Coherent and Dissipative Coupling in a Magnetomechanical System.

Hybrid elastic and spin waves hold promises for energy-efficient and versatile generation and detection of magnetic signals, with potentially long coherence times. Here we report on the combined elastic and magnetic dynamics in a one-dimensional magnetomechanical crystal composed of an array of magnetic nanostripes. Phononic and magnonic modes are impulsively excited by an optical ultrafast trigger and their decay is monitored by time-resolved magneto-optical Kerr effect. Complementary Brillouin light scattering measurements and micromagnetic simulations concur in a unified picture, in which the strength and degree of mixing of coherent and dissipative coupling of the quasiparticles are determined quantitatively.

Tunable 2-D magnonic crystals: effect of packing density.

Magnonic crystals, periodic arrays of magnetic structures, have emerged as a promising platform for manipulating and controlling spin waves in magnetic materials. Magnetic antidot nanostructures, representing 2-D magnonic crystals, are versatile platforms for controlling and manipulating magnons. In this work, we systematically investigate the effects of inter-hole spacing and lattice (rhombic and honeycomb) arrangements on the dynamic properties of Ni80Fe20 antidot structures. The dynamic responses of antidot lattices of fixed hole diameter (d = 280 nm) and inter-hole spacing (s) between 90 and 345 nm are investigated using broadband ferromagnetic spectroscopy. Multiple resonance modes sensitive to s are observed due to the inhomogeneous internal field distribution induced by the presence of holes. There is a marked variation in mode frequency, mode intensity and the number of modes for rhombic antidot lattice as the inter-hole spacing and applied field direction are varied. Our experimental results are in good agreement with micromagnetic simulations. Our findings may find application in the design of magnonic-based devices.

The 2021 Magnonics Roadmap.

Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.

Linear chains of nanomagnets: engineering the effective magnetic anisotropy.

This paper investigates the control of effective magnetic anisotropy in Permalloy linear chain arrays, achieved by tuning the symmetry arrangement of the ellipsoidal nanomagnets and the film thickness. When the ellipsoidal nanomagnets are coupled along their easy axis, stronger effective magnetic anisotropy is achieved compared to when the nanomagnets are coupled along their hard axis. A clear transition from a single domain state to a combination of complex flux closure states such as a vortex or double vortices is observed at different applied field angles when the film thickness is varied in the range from 20 nm to 100 nm. Tunable microwave absorption spectra, obtained by ferromagnetic resonance spectroscopy, established the complex interplay between the shape anisotropy and magnetostatic interactions, which becomes more intriguing at different film thicknesses and applied field angles. The micromagnetic simulations are in good agreement with the experimental results. Our results demonstrate possible ways of manipulating the effective magnetic anisotropy in arrays of nanomagnets for magnonic and microwave applications.

Interplay between intra- and inter-nanowires dynamic dipolar interactions in the spin wave band structure of Py/Cu/Py nanowires.

We have studied both experimentally and theoretically the reprogrammable spin wave band structure in Permalloy(10 nm)/Cu(5 nm)/Permalloy(30 nm) nanowire arrays of width w = 280 nm and inter-wire separation in the range from 80 to 280 nm. We found that, depending on the inter-wire separation, the anti-parallel configuration, where the magnetizations of the two Permalloy layers point in opposite directions, is stabilized over specific magnetic field ranges thus enabling us to directly compare the band structure with that of the parallel alignment. We show that collective spin waves of the Bloch type propagate through the arrays with different magnonic bandwidths as a consequence of the interplay between the intra- and inter-nanowire dynamic dipolar interactions. A detailed understanding, e.g. whether they have a stationary or propagating character, is achieved by considering the phase relation (in-phase or out-of-phase) between the dynamic magnetizations in the two ferromagnetic layers and their average value. This work opens the path to magnetic field-controlled reconfigurable layered magnonic crystals that can be used for future nanoscale magnon spintronic devices.

Origin of relationship between ferromagnetic response and damage in stretched systems.

This article presents a study whose purpose is to elucidate the damage effects in thin films on their magnetic response. Co40Fe40B20 and Ni80Fe20 films of different nanometric thicknesses were stretched by more than 10% and in situ probed by atomic force microscopy measurements to determine their irreversible mechanical behavior (multi-cracking, buckling). Once these phenomena have been well identified, magnetic behavior of these stretched systems has been studied by ferromagnetic resonance to measure resulting magnetic anisotropy and damping evolutions. All of these experimental studies show that the magnetic properties are mainly affected by the stresses generated during the damage but not by the local discontinuities induced by the numerous cracks and buckles. This is in particular confirmed by the almost zero sensitivity to the damage of the magnetic properties of Ni80Fe20 alloy which is known for its vanishing magnetostriction.

Robust electric-field tunable opto-electrical behavior in Pt-NiO-Pt planar structures.

