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Magnetic skyrmions are topologically-distinct swirls of magnetic moments which display particle-like behaviour, including the ability to undergo thermally-driven diffusion. In this paper we study the thermally activated motion of arrays of skyrmions using temperature dependent micromagnetic simulations where the skyrmions form spontaneously. In particular, we study the interaction of skyrmions with grain boundaries, which are a typical feature of sputtered ultrathin films used in experimental devices. We find the interactions lead to two distinct regimes. For longer lag times the grains lead to a reduction in the diffusion coefficient, which is strongest for grain sizes similar to the skyrmion diameter. At shorter lag times the presence of grains enhances the effective diffusion coefficient due to the gyrotropic motion of the skyrmions induced by their interactions with grain boundaries. For grain sizes significantly larger than the skyrmion diameter clustering of the skyrmions occurs in grains with lower magnetic anisotropy., 7 pages, 5 figures

The possibility of tuning the magnetic properties of materials with voltage (converse magnetoelectricity) or generating electric voltage with magnetic fields (direct magnetoelectricity) has opened new avenues in a large variety of technological fields, ranging from information technologies to healthcare devices and including a great number of multifunctional integrated systems, such as mechanical antennas, magnetometers, and radio frequency (RF) tunable inductors, which have been realized due to the strong strain-mediated magnetoelectric (ME) coupling found in ME composites. The development of single-phase multiferroic materials (which exhibit simultaneous ferroelectric and ferromagnetic or antiferromagnetic orders), multiferroic heterostructures, as well as progress in other ME mechanisms, such as electrostatic surface charging or magneto-ionics (voltage-driven ion migration), have a large potential to boost energy efficiency in spintronics and magnetic actuators. This article focuses on existing ME materials and devices and reviews the state of the art in their performance. The most recent progress on different ME devices based on ME heterostructures is presented but with a larger emphasis on ME antennas and sensors due to the significant advances achieved in these applications. The rapid development of mechanically actuated ME antennas has been observed over the past several years, producing ME antennas that are miniaturized by 1-2 orders compared to conventional antenna size. Magnetic sensors based on simple ME composites are potentially promising alternatives to conventional magnetometers due to their very good detectivity (, The work of Patrick Hayes, Viktor Schell, Eckhard Quandt, and Jeffrey McCord was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) through the Collaborative Research Center CRC 1261 "Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics." The work of Pedro Martins was supported in part by the Fundacao para a Ciencia e Tecnologia (FCT) in the framework of the Strategic Funding under Grant UID/FIS/04650/2020 and under Project PTDC/BTMMAT/28237/2017 and Project PTDC/EMD-EMD/28159/2017, in part by the Fundacao para a Ciencia e Tecnologia (FCT) for the contract under the Stimulus of Scientific Employment, Individual Support-2017 Call, under Grant CEECIND/03975/2017, in part by the Spanish State Research Agency (AEI), in part by the European Regional Development Fund (ERFD) under Project PID2019-106099RB-C43/AEI/10.13039/501100011033, and in part by the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK, and PIBA (Grant PIBA-2018-06) Programs. The work of Marisa Medarde was supported in part by the Swiss National Science Foundation under Grant 200021-141334 and Grant 206021_139082, in part by the Swiss National Center of Competence in Research MARVEL (Computational Design and Discovery of Novel Materials) under Grant 1NF40_182892, in part by the European Community's 7th Framework Program under Grant 290605 (COFUND:PSI-FELLOW). The work of Senentxu Lanceros-Mendez was supported in part by the Fundacao para a Ciencia e Tecnologia (FCT) in the framework of the Strategic Funding under Grant UID/FIS/04650/2020 and under Project PTDC/BTM-MAT/28237/2017 and Project PTDC/EMD-EMD/28159/2017, in part by the Spanish State Research Agency (AEI), in part by the European Regional Development Fund (ERFD) under Project PID2019-106099RB-C43/AEI/10.13039/501100011033, and in part by the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK, and PIBA (Grant PIBA-2018-06) Programs. The work of Sebastiaan van Dijken was supported

openaire: EC/H2020/745115/EU//OPLD Organic light-emitting diodes (OLEDs) have been established as versatile light sources that allow for easy integration in large-area surfaces and flexible substrates. In addition, the low fabrication cost of OLEDs renders them particularly attractive as general lighting sources. Current methods for the fabrication of white-light OLEDs rely on the combination of multiple organic emitters and/or the incorporation of multiple cavity modes in a thick active medium. These architectures introduce formidable challenges in both device design and performance improvements, namely, the decrease of efficiency with increasing brightness (efficiency roll-off) and short operational lifetime. Here we demonstrate, for the first time, white-light generation in an OLED consisting of a sub-100 nm thick blue single-emissive layer coupled to the photonic Bragg modes of a dielectric distributed Bragg reflector (DBR). We show that the Bragg modes, although primarily located inside the DBR stack, can significantly overlap with the emissive layer, thus efficiently enhancing emission and outcoupling of photons at selected wavelengths across the entire visible light spectrum. Moreover, we show that color temperature can be tuned by the DBR parameters, offering great versatility in the optimization of white-light emission spectra.

