Charge-to-Spin Conversion (CSC) in Spintronics

• Charge-to-spin conversion (CSC) is the process converting charge currents into spin currents via mechanisms such as spin Hall effect, Rashba–Edelstein responses, and exchange-driven splitting.
• Experimental methodologies like ST-FMR and nonlocal spin valve measurements quantify CSC efficiencies in platforms including oxide q-2DEGs, topological insulators, and van der Waals heterostructures.
• Device geometry, material symmetry, and interface engineering enable unconventional CSC channels and enhanced conversion performance beyond traditional spin–orbit coupling.

Searching arXiv for the cited CSC papers to ground the synthesis. Charge-to-spin conversion (CSC) is the process by which a charge current $J_c$ generates a spin current $J_s$ or a nonequilibrium spin accumulation. In spintronics and spin–orbitronics, CSC appears in bulk metals through spin Hall physics, at interfaces through Rashba–Edelstein responses, in topological surface states through spin–momentum locking, in oxide quasi-two-dimensional electron gases through defect-enabled interfacial SOC, in orbital systems without heavy elements, and in altermagnets even in the absence of spin–orbit coupling (Shashank et al., 2022, Kondou et al., 2015, Kim et al., 2020, Dong et al., 12 Jun 2025).

1. Definitions, figures of merit, and reciprocal structure

The most common bulk definition writes the conversion efficiency as

$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$

with $J_s$ the spin current density injected into an adjacent ferromagnet and $J_c$ the charge current density in the active layer. In oxide q-2DEGs, where the conducting layer thickness $t$ is not precisely known and is of order a few nm, the central figure of merit becomes the thickness-normalized quantity $\theta_{\mathrm{cs}}/t$ in $\mathrm{nm}^{-1}$ (Shashank et al., 2022).

Interfacial CSC is often expressed through two-dimensional quantities. In topological-insulator surface states, the interfacial coefficient

$q_{ICS}=\frac{J_s}{j_c}$

relates a three-dimensional spin current density to a two-dimensional charge current density $j_c$, thereby avoiding ambiguity about the effective thickness of the surface conducting layer (Kondou et al., 2015). In WTe$J_s$0, the CSC magnitude is parameterized through an effective Edelstein length,

$J_s$1

which the authors interpret as encoding the charge-to-spin conversion efficiency of the WTe$J_s$2 layer (Zhao et al., 2020).

In nonrelativistic altermagnets, the analogous ratio is expressed either as an anisotropic conversion ratio

$J_s$3

or as a spin-splitting angle such as

$J_s$4

emphasizing that CSC can be current-direction dependent and need not be framed by a conventional spin Hall angle (Dou et al., 9 Dec 2025, Dong et al., 12 Jun 2025).

Across these definitions, the reciprocal relation to spin-to-charge conversion is central. Several studies explicitly treat inverse charge–spin conversion and CSC as Onsager-reciprocal descriptions of the same underlying response, so the same symmetry constraints and efficiency parameters often govern both directions of interconversion (Zhao et al., 2020, Ingla-Aynés et al., 2022).

2. Microscopic mechanisms

The canonical relativistic mechanism is the spin Hall effect (SHE), in which a longitudinal charge current produces a transverse spin current. In the notation used for WTe$J_s$5,

$J_s$6

while the inverse process is written

$J_s$7

This bulk picture remains the baseline description in metallic NbSe$J_s$8, WTe$J_s$9, oxide q-2DEGs, and topological-insulator heterostructures (Zhao et al., 2020, Hoque et al., 2022, Kondou et al., 2015).

The complementary interfacial mechanism is the Rashba–Edelstein effect. In STO-based q-2DEGs, the interfacial inversion asymmetry and SOC are described by

$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$0

and a charge current produces a nonequilibrium spin accumulation interpreted as a direct Edelstein effect or as an interfacial counterpart of a spin Hall effect (Shashank et al., 2022). In graphene/WSe$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$1, first-principles modelling uses a Dirac Hamiltonian with a staggered potential $\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$2, Rashba SOC $\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$3, valley-Zeeman SOC $\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$4, Kane–Mele SOC $\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$5, and a Rashba angle $\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$6; this yields conventional REE, unconventional REE, and SHE within the same proximitized graphene bands (Lee et al., 2022).

