Peter M. Oppeneer

We theoretically investigate the ultrafast generation of spin and orbital magnetism in a two-dimensional gapped Dirac system with spin-orbit coupling. This system is representative of two-dimensional hexagonal semiconductors, such as transition-metal dichalcogenides that exhibit valley-selective optical selection rules arising from the valley-contrasting magnetic texture of their band structure. Using a time-dependent density-matrix formalism, we demonstrate that circularly polarized laser pulses generate nonequilibrium magnetization under both resonant and multiphoton resonant conditions. We show that the induced spin and orbital magnetic moments can be distinctly controlled via the photon energy and polarization of the driving field. Furthermore, spin and orbital dynamics originate from fundamentally different light-matter coupling mechanisms, leading to qualitatively dissimilar temporal behaviors. The orbital magnetic moment couples directly to the external electric field, resulting in faster dynamics and pronounced Rabi-like oscillations, whereas the spin response develops gradually through spin-orbit coupling. Consequently, orbital dynamics is significantly more sensitive to electron-hole dephasing than the spin response. Our results highlight the importance of properly accounting for orbital contributions in future technologies that utilize femtosecond control of magnetism.

The authors pursue an interesting idea, to derive the electron's anomalous magnetic moment from classical fields. This is certainly not a trivial matter. The authors carefully state that their work is not intended to replace the results of previous q...

Orbital angular momentum (OAM) has emerged as an important information carrier in solids, yet how scattering influences its transport remains poorly understood. Here, we examine scattering-dependent OAM transport in single-layer, highly disordered, nanocrystalline Mn films. We quantify current-induced orbital accumulation using longitudinal magneto-optical Kerr effect measurements while systematically varying the disorder strength as gauged by their resistivity. The Kerr signal is undetectable in the least resistive films but increases sharply in more resistive films, implying that sufficiently strong disorder scattering enhances OAM transport. Thickness-dependent measurements yield orbital relaxation lengths on the order of 10 nm, far exceeding the mean free path. Moreover, the orbital accumulation exhibits an anomalous sign, which is attributed to an intrinsic boundary contribution associated with the crystal field. These results, supported by first-principles calculations, illustrate that electron scattering may even enhance orbital transport, establishing scattering control as a practical knob for boosting the orbitronic functionality.

Terahertz radiation pulses can be generated efficiently through femtosecond laser excitation of a ferromagnetic/nonmagnetic heterostructure, wherein an ultrafast laser-induced spin current results in an electromagnetic THz pulse due to spin-charge conversion. It is, however, still poorly understood how the THz emission amplitude and its bandwidth in the frequency regime can be optimized. Here, we perform a systematic analysis of the THz emission from various magnetic heterostructures. The dynamics of the spin current is described by the semiclassical, superdiffusive spin-transport model, and the energy dependence of the spin Hall effect of hot electrons is taken into account, leading to emission profiles for Co ( 2 nm ) / Pt ( 4 nm ) bilayer in good agreement with experiment. To identify the optimal conditions for THz emission, we study the properties of the emitted THz wave profile by systematically varying the layer thicknesses of metallic bilayers, their interfacial spin-current transmission properties, their materials’ dependence, and the influence of the pump laser-pulse width, allowing us to give optimization guidelines. We find that thin nonmagnetic layer thicknesses of 5–6 nm provide the largest bandwidth in the case of Co / Pt and that the peak frequency of the THz emission depends only on the geometry of the emitter and not on the laser pulse width. The THz bandwidth is conversely found to depend on several factors such as exciting laser pulse width, layers’ thicknesses, and interface transmission-reflection properties, with the limitation that an increase in the bandwidth by tuning the interface properties comes with a trade-off in the energy efficiency of the emitter. Finally, we propose a double pulse excitation protocol of a trilayer system that could provide broadband THz emission with a large bandwidth. Our results contribute to establishing guidelines for optimizing spintronic THz generation.