Magnetoelectric coupling in multiferroic heterostructures offers a promising platform for electric-field control of magnonic devices based on low-power spin-wave transport. Here, electric-field manipulation of the amplitude and phase of propagating spin waves in a ferromagnetic Fe film on top of a ferroelectric BaTiO3 substrate is demonstrated experimentally. Electric-field effects in this composite material system are mediated by strain coupling between alternating ferroelectric stripe domains with in-plane and perpendicular polarization and fully correlated magnetic anisotropy domains with differing spin-wave transport properties. The propagation of spin waves across the strain-induced magnetic anisotropy domains of the Fe film is directly imaged and it is shown how reversible electric-field-driven motion of ferroelectric domain walls and pinned anisotropy boundaries turns the spin-wave signal on and off. Furthermore, linear electric-field tuning of the spin-wave phase by altering the width of strain-coupled stripe domains is demonstrated. The results provide a new route toward energy-efficient reconfigurable magnonics.

openaire: EC/H2020/948260 /EU//PLAS-OLED The nanoscale mode volumes of surface plasmon polaritons have enabled plasmonic lasers and condensates with ultrafast operation1–4. Most plasmonic lasers are based on noble metals, rendering the optical mode structure inert to external fields. Here we demonstrate active magnetic-field control over lasing in a periodic array of Co/Pt multilayer nanodots immersed in an IR-140 dye solution. We exploit the magnetic nature of the nanoparticles combined with mode tailoring to control the lasing action. Under circularly polarized excitation, angle-resolved photoluminescence measurements reveal a transition between the lasing action and non-lasing emission as the nanodot magnetization is reversed. Our results introduce magnetization as a means of externally controlling plasmonic nanolasers, complementary to modulation by excitation5, gain medium6,7 or substrate8. Further, the results show how the effects of magnetization on light that are inherently weak can be observed in the lasing regime, inspiring studies of topological photonics9–11.

Voltage manipulation of skyrmions is a promising path towards low-energy spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt heterostructure are observed experimentally. The results show that the skyrmion density can be both enhanced and depleted by the application of an electric field, along with the ability, at certain magnetic fields to completely switch the skyrmion state on and off. Further, a zero magnetic field skyrmion state can be stablized under a negative bias voltage using a defined voltage and magnetic field sequence. The voltage effects measured here occur on a few-second timescale, suggesting an origin in voltage-controlled magnetic anisotropy rather than ionic effects. By investigating the skyrmion nucleation rate as a function of temperature, we extract the energy barrier to skyrmion nucleation in our sample. Further, micromagnetic simulations are used to explore the effect of changing the anisotropy and Dzyaloshinskii-Moriya interaction on skyrmion density. Our work demonstrates the control of skyrmions by voltages, showing functionalities desirable for commercial devices., Comment: 5 pages, 5 figures

Active control over the concentration or distribution of oxygen vacancies in transition metal oxides enables manipulation of their structural, magnetic, electronic transport, and optical properties. Topotactic oxidation or reduction reactions involving annealing under different ambient conditions are often used to manipulate the oxygen concentration. In this paper, using in situ scanning transmission electron microscopy, the authors demonstrate a reversible structural phase transition in perovskite La${}_{0.5}$Sr${}_{0.5}$CoO${}_{3\ensuremath{-}\ensuremath{\delta}}$ films during heating/cooling cycles while the environment and oxygen vacancy concentration are kept constant. Switching between two oxygen-deficient structures is shown to arise from a local reordering of the oxygen vacancies by thermal strain imposed by the substrate. This approach presents new opportunities for switchable ionic devices.

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The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a skyrmion state using both bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy. A version of the Bloch law explicitly for thin film geometries is derived to extract the exchange stiffness. The strength of the Dzyaloshinskii-Moriya interaction, $D$, is found to have a dependence on the saturation magnetization, ${M}_{s}$ of $D\ensuremath{\propto}{M}_{s}^{1.86\ifmmode\pm\else\textpm\fi{}0.16}$. Further, by extracting the uniaxial anisotropy ${K}_{u}$ and the exchange stiffness $A$, we find that $D\ensuremath{\propto}{K}_{u}^{1.02\ifmmode\pm\else\textpm\fi{}0.11}$ and $D\ensuremath{\propto}{A}^{0.95\ifmmode\pm\else\textpm\fi{}0.07}$. Skyrmion radii are also used to extract the strength of the Dzyaloshinskii-Moriya interaction which is compared to that derived from measurements of stripe domains. The origins of the correlations between material parameters are discussed and consequences of these relationships for skyrmion devices are considered.

We demonstrate a tenfold resonant enhancement of magneto-optical effects in perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots mediated by the excitation of optimized plasmon modes. Two magnetoplasmonic systems are considered: square arrays of ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots on glass and identical arrays on a $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$ bilayer. On glass, the optical and magneto-optical spectra of the nanodot arrays are dominated by the excitation of a surface lattice resonance (SLR), whereas on $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_{2}$, a narrow surface plasmon polariton (SPP) resonance tailors the spectra further. Both the SLR and SPP modes are magneto-optically active leading to an enhancement of the Kerr angle. We detail the dependence of optical and magneto-optical spectra on the number of Co/Pt bilayer repetitions, the nanodot diameter, and the array period, offering design rules on how to maximize and spectrally tune the magneto-optical response of perpendicularly magnetized ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ nanodots.