CSC can also arise from orbital rather than spin transport. In ferromagnet/Cu/Al$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$7O$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$8 trilayers, the proposed sequence is charge-to-orbital conversion at the Cu/Al$\theta_{\mathrm{cs}} \equiv \frac{J_s}{J_c},$9O$J_s$0 interface, orbital transport through Cu, and orbital-to-spin conversion in the ferromagnet via

$J_s$1

with the measured torque attributed to this orbital route rather than to a conventional heavy-metal SHE (Kim et al., 2020).

A more radical departure from SOC-driven CSC appears in altermagnets. There, spin current generation survives in the absence of SOC and is tied to nonrelativistic spin splitting of the band structure. The spin conductivity is written

$J_s$2

and in the collinear limit becomes proportional to the difference of spin-resolved charge conductivities, so CSC follows from exchange-driven spin splitting rather than from relativistic spin–orbit entanglement (Dou et al., 9 Dec 2025, Dong et al., 12 Jun 2025).

Chiral systems and single-electron devices extend the concept further. In chiral molecules, charge–spin conversion appears as CISS, while the reciprocal ICISS is interpreted as spin-dependent deflection caused by the interaction between electron spin and chiral structure (Zhang et al., 4 Sep 2025). In a gated InSb nanowire quantum dot, a time-dependent Rashba pulse drives spin-dependent displacement of a single-electron wavefunction; the paper states that an inverse pulse sequence can, in principle, map a prepared charge superposition into a spin superposition, thereby extending CSC to the coherent single-particle limit (Pawłowski et al., 2018).

3. Principal material platforms

Topological and semimetallic van der Waals materials have become a major CSC platform. In WTe$J_s$3/graphene hybrid devices, few-layer Td-WTe$J_s$4 functions as a spin absorber and spin-to-charge converter with a very low WTe$J_s$5/graphene interface resistance of approximately $J_s$6, and the extracted effective Edelstein length is

$J_s$7

at room temperature (Zhao et al., 2020). In graphene/NbSe$J_s$8, current-induced spin polarization is observed up to room temperature, with an effective spin polarization of NbSe$J_s$9 of about $J_c$0, a reported $J_c$1, and an estimated $J_c$2 between $J_c$3 and $J_c$4 depending on the assumed $J_c$5 (Hoque et al., 2022).

Graphene/TMD proximity systems add a tunable two-dimensional route. Twisted graphene/WSe$J_c$6 shows that both the spin Hall and standard Rashba–Edelstein efficiencies are optimized at or near $J_c$7 twisting, while chiral intermediate angles allow an unconventional Edelstein response with electrically generated spin densities collinear to the applied electric field (Lee et al., 2022). In graphene/MoTe$J_c$8, low crystal symmetry and low interface resistance make it possible to access orthogonal and non-orthogonal spin–charge interconversion components in both standard and 3D-current configurations (Ontoso et al., 2022).

Oxide q-2DEGs form a distinct class. At SrTiO$J_c$9/AlN and SrTiO$t$0/Al$t$1O$t$2 interfaces, oxygen-vacancy-induced q-2DEGs yield $t$3 and $t$4, respectively, at room temperature, with the larger value assigned to oxygen-vacancy-enabled enhancement of Rashba-like SOC (Shashank et al., 2022).

Topological insulator surface states provide an interfacial benchmark. In $t$5, the interfacial coefficient $t$6 remains in the range $t$7–$t$8 in the bulk-insulating regime away from the Dirac point, but is sharply suppressed near the Dirac point, a result attributed to degeneracy of surface spins or instability of the helical spin structure (Kondou et al., 2015).

CSC is not confined to heavy-element systems. In CoFe/Cu/Al$t$9O$\theta_{\mathrm{cs}}/t$0, an effective torque efficiency $\theta_{\mathrm{cs}}/t$1 is reported in the as-deposited state and $\theta_{\mathrm{cs}}/t$2 after $\theta_{\mathrm{cs}}/t$3C annealing, despite the absence of heavy elements in the trilayer (Kim et al., 2020). In atomically thin bismuth confined between SiC and epitaxial graphene, ST-FMR detects an in-plane spin polarization perpendicular to the charge current, and the ratio of the in-plane to out-of-plane torque is 3.75 times higher than in hydrogenated graphene control samples (Yánez-Parreño et al., 13 Jan 2025).