Capacitor-like metal-NiO-metal structures have attracted large interest in non-volatile memory applications based on electric field control of resistance, known as resistive switching (RS). Formation of conducting nanofilaments by the application of an electric field (electroformation) is considered an important pre-requisite for RS. Besides RS, due to the wide band gap and p-type semiconducting nature, NiO has been used to fabricate heterojunctions for photodetector applications. However, very little is known about the electrical and opto-electrical properties of NiO films in planar structure. Here, we demonstrate intriguing photoresponse and electrical behavior in electroformed Pt-NiO-Pt planar structures. While the pristine devices show ohmic electrical behavior and negligible photoresponse, the electroformed devices exhibit a nonlinear rectification behavior and a remarkable photoresponse at low voltage biases. More interestingly, the devices show a dramatic change of sign of rectification under light illumination at higher voltage biases. A polarity dependent and robust gain phenomenon is demonstrated in these devices. The large sensitivity, fast response, simple design and ease of preparation of these planar structures make them attractive for integration with current circuit technologies and various novel opto-electrical applications.

Giant moving vortex mass in thick magnetic nanodots.

Magnetic vortex is one of the simplest topologically non-trivial textures in condensed matter physics. It is the ground state of submicron magnetic elements (dots) of different shapes: cylindrical, square etc. So far, the vast majority of the vortex dynamics studies were focused on thin dots with thickness 5-50 nm and only uniform across the thickness vortex excitation modes were observed. Here we explore the fundamental vortex mode in relatively thick (50-100 nm) dots using broadband ferromagnetic resonance and show that dimensionality increase leads to qualitatively new excitation spectra. We demonstrate that the fundamental mode frequency cannot be explained without introducing a giant vortex mass, which is a result of the vortex distortion due to interaction with spin waves. The vortex mass depends on the system geometry and is non-local because of important role of the dipolar interaction. The mass is rather small for thin dots. However, its importance increases drastically with the dot thickness increasing.

Synthesis and Characterization of Cobalt/Palladium Multilayer Film and Nanodiscs on Polyethylene Terephthalate Substrate.

Cobalt/Palladium (Co/Pd) multilayer film and nanodisc samples were fabricated on polyethylene terephthalate (PET) substrates. The effects of surface roughness and grain size of PET substrate, the Co/Pd layer and the Au intermediate layer on the magnetic properties of these samples were investigated. We observed that the coercivity for Co/Pd films deposited directly on a smoother PET substrate is significantly smaller when compared with Co/Pd films deposited at the same time on Au buffer layer. The patterned Co/Pd nanodisc array exhibited a larger coercivity than the corresponding continuous film due to lower probability of finding nucleation sites in reduced film area.

Band diagram of spin waves in a two-dimensional magnonic crystal.

The dispersion curves of collective spin-wave excitations in a magnonic crystal consisting of a square array of interacting saturated nanodisks have been measured by Brillouin light scattering along the four principal directions of the first Brillouin zone. The experimental data are successfully compared to calculations of the band diagram and of the Brillouin light scattering cross section, performed through the dynamical matrix method extended to include the dipolar interaction between the disks. We found that the fourfold symmetry of the geometrical lattice is reduced by the application of the external field and therefore equivalent directions of the first Brillouin zone are characterized by different dispersion relations of collective spin waves. The dispersion relations are explained through the introduction of a bidimensional effective wave vector that characterizes each mode in this magnonic metamaterial.

Magnonic crystal as a medium with tunable disorder on a periodical lattice.

We show that periodic magnetic nanostructures represent a perfect system for studying excitations on disordered periodical lattices because of the possibility of controlled variation of the degree of disorder by varying the applied magnetic field. Magnetic force microscopy images and ferromagnetic resonance (FMR) data collected inside minor hysteresis loops for a periodic array of Permalloy nanowires were used to demonstrate correlation between the type of FMR response and the degree of disorder of the magnetic ground state.

Magnetic field dependence of the lowest-frequency edge-localized spin wave mode in a magnetic nanotriangle.

An understanding of the spin dynamics of nanoscale magnetic elements is important for their applications in magnetic sensing and storage. Inhomogeneity of the demagnetizing field in a non-ellipsoidal magnetic element results in localization of spin waves near the edge of the element. However, relative little work has been carried out to investigate the effect of the applied magnetic fields on the nature of such localized modes. In this study, micromagnetic simulations are performed on an equilateral triangular nanomagnet to investigate the magnetic field dependence of the mode profiles of the lowest-frequency spin wave. Our findings reveal that the lowest-frequency mode is localized at the base edge of the equilateral triangle. The characteristics of its mode profile change with the ground state magnetization configuration of the nanotriangle, which, in turn, depends on the magnitude of the in-plane applied magnetic field.

Nanostructured magnonic crystal with magnetic-field tunable bandgap.

Most experimental investigations into magnonic bandgaps are based on structures composed of single-constituent magnetic materials. Here we report Brillouin and numerical studies of the spin dynamics of a bi-component magnonic crystal, viz. a one-dimensional periodic array of alternating permalloy and cobalt 150 nm-wide nanostripes. Our measurements, together with those for a similar crystal composed of 250 nm-wide nanostripes, suggest that for a stripe width ratio of 1:1, the bandgap width of such magnonic arrays increases with crystal lattice constant. The bandgap parameters are strongly dependent on external magnetic field. This magnetic-field tunability of the bandgap is expected to be a crucial property of devices based on magnonic crystals. The agreement between numerical calculations, based on finite element analysis, and the experimental data is generally good.

