Stable π-Extended Thio[7]helicene-Based Diradical with Predominant Through-Space Spin-Spin Coupling.

In this work, a novel π-extended thio[7]helicene scaffold was synthesized, where the α-position of the thiophene unit could be functionalized with bulky phenoxy radicals after considerable synthetic attempts. This open-shell helical diradical, ET7H-R, possesses high stability in the air, nontrivial π conjugation, persistent chirality, and a high diradical character (y0 of 0.998). The key feature is a predominant through-space spin-spin coupling (TSC) between two radicals at the helical terminals. Variable-temperature continuous-wave electron spin resonance (cw-ESR) and superconducting quantum interference device (SQUID) magnetometry in the solid state reveal a singlet ground state with a nearly degenerate triplet state of ET7H-R. These results highlight the significance of a stable helical diradicaloid as a promising platform for investigating intramolecular TSCs.

Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2.

Altermagnets are an emerging elementary class of collinear magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of time-reversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO2 by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic structure basis for a family of interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

Enhanced thermally-activated skyrmion diffusion with tunable effective gyrotropic force.

Magnetic skyrmions, topologically-stabilized spin textures that emerge in magnetic systems, have garnered considerable interest due to a variety of electromagnetic responses that are governed by the topology. The topology that creates a microscopic gyrotropic force also causes detrimental effects, such as the skyrmion Hall effect, which is a well-studied phenomenon highlighting the influence of topology on the deterministic dynamics and drift motion. Furthermore, the gyrotropic force is anticipated to have a substantial impact on stochastic diffusive motion; however, the predicted repercussions have yet to be demonstrated, even qualitatively. Here we demonstrate enhanced thermally-activated diffusive motion of skyrmions in a specifically designed synthetic antiferromagnet. Suppressing the effective gyrotropic force by tuning the angular momentum compensation leads to a more than 10 times enhanced diffusion coefficient compared to that of ferromagnetic skyrmions. Consequently, our findings not only demonstrate the gyro-force dependence of the diffusion coefficient but also enable ultimately energy-efficient unconventional stochastic computing.

Fiber-tip spintronic terahertz emitters.

Spintronic terahertz emitters promise terahertz sources with an unmatched broad frequency bandwidth that are easy to fabricate and operate, and therefore easy to scale at low cost. However, current experiments and proofs of concept rely on free-space ultrafast pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe designs are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. Using single-mode fiber waveguiding, the newly developed solution naturally leads to a simple and straightforward terahertz near-field imaging system with a 90%-10% knife-edge-response spatial resolution of 30 µm.

Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry.

Metallic spintronic terahertz (THz) emitters have become well-established for offering ultra-broadband, gapless THz emission in a variety of excitation regimes, in combination with reliable fabrication and excellent scalability. However, so far, their potential for high-average-power excitation to reach strong THz fields at high repetition rates has not been thoroughly investigated. In this article, we explore the power scaling behavior of tri-layer spintronic emitters using an Yb-fiber excitation source, delivering an average power of 18.5 W (7 W incident on the emitter after chopping) at 400 kHz repetition rate, temporally compressed to a pulse duration of 27 fs. We confirm that a reflection geometry with back-side cooling is ideally suited for these emitters in the high-average-power excitation regime. In order to understand limiting mechanisms, we disentangle the effects on THz power generation by average power and pulse energy by varying the repetition rate of the laser. Our results show that the conversion efficiency is predominantly determined by the incident fluence in this high-average-power, high-repetition-rate excitation regime if the emitters are efficiently cooled. Using these findings, we optimize the conversion efficiency and reach highest excitation powers in the back-cooled reflection geometry. Our findings provide guidelines for scaling the power of THz radiation emitted by spintronic emitters to the milliwatt-level by using state-of-the-art femtosecond sources with multi-hundred-Watt average power to reach ultra-broadband, strong-field THz sources with high repetition rate.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics.

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Observation of the Orbital Rashba-Edelstein Magnetoresistance.

We report the observation of magnetoresistance (MR) that could originate from the orbital angular momentum (OAM) transport in a permalloy (Py)/oxidized Cu (Cu^{*}) heterostructure: the orbital Rashba-Edelstein magnetoresistance. The angular dependence of the MR depends on the relative angle between the induced OAM and the magnetization in a similar fashion as the spin Hall magnetoresistance. Despite the absence of elements with large spin-orbit coupling, we find a sizable MR ratio, which is in contrast to the conventional spin Hall magnetoresistance which requires heavy elements. Through Py thickness-dependence studies, we conclude another mechanism beyond the conventional spin-based scenario is responsible for the MR observed in Py/Cu^{*} structures-originated in a sizable transport of OAM. Our findings not only suggest the current-induced torques without using any heavy elements via the OAM channel but also provide an important clue towards the microscopic understanding of the role that OAM transport can play for magnetization dynamics.