Deterministic polarization reversal in ferroelectric and multiferroic films is critical for their exploitation in nanoelectronic devices. While ferroelectricity has been studied for nearly a century, major discrepancies in the reported values of coercive fields and saturation polarization persist in literature for many materials. This raises questions about the atomic‐scale mechanisms behind polarization reversal. Unconventional ferroelectric switching in ε‐Fe2O3 films, a material that combines ferrimagnetism and ferroelectricity at room temperature, is reported. High‐resolution in situ scanning transmission electron microscopy experiments and first‐principles calculations demonstrate that polarization reversal in ε‐Fe2O3 occurs around pre‐existing domain walls only, triggering local domain wall motion in moderate electric fields of 250–500 kV cm−1. Calculations indicate that the activation barrier for switching at domain walls is nearly a quarter of that corresponding to the most likely transition paths inside ε‐Fe2O3 domains. Moreover, domain walls provide symmetry lowering of the polar structure near the domain boundary, which is shown to be necessary for ferroelectric switching in ε‐Fe2O3. Local polarization reversal in ε‐Fe2O3 limits the macroscopic ferroelectric response and offers important hints on how to tailor ferroelectric properties by domain structure design in other relevant ferroelectric materials., This work was supported by the Academy of Finland (Grant Nos. 293929, 304291, 319218 and 316857) and by the European Research Council (ERC‐2012‐StG 307502 and ERC‐2018‐CoG 819623). STEM analysis was conducted at the Aalto University OtaNano‐Nanomicroscopy Center (Aalto‐NMC). ICMAB acknowledges financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and the project MAT2017‐85232‐R co‐financed with the EU FEDER program, as well as the Generalitat de Catalunya (projects 2017SGR1377 and 2017SGR765). The authors thank Jaume Gàzquez for discussions about the work and critical reading of the manuscript. K.Z.R. and M.L. gratefully acknowledge the financial support by Deutsche Forschungsgemeinschaft (DFG) through DFG‐ANR GALIMEO project (LE 2504/2‐1), as well as the support of Jülich Supercomputing Centre (JSC, project jiff38) and JARA‐HPC Partition (projects jara0081 and jara0126). R.C.Y. and X.X.G. acknowledge the financial support by the National Key Research Program of China (Grant No. 2017YFA0206200) and the National Natural Science Foundation of China (Grant No. 11874413). We thank Prof. Nathalie Viart for valuable discussions.

Domain motion during ferroelectric switching has been recently suggested to follow scale-invariant avalanche dynamics. An interesting question concerns the dynamics of ferroelastic materials where the bulk material is nonpolar, while the polarity arises at domain walls only. We tackle this issue by investigating the dynamics of ferroelastic twins in SrTi O 3 where the movement of domains is driven mainly by the anisotropic dielectric response at low temperatures. We find that the dynamics of the twin reconfiguration under electric field proceeds by jerks, where the energy distribution is power-law distributed, indicating avalanche dynamics. Avalanche exponents are sensitive to the complexity of the twin pattern structure, reflecting glassiness when twins are interwoven and forming junctions at the intersections between domain walls. This “glassy” behavior is attributed to the pinning originated by these self-generated defects during jamming between twins., We acknowledge financial support from EPSRC (Grant No. EP/P024904/1) and support from the Spanish Ministerio de Ciencia, Innovación y Universidades through Grant No. MAT2017-85232-R (AEI/FEDER, EU), Severo Ochoa SEV-2015-0496, the Generalitat de Catalunya through Grant No. 2017 301 SGR1377, and the Academy of Finland (Grant No. 316857).

We present a comprehensive investigation of propagating spin waves in nanometer-thick yttrium iron garnet (YIG) films. We use broadband spin-wave spectroscopy with integrated coplanar waveguides (CPWs) and antennas on top of continuous and patterned YIG films to characterize spin waves with wave vectors up to 10 $\mathrm{rad}/\ensuremath{\mu}\mathrm{m}$. All films are grown by pulsed laser deposition. From spin-wave transmission spectra, parameters such as the Gilbert damping constant, spin-wave dispersion relation, group velocity, relaxation time, and decay length are derived, and their dependence on magnetic bias field strength and angle is systematically gauged. For a 40-nm-thick YIG film, we obtain a damping constant of $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and a maximum decay length of 1.2 mm. We show a strong variation of spin-wave parameters with wave vector, magnetic field strength, and field angle. The properties of spin waves with small wave vectors change considerably with in-plane magnetic bias field up to 30 mT and magnetic field angle beyond ${20}^{\ensuremath{\circ}}$. We also compare broadband spin-wave spectroscopy measurements on 35-nm-thick YIG films with integrated CPWs and antennas and demonstrate that both methods provide similar spin-wave parameters.

Ferroelectric tunnel junctions (FTJs) are promising candidates for nonvolatile memories and memristor-based computing circuits. Thus far, most research has focused on FTJs with a perovskite oxide ferroelectric tunnel barrier. As the need for high-temperature epitaxial film growth challenges the technological application of such inorganic junctions, more easily processable organic ferroelectrics can serve as alternative if large tunneling electroresistance (TER) and good switching durability would persist. This study reports on the performance of FTJs with a spin-coated ferroelectric P(VDF-TrFE) copolymer tunnel barrier. The use of three different bottom electrodes, indium tin oxide (ITO), La0.67Sr0.33MnO3, (LSMO), and Nb-doped SrTiO3 (STO) are compared and it is shown that the polarity and magnitude of the TER effect depend on their conductivity. The largest TER of up to 107% at room temperature is measured on FTJs with a semiconducting Nb-doped STO electrode. This large switching effect is attributed to the formation of an extra barrier over the space charge region in the substrate. The organic FTJs exhibit good resistance retention and switching endurance up to 380 K, which is just below the ferroelectric Curie temperature of the P(VDF-TrFE) barrier.

Domains in BaTiO$_3$ induces a regular modulation of uniaxial magnetic anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the domain structures of the CoFeB wedge film and BaTiO$_3$ substrate correlate fully and straight ferroelectric domain boundaries in BaTiO$_3$ pin magnetic domain walls in CoFeB. We use x-ray photoemission electron microscopy and magneto-optical Kerr effect microscopy to characterize the spin structure of the pinned domain walls. In a rotating magnetic field, abrupt and reversible transitions between two domain wall types occur, namely, narrow walls where the magnetization vectors align head-to-tail and much broader walls with alternating head-to-head and tail-to-tail magnetization configurations. We characterize variations of the domain wall spin structure as a function of magnetic field strength and CoFeB film thickness and compare the experimental results with micromagnetic simulations., 5 pages, 5 figures

Resistive switching in transition metal oxides involves intricate physical and chemical behaviours with potential for non-volatile memory and memristive devices. Although oxygen vacancy migration is known to play a crucial role in resistive switching of oxides, an in-depth understanding of oxygen vacancy-driven effects requires direct imaging of atomic-scale dynamic processes and their real-time impact on resistance changes. Here we use in situ transmission electron microscopy to demonstrate reversible switching between three resistance states in epitaxial La2/3Sr1/3MnO3 films. Simultaneous high-resolution imaging and resistance probing indicate that the switching events are caused by the formation of uniform structural phases. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive transitions. Our findings open prospects for ionotronic devices based on dynamic control of physical properties in complex oxide nanostructures.

