Ultrafast Demagnetization Control in Magnetophotonic Surface Crystals.

Magnetic memory combining plasmonics and magnetism is poised to dramatically increase the bit density and energy efficiency of light-assisted ultrafast magnetic storage, thanks to nanoplasmon-driven enhancement and confinement of light. Here we devise a new path for that, simultaneously enabling light-driven bit downscaling, reduction of the required energy for magnetic memory writing, and a subtle control over the degree of demagnetization in a magnetophotonic surface crystal. It features a regular array of truncated-nanocone-shaped Au-TbCo antennas showing both localized plasmon and surface lattice resonance modes. The ultrafast magnetization dynamics of the nanoantennas show a 3-fold resonant enhancement of the demagnetization efficiency. The degree of demagnetization is further tuned by activating surface lattice modes. This reveals a platform where ultrafast demagnetization is localized at the nanoscale and its extent can be controlled at will, rendering it multistate and potentially opening up so-far-unforeseen nanomagnetic neuromorphic-like systems operating at femtosecond time scales controlled by light.

Giant magneto-refractive effect in mid-infrared second-harmonic generation from plasmonic antennas.

Active modulation of nonlinear-optical response from metallic nanostructures can be realized with an external magnetic field. We report a resonant 20% magneto-refractive modulation in second-harmonic generation (SHG) from spintronic multilayer antennas in the mid-infrared. We discuss mechanisms of this modulation and show that it cannot be explained by an unequal enhancement of the electromagnetic field. Instead, we propose a novel, to the best of our knowledge, contribution to the nonlinear susceptibility, which relies on the spin-dependent electron mean free path. In contrast to magneto-optics in ferromagnets, our approach allows simultaneous observation of the enhanced SHG and its large modulation.

Direct IR Spectroscopic Detection of a Low-Lying Electronic State in a Metal Carbide Cluster.

The electronic structure of metal clusters is notoriously difficult to detect spectroscopically, due to rapid relaxation into the ground state following excitation. We have used IR multiple photon excitation to identify a low-lying electronic state in a tantalum carbide cluster. The electronic excitation is found at 458 cm-1 , and is confirmed by experiments on isotopically labeled clusters. Time-dependent density functional theory (TD-DFT) calculations confirm the current assignment, but a second predicted electronic state was not observed.

Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo--Competition with Nanoscale Heterogeneity.

Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the material's heterogeneity.

Orbit and spin resolved magnetic properties of size selected [ConRh]⁺ and [ConAu]⁺ nanoalloy clusters.

Bi-metallic nanoalloys of mixed 3d-4d or 3d-5d elements are promising candidates for technological applications. The large magnetic moment of the 3d materials in combination with a high spin-orbit coupling of the 4d or 5d materials give rise to a material with a large magnetic moment and a strong magnetic anisotropy, making them ideally suitable in for example magnetic storage devices. Especially for clusters, which already have a higher magnetic moment compared to the bulk, these alloys can profit from the cooperative role of alloying and size reduction in order to obtain magnetically stable materials with a large magnetic moment. Here, the influence of doping of small cobalt clusters on the spin and orbital magnetic moment has been studied for the cations [Co(8-14)Au](+) and [Co(10-14)Rh](+). Compared to the undoped pure cobalt [Co(N)](+) clusters we find a significant increase in the spin moment for specific Co(N-1)Au(+) clusters and a very strong increase in the orbital moment for some Co(N-1)Rh(+) clusters, with more than doubling for Co12Rh(+). This result shows that substitutional doping of a 3d metal with even just one atom of a 4d or 5d metal can lead to dramatic changes in both spin and orbital moment, opening up the route to novel applications.

Multiferroic rhodium clusters.

Simultaneous magnetic and electric deflection measurements of rhodium clusters (Rh(N), 6 ≤ N ≤ 40) reveal ferromagnetism and ferroelectricity at low temperatures, while neither property exists in the bulk metal. Temperature-independent magnetic moments (up to 1 µ(B) per atom) are observed, and superparamagnetic blocking temperatures up to 20 K. Ferroelectric dipole moments on the order of 1D with transition temperatures up to 30 K are observed. Ferromagnetism and ferroelectricity coexist in rhodium clusters in the measured size range, with size-dependent variations in the transition temperatures that tend to be anticorrelated in the range n = 6-25. Both effects diminish with size and essentially vanish at N = 40. The ferroelectric properties suggest a Jahn-Teller ground state. These experiments represent the first example of multiferroic behavior in pure metal clusters.

Spectroscopy of phononic switching of magnetization in the terahertz gap.

All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.

Laser-induced magnetization dynamics and reversal in ferrimagnetic alloys.