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni81 Fe19 , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

Assembly of iron oxide nanosheets at the air-water interface by leucine-histidine peptides.

The fabrication of inorganic nanomaterials is important for a wide range of disciplines. While many purely inorganic synthetic routes have enabled a manifold of nanostructures under well-controlled conditions, organisms have the ability to synthesize structures under ambient conditions. For example, magnetotactic bacteria, can synthesize tiny 'compass needles' of magnetite (Fe3O4). Here, we demonstrate the bio-inspired synthesis of extended, self-supporting, nanometer-thin sheets of iron oxide at the water-air interface through self-assembly using small histidine-rich peptides.

Harnessing Orbital-to-Spin Conversion of Interfacial Orbital Currents for Efficient Spin-Orbit Torques.

Current-induced spin-orbit torques (SOTs) allow for the efficient electrical manipulation of magnetism in spintronic devices. Engineering the SOT efficiency is a key goal that is pursued by maximizing the active interfacial spin accumulation or modulating the nonequilibrium spin density that builds up through the spin Hall and inverse spin galvanic effects. Regardless of the origin, the fundamental requirement for the generation of the current-induced torques is a net spin accumulation. We report on the large enhancement of the SOT efficiency in thulium iron garnet (TmIG)/Pt by capping with a CuO_{x} layer. Considering the weak spin-orbit coupling (SOC) of CuO_{x}, these surprising findings likely result from an orbital current generated at the interface between CuO_{x} and Pt, which is injected into the Pt layer and converted into a spin current by strong SOC. The converted spin current decays across the Pt layer and exerts a "nonlocal" torque on TmIG. This additional torque leads to a maximum colossal enhancement of the SOT efficiency of a factor 16 for 1.5 nm of Pt at room temperature, thus opening a path to increase torques while at the same time offering insights into the underlying physics of orbital transport, which has so far been elusive.

Phonon Bridge Effect in Superlattices of Thermoelectric TiNiSn/HfNiSn With Controlled Interface Intermixing.

The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3 ω -method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition.

Electric-Field Control of Spin-Orbit Torques in Perpendicularly Magnetized W/CoFeB/MgO Films.

Controlling magnetism by electric fields offers a highly attractive perspective for designing future generations of energy-efficient information technologies. Here, we demonstrate that the magnitude of current-induced spin-orbit torques in thin perpendicularly magnetized CoFeB films can be tuned and even increased by electric-field generated piezoelectric strain. Using theoretical calculations, we uncover that the subtle interplay of spin-orbit coupling, crystal symmetry, and orbital polarization is at the core of the observed strain dependence of spin-orbit torques. Our results open a path to integrating two energy efficient spin manipulation approaches, the electric-field-induced strain and the current-induced magnetization switching, thereby enabling novel device concepts.

Propagation Length of Antiferromagnetic Magnons Governed by Domain Configurations.

The compensated magnetic order and characteristic terahertz frequencies of antiferromagnetic materials make them promising candidates to develop a new class of robust, ultrafast spintronic devices. The manipulation of antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic phenomena such as spin-superfluidity, requiring an efficient propagation of spin-waves in thin films. However, the reported decay length in thin films has so far been limited to a few nanometers. In this work, we achieve efficient spin-wave propagation over micrometer distances in thin films of the insulating antiferromagnet hematite with large magnetic domains while evidencing much shorter attenuation lengths in multidomain thin films. Through transport and magnetic imaging, we determine the role of the magnetic domain structure and spin-wave scattering at domain walls to govern the transport. We manipulate the spin transport by tailoring the domain configuration through field cycle training. For the appropriate crystalline orientation, zero-field spin transport is achieved across micrometers, as required for device integration.

Thermal skyrmion diffusion used in a reshuffler device.

