Ultrafast Magneto-Photocurrents

• Ultrafast magneto-photocurrents are transient spin and charge currents generated in solids via femtosecond optical excitation in magnetic environments with broken symmetry or strong spin–orbit coupling.
• They are detected through THz emission spectroscopy, revealing critical insights into spin-charge conversion, carrier dynamics, and interfacial properties.
• This research underpins advances in THz spintronics and all-optical magnetic control, paving the way for novel ultrafast on-chip devices.

Ultrafast magneto-photocurrents are transient charge and spin currents generated on femtosecond timescales in solids via optical excitation in the presence of magnetic order, broken symmetry, or spin–orbit coupling. These currents are typically probed through the concomitant emission of electromagnetic radiation in the THz regime. Central to recent research are interfacial and bulk mechanisms—vertical spin pumping, inverse spin–orbit torque, photon drag, and spin-galvanic effects—that convert optically driven electronic, spin, or excitonic excitations into directed charge flow and THz transients. Their study provides insight into ultrafast spin-charge conversion, carrier dynamics, and interfacial symmetry, and underpins advances in spintronics, THz optoelectronics, and ultrafast magnetic control.

1. Fundamental Mechanisms and Theoretical Frameworks

Ultrafast magneto-photocurrents emerge through several primary microscopic processes, distinguished by material symmetry, magnetic configuration, and light polarization:

• Spin Pumping and Inverse Spin Hall Effect (ISHE): In ferromagnet|heavy metal (F|HM) stacks, an optical pulse induces sub-picosecond demagnetization of the FM, launching a vertical spin current $J_s(t)$ into the adjacent HM (e.g., Pt), which is converted via ISHE into an in-plane charge current $J_c(t)$, resulting in THz emission (Medapalli et al., 2020, Rouzegar et al., 1 Jul 2025). The relation $$J_s(t) = (\hbar/2e)g_\mathrm{eff}^{\uparrow\downarrow}\,\mathrm{d}m_z/\mathrm{d}t$$ describes spin injection, while $$J_c(\omega) = \theta_\mathrm{SH}(2e/\hbar)\lambda_\mathrm{sf}\tanh(d_\mathrm{Pt}/2\lambda_\mathrm{sf})[J_s(\omega)\times \hat\sigma]$$ quantifies ISHE conversion.
• Inverse Spin-Orbit Torque (ISOT): Circularly polarized light, via the inverse Faraday effect and spin–orbit coupling at inversion-asymmetric interfaces, exerts ultrafast torque on magnetization. The resulting photocurrent is described as $$J_i^\mathrm{ISOT}(t) = (e/\hbar)\mathrm{d}/\mathrm{d}t[\chi_{ijk}m_jE_k^\mathrm{pump}(t)]$$ where $\chi_{ijk}$ is a third-rank tensor dictated by interfacial symmetry (Medapalli et al., 2020, Huisman et al., 2015).
• Photon Drag and Photogalvanic Effects: In centrosymmetric bulk topological magnets such as Mn$_3$Sn, photocurrents are governed by the photon drag effect rather than photogalvanic effects. For the bulk, only $\chi^{(3)}$ tensors (photon drag) contribute, with the circular photon-drag component (CPDE) controlled by light helicity and incident wavevector (Hamara et al., 2023).
• Dresselhaus Spin-Galvanic Effect (SGE): In non-centrosymmetric ferromagnets (e.g., NiMnSb), ultrafast spin accumulation induced by optical heating transfers angular momentum into Dresselhaus spin–momentum-locked states, creating a rapidly relaxing charge current $j_D$ governed by $\tau_p\sim10$ fs (momentum relaxation) and scaling with film thickness (Tong et al., 1 Jul 2025).
• Exciton-Mediated Photocurrent and Spin Torque: In 2D antiferromagnets, ultrafast photocurrents (from laser pulses above the gap) can exert spin-transfer torque on local moments via nonequilibrium Green’s function–coupled Landau-Lifshitz-Gilbert dynamics. The frequency and decay of the resulting magnonic oscillations imprint themselves on the pumped charge/THz emission (Varela-Manjarres et al., 3 Jul 2025).