Spin re-orientation in magnetostatically coupled Ni(80)Fe(20) ellipsoidal nanomagnets.

We investigate the influence of magnetostatic coupling on the spin configurations and magnetization reversal mechanism in a one-dimensional linear chain of densely packed Ni(80)Fe(20) ellipsoidal nanomagnets arranged in two basic configurations (elements coupled along the major or minor axes). Using magnetic force microscopy (MFM) we observed that for geometrically identical ellipsoidal nanomagnets the magnetic states at remanence are strongly dependent on the arrangement of the ellipsoid due to competition between the inherent shape and configuration anisotropies. When the elements are coupled along the major axis, the individual elements adopt a single domain magnetic state at remanence for field applied along the linear chain. This is in contrast with a wide range of magnetic states (single vortex states, double vortex states and modified single domain states) observed for elements coupled along the minor axis and also isolated elements. We have conducted a detailed investigation on the magnetization reversal mechanisms for both configurations and have correlated our experimental results with micromagnetic simulations.

Controlling the magnetization reversal in exchange-biased Co/CoO elongated nanorings.

We report on the control of magnetization reversal in exchange-biased Co/CoO nanorings resulting from the competition between field-cooling-induced unidirectional anisotropy at the Co/CoO interface and shape anisotropy of the elongated Co nanorings. We observed that the magnetization reversal mechanisms and magnitudes of exchange bias fields are strongly dependent on the strength and orientation of the cooling field relative to the major axis of the nanorings. Our results demonstrate a convenient technique to control the magnetization reversal modes in ferromagnetic nanorings.

Magnetoresistance behavior of ferromagnetic nanorings in a ring-wire hybrid configuration.

We present a technique for probing the magnetic configurations in ferromagnetic rings electrically without placing the electrical contact leads directly on the nanorings, but on wires attached to the ring structures. The magnetic configurations in pseudo spin valve rectangular and elliptical rings of width in the range from 60 to 300 nm have been systematically mapped using this technique. The giant magnetoresistance (GMR) responses for both the rings exhibit distinct switching fields and features corresponding to identifiable magnetization states in different segments of the nanostructures.

Field evolution of the magnetic normal modes in elongated permalloy nanometric rings.

The eigenmode spectrum of elongated permalloy rings with relatively wide arms is investigated by combined Brillouin light scattering and ferromagnetic resonance measurements as a function of the applied field intensity, encompassing both vortex and onion ground states. To reproduce the frequencies and the spatial profiles of the measured modes we performed micromagnetic simulations which solve the discretized Landau-Lifshitz-Gilbert equation in the time domain and calculate locally the Fourier transform. This allowed us to correlate the field dependence of different modes to their localization inside different portions of the rings. With the rings in the vortex ground state, in addition to radial, fundamental, and azimuthal modes, a localized mode, existing in the element portions where the internal field assumes its minima, has been measured and identified. This latter mode, whose frequency decreases for increasing field intensity, becomes soft near the transition from vortex to onion state and determines the change in symmetry of the magnetic ground state. After the transition, it is replaced by two edge modes, localized on the internal and external boundary of the rings, respectively.

Magnetic antidot nanostructures: effect of lattice geometry.

We investigate the effect of lattice geometry on the magnetic anisotropy and transport properties of Ni80Fe20 antidot nanostructures. The structures were fabricated using deep ultra-violet lithography at 248 nm exposure wavelength. For an antidot array with a square lattice, a fourfold magnetic anisotropy with alternating hard axis and easy axis every 45° was observed. The honeycomb and rhomboid antidot lattice, however, both show a sixfold anisotropy, conforming well to the symmetry of their respective lattices. The magnetic hysteresis and micromagnetic simulation of the spin states at remanence show that the magnetization reversal process is very sensitive to the lattice arrangement of the holes. From the magnetotransport measurements, both the current density distribution and the magnetoresistance behaviour are markedly dependent on the antidot lattice geometry, in agreement with our transport simulations.

Magnetic properties of large area cobalt nanomagnets.

We have investigated the magnetization reversal process of cobalt nanomagnetic dot arrays with thickness t in the range from 5 to 90 nm and diameter d in the range from 150 to 250 nm. Large area cobalt nanomagnets were fabricated on Si(100) substrate using deep ultra-violet lithography at 248 nm exposure wavelength. We observed that the magnetic properties of the nanomagnets strongly depend on the thickness and diameter of Co dots due to the effect of the demagnetizing field. The onset of the formation of a magnetization vortex at remanence was found at a phase boundary of thickness and size (e.g., t = 20 nm, d = 250 nm; t = 40 nm, d = 150 nm). Above this boundary, vortex annihilation and nucleation fields markedly varied with the thickness and diameter of the dots. Our experimental results are in good agreement with a simple micromagnetic modelling.