Contact

Dr. Igor Ilyakov

i.ilyakovAthzdr.de
Phone: +49 351 260 3616

Dr. Ruslan Salikhov

r.salikhovAthzdr.de
Phone: +49 351 260 3758

THz Spintronics: Ultrafast Magnetization Dynamics and Spin Transport

THz spintronics exploits ultrafast control of electron spin angular momentum to advance magnetic memories, sensors, THz emitters, and information technologies operating at THz frequencies, i.e. on sub-picosecond time scales. At TELBE, this research is enabled by a unique combination of intense, phase-stable THz pulses, high-repetition rates, and precise synchronization with femtosecond laser probes, providing an ideal platform for studying coherent spin dynamics on picosecond and sub-picosecond timescales with outstanding sensitivity. In contrast to conventional optical excitation, THz electric fields coherently drive charge and spin currents, spin-orbit torques, and magnetization dynamics without introducing high-energy electronic excitations. This provides exceptional opportunities to investigate the microscopic mechanisms governing ultrafast spin transport and spin-charge conversion in magnetic materials and heterostructures.

THz nonlinear spectroscopy of spin transport and spin-orbit phenomena

Using THz harmonic generation spectroscopy, we investigate ultrafast spin-dependent transport and nonlinear spintronic phenomena in magnetic thin-film heterostructures. Here, the oscillating THz electric field drives either ultrafast spin Seebeck currents across magnetic heterostructures [1] or lateral charge currents that generate interfacial spin accumulation through spin Hall and spin-Edelstein effects [2, 3]. The resulting spin-dependent transport and modulation of the interface resistance produces nonlinear THz signals, providing direct access to coherent spin-current generation, spin-charge interconversion, and ultrafast magnetoresistance on picosecond timescales.

 

Foto: Nonlinear THz Spintronics ©Copyright: Dr. Igor Ilyakov

Figure 1: a Schematic representation of the THz conversion process. The THz-pump pulse with central frequency Ω (green curve) illuminates the Pt/Py/Ta sample, resulting in an electrical current jE in the whole heterostructure. The Py layer is magnetized along the z-axis (in plane). An ultrafast spin current, js (blue arrows in the Py layer) is generated via the spin Seebeck effect and results in an electrical current jISHE in the Pt and Ta layers (blue arrows) via the ISHE. The jISHE in both heavy metals causes emission of THz radiation at each half of the period of the incident THz wave resulting in second harmonic (Ω + Ω) and rectified zero frequency generation (Ω − Ω). b Time-domain and c corresponding FFT spectra of the near-field signals from the Pt(2 nm)/Py(2 nm)/Ta(3 nm) sample exposed to the 0.5 THz radiation. The green curve in (b) corresponds to the fundamental radiation (Ω) and is scaled by 3 × 10−4. The gray curve corresponds to the total measured near-field signal and contains both TSHG and TOR signals, which afterward are separated by implementing FFT filters (violet and red curves).

Our work has demonstrated efficient THz-driven spin-current generation in ferromagnet/heavy-metal heterostructures, enabling spintronic THz frequency conversion, second-harmonic generation, and optical rectification (Fig. 1). We have further established THz-frequency unidirectional spin Hall magnetoresistance as an ultrafast mechanism for electrically reading magnetic states, revealing the central role of electron-magnon spin-flip scattering in picosecond spin transport. Extending these studies beyond magnetic heterostructures, we discovered a universal nonlinear response in transition metals governed by spin-orbit interaction, linking THz third-harmonic generation directly to spin Hall conductivity and providing new insight into nonequilibrium spin and orbital polarization dynamics [4].

These experiments establish THz nonlinear spectroscopy as a powerful tool for studying ultrafast spin transport, spin-orbit coupling, and charge-spin conversion in materials relevant for next-generation spintronic and orbitronic devices.

THz pump–optical probe spectroscopy of ultrafast magnetization dynamics

THz pump-optical probe spectroscopy combines intense THz excitation with synchronized femtosecond laser pulses to directly observe coherent magnetization dynamics. Depending on the material system, THz fields excite magnetization either directly through magnetic-field torques or indirectly via interfacial spin-orbit torques generated by ultrafast spin currents. Time-resolved magneto-optical detection follows the ensuing spin dynamics with sub-picosecond temporal resolution.

