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Discovery of a New Type of Magnetoresistance

Unconventional unidirectional magnetoresistance in heterostructures of a topological semimetal and a ferromagnet

Currently, spin-orbit torque based two-terminal magnetic memory device, like commercially available magnetic-tunnel-junction-based magnetic memory devices, is missing due to the lack of identified magnetoresistance phenomena to distinguish the up and down states of a perpendicular polarized magnet in bilayer heterostructure.

  • Unidirectional magnetoresistance (UMR) is a change in the longitudinal resistance of a heterostructure, composed of a spin-source material and a magnetic layer, due to magnetization reversal and its interaction with non-equilibrium spin accumulation.

  • Atomically clean bilayer heterostructures consisting of a low-symmetry semimetal (WTe2) and a ferromagnetic semiconductor (Cr2Ge2Te6, CGT) was employed.

  • Unconventional UMR originates from the interplay of crystal symmetry-breaking in WTe2 and magnetic exchange interaction across the WTe2/CGT interface.

  • This work was published in Kao et al., Nature Materials in March 2025.

Figure 1. (a) Atomically clean WTe2/CGT interface. (b) Left: Generation of nonequilibrium spin accumulation with an out-of-plane spin polarization (𝝈_𝒛), when a charge current (𝐽) is applied along the a-axis of WTe2. Right: Concept of unconventional UMR in heterostructures, i.e., change of longitudinal resistance (𝑅_𝑈𝑀𝑅) depending on the relative orientation of out-of-plane magnetization (𝒎_𝒛) and spin polarization (𝝈_𝒛). (c) A schematic showing the measurement configuration to read the out-of-plane magnetic state of CGT employing 2-point longitudinal resistance (𝑅_𝑥𝑥). (d) UMR signal measured with +/- current. The step-like change in 2-point 𝑅_𝑥𝑥 is due to UMR switching sign when the current is reversed, with symmetric and antisymmetric components shown (lower panel).
Figure 1. (a) Atomically clean WTe2/CGT interface. (b) Left: Generation of nonequilibrium spin accumulation with an out-of-plane spin polarization (𝝈_𝒛), when a charge current (𝐽) is applied along the a-axis of WTe2. Right: Concept of unconventional UMR in heterostructures, i.e., change of longitudinal resistance (𝑅_𝑈𝑀𝑅) depending on the relative orientation of out-of-plane magnetization (𝒎_𝒛) and spin polarization (𝝈_𝒛). (c) A schematic showing the measurement configuration to read the out-of-plane magnetic state of CGT employing 2-point longitudinal resistance (𝑅_𝑥𝑥). (d) UMR signal measured with +/- current. The step-like change in 2-point 𝑅_𝑥𝑥 is due to UMR switching sign when the current is reversed, with symmetric and antisymmetric components shown (lower panel).
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Unconventional unidirectional magnetoresistance in heterostructures of a topological semimetal and a ferromagnet

Nature Materials
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