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Inverse Spin Hall EffectEdit

The Inverse Spin Hall Effect (ISHE) is a fundamental transport phenomenon in which a spin current propagating through a material with appreciable spin-orbit coupling generates a transverse electrical signal. It is the reciprocal counterpart to the Spin Hall Effect (Spin Hall Effect), and together these processes embody a symmetry in which spin and charge degrees of freedom can be interconverted under the right conditions. ISHE provides a practical readout channel for spin-based signals, enabling electrical detection of spin polarization in a variety of platforms, from heavy metals to emerging quantum materials.

In practice, spin currents can be created by injecting spins from a ferromagnet, by spin pumping at a ferromagnet–nonmagnetic interface, or by thermal gradients that drive spin transport (the spin Seebeck effect). The resulting voltages are typically small and require careful experimental design to separate the ISHE signal from other spin- and charge-related effects, but the basic mechanism is robust: a spin current flowing through a material with strong spin-orbit coupling induces a transverse charge current or voltage proportional to the spin current and to the material’s spin Hall response. See for example studies in Platinum and other heavy metals, as well as in Topological insulators and some semiconductors.

Over the years, researchers have refined the understanding of ISHE and its relation to the direct Spin Hall Effect. In some materials, the signal appears to be governed largely by intrinsic band-structure properties tied to Berry curvature, while in others extrinsic mechanisms related to impurity scattering—such as Skew scattering and Side-jump processes—play a dominant role. In low-dimensional systems or at interfaces with strong spin-orbit coupling, signals comparable to ISHE can arise from the inverse Edelstein effect (often discussed as the Rashba–Edelstein mechanism), which can complicate interpretation of experimental data. These debates reflect the broader challenge of disentangling universal symmetry considerations from material-specific details in spin–orbit–coupled systems, and they continue to shape experimental design and theoretical modeling.

Physical principles

Spin-charge interconversion

ISHE converts a flow of spin angular momentum into a transverse charge current. If a spin current J_s with polarization direction p travels through a material, a perpendicular charge current J_c is generated with a magnitude proportional to the spin Hall angle θ_SH of the material: J_c ∝ θ_SH (J_s × p). The sign and magnitude depend on the details of spin-orbit coupling and scattering processes, as well as the geometry of the device. See Spin current and Spin Hall angle for related concepts.

Intrinsic vs extrinsic mechanisms

  • Intrinsic contributions arise from the electronic band structure and Berry curvature, independent of impurity scattering, and can dominate in clean, well-characterized systems.
  • Extrinsic contributions come from impurity-related processes such as skew scattering and side-jump mechanisms, which depend on the impurity concentration and scattering potential.
  • The balance between intrinsic and extrinsic components varies across materials (e.g., Pt, Ta, and other heavy metals; certain semiconductors; and topological materials), and a full description often requires both perspectives. See Berry curvature, Skew scattering, and Side-jump for background.

Reciprocal relationship and related effects

ISHE is the reciprocal of the direct Spin Hall Effect: a charge current can generate a transverse spin current, just as a spin current can generate a transverse charge current. Onsager reciprocity underpins this symmetry. In two-dimensional or interface-dominated systems, related phenomena such as the Rashba–Edelstein effect and its inverse counterpart (the inverse Edelstein effect) can contribute to spin–charge conversion signatures, adding interpretive complexity in some devices. See Rashba–Edelstein effect and Spin Hall Effect for context.

Spin dynamics and diffusion

Spin currents in solids decay over the spin-diffusion length and can be spatially distributed according to the material’s spin relaxation properties. In layered devices, the measured ISHE voltage reflects the integrated effect of spin injection, diffusion, and conversion across the stack. See Spin diffusion for related concepts.

Materials and device architectures

Metallic systems

Heavy metals with strong spin-orbit coupling, such as platinum, are common platforms for ISHE studies. In spin-pumping experiments, a ferromagnetic layer (for example, NiFe or CoFeB) can inject a spin current into an adjacent metal; the ISHE voltage is then measured across the metal layer. Other metals like Tantalum and Tungsten can exhibit sizable, sometimes opposite-sign spin Hall responses depending on phase (alpha vs beta). See Platinum, Tantalum, and W (tungsten) for material-specific discussions.

Semiconductors and topological materials

ISHE has been explored in semiconductors (e.g., GaAs, InAs) where spin currents can be generated by optical or electrical means and detected via transverse voltages. Proximity effects and engineered interfaces enable spin–orbit coupling in otherwise weakly spin–orbit coupled materials. Topological insulators and certain oxide interfaces also provide strong spin–orbit coupling, yielding ISHE signals that reflect their unique surface or interface states. See Gallium arsenide, Topological insulator, and Platinum for related material discussions.

Two-dimensional systems and interfaces

Two-dimensional materials and heterostructures with engineered spin–orbit coupling expand the ISHE platform beyond conventional metals. Graphene with proximity-induced spin–orbit coupling and van der Waals heterostructures offer tunable scenarios for spin–charge conversion. See Graphene and Van der Waals heterostructure for context.

Experimental methods and challenges

Common measurement geometries

  • Spin pumping: a ferromagnetic layer excited by microwave radiation at ferromagnetic resonance injects a spin current into a neighboring nonmagnetic layer; the ISHE then converts part of that spin current into a measurable voltage. See Ferromagnetic resonance and Spin pumping.
  • Nonlocal spin valve: spin-polarized currents are injected in one region and, after diffusion, converted to a transverse voltage via ISHE in a separate region. See Nonlocal spin valve.
  • Spin Seebeck–ISHE configurations: thermal gradients generate spin currents, which are detected through ISHE, requiring careful control of spurious thermolectric effects. See Spin Seebeck effect and Planar Hall effect for potential background signals.

Challenges and interpretation

  • Distinguishing ISHE from inverse Edelstein effect signals in low-dimensional or interface-dominated systems can be nontrivial.
  • Background voltages from thermoelectric effects, galvanic potentials, or magnetoresistive contributions require careful control experiments and systematic variation of geometry, temperature, and magnetic field orientation.
  • Quantitative extraction of the spin Hall angle and spin diffusion length relies on models of spin transport and interfaces, which may differ between material systems.

Theoretical perspectives and debates

  • The relative importance of intrinsic versus extrinsic mechanisms continues to be a central topic, especially in newly synthesized materials and complex alloys.
  • In some materials, Berry-curvature-based intrinsic contributions appear to be robust, while in others, impurity-driven scattering dominates, leading to different scaling with resistivity and temperature.
  • The interpretation of signals in two-dimensional systems and at oxide interfaces remains nuanced due to competing interfacial effects and the proximity-induced spin–orbit coupling that can mimic or mask ISHE signatures.
  • Ongoing work seeks to unify the phenomenology across metals, semiconductors, and topological materials, clarifying how crystal structure, dimensionality, and disorder shape the observed ISHE response.

See also