2. Experimental Methodologies and Detection Schemes

Typical experiments employ pump–THz emission spectroscopy, integrating advanced sample geometries, polarization control, and time-resolved detection:

• Sample Designs: Ferromagnet|heavy-metal bilayers (e.g., FeRh/Pt, Co/Pt), inversion-asymmetric ferromagnets (NiMnSb), centrosymmetric antiferromagnets (Mn$_3$Sn), III-V semiconductors (GaAs, In$_r$Bi$_{1-r}$)$_2$Se$_3$), and 2D van der Waals antiferromagnets (CrSBr, NiPS$_3$, MnPS$_3$) are used (Medapalli et al., 2020, Tong et al., 1 Jul 2025, Hamara et al., 2023, Schmidt et al., 2016, In et al., 3 Jan 2026, Varela-Manjarres et al., 3 Jul 2025).
• Optical Excitation: Femtosecond Ti:Sapphire lasers (pulse duration 10–100 fs, λ = 800 nm, fluence up to 10 mJ/cm²) provide variable linear or circular polarization. Control over pulse parameters (duration, polarization, spot size) accesses the regime of interest for both spin and momentum dynamics (Medapalli et al., 2020, Mou et al., 2024).
• THz Emission Detection: Emitted THz pulses (bandwidth up to 10 THz) are focused and detected via electro-optic sampling (ZnTe or GaP crystals), with time-domain resolution down to 50 fs. Azimuthal and field-dependent measurements enable decomposition of symmetry contributions to the photocurrent, distinguishing surface vs. bulk effects, and linear vs. circular photogalvanic or drag response (Medapalli et al., 2020, Schmidt et al., 2015).
• Analysis Protocols: Magnetic field reversal, in-plane sample rotation, and polarization modulation isolate helicity-dependent and -independent currents; time-domain traces yield rise/decay dynamics. Angular-resolved measurements facilitate symmetry decomposition (Rashba vs. Dresselhaus) (Tong et al., 1 Jul 2025, Schmidt et al., 2016).

3. Materials Systems and Symmetry Considerations

Distinct materials exhibit qualitatively different ultrafast magneto-photocurrent mechanisms due to their symmetry, electronic structure, and magnetic order:

Materials System	Dominant Photocurrent Mechanism	Symmetry Features
FeRh/Pt, Co/Pt interfaces	ISHE spin pumping, ISOT	Interfacial C$_{4v}$, strong SOC
NiMnSb (half-metallic Heusler)	Dresselhaus bulk SGE	mm2, bulk inversion symmetry broken
Mn$_3$Sn (Weyl antiferromagnet)	Circular photon drag (CPDE)	Centrosymmetric bulk (6/mmm)
GaAs	Linear/circular magneto-photocurrent, surface ISHE	Zincblende, C$_{\infty v}$ surfaces
Bi$_2$Se$_3$ (topological insulator)	Spin–velocity locked SCC in TSS, magnetic-field-driven	Topological surface states, Dirac
2D AF semiconductors (CrSBr, MnPS$_3$)	Exciton-photocurrent driven spin torque, magnon pumping	AF order, interlayer exchange

Surface states, interface symmetry, and the presence or absence of inversion symmetry crucially determine whether the photocurrent is generated in the bulk, at the surface/interface, or via topological features (e.g., Dirac cones).

4. Ultrafast Dynamics, Timescales, and Quantitative Metrics

Transient magneto-photocurrents exhibit rise and decay governed by characteristic timescales and relaxation channels:

