Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys.

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Energy Efficient Single Pulse Switching of [Co/Gd/Pt]N Nanodisks Using Surface Lattice Resonances.

The impact of plasmonic surface lattice resonances on the magneto-optical properties and energy absorption efficiency has been studied in arrays of [Co/Gd/Pt]N multilayer nanodisks. Varying the light wavelength, the disk diameter, and the period of the array, it is demonstrated that surface lattice resonances allow all-optical single pulse switching of [Co/Gd/Pt]N nanodisk arrays with an energy 400% smaller than the energy needed to switch a continuous [Co/Gd/Pt]N film. Moreover, the magneto-optical Faraday effect is enhanced at the resonance condition by up to 5,000%. The influence of the disk diameter and array period on the amplitude, width and position of the surface lattice resonances is in qualitative agreement with theoretical calculations and opens the way to designing magnetic metasurfaces for all-optical magnetization switching applications.

Quantum Advantage in a Molecular Spintronic Engine that Harvests Thermal Fluctuation Energy.

Recent theory and experiments have showcased how to harness quantum mechanics to assemble heat/information engines with efficiencies that surpass the classical Carnot limit. So far, this has required atomic engines that are driven by cumbersome external electromagnetic sources. Here, using molecular spintronics, an implementation that is both electronic and autonomous is proposed. The spintronic quantum engine heuristically deploys several known quantum assets by having a chain of spin qubits formed by the paramagnetic Co center of phthalocyanine (Pc) molecules electronically interact with electron-spin-selecting Fe/C60 interfaces. Density functional calculations reveal that transport fluctuations across the interface can stabilize spin coherence on the Co paramagnetic centers, which host spin flip processes. Across vertical molecular nanodevices, enduring dc current generation, output power above room temperature, two quantum thermodynamical signatures of the engine's processes, and a record 89% spin polarization of current across the Fe/C60 interface are measured. It is crucially this electron spin selection that forces, through demonic feedback and control, charge current to flow against the built-in potential barrier. Further research into spintronic quantum engines, insight into the quantum information processes within spintronic technologies, and retooling the spintronic-based information technology chain, can help accelerate the transition to clean energy.

Tunable Stochasticity in an Artificial Spin Network.

Metamaterials present the possibility of artificially generating advanced functionalities through engineering of their internal structure. Artificial spin networks, in which a large number of nanoscale magnetic elements are coupled together, are promising metamaterial candidates that enable the control of collective magnetic behavior through tuning of the local interaction between elements. In this work, the motion of magnetic domain-walls in an artificial spin network leads to a tunable stochastic response of the metamaterial, which can be tailored through an external magnetic field and local lattice modifications. This type of tunable stochastic network produces a controllable random response exploiting intrinsic stochasticity within magnetic domain-wall motion at the nanoscale. An iconic demonstration used to illustrate the control of randomness is the Galton board. In this system, multiple balls fall into an array of pegs to generate a bell-shaped curve that can be modified via the array spacing or the tilt of the board. A nanoscale recreation of this experiment using an artificial spin network is employed to demonstrate tunable stochasticity. This type of tunable stochastic network opens new paths toward post-Von Neumann computing architectures such as Bayesian sensing or random neural networks, in which stochasticity is harnessed to efficiently perform complex computational tasks.

Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering.

While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction (DMI) are candidates as novel information carriers, their dynamics on the fs-ps timescale is little known. Since with the bulk Heisenberg exchange and the interfacial DMI two distinct exchange mechanisms are at play, the ultrafast dynamics of the chiral order needs to be ascertained and compared to the dynamics of the conventional collinear order. Using an XUV free-electron laser we determine the fs-ps temporal evolution of the chiral order in domain walls in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe experiment. Upon demagnetization we observe that the dichroic (CL-CR) signal connected with the chiral order correlator mzmx in the domain walls recovers significantly faster than the (CL + CR) sum signal representing the average collinear domain magnetization mz2 + mx2. We explore possible explanations based on spin structure dynamics and reduced transversal magnetization fluctuations inside the domain walls and find that the latter can explain the experimental data leading to different dynamics for collinear magnetic order and chiral magnetic order.