We have studied low-energy configurations in two-dimensional arrays consisting of Ising-type dipolar coupled nanomagnets lithographically defined onto a two-dimensional Cairo lattice, thus dubbed the dipolar Cairo lattice. Employing synchrotron-based photoemission electron microscopy (PEEM), we perform real-space imaging of moment configurations achieved after thermal annealing. These states are then characterized in terms of vertex populations, spin- and emergent magnetic charge correlations, and a topology-enforced emergent ice rule. The results reveal a strong dominance of short-range correlations and the absence of long-range order, reflecting the high degree of geometrical spin frustration present in this example of an artificial frustrated spin system.

Activated by action potentials and Ca2+ ion migration, neurotransmitters in biological synapses are released from vesicles at the presynaptic membrane to the cleft and bonded to receptors on the postsynaptic membrane. The bonded neurotransmitters modify the electrochemical properties of the postsynaptic membrane and, thereby, the synaptic plasticity, which forms the basis for learning, memory, emotion, cognition, and consciousness. Here, the oxygen vacancy transport in Au/SrTiO3 (STO)/La0.67Sr0.33MnO3 (LSMO) tunnel junctions is exploited to mimic neurotransmission processes in an artificial ionic electronic device. Using voltage pulses of varying number, amplitude, and polarity, it is demonstrated that reversible oxygen vacancy migration across the STO/LSMO interface provides stable multilevel resistance switching for octal memory devices and resembles the quantal, stochastic, and excitatory or inhibitory nature of neurotransmitter release dynamics. Moreover, fundamental synaptic behaviors including long‐term potentiation/depression and various types of spike‐timing‐dependent plasticity characteristics are emulated, opening a promising biorealistic approach to the design of neuromorphic devices.

We report on lasing at visible wavelengths in arrays of ferromagnetic Ni nanodisks overlaid with an organic gain medium. We demonstrate that by placing an organic gain material within the mode volume of the plasmonic nanoparticles both the radiative and, in particular, the high ohmic losses of Ni nanodisk resonances can be compensated. Under increasing pump fluence, the systems exhibit a transition from lattice-modified spontaneous emission to lasing, the latter being characterized by highly directional and sub-nanometer line width emission. By breaking the symmetry of the array, we observe tunable multimode lasing at two wavelengths corresponding to the particle periodicity along the two principal directions of the lattice. Our results are relevant for loss-compensated magnetoplasmonic devices and topological photonics.

Magnetization reversal is investigated in the presence of anisotropy dispersion in weakly disordered CoxPd100-x(x=23,36,43) thin films. A fourfold in-plane magnetic anisotropy is observed for x=23, whereas an increase of the Co:Pd ratio induces a uniaxial anisotropy in x=36 and 43 films. Anomalous magnetization reversal appears along the hard axes (referred to as collapse of hard axes) in x=23 and 36 films, but disappears for x=43. The results are consistent with the two-grain Stoner-Wohlfarth model. It is argued that the collapse of the hard axes can be controlled by intergranular exchange interaction. Quadratic magneto-optical Kerr effect (QMOKE) is also observed in the films and is found to weaken if the in-plane anisotropy is lowered through the Co:Pd ratio. QMOKE is observed for magnetic field orientations near the hard axes but not exactly along it, in agreement with the two-grain Stoner-Wohlfarth model.

Control of spin waves in magnonic crystals is essential for magnon-based computing. Crystals made of ferromagnetic metals offer versatility in band structure design, but strong magnetic damping restricts their transmission efficiency. Yttrium iron garnet (YIG) with ultralow damping is the palpable alternative, yet its small saturation magnetization limits dipolar coupling between discrete units. Here, we experimentally demonstrate low-loss spin-wave manipulation in magnonic crystals of physically separated nanometer-thick YIG stripes. We enhance the transmission of spin waves in allowed minibands by filling the gaps between YIG stripes with CoFeB. Thus-formed magnonic crystals exhibit tunable bandgaps of 50–200 MHz with nearly complete suppression of the spin-wave signal. We also show that Bragg scattering on only two units produces clear frequency gaps in spin-wave transmission spectra. The integration of strong ferromagnets in nanometer-thick YIG-based magnonic crystals provides effective spin-wave manipulation and low-loss propagation, a vital parameter combination for magnonic technologies., Control of spin wave transport in magnonic crystals is vital for magnonic devices. Here the authors show low-loss spin-wave manipulation in nanometer thick magnonic crystals of discrete YIG stripes separated by air or CoFeB filled grooves exhibiting tunable bandgaps of 50–200 MHz.

This report is on the growth of Fe3O4 nanofilms on Al2O3(0001) and MgO(001) substrates with and without the presence of an external magnetic field using a pulsed injection metallorganic chemical vapour deposition (PI MOCVD) technique. The effects of growing magnetic oxide nanofilms in a 1 T field have been examined using various instrumental methods. It was found that the application of a magnetic field during PI MOCVD does not drastically alter the crystalline texture, surface morphology, and film thickness, but it significantly modifies the Fe3O4 film magnetisation and coercive field. Moreover, it was shown that the application of a 1 T field during the cooling of the sample also improves the magnetic properties. We believe that the large external field orients the magnetic spin structure at high temperatures (during growth or the initial stages of cool down) and that cooling through local magnetic ordering temperatures at Fe3O4 defect sites subsequently favours a ferromagnetic spin alignment. The control of magnetic properties of magnetite nanofilms by the application of magnetic fields during growth opens up new routes towards the fabrication and application of magnetic thin film devices.