• Spin Pumping and Demagnetization: FeRh shows $\tau_\mathrm{demag}\sim100$–$200$ fs, with spin current injection peaking at $J_s\sim10^6$–$10^7$ A/m² and THz emission bandwidth up to 3 THz (Medapalli et al., 2020).
• ISOT-induced Photocurrent: The ISOT current constitutes only 10–20% of the demagnetization-driven one but is fully helicity- and magnetization-reversible (amplitude $\sim10^4$–$10^5$ A/m²) (Medapalli et al., 2020, Huisman et al., 2015).
• Electron Heating: Across F|HM systems, the ultrafast electron temperature rise dominates spin-current generation, with universal THz temporal profiles (peak $\sim0.2$ ps, decay $\sim1$ ps), independent of pump photon energy (Rouzegar et al., 1 Jul 2025).
• Dresselhaus SGE: In NiMnSb, the Dresselhaus photocurrent rises within $\tau_p\sim7$–$10$ fs (electron scattering) and decays over $T_\mathrm{ep}\sim0.25$ ps (electron–phonon cooling), not spin-lattice relaxation (Tong et al., 1 Jul 2025).
• Topological Insulator SCC: Bi$_2$Se$_3$ exhibits a TSS rise time $T_\mathrm{TSS}\sim100$ fs and a thickness-dependent drift decay $T_j\sim100$–$500$ fs. THz emission amplitude scales linearly with applied field and is quenched by indium-induced gap opening in the TSS (In et al., 3 Jan 2026).
• Photon Drag: In Mn$_3$Sn, CPDE and LPDE amplitudes are comparable; THz emission is helicity-tunable and persists under field reversal, indicating topologically robust, magnetization-independent generation (Hamara et al., 2023).

5. Control Parameters and Dependence on External Perturbations

Magneto-photocurrent generation is tunable via external magnetic field, light polarization, sample temperature, photon energy, and sample orientation:

• Magnetic Field Dependence: Most mechanisms (demagnetization-driven spin pumping, surface ISHE in GaAs, SCC in Bi$_2$Se$_3$) scale linearly with applied field, reversing sign with $B\rightarrow -B$ or $M\rightarrow -M$ (Medapalli et al., 2020, In et al., 3 Jan 2026, Schmidt et al., 2015).
• Polarization Control: Helicity-dependent currents (ISOT, CPDE) flip sign upon changing light helicity or magnetization; linear-polarization mechanisms yield non-helicity current contributions (e.g., vertical spin pumping) (Medapalli et al., 2020, Hamara et al., 2023).
• Photon Energy Insensitivity: Across FM/HM stacks, TSC dynamics are unaffected by photon energy (1.5 vs. 3 eV), suggesting dominance of ultrafast electron heating over resonant excitation mechanisms (Rouzegar et al., 1 Jul 2025).
• Temperature and Magnetic Phase: In FeRh, the FM fraction (temperature-dependent) controls photocurrent amplitude; ISOT and spin-pumping vanish in the AFM phase (Medapalli et al., 2020).
• Thickness and Volume Scaling: In bulk SGE (NiMnSb), sheet current amplitude scales linearly with film thickness, reflecting the volume-traversing nature of Dresselhaus conversion; interface mechanisms are thickness-independent (Tong et al., 1 Jul 2025).

6. Implications, Applications, and Future Research Directions

Ultrafast magneto-photocurrents are central to the development of THz spintronic devices and contact-free probes of spin-charge conversion:

• On-chip THz Emitters and Detectors: Interfacial spin-pumping, ISOT, and bulk SGE mechanisms enable integration of broadband, polarization-tunable THz sources in magnetic nanodevices (Medapalli et al., 2020, Tong et al., 1 Jul 2025, Mou et al., 2024).
• Spin-Orbit Torque Control: All-optical switching and control of interfacial spin–orbit torques may lead to femtosecond magnetic memory and logic applications without external fields (Huisman et al., 2015).
• Non-invasive SCC Probes: THz emission spectroscopy in topological insulators quantifies spin–charge conversion in TSS without magnetic contacts, applicable to paramagnetic and van der Waals systems (In et al., 3 Jan 2026).
• Ultrafast Carrier and Spin Dynamics: Linear surface magneto-photocurrents in GaAs serve as intrinsic probes of momentum and spin anisotropy relaxation, revealing electron–phonon and hole–band scattering channels (Schmidt et al., 2016, Schmidt et al., 2015).
• Photon Drag in Topological Antiferromagnets: Helicity-tunable ultrafast currents in Weyl magnets enable chiral THz emission, robust against magnetic disorder and stray fields (Hamara et al., 2023).
• Magnon Excitation and Charge Pumping: In 2D AF semiconductors, ultrafast photocurrents not only induce magnonic precession but also pump charge into contacts, offering direct probes of magnon–exciton interplay (Varela-Manjarres et al., 3 Jul 2025).

Key future directions involve tailoring interfacial and bulk symmetry for enhanced spin–charge conversion, optimizing electron heating and absorption pathways, and exploiting topological features for robust, scalable THz functionality in spintronic architectures.