Ultrafast control of lattice motion in metals is a central challenge for high-frequency strain engineering and spintronic applications. Coherent strain control at terahertz (THz) frequencies in metals has remained elusive because free electrons are expected to delocalize energy beyond the optical penetration depth, preventing rapid and efficient stress generation. Here we show that robust and cost-effective metal–metal superlattices (SLs), where periodic repetitions of bilayers — each layer a few atoms thick — are deposited by sputtering, constitute thermoacoustic metamaterials that overcome this limitation. We combine femtosecond X-ray diffraction with mode-resolved density-functional theory and two-temperature modeling to show that electron pressure, rather than phonon stress, drives a large-amplitude coherent terahertz (1 THz) lattice oscillation in sputtered Pt/Cu superlattices. We establish electron pressure as an engineerable, dominant actuation mechanism in metallic metamaterials which can be tailored by the pitch and the constituent materials of the sputtered SL structure, enabling applications such as ultrafast strain-mediated antiferromagnetic spintronic devices.

Orbital angular momentum (OAM) is getting more attention as an important information carrier in solids, yet it remains poorly understood how its transport is influenced by scattering. While previous studies on OAM transport examined epitaxial and polycrystalline systems with less frequent scattering, we examine single-layer amorphous Mn films with more frequent scattering. We quantify OAM transport using longitudinal magneto-optical Kerr effect measurements while systematically varying the film thickness and disorder, as gauged by its resistivity (3.6, 5.0, and 7.0 μΩ·m). The complex Kerr angle is undetectable in the relatively weakly resistive films (3.6 μΩ·m), implying the suppression of OAM transport in weakly disordered films. However, in more resistive films (5.0 μΩ·m and 7.0 μΩ·m), the Kerr angle enhances significantly, corresponding to orbital relaxation lengths of 10–15 nm, implying that sufficiently strong disorder scattering enhances OAM transport. Another important source of scattering is the boundary scattering at film surfaces. We find that the sign of the orbital accumulation is abnormal, which is attributed to the interplay between the boundary scattering and the crystal field. Thus, our result illustrates that electron scattering is not necessarily harmful and may even enhance the orbital transport, establishing scattering control as a practical knob for boosting orbitronic functionality.

Charge-to-orbital conversion via the orbital Rashba-Edelstein effect represents a key functionality for orbitronics but has been challenging to identify. Here, we combine first-principles density functional theory, linear-response theory, and magneto-optical modeling to reveal how this effect can be detected optically through the quadratic magneto-optical Voigt effect in magnetic/nonmagnetic heavy-metal bilayers. We find that, in a cobalt/platinum bilayer, the current-induced orbital angular momentum can exceed the spin contribution by nearly a factor of three and produce a strong optical signal in addition to the equilibrium Voigt effect. Our atom-resolved study reveals that the platinum layer, despite being nominally nonmagnetic, can contribute strongly because of proximity-induced and current-induced magnetic moments. These results establish magneto-optical detection as a route to probe interfacial orbital phenomena in magnetic heterostructures.

The ultrafast magnetization dynamics of an epitaxial Fe / CoO bilayer on Ag(001) is examined in an element-resolved way by resonant soft-x-ray reflectivity. The transient magnetic linear dichroism at the Co L 2 edge and the magnetic circular dichroism at the Fe L 3 edge measured in reflection in a pump–probe experiment with 120 fs temporal resolution show the loss of antiferromagnetic and ferromagnetic order in CoO and Fe, respectively, both within 300 fs after excitation with 60 fs light pulses of 800 and 400 nm wavelengths. A comparison to spin-dynamics simulations using an atomistic spin model shows that direct energy transfer from the laser-excited electrons in Fe to the magnetic moments in CoO provides the dominant demagnetization channel in the case of 800-nm excitation.

The manipulation of magnetization in ferromagnetic metals (FMs) through orbital torque (OT) has emerged as a promising route for energy-efficient magnetic devices without relying on heavy metals. While Ti and Cu are among the most extensively studied light nonmagnetic metals (NMs) for OT devices, theoretical calculations of the resulting torque have remained limited. Here, we present a systematic and quantitative theoretical study of current-induced torques in Ti/FM and Cu/FM (FM = Co, Ni) bilayers using realistic tight-binding models derived from ab initio electronic structures. We find that the torque in Ti/FM is larger for Ni than for Co, but this trend does not necessarily hold in Cu/FM, revealing that the FM dependence of OT is not universal but varies with the orbital current source. Moreover, the dependence of OT on NM thickness clearly indicates its NM bulk origin in both Ti- and Cu-based systems. Notwithstanding, the quantitative characteristics of OT cannot be explained by a simplified picture based on the individual bulk properties of the NM or FM layers. These results provide microscopic insight and practical guidance for designing light-metal-based orbitronic devices.