Coherent Resonant Tunneling through Double Metallic Quantum Well States.

Study of resonant tunneling through multimetallic quantum well (QW) structure is not only important for the fundamental understanding of quantum transport but also for the great potential to generate advanced functionalities of spintronic devices. However, it remains challenging to engineer such a structure due to the short electron phase coherence length in metallic QW system. Here, we demonstrate the successful fabrication of double-QW structure in a single fully epitaxial magnetic tunnel junction (MTJ) heterostructure, where two Fe QW layers are sandwiched between three MgAlO x tunnel barriers. We show clear evidence of the coherent resonant tunneling through the discrete QW states in the two QWs. The coherent resonant tunneling condition is fulfilled only when the middle barrier between the two QWs is thin enough and available QW states are present simultaneously in both QWs under a certain bias. Compared to the single QW structure, the resonant tunneling in double-QW MTJ produces strong conductivity oscillations with much narrower peak width (about half) owing to the enhanced energy filtering effect. This study presents a comprehensive understanding of the resonant tunneling mechanism in MTJ with multiple QWs, which is essential for future development of new spintronic devices operating in the quantum tunneling regime.

Linking Electronic Transport through a Spin Crossover Thin Film to the Molecular Spin State Using X-ray Absorption Spectroscopy Operando Techniques.

One promising route toward encoding information is to utilize the two stable electronic states of a spin crossover molecule. Although this property is clearly manifested in transport across single molecule junctions, evidence linking charge transport across a solid-state device to the molecular film's spin state has thus far remained indirect. To establish this link, we deploy materials-centric and device-centric operando experiments involving X-ray absorption spectroscopy. We find a correlation between the temperature dependencies of the junction resistance and the Fe spin state within the device's [Fe(H2B(pz)2)2(NH2-phen)] molecular film. We also factually observe that the Fe molecular site mediates charge transport. Our dual operando studies reveal that transport involves a subset of molecules within an electronically heterogeneous spin crossover film. Our work confers an insight that substantially improves the state-of-the-art regarding spin crossover-based devices, thanks to a methodology that can benefit device studies of other next-generation molecular compounds.

Probing a Device's Active Atoms.

Materials science and device studies have, when implemented jointly as "operando" studies, better revealed the causal link between the properties of the device's materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a device's operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junction's (MTJ's) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite FeO bonds parallel to the MTJ's interfaces. This explicit link between the device's spintronic performance and these FeO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.

Ferroelectric Control of Organic/Ferromagnetic Spinterface.

Organic multiferroic tunnel junctions based on La0.6 Sr0.4 MnO3 /poly(vinylidene fluoride) (PVDF)/Co structures are fabricated. The tunneling magneto-resistance sign can be changed by electrically switching the ferroelectric polarization of PVDF barrier. It is demonstrated that the spin-polarization of the PVDF/Co spinterface can be actively controlled by tuning the ferroelectric polarization of PVDF. This study opens new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier.

Fragmentation of magnetism in artificial kagome dipolar spin ice.

Geometrical frustration in magnetic materials often gives rise to exotic, low-temperature states of matter, such as the ones observed in spin ices. Here we report the imaging of the magnetic states of a thermally active artificial magnetic ice that reveal the fingerprints of a spin fragmentation process. This fragmentation corresponds to a splitting of the magnetic degree of freedom into two channels and is evidenced in both real and reciprocal space. Furthermore, the internal organization of both channels is interpreted within the framework of a hybrid spin-charge model that directly emerges from the parent spin model of the kagome dipolar spin ice. Our experimental and theoretical results provide insights into the physics of frustrated magnets and deepen our understanding of emergent fields through the use of tailor-made magnetism.