arXiv subjects

T. Kampfrath

Publications and source records attributed to T. Kampfrath.

At least 19 recordsLinked to original sources

arXiv2026

Controlling Terahertz Spintronic Photocurrents in 2D-Semiconductor|Ferromagnet Heterostructures through a Functional Hybrid Interface

A profound understanding of terahertz (THz) spin and charge currents in heterostructures involving ferromagnets (FMs) and two-dimensional (2D) materials promises emerging applications in high-speed sensing and data processing. Yet, ultrafast experimental insights remain very limited. Here, we study the efficient photo-generation of THz spin and charge currents in bilayers made from the transition-metal dichalcogenide (TMD) MoS2 and the FM Co. We find that the efficiency of current generation strongly depends on the pump photon energy, as previously reported. Surprisingly, however, we observe that the current dynamics remain identical for pump photon energies above and below the MoS2 band gap. Supported by ab-initio calculations, we conclude that an interfacial hybrid metallic layer forms at the MoS2/Co boundary that has a pronounced photon-energy-dependent absorptance. Thus, the hybrid interfacial layer effectively acts like a pump-energy transducer that increases the spin-current generated in the nearby Co. Our results uncover the vital role of interfacial hybridization as a yet unexplored mechanism for efficient generation of ultrafast photocurrents in 2D-TMD|FM structures.

cond-mat.mes-hall↗

arXiv2025

Spin-to-charge-current conversion in altermagnetic candidate RuO$_2$ probed by terahertz emission spectroscopy

Using the THz emission spectroscopy, we investigate ultrafast spin-to-charge current conversion in epitaxial thin films of the altermagnetic candidate RuO$_2$. We perform a quantitative analysis of competing effects that can contribute to the measured anisotropic THz emission. These include the anisotropic inverse spin splitter and spin Hall effects in RuO$_2$, the anisotropic conductivity of RuO$_2$, and the birefringence of the TiO$_2$ substrate. We observe that the leading contribution to the measured signals comes from the anisotropic inverse spin Hall effect, with an average spin-Hall angle of $2.4\times 10^{-3}$ at room temperature. In comparison, a possible contribution from the altermagnetic inverse spin-splitter effect is found to be approximately $2-4\times 10^{-4}$. Our work stresses the importance of carefully disentangling spin-dependent phenomena that can be generated by the unconventional altermagnetic order, from the effects of the relativistic spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2025

Observation of Ultrafast Coherent and Incoherent Spin Torques via Terahertz Spin-Hall Magnetoresistance

Spin Hall magnetoresistance (SMR) is a cornerstone effect in spintronics, enabling the electrical readout of magnetic states through spin-orbit coupling at interfaces. Despite its widespread use, experimental studies of SMR are limited to static (DC) or low-frequency (GHz) regimes, leaving the ultrafast dynamics of interfacial spin transport in the terahertz (THz) frequency window entirely unexplored. Here, we report the observation of SMR spanning from DC to 1.5 THz in the model SMR system YIG/Pt, bridging the divide between electronic transport and THz optics. We demonstrate that the SMR response exhibits a pronounced low-pass behavior, decreasing significantly with frequency and vanishing above 1.5 THz. Through a dynamic model, we reveal that this unexpected spectral decay arises from a fundamental competition between two distinct interfacial spin-magnetization coupling mechanisms: the frequency-independent coherent spin torque, and an incoherent torque mediated by thermal magnons. Our analysis shows that the reduced THz SMR signal reflects the nonzero response time of the magnon chemical potential, which facilitates the increased outflow of incoherent magnons through the interface at high frequencies. These results resolve the microscopic nature of spin-torque generation at ultrafast timescales. Crucially, this work establishes THz SMR as a powerful, non-contact methodology capable of probing spin-magnon coupling in magnetic thin films, providing the essential tool for advancing the next generation of spintronic devices, particularly those based on high-frequency antiferromagnets and emerging altermagnetic materials.

cond-mat.mes-hall↗

arXiv2025

Efficient broadband terahertz generation by above band-gap excitation of the pyroelectric ZnSnN2

Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 to above 30 THz. Our results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN2. Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. Our study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

cond-mat.mtrl-sci↗

arXiv2023

Broadband spintronic detection of the absolute field strength of terahertz electromagnetic pulses

