Nonlinear effects in the propagation of optically generated magnetostatic volume mode spin waves

Recent experimental work has demonstrated optical control of spin wave emission by tuning the shape of the optical pulse (Satoh et al.\ Nature Photonics, 6, 662 (2012)). We reproduce these results and extend the scope of the control by investigating nonlinear effects for large amplitude excitations. We observe an accumulation of spin wave power at the center of the initial excitation combined with short-wavelength spin waves. These kind of nonlinear effects have not been observed in earlier work on nonlinearities of spin waves. Our observations pave the way for the manipulation of magnetic structures at a smaller scale than the beam focus, for instance in devices with all-optical control of magnetism.Comment: Added new figures to further illustrate the nonlinear effects to show time evolution and spectral flow. Added references. Changed perspective on nonlinear effects w.r.t. applicability of NSE. Added acknowledgemen

Supervised learning of an opto-magnetic neural network with ultrashort laser pulses

The explosive growth of data and its related energy consumption is pushing the need to develop energy-efficient brain-inspired schemes and materials for data processing and storage. Here, we demonstrate experimentally that Co/Pt films can be used as artificial synapses by manipulating their magnetization state using circularly-polarized ultrashort optical pulses at room temperature. We also show an efficient implementation of supervised perceptron learning on an opto-magnetic neural network, built from such magnetic synapses. Importantly, we demonstrate that the optimization of synaptic weights can be achieved using a global feedback mechanism, such that the learning does not rely on external storage or additional optimization schemes. These results suggest there is high potential for realizing artificial neural networks using optically-controlled magnetization in technologically relevant materials, that can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure

Terahertz emission spectroscopy of laser-induced spin dynamics in TmFeO3 and ErFeO3 orthoferrites

Copyright © 2014 American Physical SocietyUsing the examples of laser-induced spin-reorientation phase transitions in TmFeO3 and ErFeO3 orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of ∼0.3–0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the Fe3+ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the 6A1 ground state of the Fe+3 ions occupying rare-earth positions.European Commission's 7th Framework Program (FP7/2007–2013)Engineering and Physical Sciences Research Council (EPSRC)Netherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)ERCRussian GovernmentRFB

Spin-photo-currents generated by femtosecond laser pulses in a ferrimagnetic GdFeCo/Pt bilayer

© 2017 Author(s). Using THz emission spectroscopy, we detect spin-photo-currents from a ferrimagnetic amorphous alloy GdFeCo to an adjacent Pt capping layer. The currents are generated upon excitation of a GdFeCo/Pt heterostructure with femtosecond laser pulses. It is found that the polarization of the spin-polarized current is determined by magnetic sublattice sensitivity rather than the total magnetization, allowing for spin-polarized current generation when the net magnetization is zero

A compartive study of the retentive capability of the Sydney mini-screw with 6mm orthodontic anchorage miniscrews in the tibia and femur of New Zealand rabbits by removal torque test

Aim: To investigate the retentive capability of the Sydney Mini-screw with injectable bone cement by removal torque. Method: 16 New Zealand White rabbits were divided evenly into 2 groups, T1 0 week to assess primary stability and T2 8 weeks to test secondary stability. Three groups of miniscrews Sydney Mini-screw with Cement (SMSC) N=12, Sydney Miniscrew without cement (SMS) N=10 and control Aarhus (CA) 6mm screw N=10 were placed randomly and evenly between the right and left tibial and femoral sites. The SMSC and SMS required predrilling of a pilot hole and the SMSC had injectable bone cement PRODENSE. Removal torque was measured and Friedman's Test and two-sample t-test were used for statistical analysis, where appropriate. Results: Removal torque values at T1 for CA, SMS, SMSC were not significantly different (p=0.072) but were significantly different at T2 (p=0.012). Only SMS (p=0.006) showed statistically significant difference between T1 and T2. The different surgical locations at T2 did not statistically differ from each other either (p=0.948). Conclusion: Sydney Miniscrew with and without cement had significantly higher secondary stability and had a trend towards increased primary compared to a normal control miniscrew. More research is required with an increased sample size

THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing

Ultrafast demagnetization of Co/Pt heterostructures induced by a femtosecond 800-nm laser pulse launches a spin current from Co to Pt and subsequent conversion of the spin current to a charge current in the Pt layer due to the inverse spin-Hall effect. At the same time, due to the spin-dependent photogalvanic effect, a circularly polarized femtosecond laser pulse also generates a photocurrent at the Co/Pt interface. Both ultrashort photocurrent pulses are effectively detected in a contactless way by measuring the THz radiation they emit. Here we aim to understand how the properties of the Co/Pt interface affect the photocurrents in the bilayers. By varying the interfacial roughness, crystal structure, and interfacial intermixing, as well as having an explicit focus on the cases when THz emissions from these two photocurrents reveal opposite trends, we identify which interface properties play a crucial role for the photocurrents. In particular, we show that by reducing the roughness, the THz emission due to the spin-dependent photogalvanic effect reduces to zero while the strength of the THz emission from the photocurrent associated with the inverse spin-Hall effect increases by a factor of 2. On the other hand, while intermixing strongly enhances the THz emission from the inverse spin-Hall effect by a factor of 4.2, THz emission related to the spin-dependent photogalvanic effect reveals the opposite trend. These findings indicate that microstructural properties of the Co-Pt interface play a decisive role in the generation of photocurrents

Ultrafast all-optical control of the magnetization in magnetic dielectrics

The purpose of this review is to summarize the recent progress on laser-induced magnetization dynamics in magnetic dielectrics. Due to the slow phonon–magnon interaction in these materials, direct thermal effects of the laser excitation can only be seen on the time scale of almost a nanosecond and thus are clearly distinguished from the ultrafast nonthermal effects. However, via the crystal field, laser pulses are shown to indirectly modify the magnetic anisotropy in rare-earth orthoferrites and lead to the spin reorientation within a few picoseconds. More interesting, however, are the direct nonthermal effects of light on spin systems. We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on a femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites. In addition, circularly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. An all-optical scheme of excitation and detection of different antiferromagnetic resonance modes with frequencies of up to 500 GHz will be discussed as well. The reported effects open new and exciting possibilities for ultrafast manipulation of spins by light, and provide new insight into the physics of magnetism on ultrafast time scales

Een schijn van Voodoo. Culturele achtergronden van de handel in Nigeriaanse meisjes voor de Nederlandse prostitutie: een verkenning (vierde druk)

Dit rapport is het resultaat van een verkennend onderzoek naar de culturele achtergronden van de handel in Nigeriaanse meisjes die naar Nederland komen om in de prostitutie te gaan werken. Een belangrijke overeenkomst in de verhalen die deze meisjes de Nederlandse politie vertellen betreft de 'voodoo'-rituelen waaraan zij voor vertrek naar Nederland worden onderworpen. In het rapport komen de volgende vragen aan de orde: wat doen handelaren met denkbeelden van 'voodoo'? Hoe creëren zij daarmee een sfeer van angst en intimidatie? Hoe komt het dat dit voor de meisjes evenzeer een werkelijkheid wordt als dat in de sinistere betekenis van het woord ook voor velen in de Nederlandse samenleving het geval is? Het rapport stelt dat 'voodoo' in de Nigeriaanse praktijk niet bestaat (er is wel een aantal andere rituele praktijken waar op wordt ingegaan) en in de Nederlandse situatie een goed begrip van de omstandigheden verhindert. Het rapport behandelt de sociaal-economische situatie in Nigeria en Edo State, het district waar veel van de betrokken meisjes vandaan komen; de positie van vrouwen in Nigeria t.a.v. huwelijk, seksualiteit, prostitutie, en de culturele achtergronden daarvan; aspecten van religie in West Afrika en Edo; een gevalsstudie; en het ongedefinieerde gebruik van het begrip 'voodoo' door Nederlandse hulpverleningsinstanties  ASC – Publicaties niet-programma gebonde

Ultrafast optical modification of exchange interactions in iron oxides

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.European Commission’s 7th Framework Program (FP7/2007–2013)EPSRCNetherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)European Research CouncilRussian Ministry of Education and ScienceRFBR-NSFC projectBureau of International CooperationNSFC projectNSFC-NWOEU Seventh Framework ProgramNWO by a Rubicon grantEuropean Commission (FP7-ICT-2013-613024–GRASP

Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces?

Femtosecond pump-probe second-harmonic generation (SHG) and transient linear reflectivity measurements were carried out on polycrystalline Cu, Ag and Au in air to analyze whether the electron temperature affects Fresnel factors or nonlinear susceptibilities, or both. Sensitivity to electron temperatures was attained by using photon energies near the interband transition threshold. We find that the nonlinear susceptibility carries the electron temperature dependence in case of Ag and Au, while for Cu the dependence is in the Fresnel factors. This contrasting behavior emphasizes that SHG is not a priori sensitive to electron dynamics at surfaces or interfaces, notwithstanding its cause.Comment: 11 pages, 4 figure