Subwavelength metallic particles support plasmon resonances that allow extreme confinement of light down to the nanoscale. Irradiation with left- and right hand circularly polarized light results in the excitation of circular plasmon modes with opposite helicity. The Lorenz force lifts the degeneracy of the two modes in magnetic nanoparticles. Consequently, the confinement and frequency of localized surface plasmon resonances can be tuned by an external magnetic field. In this paper, we experimentally demonstrate this effect for nickel nanoparticles using magnetic circular dichroism (MCD). Besides, we show that non-local surface lattice resonances in periodic arrays of the same nanoparticles significantly enhance the MCD signal. A numerical model based on the modified long wavelength approximation is used to reproduce the main features in the experimental spectra and provide design rules for large MCD effects in sensing applications.

We report on bandgap tuning in magnonic crystals made of nanometer-thick yttrium iron garnet (YIG) films with CoFeB-filled grooves via a variation of the groove depth, lattice constant, and film thickness. Using broadband spin-wave spectroscopy, we demonstrate bandgap widening in a 260-nm-thick YIG crystal when the grooves are deepened from half to full film thickness. Importantly, low-loss spin-wave transmission in the allowed bands of the magnonic crystal is almost unaffected by the patterning of fully discrete YIG stripes. Downscaling of the YIG film thickness to 35 nm decreases the bandgap size through a flattening of the spin-wave dispersion relation. We show that a reduction in the lattice constant effectively compensates for this trend. Our experimental results are corroborated by micromagnetic simulations, providing relevant information for the design of ultrathin YIG-based magnonic crystals with optimized bandgaps and spin-wave transmission properties.

Magnetic domain walls are pinned strongly by abrupt changes in magnetic anisotropy. When driven into oscillation by a spin-polarized current, locally pinned domain walls can be exploited as tunable sources of short-wavelength spin waves. Here, we develop an analytical framework and discrete Heisenberg model to describe the static and dynamic properties of pinned domain walls with a head-to-tail magnetic structure. We focus on magnetic domain walls that are pinned by ${90}^{\ensuremath{\circ}}$ rotations of uniaxial magnetic anisotropy. Our model captures the domain wall response to a spin-transfer torque that is exerted by an electric current. Model predictions of the domain wall resonance frequency and its evolution with magnetic anisotropy strength and external magnetic field are compared to micromagnetic simulations.

A critical factor for electronics based on inorganic layered crystals stems from the electrical contact mode between the semiconducting crystals and the metal counterparts in the electric circuit. Here, a materials tailoring strategy via nanocomposite decoration is carried out to reach metallic contact between MoS2 matrix and transition metal nanoparticles. Nickel nanoparticles (NiNPs) are successfully joined to the sides of a layered MoS2 crystal through gold nanobuffers, forming semiconducting and magnetic NiNPs@MoS2 complexes. The intrinsic semiconducting property of MoS2 remains unchanged, and it can be lowered to only few layers. Chemical bonding of the Ni to the MoS2 host is verified by synchrotron radiation based photoemission electron microscopy, and further proved by first- principles calculations. Following the system's band alignment, new electron migration channels between metal and the semiconducting side contribute to the metallic contact mechanism, while semiconductor–metal heterojunctions enhance the photocatalytic ability.

Strontium titanate (${\mathrm{SrTiO}}_{3}$) is the quintessential material for oxide electronics. One of its hallmark features is the transition, driven by antiferrodistortive (AFD) lattice modes, from a cubic to a ferroelastic low-temperature phase. Here we investigate the evolution of the ferroelastic twin walls upon application of an electric field. Remarkably, we find that the dielectric anisotropy of tetragonal ${\mathrm{SrTiO}}_{3}$, rather than the intrinsic domain wall polarity, is the main driving force for the motion of the twins. Based on a combined first-principles and Landau-theory analysis, we show that such anisotropy is dominated by a trilinear coupling between the polarization, the AFD lattice tilts, and a previously overlooked antiferroelectric (AFE) mode. We identify the latter AFE phonon with the so-called ``$R$ mode'' at $\ensuremath{\sim}440\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, which was previously detected in IR experiments, but whose microscopic nature was unknown.

We report on the manipulation of magnetization by femtosecond laser pulses in a periodic array of cylindrical nickel nanoparticles. By performing experiments at different wavelength, we show that the excitation of collective surface plasmon resonances triggers demagnetization in zero field or magnetic switching in a small perpendicular field. Both magnetic effects are explained by plasmon-induced heating of the nickel nanoparticles to their Curie temperature. Model calculations confirm the strong correlation between the excitation of surface plasmon modes and laser-induced changes in magnetization., 4 pages, 4 figures

Active manipulation of spin waves is essential for the development of magnon-based technologies. Here, we demonstrate programmable spin-wave filtering by resetting the spin structure of pinned 90° Néel domain walls in a continuous CoFeB film with abrupt rotations of uniaxial magnetic anisotropy. Using micro-focused Brillouin light scattering and micromagnetic simulations, we show that broad 90° head-to-head or tail-to-tail magnetic domain walls are transparent to spin waves over a broad frequency range. In contrast, magnetic switching to a 90° head-to-tail configuration produces much narrower and strongly reflecting domain walls at the same pinning locations. Based on these results, we propose a magnetic spin-wave valve with two parallel domain walls. Switching the spin-wave valve from an open to a closed state changes the transmission of spin waves from nearly 100 to 0%. Active control over spin-wave transport through programmable domain walls could be utilized in magnonic logic devices or non-volatile memory elements., Magnon-based spintronic devices crucially rely on the capability of spin wave manipulation. Here the authors achieve active control of spin wave transmission by programming a pinned 90 degree Néel domain wall in a continuous CoFeB/BaTiO3 film with abrupt rotations of uniaxial magnetic anisotropy.