Experiments at TELBE have provided the first direct observation of intrinsic inertial spin dynamics (magnetization nutation) in ferromagnets [5], establishing the fundamental timescale of angular momentum relaxation. By exploiting interfacial spin-orbit torques, we demonstrated efficient excitation of exchange magnons [6] with nanometer wavelengths and frequencies approaching 1 THz, overcoming the momentum mismatch that traditionally limits optical access to high-energy spin waves (Fig. 2). We also revealed that ultrafast spin-orbit torques can be generated in heavy-metal-free oxide-engineered ferromagnets, showing that atomic-scale interface oxidation enables highly efficient picosecond magnetization control while offering a sustainable alternative to conventional heavy-metal heterostructures [3].

Foto: THz pump–optical probe Spintronics ©Copyright: Dr. Igor Ilyakov

Figure 2: (left) Schematic of the experiment. The single-cycle terahertz pulse is incident on the Ta/Py/Pt sample, which excites standing SWR modes in the Py layer. The z-axis dynamic component of the Py magnetization is detected using a 100 fs laser-probe pulse via the Faraday rotation effect. (right) Resonance frequency of the n = 2 mode as a function of Py thickness (black circles). The red line is a fit to all data points, representing the universal 1/dPy2 behaviour for n = 2 modes in Py films with different thickness.

Beyond ferromagnetic systems, THz pump-probe studies at TELBE explore coherent spin-lattice interactions in antiferromagnets. By realizing magnon-phonon Fermi resonance in CoF₂, controlled nonlinear energy exchange between magnetic and lattice excitations was demonstrated, providing new opportunities for manipulating coupled quasiparticles in antiferromagnetic spintronic materials [7].

Opportunities for external collaborators

We have established a comprehensive THz toolbox for investigating spintronic phenomena on picosecond timescales. By combining nonlinear THz spectroscopy with ultrafast THz pump-optical probe techniques, our experimental platform bridges conventional magnetotransport, ferromagnetic resonance, and THz emission spectroscopy. It enables quantitative studies of spin-orbit torques, spin-charge interconversion, and spin transport across a wide range of magnetic and non-magnetic materials, providing new opportunities for emerging research directions including orbitronics, antiferromagnetic spintronics, and altermagnetism.

We welcome collaborations with researchers interested in ultrafast spin dynamics, THz spin transport, spin-orbit torques, magnonics, orbitronics, and spin-lattice interactions. Our experimental platform combines the unique properties of TELBE's high-field, phase-stable THz pulses with state-of-the-art nonlinear THz spectroscopy and ultrafast optical pump-probe techniques, enabling investigations of a broad range of magnetic materials and heterostructures under well-controlled excitation conditions.

Potential user projects include:

  • THz-driven spin-current generation and spin-charge conversion.
  • Nonlinear THz spectroscopy of spin-orbit and orbital transport phenomena.
  • Ultrafast spin-orbit torque dynamics and magnetization switching.
  • Excitation and detection of coherent magnon modes from GHz to THz frequencies.
  • Spin-lattice coupling, magnon-phonon interactions, and nonequilibrium dynamics in ferro- and antiferromagnets.
  • Development and characterization of materials for THz spintronic and magnonic technologies.

References

[1] I. Ilyakov, A. Brataas, T. V. A. G. Oliveira, et al., Efficient ultrafast field-driven spin current generation for spintronic terahertz frequency conversion. Nat. Commun. 14, 7010 (2023).

[2] R. Salikhov, I. Ilyakov, A. Reinold, et al., Ultrafast unidirectional spin Hall magnetoresistance driven by terahertz light field. Nat. Commun. 16, 2249 (2025). [press release]

[3] K. Jin, S. Kober, I. Ilyakov et al., Ultrafast spin-orbit torques and spin-charge interconversion at oxidation-tailored NiFe/oxide interfaces. preprint (2026).

[4] Salikhov, R., Lysne, M., Werner, P. et al. Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals. npj Spintronics 3, 3 (2025). 

[5] Neeraj, K., Awari, N., Kovalev, S. et al. Inertial spin dynamics in ferromagnets. Nat. Phys. 17, 245–250 (2021).

[6] R. Salikhov, I. Ilyakov, L. Körber, et al., Coupling of terahertz light with nanometre-wavelength magnon modes via spin-orbit torque. Nat. Phys. 19, 529–535 (2023). [press release]

[7] Metzger, T.W.J., Grishunin, K.A., Reinhoffer, C. et al. Magnon-phonon Fermi resonance in antiferromagnetic CoF2. Nat Commun 15, 5472 (2024).