We demonstrate detection of broadband intense terahertz electromagnetic pulses by Zeeman-torque sampling (ZTS). Our approach is based on magneto-optic probing of the Zeeman torque the terahertz magnetic field exerts on the magnetization of a ferromagnet. Using an 8 nm thick iron film as sensor, we detect pulses from a silicon-based spintronic terahertz emitter with bandwidth 0.1-11 THz and peak field >0.1 MV/cm. Static calibration provides access to absolute transient THz field strengths. We show relevant added values of ZTS compared to electro-optic sampling (EOS): an absolute and echo-free transfer function with simple frequency dependence, linearity even at high terahertz field amplitudes, the straightforward calibration of EOS response functions and the modulation of the polarization-sensitive direction by an external AC magnetic field. Consequently, ZTS has interesting applications even beyond the accurate characterization of broadband high-field terahertz pulses for nonlinear terahertz spectroscopy.

physics.optics↗

arXiv2023

Terahertz spin conductance probes of coherent and incoherent spin tunneling through MgO tunnel junctions

We study femtosecond spin currents through MgO tunneling barriers in CoFeB(2 nm)|MgO($d$)|Pt(2 nm) stacks by terahertz emission spectroscopy. To obtain transport information independent of extrinsic experimental factors, we determine the complex-valued spin conductance $\tilde{G}_d (ω)$ of the MgO layer (thickness d= 0-6 Å over a wide frequency range $(ω/2π=$ 0.5-8 THz). In the time $(t)$ domain,$ G_d (t)$ has an instantaneous and delayed component that point to (i) spin transport through Pt pinholes in MgO, (ii) coherent spin tunneling and (iii) incoherent resonant spin tunneling mediated by defect states in MgO. A remarkable signature of (iii) is its relaxation time that grows monotonically with $d$ to as much as 270 fs at $d= 6$ Å, in full agreement with an analytical model. Our results indicate that terahertz spin conductance spectroscopy will yield new and relevant insights into ultrafast spin transport for a wide range of materials.

physics.app-ph↗

arXiv2022

Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin-phonon interactions

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modelling, two excitation processes of magnetization dynamics in NiO, an off-resonant instantaneous optical spin torque and a strain-wave-induced THz torque induced by ultrafast Pt excitation. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2022

Circular photogalvanic effects in topological insulator/ferromagnet hybrid structures

We study laser driven spin-current effects at ferromagnet/topological-insulator interfaces by two complementary experimental approaches. The DC photocurrent is studied in ferromagnet/topological-insulator bilayers with high spatial resolution. Dynamic interface currents are explored via the emission of terahertz radiation emitted by these currents with high temporal resolution. From our experiments, we reveal a lateral and dynamic interaction of the ferromagnet and the topological insulator interface.

cond-mat.mes-hall↗

arXiv2021

Terahertz spin currents and inverse spin Hall effect in thin-film heterostructures containing complex magnetic compounds

Terahertz emission spectroscopy of ultrathin multilayers of magnetic and heavy metals has recently attracted much interest. This method not only provides fundamental insights into photoinduced spin transport and spin-orbit interaction at highest frequencies but has also paved the way to applications such as efficient and ultrabroadband emitters of terahertz electromagnetic radiation. So far, predominantly standard ferromagnetic materials have been exploited. Here, by introducing a suitable figure of merit, we systematically compare the strength of terahertz emission from X/Pt bilayers with X being a complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt (DyCo$_5$), gadolinium iron (Gd$_{24}$Fe$_{76}$), Magnetite (Fe$_3$O$_4$) and iron rhodium (FeRh). We find that the performance in terms of spin-current generation not only depends on the spin polarization of the magnet's conduction electrons but also on the specific interface conditions, thereby suggesting terahertz emission spectroscopy to be a highly surface-sensitive technique. In general, our results are relevant for all applications that rely on the optical generation of ultrafast spin currents in spintronic metallic multilayers.

cond-mat.mtrl-sci↗

arXiv2021

Optically gated terahertz-field-driven switching of antiferromagnetic CuMnAs

We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic CuMnAs thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the time scale of the suppression. As we do not observe a direct impact of the optical pulse on the state of CuMnAs, all observed effects are primarily mediated by the substrate. The sample region of suppressed resistance switching is given by the optical spot size, thereby making our scheme potentially applicable for transient low-power masking of structured areas with feature sizes of ~100 nm and even smaller.