Magnetic skyrmions in thin films can be efficiently displaced with high speed by using spin-transfer torques1,2 and spin-orbit torques3-5 at low current densities. Although this favourable combination of properties has raised expectations for using skyrmions in devices6,7, only a few publications have studied the thermal effects on the skyrmion dynamics8-10. However, thermally induced skyrmion dynamics can be used for applications11 such as unconventional computing approaches12, as they have been predicted to be useful for probabilistic computing devices13. In our work, we uncover thermal diffusive skyrmion dynamics by a combined experimental and numerical study. We probed the dynamics of magnetic skyrmions in a specially tailored low-pinning multilayer material. The observed thermally excited skyrmion motion dominates the dynamics. Analysing the diffusion as a function of temperature, we found an exponential dependence, which we confirmed by means of numerical simulations. The diffusion of skyrmions was further used in a signal reshuffling device as part of a skyrmion-based probabilistic computing architecture. Owing to its inherent two-dimensional texture, the observation of a diffusive motion of skyrmions in thin-film systems may also yield insights in soft-matter-like characteristics (for example, studies of fluctuation theorems, thermally induced roughening and so on), which thus makes it highly desirable to realize and study thermal effects in experimentally accessible skyrmion systems.

Microstructure Design for Fast Lifetime Measurements of Magnetic Tunneling Junctions.

The estimation of the reliability of magnetic field sensors against failure is a critical point concerning their application for industrial purposes. Due to the physical stochastic nature of the failure events, this can only be done by means of a statistical approach which is extremely time consuming and prevents a continuous observation of the production. Here, we present a novel microstructure design for a parallel measurement of the lifetime characteristics of a sensor population. By making use of two alternative designs and the Weibull statistical distribution function, we are able to measure the lifetime characteristics of a CoFeB/MgO/CoFeB tunneling junction population. The main parameters governing the time evolution of the failure rate are estimated and discussed and the suitability of the microstructure for highly reliable sensor application is proven.

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 <27 fs 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 js arises on the same ~100 fs 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 js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

Complex Terahertz and Direct Current Inverse Spin Hall Effect in YIG/Cu1-xIrx Bilayers Across a Wide Concentration Range.

We measure the inverse spin Hall effect of Cu1-xIrx thin films on yttrium iron garnet over a wide range of Ir concentrations (0.05 ⩽ x ⩽ 0.7). Spin currents are triggered through the spin Seebeck effect, either by a continuous (dc) temperature gradient or by ultrafast optical heating of the metal layer. The spin Hall current is detected by electrical contacts or measurement of the emitted terahertz radiation. With both approaches, we reveal the same Ir concentration dependence that follows a novel complex, nonmonotonous behavior as compared to previous studies. For small Ir concentrations a signal minimum is observed, whereas a pronounced maximum appears near the equiatomic composition. We identify this behavior as originating from the interplay of different spin Hall mechanisms as well as a concentration-dependent variation of the integrated spin current density in Cu1-xIrx. The coinciding results obtained for dc and ultrafast stimuli provide further support that the spin Seebeck effect extends to terahertz frequencies, thus enabling a transfer of established spintronic measurement schemes into the terahertz regime. Our findings also show that the studied material allows for efficient spin-to-charge conversion even on ultrafast time scales.

Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study.

Two stable nitronyl nitroxide free radicals {R 1 = 4'-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R 2 = 2-(2'-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac)2 (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal-organic complexes Co(hfac)2(NNPhOMe)2, 1; Co(hfac)2(NNT)2·(H2O), 2; Mn(hfac)2(NNPhOMe)·x(C7H16), 3; and Mn(hfac)2(NNT)2, 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P1̅, space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P21/c space group in which the N-O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/K B) are derived using isotropic spin Hamiltonian H = -2J∑(S metal S radical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the 1, 2, 3, and 4 metal-organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal-radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.

CADEM: calculate X-ray diffraction of epitaxial multilayers.

Epitaxial multilayers and superlattice (SL) structures are gaining increasing importance as they offer the opportunity to create artificial crystals with new functionalities. These crystals deviate from the parent bulk compounds not only in terms of the lattice constants but also in the symmetry classification, which renders calculation of their X-ray diffraction (XRD) patterns tedious. Nevertheless, XRD is essential to get information on the multilayer/SL structure such as, for example, out-of-plane lattice constants, strain relaxation and period length of the crystalline SL. This article presents a powerful yet simple program, based on the general one-dimensional kinematic X-ray diffraction theory, which calculates the XRD patterns of tailor-made multilayers and thus enables quantitative comparison of measured and calculated XRD data. As the multilayers are constructed layer by layer, the final material stack can be entirely arbitrary. Moreover, CADEM is very flexible and can be straightforwardly adapted to any material system. The source code of CADEM is available as supporting material for this article.

Origin of the spin Seebeck effect in compensated ferrimagnets.

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.