Circular Spin Currents

Updated 26 July 2025

Circular spin currents are persistent flows of spin angular momentum along closed-loop paths, emerging from symmetry-breaking and quantum interference effects.
They manifest in diverse systems such as mesoscopic rings, molecular circuits, and magnetic nanostructures, with mechanisms including spin–orbit coupling and the spin Hall effect.
Recent advances enable precise experimental detection and control of these currents, promising innovative applications in ultrafast spintronics and quantum information processing.

Circular spin currents, defined as persistent or dynamically generated flows of spin angular momentum with closed-loop topology, represent a fundamental aspect of spin transport in condensed matter systems, mesoscopic and molecular circuits, and magnetic nanostructures. Unlike conventional charge currents, circular spin currents can manifest even in the absence of net charge motion, and may arise in systems ranging from quantum dots and molecular rings to antiferromagnetic insulators and magnetic textures under various driving mechanisms. The generation, measurement, and control of such currents underpin much of modern spintronics, with implications for quantum information, ultrafast devices, and new paradigms of coherent manipulation.

1. Fundamental Concepts and Mechanisms

A circular spin current is a local or global flow of spin angular momentum (typically quantified as the difference in current flow between spin-up and spin-down species or as a magnon current in magnets) along a closed path. The general form is:

$\mathbf{J}_s(\mathbf{r}) = \mathbf{j}_\uparrow(\mathbf{r}) - \mathbf{j}_\downarrow(\mathbf{r})$

Microscopically, circular spin currents may arise via:

Quantum interference in mesoscopic rings or molecular circuits (1006.1729, Patra, 2022)
Spin–orbit coupling induced band mixing (Bree et al., 2014, Bree et al., 2014)
Magneto-geometric effects in systems with broken symmetry (chirality, parity breaking) (Chen et al., 2023, Flores et al., 29 May 2025)
Spin Hall and related transverse anomalous effects (1107.3121, Bazaliy et al., 2016)
Magnon-mediated transport in magnetic insulators subjected to external fields or ultrafast optical pulses (Boström et al., 2021, Hegstad et al., 24 Jul 2025)
Spin texture dynamics in nanostructures (skyrmions, skyrmioniums) (Assis et al., 5 Feb 2024)

The distinguishing feature of a circular spin current is that its topology—i.e., the path along which spin flows—encloses a nontrivial loop, often resulting in measurable magnetic or electrical consequences.

2. Circular Spin Currents in Mesoscopic and Molecular Rings

Tight-binding and second-quantized approaches establish that spin (and charge) currents in rings are acutely sensitive to symmetry, coupling, and external perturbations:

In antiferromagnetic (AF) rings with perfectly compensated spin structure, symmetry between spin-up and spin-down sub-Hamiltonians yields complete cancellation of persistent spin currents (Karmakar et al., 6 Apr 2025).
Coupling a nonmagnetic (NM) chain to the AF ring breaks the symmetry, resulting in spin-channel separation—a net persistent spin current appears, which is tunable via the ring-chain coupling strength. Both charge and spin current amplitudes can be modulated by this coupling and by temperature, with non-monotonic dependence (Majhi et al., 22 Oct 2024, Karmakar et al., 6 Apr 2025).
The presence of quasi-periodicity or disorder (e.g., via an Aubry-André-Harper potential in a side-coupled chain) leads to energy level asymmetry and strong oscillatory behavior in circular spin currents, often linked to disorder-induced localization or interference effects (Majhi et al., 22 Oct 2024).
In quantum rings with Rashba spin–orbit interaction (RSOI), setting the ring-lead configuration (symmetric vs. asymmetric) and tuning the RSOI parameter α (for example, electrically via a gate) enables complete electrical control of pure spin vs. charge circular currents (Patra, 2022).

The net persistent spin current is typically calculated by summing state currents up to the chemical potential, or by integrating over the bias window in a transport scenario:

$I_S = \sum_{n}(I_{n,\uparrow} - I_{n,\downarrow}) \quad \text{or} \quad \int J_S(E) f(E) dE$

3. Spin and Charge Transport: Interference, Topology, and Symmetry Breaking

A recurrent theme is that circular spin currents require a mechanism breaking the symmetry between spin channels or introducing a nontrivial geometric phase:

In molecular rings or quantum dots, spin–orbit (SO) interaction admixes conduction and valence states, resulting in orbital current loops with maxima midway between the dot center and edge; these are tied to the effective g-factor and spatially separated from spin magnetization (Bree et al., 2014, Bree et al., 2014).
In chiral molecules, the helical geometry combined with SO coupling and applied magnetic field yields persistent spin currents that can be tuned between trivial (vanishing or strictly antisymmetric) and nontrivial (finite and robust) regimes by field orientation or molecular configuration (Chen et al., 2023).
In mesoscopic devices, spin current injection into ferromagnet|normal-metal hybrids can generate closed-loop electric currents due to gradients in spin accumulation and spin-channel dependent conductivities. This leads to novel voltage distributions and enhances spin accumulation tails well beyond naive expectations (Bazaliy et al., 2016).
In ring/chain hybrid systems with additional disorder or quasi-periodic potentials, the spectrum of flux-sensitive vs. insensitive energy levels and their interference governs the scaling and even the sign reversal of circular spin currents (Majhi et al., 22 Oct 2024).

