A major advance in spintronics came with the discovery of unusual magnetoresistance (UMR). In this effect, the electrical resistance of a heavy metal changes when it is placed next to a magnetic insulator and the magnetization rotates within a plane that is perpendicular to the direction of the electric current.
This observation motivated the introduction of spin Hall magnetoresistance (SMR), a framework that quickly became widely accepted. SMR has since been used to explain UMR across many experimental settings, including standard magnetoresistance tests, spin-torque ferromagnetic resonance experiments, harmonic Hall voltage measurements, magnetic field sensors, and techniques for controlling magnetization or Néel-vector switching.
Over time, however, experiments revealed a broader and more puzzling picture. UMR was found to occur in many magnetic systems, even when no spin Hall material was present. Because the effect also appears in systems where SMR cannot apply (e.g., those without a spin Hall effect), researchers proposed a variety of alternative spin-current-related magnetoresistance (MR) models. These include Rashba-Edelstein MR, spin-orbit MR, anomalous Hall MR, orbital Hall MR, crystal-symmetry MR, orbital Rashba-Edelstein MR, and Hanle MR, all aimed at explaining the “SMR-like” signals seen in specific materials.
More recently, Prof. Lijun Zhu from the Institute of Semiconductors, Chinese Academy of Sciences, working with Prof. Xiangrong Wang from the Chinese University of Hong Kong, reported clear experimental results pointing to a different origin of universal UMR. Their work shows that the effect arises from electron scattering at material interfaces, controlled by the magnetization and the interfacial electric field. This process is known as two-vector magnetoresistance and does not rely on spin currents.
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