Finally, altermagnets establish a nonrelativistic materials class. Density-functional and transport calculations on RuO$\theta_{\mathrm{cs}}/t$4, Mn$\theta_{\mathrm{cs}}/t$5Si$\theta_{\mathrm{cs}}/t$6, KRu$\theta_{\mathrm{cs}}/t$7O$\theta_{\mathrm{cs}}/t$8, CuF$\theta_{\mathrm{cs}}/t$9, and related compounds yield spin-splitting angles from $\mathrm{nm}^{-1}$0 to $\mathrm{nm}^{-1}$1, significantly larger than the spin-Hall angle typically observed in the anomalous spin-Hall effect, where the spin-Hall angle is generally less than $\mathrm{nm}^{-1}$2 (Dong et al., 12 Jun 2025).

4. Experimental methodologies and quantification

Spin-torque ferromagnetic resonance (ST-FMR) is the dominant quantitative method in metallic and oxide CSC systems. The measured mixing voltage is decomposed as

$\mathrm{nm}^{-1}$3

with a symmetric Lorentzian from the damping-like torque and an antisymmetric Lorentzian from the Oersted-field or field-like torque. In STO-based q-2DEGs, the angular dependence is not purely $\mathrm{nm}^{-1}$4, so the analysis protocol decomposes

$\mathrm{nm}^{-1}$5

$\mathrm{nm}^{-1}$6

and uses the ratio $\mathrm{nm}^{-1}$7 to isolate the canonical torque symmetry before extracting $\mathrm{nm}^{-1}$8 (Shashank et al., 2022).

Nonlocal spin-valve and Hanle methods dominate in graphene-based van der Waals devices. In WTe$\mathrm{nm}^{-1}$9/graphene, spin-polarized current injected into graphene diffuses to the WTe$q_{ICS}=\frac{J_s}{j_c}$0 overlap, where CSC is detected as a nonlocal voltage

$q_{ICS}=\frac{J_s}{j_c}$1

The signal is decomposed into

$q_{ICS}=\frac{J_s}{j_c}$2

and the antisymmetric component is fitted with a diffusive Hanle model with finite spin absorption at WTe$q_{ICS}=\frac{J_s}{j_c}$3 (Zhao et al., 2020). In graphene/NbSe$q_{ICS}=\frac{J_s}{j_c}$4, nonlocal spin-switch and Hanle spin precession measurements provide the current-induced spin polarization of NbSe$q_{ICS}=\frac{J_s}{j_c}$5 and the graphene spin transport parameters under CSC injection (Hoque et al., 2022).

Quantification can also be performed through reciprocal detection schemes. In a three-terminal mesoscopic conductor with Rashba SOC, a single-channel quantum point contact operated as a voltage probe yields a charge current $q_{ICS}=\frac{J_s}{j_c}$6 whose zero-field derivative with respect to an in-plane Zeeman field is proportional to the spin current: $q_{ICS}=\frac{J_s}{j_c}$7 Although developed for spin-to-charge conversion, this framework is relevant to CSC because the same spin–orbit-coupled cavity generates the mesoscopic spin currents from an applied charge bias (Stano et al., 2010).

The same reciprocity logic underlies omnidirectional SCC experiments in graphene/NbSe$q_{ICS}=\frac{J_s}{j_c}$8, where spin precession is used to prepare $q_{ICS}=\frac{J_s}{j_c}$9-, $j_c$0-, and $j_c$1-polarized spin populations and the corresponding SCC amplitudes are disentangled by symmetry under field reversal. In linear response, the measured tensor components map directly onto the reciprocal CSC channels (Ingla-Aynés et al., 2022).

5. Symmetry, geometry, and unconventional CSC

A central development in CSC research is the move beyond the mutually orthogonal geometry of conventional SHE. Device geometry, crystal symmetry, and interface twist can all create additional allowed tensor components.

In WTe$j_c$2/graphene, the WTe$j_c$3 flake is deliberately tilted by an angle $j_c$4 relative to the graphene spin channel. This produces a mixture of sine-like and cosine-like Hanle responses with

$j_c$5

leading to

$j_c$6

The extracted $j_c$7 matches the optical geometry and establishes a purely geometrical handle on the phase and sign of the CSC signal (Zhao et al., 2020).

In twisted graphene/WSe$j_c$8, symmetry breaking by twist permits a finite Rashba angle $j_c$9 and hence non-orthogonal spin–momentum locking. The result is an unconventional Rashba–Edelstein effect with $J_s$00 for $J_s$01, so the electrically generated spin density can be collinear with the applied electric field. The unconventional component vanishes at $J_s$02 and $J_s$03, where mirror symmetries enforce $J_s$04 (Lee et al., 2022).

