N2 the system is time‑reversal symmetric and shows no spontaneous Hall effect. In this intermediate regime the longitudinal resistance shows a strain‑switchable temperature dependence, which the authors associate with a strain‑induced nematic domain selection that breaks the six‑fold rotational symmetry.

Overall, the study demonstrates that (i) uniaxial strain linearly tunes exchange interactions and magnetic anisotropy in vdW antiferromagnets, (ii) the resulting piezomagnetic magnetization modifies the solid angle of the spin texture, thereby controlling the Berry curvature and associated spontaneous Hall response, and (iii) these effects are robust across different materials and strain directions. The ability to manipulate both magnetic order and topological transport properties via modest mechanical deformation highlights vdW non‑coplanar antiferromagnets as a versatile platform for strain‑tronic and spin‑tronic devices, such as strain‑controlled memory elements or logic gates where information is encoded in Berry‑curvature‑driven Hall signals. Future work may explore dynamic strain actuation, multi‑axis strain engineering, and integration with other 2D functional layers to realize practical, low‑energy spintronic architectures.