physics.app-ph↗

arXiv2021

Ultrafast demagnetization of iron induced by optical vs terahertz pulses

We study ultrafast magnetization quenching of ferromagnetic iron following excitation by an optical vs a terahertz pump pulse. While the optical pump (photon energy of 3.1 eV) induces a strongly nonthermal electron distribution, terahertz excitation (~4 meV) results in a quasi-thermal perturbation of the electron population. The pump-induced spin and electron dynamics are interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure allows us to push the time resolution down to 130 fs, even though the driving terahertz pulse is more than 0.5 ps long. Remarkably, the MOKE signals exhibit an almost identical time evolution for both optical and terahertz pump pulses, despite the three orders of magnitude different number of excited electrons. We are able to quantitatively explain our results using a model based on quasi-elastic spin-flip scattering. It shows that in the small-perturbation limit, the rate of demagnetization of a metallic ferromagnet is proportional to the excess energy of the electrons, independent of the precise shape of their distribution. Our results reveal that the dynamics of ultrafast demagnetization and of the closely related terahertz spin transport do not depend on the pump photon energy.

physics.optics↗

arXiv2021

Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we use broadband terahertz emission spectroscopy to reliably measure both processes in one setup. We find that the rate of UDM of a single ferromagnetic metal film F has the same time evolution as the flux of TST from F into an adjacent normal-metal layer N. This remarkable agreement shows that UDM and TST are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model consistently and quantitatively explains our observations. It reveals that both UDM in F and TST in the F|N stack arise from a generalized spin voltage, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between F and N are minor and that the spin-current amplitude can, in principle, be increased by one order of magnitude. In general, our findings allow one to apply the vast knowledge of UDM to TST, thereby opening up new pathways toward large-amplitude terahertz spin currents and, thus, energy-efficient ultrafast spintronic devices.

cond-mat.mes-hall↗

arXiv2018

Observation of spin Nernst photocurrents in topological insulators

The theoretical prediction of topological insulators in 2007 triggered tremendous interest. They are of fundamental interest because of their topological twist in k-space, which comes along with unidirectional, spin-polarized surface-state currents, required for spin-optoelectronics. This property makes topological insulators on one hand perfect materials for optically generated, ultrafast spin-bunches spin-current sources for the generation of THz radiation. On the other hand, those spin-polarized surface-state currents when generated by a voltage lead to large spin Hall effects, or when generated by a temperature gradient to the thermal analogue, the spin Nernst effect. Both mutually convert charge/ heat currents into transverse spin currents leading to spin accumulations. By connecting both research fields, we show the evidence of heat-transport related spin Hall effects that can be extracted from opto-transport experiments. This heat-driven spin Nernst effect drives a transverse spin-current and affects the optical spin-orientation in the three-dimensional topological insulator. This manifests as a modification of the circular polarization-dependent photocurrent. We illuminate the detailed thermocurrent distribution, including the influence of edges and contacts, in spatially resolved current maps.

cond-mat.mes-hall↗

arXiv2018

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto $j_\textrm{s}$ because their spins have a correlation time of only ~4fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

cond-mat.mes-hall↗

arXiv2018

Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

We systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic Fe layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially at for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO2 and SiO2, designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of of up to two in field.

physics.optics↗

arXiv2018

Magnetic field dependence of antiferromagnetic resonance in NiO

We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles.

cond-mat.mtrl-sci↗

arXiv2017

THz electrical writing speed in an antiferromagnetic memory

The speed of writing of state-of-the-art ferromagnetic memories is physically limited by an intrinsic GHz threshold. Recently, an alternative research direction has been initiated by realizing memory devices based on antiferromagnets in which spin directions periodically alternate from one atomic lattice site to the next. In our work we experimentally demonstrate at room temperature that the speed of reversible electrical writing in a memory device can be scaled up to THz using an antiferromagnet. Efficient current-induced spin-torque mechanism is responsible for the switching in our memory devices throughout the twelve orders of magnitude range of writing speeds from Hz to THz. Our work opens the path towards the development of memory-logic technology reaching the elusive THz band.

physics.app-ph↗

arXiv2017

Antiferromagnetic opto-spintronics: Part of a collection of reviews on antiferromagnetic spintronics

Control and detection of spin order in ferromagnets is the main principle allowing storing and reading of magnetic information in nowadays technology. The large class of antiferromagnets, on the other hand, is less utilized, despite its very appealing features for spintronics applications. For instance, the absence of net magnetization and stray fields eliminates crosstalk between neighbouring devices and the absence of a primary macroscopic magnetization makes spin manipulation in antiferromagnets inherently faster than in ferromagnets. However, control of spins in antiferromagnets requires exceedingly high magnetic fields, and antiferromagnetic order cannot be detected with conventional magnetometry. Here we provide an overview and illustrative examples of how electromagnetic radiation can be used for probing and modification of the magnetic order in antiferromagnets. Spin pumping from antiferromagnets, propagation of terahertz spin excitations, and tracing the reversal of the antiferromagnetic and ferroelectric order parameter in multiferroics are anticipated to be among the main topics defining the future of this field.

cond-mat.mtrl-sci↗