We model tunneling currents through step barrier structures representative of ferroelectric tunnel junctions wherein one of the electrodes contributes to the barrier potential profile or an extra layer is grown between the ferroelectric barrier and one of the electrodes. We study current density-voltage (J-V) and tunneling electroresistance (TER) curves using the Tsu-Esaki formula with numerically calculated transmission. This method is computationally robust, and the same results cannot be obtained with the standard methods usually applied for interpreting experimental I-V curves. Our results predict that the effect of resonant tunneling produces asymmetry in the J-V curves and negative differential resistance characteristics. We show that the asymmetry of the J-V curves can be tuned by adjusting the barrier heights and widths and that changing the extra barrier width affects the asymmetry the most. The barrier widths and the main barrier height affect the magnitude of the tunneling current the most. The change in the tilt of the main barrier is suggested to produce a significant TER of the order of 102. Our numerical method provides a systematic way to study trends in tunneling currents through step barrier structures across a wide range of barrier parameters and bias voltages, in contrast to the necessity of employing several different approximations when using analytical formulae. Therefore, our method provides the means for interpreting existing and future experiments and can be used as a tool for designing new devices with desired functionalities.

openaire: EC/FP7/307502/EU//E-CONTROL We report on the generation and confinement of short-wavelength spin waves in a continuous film with periodically modulated magnetic anisotropy. The concept, which is demonstrated for strain-coupled Co40Fe40B20/BaTiO3 heterostructures, relies on abrupt rotation of magnetic anisotropy at the boundaries of magnetic stripe domains. In combination with an external bias field, this modulation of magnetic anisotropy produces a lateral variation of the effective magnetic field, leading to local spin-wave excitation when irradiated by a microwave magnetic field. In domains with small effective field, spin waves are perfectly confined by the spin gap in neighboring domains. In contrast, standing spin waves in domains with large effective field radiate into neighboring domains. Using micromagnetic simulation, we show that the wavelength of emitted spin waves is tunable from a few micrometers down to about 100 nm by rotation of the bias field. Importantly, the orientation of the wave front remains fixed. We also demonstrate that dynamic fluctuations of the effective magnetic field produce exchange-dominated spin waves at single-anisotropy boundaries. The multiferroic heterostructures presented here enable the use of global excitation fields from a microwave antenna to emit tunable spin waves from a nanometer-wide line source at well-defined locations of a continuous ferromagnetic film.

We investigate the spin Hall effect in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with Ta underlayers thicker than the spin diffusion length. The crystallographic structures of the Ta layer and Ta/CoFeB interface are examined in detail using X-ray diffraction and transmission electron microscopy. The thinnest Ta underlayer is amorphous, whereas for thicker Ta layers a disoriented tetragonal beta-phase appears. Effective spin-orbit torques are calculated based on harmonic Hall voltage measurements performed in a temperature range between 15 and 300 K. To account for the temperature dependence of damping-like and field-like torques, we extend the spin diffusion model by including an additional contribution from the Ta/CoFeB interface. Based on this approach, the temperature dependence of the spin Hall angle in the Ta underlayer and at Ta/CoFeB interface are determined separately. The results indicate an almost temperature-independent spin Hall angle of theta_SH-N = -0.2 in Ta and a strongly temperature-dependent theta_SH-I for the intermixed Ta/CoFeB interface., Comment: 11 pages, 10 figures

Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow magnetic damping are relevant for magnon-based spintronics and low-power wave-like computing. The excitation frequency of spin waves in YIG is rather low in weak external magnetic fields because of its small saturation magnetization, which limits the potential of YIG films for high-frequency applications. Here, we demonstrate how exchange-coupling to a CoFeB film enables efficient excitation of high-frequency perpendicular standing spin waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense PSSW modes up to 10th order. Strong hybridization between the PSSW modes and the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing behavior in broadband spin-wave spectra. A dynamic exchange torque at the YIG/CoFeB interface explains the excitation of PSSWs. The localized torque originates from exchange coupling between two dissimilar magnetization precessions in the YIG and CoFeB layers. As a consequence, spin waves are emitted from the YIG/CoFeB interface and PSSWs form when their wave vector matches the perpendicular confinement condition. PSSWs are not excited when the exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer. Micromagnetic simulations confirm the exchange-torque mechanism., Comment: 9 pages, 6 figures

Magnetoplasmonics is an emerging field of intense research on materials combining magnetic and plasmonic functionalities. The novel optical and magneto-optical (MO) properties displayed by these materials could allow the design of a new class of magnetically controllable optical nano-devices. In this work, we investigate the effects of a non-absorbing (insulating) substrate on the MO activity of pure ferromagnetic disk-shaped nanostructures supporting localized plasmon resonances. We show that the red-shift of the localized plasmon resonance, related to the modification of the localization of the electromagnetic field due to the substrate, is not the only effect that the substrate has on the MO response. We demonstrate that the reflectivity of the substrate itself plays a key role in determining the MO response of the system. We discuss why it is so and provide a description of the modeling tools suitable to take into account both effects. Understanding the role of the substrate will permit a more aware design of magnetoplasmonic nanostructured devices for future biotechnological and optoelectronic applications. Ferromagnetic nickel nanodisk in vacuum (left) and on a non-absorbing substrate (right), illuminated by linearly polarized light. The polarization of the reflected field is changed in the first case due to a combination of intrinsic magneto-optical properties and the nanoconfinement of the material. In the second case, the polarization of the reflected light is affected also by the presence of the substrate. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

openaire: EC/FP7/340748/EU//CODE We report on the optical and magneto-optical response of hybrid plasmonic lattices that consist of pure nickel and gold nanoparticles in a checkerboard arrangement. Diffractive far-field coupling between the individual emitters of the lattices results in the excitation of two orthogonal surface lattice resonance modes. Local analyses of the radiation fields indicate that both the nickel and gold nanoparticles contribute to these collective resonances and, thereby, to the magneto-optical activity of the hybrid arrays. The strong effect of noble metal nanoparticles on the magneto-optical response of hybrid lattices opens up new avenues for the realization of sensitive and tunable magneto-plasmonic nanostructures.