4. Magnetic Texture Dynamics and Magnon Currents

In magnetic insulators and nanostructures, circular spin currents are manifested as persistent flows of angular momentum, often mediated by collective excitations:

In Corbino disks, the dynamics of non-collinear spin textures (Bloch vs. Néel skyrmions, skyrmioniums) are governed by their helicity and topological charge under spin–orbit and spin transfer torques. While Bloch skyrmions under radial currents execute robust circular motion via the Skyrmion Hall effect, Néel skyrmions can be trapped at the device edge; skyrmioniums (with zero topological charge) exhibit even higher rotational frequencies and are less prone to edge annihilation, thus favorable for spin torque nano-oscillators (STNOs) (Assis et al., 5 Feb 2024).
Ultrafast coherent magnon spin currents in antiferromagnets can be generated by pulsed excitation of counter-propagating magnon pairs. When parity symmetry is broken, superpositions of pairs yield a net spin current. Importantly, by employing two orthogonal polarizations, a circular (rotational) magnon spin current can be engineered—this scenario is not achievable in steady-state magnon currents where spin angular momentum nominally cancels (Hegstad et al., 24 Jul 2025).
The magnon circular photogalvanic effect allows generation of directed magnon spin currents in antiferromagnetic insulators using circularly polarized light via stimulated Raman processes. Here, the spin current is directly linked to the incident light's chirality and angle, and can be measured via the inverse spin Hall voltage in adjacent metal contacts (Boström et al., 2021).

5. Experimental Detection and Functional Control

Emerging methods and device architectures increasingly enable direct probing and manipulation of circular spin currents:

Quantum point contacts (QPCs) offer a means to convert mesoscopic spin current to charge current: the odd-in-B component of the QPC signal is proportional to the underlying spin current, enabling both detection and quantitative measurement (1012.1831).
In chiral molecules, spin-polarized photoelectron detection in circular dichroism (PECD) experiments reveals enantio-sensitive, multipolar spin current patterns—including spin-polarization vortices and spin sources/sinks, which change sign for opposite enantiomers and encode chirality (Flores et al., 29 May 2025).
In quantum wells or nanostructures with engineered confining potentials, measurement of the magnetic field distribution and evanescent current beyond geometric boundaries tests the distributed nature of the spin current, supporting a unified four-current (charge + spin) quantum model (Gao et al., 20 Mar 2024).
Spin–electric stripes, unambiguously tied to circular spin current, manifest as “edge” domains in 2D conductors, with robust 100% spin polarization maintained at the boundaries under suppressed spin relaxation (1107.3121).

6. Prospects, Challenges, and Theoretical Outlook

The theoretical landscape of circular spin currents has broadened with the integration of topological, geometric, and ultrafast dynamical elements:

Electrical and optical control of spin current topology (e.g., creating and steering circular vs. linear currents) suggests potential for highly coherent spin logic, reconfigurable memory, and all-optical switching at THz rates (Hegstad et al., 24 Jul 2025, Wu et al., 2020, Boström et al., 2021).
The interplay between drive geometry, symmetry breaking, disorder, and topological invariants underpins the robustness and directionality of circular spin currents, which is critical for practical device miniaturization and resilience (Majhi et al., 22 Oct 2024, Patra, 2022).
Advances in materials synthesis (e.g., 2D van der Waals antiferromagnets, quantum heterostructures) and hybrid device architectures (magnetic rings with side-coupled chains, Corbino disks) provide platforms for experimental realization and further exploration.
Challenges remain in direct spatial and temporal imaging of such currents, deconvolving spin, charge, and magnon contributions especially in the presence of spin relaxation, and integrating quantum spin currents into scalable logic or memory architectures.

Table: Physical Mechanisms for Circular Spin Currents

System Type	Key Mechanism	Representative Papers
Mesoscopic rings / molecular wires	Quantum interference + SOI	(1006.1729, Patra, 2022)
Magnetic nanostructures	Skyrmion Hall effect, SOT, STT	(Assis et al., 5 Feb 2024)
AFM rings + chains	Symmetry breaking via coupling	(Majhi et al., 22 Oct 2024, Karmakar et al., 6 Apr 2025)
Chiral molecules	Chiral-induced SOC + field	(Chen et al., 2023, Flores et al., 29 May 2025)
AFMs (magnon circuits)	Multimode coherent excitation	(Boström et al., 2021, Hegstad et al., 24 Jul 2025)
2D conductors, QWs	Spin Hall effect (spin–electric)	(1107.3121)

Conclusion

Circular spin currents encompass a broad range of physical effects, from quantum coherence in nanoscale rings and topological magnetic textures to ultrafast, optically generated magnon flows in antiferromagnets. Their generation almost always requires the interplay between symmetry breaking (geometric, magnetic, or parity), quantum phase coherence, and suitable external driving (electric, magnetic, optical, or mechanical). Their detection, control, and functional exploitation are rapidly advancing, leveraging both emergent material platforms and novel measurement schemes. As such, circular spin currents are central both for deepening the theoretical understanding of angular momentum transport and for developing next-generation spintronic and quantum technologies.