A dramatic discovery in spintronics is the unusual magnetoresistance (UMR), where the resistivity of a heavy metal in contact with a magnetic insulator varies with the rotation of magnetization in the plane perpendicular to the electric current. This phenomenon led to the development of the widely accepted concept of spin Hall magnetoresistance (SMR), which has since been extensively employed to interpret UMR in diverse experiments, including magnetoresistance measurements, spin-torque ferromagnetic resonance, harmonic Hall voltage, magnetic field sensing, and magnetization or Néel-vector switching.
However, experimental studies have shown that UMR appears universally in magnetic systems, regardless of whether a spin Hall material is involved. Since UMR is also observed in systems where SMR theory clearly fails (e.g., those without a spin Hall effect), a number of alternative spin-current-related magnetoresistance (MR) models have been proposed, such as Rashba-Edelstein MR, spin-orbit MR, anomalous Hall MR, orbital Hall MR, crystal-symmetry MR, orbital Rashba-Edelstein MR, and Hanle MR, in an attempt to explain the “SMR-like” signals observed in specific systems.
Recently, Prof. Lijun Zhu from the Institute of Semiconductors, Chinese Academy of Sciences, together with Prof. Xiangrong Wang from the Chinese University of Hong Kong, provided unambiguous experimental evidence that universal UMR originates from interfacial electron scattering governed by the magnetization and the interfacial electric field—a mechanism known as two-vector magnetoresistance. Importantly, this explanation avoids the complications associated with spin currents. Their results demonstrate that giant UMR also emerges in single-layer magnetic metals, exhibits higher-order contributions, and obeys a universal sum rule—all in excellent agreement with the physical origin predicted by the two-vector MR model, without invoking spin currents.
A systematic re-examination of existing literature further reveals that the most representative experimental data previously attributed to spin Hall magnetoresistance or other spin-current-related (or even unrelated) mechanisms can, in fact, be consistently explained within the framework of the two-vector MR theory. Moreover, they summarize a range of experimental and theoretical findings that strongly contradict spin-current-based MR models but are naturally accounted for by the two-vector MR mechanism.
These findings fundamentally challenge the long-standing and widely accepted SMR theory, offering the first robust experimental validation of the two-vector magnetoresistance model. By establishing a unified and universal physical origin for UMR, this work provides a simple yet comprehensive framework for understanding magnetoresistance phenomena in a broad range of spintronic systems.
This breakthrough has been recently published in National Science Review under the title “Physics Origin of Universal Unusual Magnetoresistance.”
See the article:
Physics Origin of Universal Unusual Magnetoresistance
https://doi.org/10.1093/nsr/nwaf240
Journal
National Science Review