Universal, giant and nonvolatile resistive switching is demonstrated for oxide tunnel junctions with ferroelectric PbZr0.2 Ti0.8 O3 , ferroelectric BaTiO3, and paraelectric SrTiO3 tunnel barriers. The effects are caused by reversible migration of oxygen vacancies between the tunnel barrier and bottom La2/3 Sr1/3 MnO3 electrode. The switching process, which is driven by large electric fields, is efficient down to a temperature of 5 K.

openaire: EC/H2020/665215/EU//EMOTION | openaire: EC/FP7/307502/EU//E-CONTROL Miniaturization of magnonic devices for wave-like computing requires emission of short-wavelength spin waves, a key feature that cannot be achieved with microwave antennas. In this paper, we propose a tunable source of short-wavelength spin waves based on highly localized and strongly pinned magnetic domain walls in ferroelectric-ferromagnetic bilayers. When driven into oscillation by a microwave spin-polarized current, the magnetic domain walls emit spin waves with the same frequency as the excitation current. The amplitude of the emitted spin waves and the range of attainable excitation frequencies depend on the availability of domain wall resonance modes. In this respect, pinned domain walls in magnetic nanowires are particularly attractive. In this geometry, spin wave confinement perpendicular to the nanowire axis produces a multitude of domain wall resonances enabling efficient spin wave emission at frequencies up to 100 GHz and wavelengths down to 20 nm. At high frequency, the emission of spin waves in magnetic nanowires becomes monochromatic. Moreover, pinning of magnetic domain wall oscillators onto the same ferroelectric domain boundary in parallel nanowires guarantees good coherency between spin wave sources, which opens perspectives towards the realization of Mach-Zehnder type logic devices and sensors.

We report on the temperature and layer thickness variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements reveal dissimilar temperature evolutions of longitudinal and transverse effective magnetic field components. The transverse effective field changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much stronger contribution from interface spin-orbit interactions compared to, for example, Ta. Transmission electron microscopy measurements reveal that considerable interface mixing between W and CoFeB is primarily responsible for this effect., Comment: 4 pages, 5 figures

By synergically combining experiments and simulations, we show how the excitation of lattice surface modes in ordered arrays of magnetic and optically-anisotropic nanoantennas leads to a highly enhanced and tunable Fano-like modulation of the magnetoplasmonic response.

openaire: EC/H2020/665215/EU//EMOTION | openaire: EC/FP7/307502/EU//E-CONTROL Strong coupling of magnetic domain walls onto straight ferroelastic boundaries of a ferroelectric layer enables full and reversible electric-field control of magnetic domain wall motion. In this paper, the dynamics of this new driving mechanism is analyzed using micromagnetic simulations. We show that transverse domain walls with a near-180° spin structure are stabilized in magnetic nanowires and that electric fields can move these walls with high velocities. Above a critical velocity, which depends on material parameters, nanowire geometry and the direction of domain wall motion, the magnetic domain walls depin abruptly from the ferroelastic boundaries. Depinning evolves either smoothly or via the emission and annihilation of a vortex or antivortex core (Walker breakdown). In both cases, the magnetic domain wall slows down after depinning in an oscillatory fashion and eventually comes to a halt. The simulations provide design rules for hybrid ferromagnetic-ferroelectric domain-wall-based devices and indicatethat material disorder and structural imperfections only influence Walker-breakdown-like depinning at high domain wall velocities.

The potential application of artificial spin ice in magnetic nanodevices provides a strong drive to investigate different lattice geometries. Here, we combine components of a recently investigated artificial spin ratchet with components of the prototypical square lattice to form a geometrically frustrated artificial spin ice system where Ising-type nanomagnets are arranged onto a two-dimensional square-kite lattice. Using synchrotron-based photoemission electron microscopy, we explore moment configurations achieved in this lattice geometry. Following thermal annealing, we image how a variation of the relevant lattice parameter affects magnetic ordering in four-island squares and four-island vertices during cooling through the Blocking temperature. Depending on lattice spacing, both nearly uniform and disordered spin configurations are accessible in our samples. We show that the relative energies of the building blocks of the system, which are typically used to classify lattice configurations, are not predictive of the low energy states adopted by the experimental system. To understand magnetic ordering in the square-kite lattice, longer range interactions must be considered.

Geometrical frustration occurs when entities in a system, subject to given lattice constraints, are hindered to simultaneously minimize their local interactions. In magnetism, systems incorporating geometrical frustration are fascinating, as their behavior is not only hard to predict, but also leads to the emergence of exotic states of matter. Here, we provide a first look into an artificial frustrated system, the dipolar trident lattice, where the balance of competing interactions between nearest-neighbor magnetic moments can be directly controlled, thus allowing versatile tuning of geometrical frustration and manipulation of ground state configurations. Our findings not only provide the basis for future studies on the low-temperature physics of the dipolar trident lattice, but also demonstrate how this frustration-by-design concept can deliver magnetically frustrated metamaterials., Artificial magnetic nanostructures enable the study of competing frustrated interactions with more control over the system parameters than is possible in magnetic materials. Farhan et al. present a two-dimensional lattice geometry where the frustration can be controlled by tuning the unit cell parameters.