This review discusses the recent studies of magnetization dynamics and the role of angular momentum in thin films of ferrimagnetic rare-earth-transition metal (RE-TM) alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. It has been experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 fs laser pulse, without any applied magnetic field. This switching is found to follow a novel reversal pathway, that is shown however to depend crucially on the net angular momentum, reflecting the balance of the two opposite sublattices. In particular, optical excitation of ferrimagnetic GdFeCo on a time scale pertinent to the characteristic time of the exchange interaction between the RE and TM spins, i.e. on the time scale of tens of femtoseconds, pushes the spin dynamics into a yet unexplored regime, where the two exchange-coupled magnetic sublattices demonstrate substantially different dynamics. As a result, the reversal of spins appears to proceed via a novel transient state characterized by a ferromagnetic alignment of the Gd and Fe magnetic moments, despite their ground-state antiferromagnetic coupling.Thus, optical manipulation of magnetic order by femtosecond laser pulses has developed into an exciting and still expanding research field that keeps being fueled by a continuous stream of new and sometimes counterintuitive results. Considering the progress in the development of plasmonic antennas and compact ultrafast lasers, optical control of magnetic order may also potentially revolutionize data storage and information processing technologies.

Communication: structure of magnetic lanthanide clusters from far-IR spectroscopy: Tb(n)+ (n = 5-9).

Small lanthanide clusters have interesting magnetic properties, but their structures are unknown. We have identified the structures of small terbium cluster cations Tb(n)(+) (n = 5-9) in the gas phase by analysis of their vibrational spectra. The spectra have been measured via IR multiple photon dissociation of their complexes with Ar atoms in the 50-250 cm(-1) range with an infrared free electron laser. Density functional theory calculations using a 4f-in-core effective core potential (ECP) accurately reproduce the experimental far-IR spectra. The ECP corresponds to a 4f(8)5d(1)6s(2) trivalent configuration of terbium. The assigned structures are similar to those observed in several other transition metal systems. From this, we conclude that the bonding in Tb clusters is through the interactions between the 5d and 6s electrons, and that the 4f electrons have only an indirect effect on the cluster structures.

Magnetic domain wall motion driven by an acoustic wave.

Dynamic interaction of acoustic and magnetic systems is of strong current interest, triggered by the promises of almost lossless new concepts of magnet-based information technology. In such concepts, a significant role is often given to domain walls (DW). Therefore, here we investigate how launching an acoustic shear wave, we can control the DW motion. Surprisingly, at sufficiently large amplitudes of the shear displacement, the speed of the forced DW motion can reach sizeable fraction of the speed of sound. This was shown to happen due to certain resonance conditions depending on the wave frequency, its angle of incidence, and shear displacement amplitudes, leading to a total reflection of the wave and maximizing the impact. Most interesting, strong nonlinearity appears in the interaction of the elastic and magnetic subsystems, expressed by the negative slope of the resonant reflection peak and the s-shaped dependence of the domain wall velocity on the shear displacement amplitude, typical for nonlinear systems.

Ultrafast demagnetization in a ferrimagnet under electromagnetic field funneling.

The quest to improve the density, speed and energy efficiency of magnetic memory storage has led to the exploration of new ways of optically manipulating magnetism at the ultrafast time scale, in particular in ferrimagnetic alloys. While all-optical magnetization switching is well-established on the femtosecond timescale, lateral nanoscale confinement and thus the potential significant reduction of the size of the magnetic element remains an outstanding challenge. Here we employ resonant electromagnetic energy funneling through plasmon nanoantennas to influence the demagnetization dynamics of a ferrimagnetic TbCo alloy thin film. We demonstrate how Ag nanoring-shaped antennas under resonant optical femtosecond pumping reduce the overall demagnetization in the underlying films up to three times compared to non-resonant illumination. We attribute such a substantial reduction to the nanoscale confinement of the demagnetization process. This is qualitatively supported by the electromagnetic simulations that strongly evidence the resonant optical energy-funneling to the nanoscale from the nanoantennas into the ferrimagnetic film. This observation is an important step for reaching deterministic ultrafast all-optical magnetization switching at the nanoscale in such systems, opening a route to develop nanoscale ultrafast magneto-optics.

Dual-shot dynamics and ultimate frequency of all-optical magnetic recording on GdFeCo.

Although photonics presents the fastest and most energy-efficient method of data transfer, magnetism still offers the cheapest and most natural way to store data. The ultrafast and energy-efficient optical control of magnetism is presently a missing technological link that prevents us from reaching the next evolution in information processing. The discovery of all-optical magnetization reversal in GdFeCo with the help of 100 fs laser pulses has further aroused intense interest in this compelling problem. Although the applicability of this approach to high-speed data processing depends vitally on the maximum repetition rate of the switching, the latter remains virtually unknown. Here we experimentally unveil the ultimate frequency of repetitive all-optical magnetization reversal through time-resolved studies of the dual-shot magnetization dynamics in Gd27Fe63.87Co9.13. Varying the intensities of the shots and the shot-to-shot separation, we reveal the conditions for ultrafast writing and the fastest possible restoration of magnetic bits. It is shown that although magnetic writing launched by the first shot is completed after 100 ps, a reliable rewriting of the bit by the second shot requires separating the shots by at least 300 ps. Using two shots partially overlapping in space and minimally separated by 300 ps, we demonstrate an approach for GHz magnetic writing that can be scaled down to sizes below the diffraction limit.