Low-symmetry MoTe$J_s$05 provides a direct experimental realization of non-orthogonal interconversion. In graphene/MoTe$J_s$06 heterostructures, the combination of standard and 3D-current geometries reveals three SCI components: one orthogonal and two non-orthogonal. One channel corresponds to $J_s$07, and another to $J_s$08, neither of which is available in a high-symmetry metal. The authors assign the orthogonal channel to symmetry-allowed SHE and/or EE, the $J_s$09 channel primarily to an unconventional Edelstein effect in proximitized graphene, and the $J_s$10 channel to an unconventional SHE in MoTe$J_s$11 (Ontoso et al., 2022).

Altermagnets push symmetry engineering further by allowing nonrelativistic, current-direction-dependent CSC. Group-theoretical analysis and DFT show conversion efficiencies varying from zero to several tens of percent depending on current orientation, with explicit formulas for $J_s$12 or the spin-splitting angle $J_s$13 determined by the spin point group. This establishes that the geometry of CSC need not be tied to SOC alone; exchange symmetry can play the same role (Dou et al., 9 Dec 2025, Dong et al., 12 Jun 2025).

6. Performance, limitations, and outlook

Room-temperature operation is already established across several CSC platforms. WTe$J_s$14-based CSC is robust at room temperature against gate and magnetization changes (Zhao et al., 2020). STO/AlN and STO/Al$J_s$15O$J_s$16 q-2DEGs exhibit room-temperature $J_s$17 values of $J_s$18 and $J_s$19, respectively (Shashank et al., 2022). Atomically thin Bi under graphene yields room-temperature spin-torque signatures stronger than hydrogenated-graphene controls by a factor of 3.75 in the in-plane to out-of-plane torque ratio (Yánez-Parreño et al., 13 Jan 2025). NbSe$J_s$20 generates spin polarization up to room temperature and shows significantly higher CSC at low temperature, although in the reported measurements the signal is observed only above the superconducting critical current, i.e. in the non-superconducting state of NbSe$J_s$21 (Hoque et al., 2022).

The most persistent limitation is mechanistic ambiguity. In oxide q-2DEGs, the analysis is phenomenological and does not conclusively distinguish interfacial Edelstein physics from a bulk spin Hall effect within the q-2DEG band structure (Shashank et al., 2022). In ferromagnet/Cu/Al$J_s$22O$J_s$23, ST-FMR measures torque rather than orbital current directly, so the orbital interpretation is inferred from thickness dependence, ferromagnet dependence, and interface sensitivity (Kim et al., 2020). In atomically thin Bi, the observed in-plane polarization is consistent with either spin Hall or Rashba–Edelstein effects, and the paper does not decisively separate them (Yánez-Parreño et al., 13 Jan 2025). In graphene/NbSe$J_s$24, the inferred SCC efficiencies are so large under an NbSe$J_s$25-only assumption that proximitized graphene becomes the more plausible conversion medium for several components (Ingla-Aynés et al., 2022).

Thickness, disorder, and current distribution remain recurring metrological bottlenecks. Absolute $J_s$26 in oxide q-2DEGs depends on the uncertain conducting-layer thickness $J_s$27 (Shashank et al., 2022). In mesoscopic and quantum-confined systems, orbital magnetic phases and boundary-induced electric fields alter both the magnitude and the field dependence of CSC signatures (Maiellaro et al., 19 May 2025). In altermagnets, the present efficiencies are theoretical and orientation dependent; direct experiments must still determine spin diffusion, interface transparency, and torque transfer in realistic stacks (Dong et al., 12 Jun 2025).

The field is therefore moving toward deliberately engineered CSC tensors. The data suggest three major directions. First, interface engineering—through oxygen vacancies, twist angle, strain, and low-resistance vdW contacts—acts directly on the allowed conversion channels (Shashank et al., 2022, Lee et al., 2022, Ontoso et al., 2022). Second, materials selection no longer follows only atomic number: orbital transport without heavy elements and nonrelativistic altermagnetic CSC both demonstrate large efficiencies from symmetry and band structure rather than from strong SOC alone (Kim et al., 2020, Dong et al., 12 Jun 2025). Third, CSC is broadening from conventional bulk transport into topological, chiral, and coherent quantum regimes, suggesting a unified spintronic landscape in which bulk Hall, interfacial Edelstein, orbital, chiral, and single-particle protocols are design variants of the same charge–spin interconversion problem (Kondou et al., 2015, Zhang et al., 4 Sep 2025, Pawłowski et al., 2018).