Perpendicularly magnetized layers are used widely for high-density information storage in magnetic hard disk drives and nonvolatile magnetic random access memories. Writing and erasing of information in these devices is implemented by magnetization switching in local magnetic fields or via intense pulses of electric current. Improvements in energy efficiency could be obtained when the reorientation of perpendicular magnetization is controlled by an electric field. Here, we report on reversible electric-field-driven out-of-plane to in-plane magnetization switching in Cu/Ni multilayers on ferroelectric BaTiO3 at room temperature. Fully deterministic magnetic switching in this hybrid material system is based on efficient strain transfer from ferroelastic domains in BaTiO3 and the high sensitivity of perpendicular magnetic anisotropy in Cu/Ni to electric-field-induced strain modulations. We also demonstrate that the magnetoelectric coupling effect can be used to realize 180° magnetization reversal if the out-of-plane symmetry of magnetic anisotropy is temporarily broken by a small magnetic field. An electric-field switching technique can lower the energy required to write and erase data in high-density magnetic storage devices. Perpendicularly magnetic recording is a new technology that aligns magnetic bits into vertical arrangements using specially constructed multilayer films. Normally, magnetic fields or intense bursts of electric currents are needed to modify individual bits. Tomoyasu Taniyama from the Tokyo Institute of Technology and international collaborators have discovered that growing perpendicularly magnetised copper-nickel multilayers on top of ferroelectric BaTiO3 crystals produces devices responsive to modest electric fields. Laser-based measurements and theoretical analysis revealed that this fast, reversible, and room temperature data switching method was driven by mechanical strain at the metal multilayer-ferroelectric interface. Successful results with films up to 65 nanometers thick indicate that this approach holds promise for practical spintronics applications. Reversible electric-field-driven magnetization switching between perpendicular-to-plane and in-plane orientations in Cu/Ni multilayers on ferroelectric BaTiO3 is demonstrated at room temperature. Fully deterministic magnetic switching is based on efficient strain transfer from ferroelastic domains in BaTiO3 and the high sensitivity of perpendicular magnetic anisotropy in Cu/Ni to electric-field-induced strain modulations. The magnetoelectric coupling effect can also be used to realize 180° magnetization reversal if the out-of-plane symmetry of magnetic anisotropy is temporarily broken by a small magnetic field.

Systems allowing label-free molecular detection are expected to have enormous impact on biochemical sciences. Research focuses on materials and technologies based on exploiting localized surface plasmon resonances in metallic nanostructures. The reason for this focused attention is their suitability for single-molecule sensing, arising from intrinsically nanoscopic sensing volume and the high sensitivity to the local environment. Here we propose an alternative route, which enables radically improved sensitivity compared with recently reported plasmon-based sensors. Such high sensitivity is achieved by exploiting the control of the phase of light in magnetoplasmonic nanoantennas. We demonstrate a manifold improvement of refractometric sensing figure-of-merit. Most remarkably, we show a raw surface sensitivity (that is, without applying fitting procedures) of two orders of magnitude higher than the current values reported for nanoplasmonic sensors. Such sensitivity corresponds to a mass of ~0.8 ag per nanoantenna of polyamide-6.6 (n=1.51), which is representative for a large variety of polymers, peptides and proteins.

In elastically coupled multiferroic heterostructures that exhibit full domain correlations between ferroelectric and ferromagnetic subsystems, magnetic domain walls are firmly pinned on top of ferroelectric domain boundaries. In this work, we investigate the influence of pinned magnetic domain walls on the magnetization reversal process in a ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ wedge film that is coupled to a ferroelectric ${\mathrm{BaTiO}}_{3}$ substrate via interface strain transfer. We show that the magnetic field direction can be used to select between two distinct magnetization reversal mechanisms, namely, (1) double switching events involving alternate stripe domains at a time or (2) synchronized switching of all domains. Furthermore, scaling of the switching fields with domain width and film thickness is also found to depend on the field orientation. These results are explained by considering the dissimilar energies of the two types of pinned magnetic domain walls that are formed in the system.

We report on reversible electric-field-driven magnetic domain wall motion in a Cu/Ni multilayer on a ferroelectric BaTiO$_3$ substrate. In our heterostructure, strain-coupling to ferroelastic domains with in-plane and perpendicular polarization in the BaTiO$_3$ substrate causes the formation of domains with perpendicular and in-plane magnetic anisotropy, respectively, in the Cu/Ni multilayer. Walls that separate magnetic domains are elastically pinned onto ferroelectric domain walls. Using magneto-optical Kerr effect microscopy, we demonstrate that out-of-plane electric field pulses across the BaTiO$_3$ substrate move the magnetic and ferroelectric domain walls in unison. Our experiments indicate an exponential increase of domain wall velocity with electric field strength and opposite domain wall motion for positive and negative field pulses. Magnetic fields do not affect the velocity of magnetic domain walls, but independently tailor their internal spin structure, causing a change in domain wall dynamics at high velocities., 5 pages, 4 figures

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier (0.75 nm < $t_{MgO}$ < 1.05 nm) is studied using the spin-torque diode effect. The application of an RF current into nanosized MTJs generates a DC mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different DC bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free-electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed., 6 pages, 5 figures

We report on the magnetic switching and backhopping effects due to spin-transfer-torque in magnetic tunnel junctions. Experimental data on the current-induced switching in junctions with MgO tunnel barrier reveal a random back-and-forth switching between the magnetization states, which appears when the current direction favors the parallel magnetic configuration. The effect depends on the barrier thickness $t_b$, and is not observed in tunnel junctions with very thin MgO tunnel barriers, $t_b$ $, Comment: 7 pages, 3 figures, regular journal paper