Magnetic properties of oxygen doped samarium clusters.

Here we present the results of experimental study of magnetic properties of samarium clusters doped with a single oxygen atom. In a recent theoretical study it was observed that for pure Sm clusters a transition from fully non-magnetic to weakly magnetic occurs due to a valence change occurring at a size of eight atoms. Here we found, first, that pure Sm clusters could not be synthesized due to the strong oxidation tendency of Sm and the inability to sufficiently remove oxygen from the setup. Therefore, we studied Sm[Formula: see text]O clusters. Since the oxygen contributes to the binding, the valence transition for Sm[Formula: see text]O clusters may be expected to occur for a smaller cluster size than for pure Sm clusters. Indeed from our experiments the valence transition is indicated to occur for [Formula: see text] Sm atoms instead of [Formula: see text]. Furthermore, the observed magnetic moment as function of cluster size for Sm[Formula: see text]O clusters shows a strong dependency of the magnetic moment on the cluster size. A large total magnetic moment is observed for Sm[Formula: see text]O, Sm[Formula: see text]O, Sm[Formula: see text]O and Sm[Formula: see text]O compared to the smaller moment for Sm[Formula: see text]O to Sm[Formula: see text]O and Sm[Formula: see text]O to Sm[Formula: see text]O.

Viral capsids as templates for the production of monodisperse Prussian blue nanoparticles.

The use of a viral template has allowed the synthesis of monodisperse Prussian blue nanoparticles with a diameter of 18 +/- 1.7 nm and their organization into hexagonal patterns on mica and hydrophilic carbon surfaces.

All-optical observation and reconstruction of spin wave dispersion.

To know the properties of a particle or a wave, one should measure how its energy changes with its momentum. The relation between them is called the dispersion relation, which encodes essential information of the kinetics. In a magnet, the wave motion of atomic spins serves as an elementary excitation, called a spin wave, and behaves like a fictitious particle. Although the dispersion relation of spin waves governs many of the magnetic properties, observation of their entire dispersion is one of the challenges today. Spin waves whose dispersion is dominated by magnetostatic interaction are called pure-magnetostatic waves, which are still missing despite of their practical importance. Here, we report observation of the band dispersion relation of pure-magnetostatic waves by developing a table-top all-optical spectroscopy named spin-wave tomography. The result unmasks characteristics of pure-magnetostatic waves. We also demonstrate time-resolved measurements, which reveal coherent energy transfer between spin waves and lattice vibrations.

Spectrally resolved optical probing of laser induced magnetization dynamics in bismuth iron garnet.

The spectrally resolved magnetization dynamics in bismuth iron garnet shows a fluence dependent light induced modification of the magneto-optical Faraday spectrum. It is demonstrated that the relative contributions from the tetrahedral and octahedral iron sites to the Faraday spectrum change due to the impact of the pump pulse. This change explains the observed deviation from a linear dependence of the amplitude of the oscillations on the fluence, as expected for the inverse Faraday effect.

Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative.

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

Channeling Vibrational Energy To Probe the Electronic Density of States in Metal Clusters.

We investigate the electronic density of states (DOS) of isolated neutral cobalt clusters by probing the temperature-modulated population of electronic states through UV photoionization. The temperature is controlled via resonant excitation of lattice vibrations using the free-electron laser FELICE, after which the vibrational and electronic systems equilibrate through the electron-phonon coupling, redistributing the population of electronic states. The data are analyzed by surface photoemission theory, modified to incorporate the realistic DOS.

Unusual Temperature Dependence of Magnetization and Possible Magnetic Noncollinearity in Tm and Pr Clusters.

Rare-earth metals in their bulk form possess rather similar crystallographic structures, which is due to the very similar features of their outer electronic states. On the other hand, their magnetic properties are of rich variety, which is related to the specific form of the indirect magnetic exchange interaction between the inner electronic shells. In cluster form, this interplay may lead to very unusual magnetic structures. Here we show how the magnetic moments vary with size and temperature in Tm and Pr clusters. While in Pr clusters clear atom-by-atom oscillations indicate antiferromagnetic ordering, smooth variation and anomalous temperature behavior in Tm is representative for an essentially non-collinear spin arrangement. Their electric behavior is also very different, with a metallic-like behavior of Pr and localized electronic states in Tm.

Controlling spins with light.

The interaction of sub-picosecond laser pulses with magnetically ordered materials has developed into an extremely exciting research topic in modern magnetism. From the discovery of sub-picosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of spins by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. We have recently demonstrated that one can generate ultrashort and very strong (teslas) magnetic field pulses via the so-called inverse Faraday effect. Such optically induced magnetic field pulses provide unprecedented means for the generation, manipulation and coherent control of spins on very short time scales. The basic ideas behind these so-called opto-magnetic effects will be discussed and illustrated with recent results, demonstrating the various possibilities of this new field of femto-magnetism.