Manipulation of Magnetic Skyrmion Density in Continuous Ir/Co/Pt Multilayers

We show that magnetic skyrmions can be stabilised at room temperature in continuous [Ir/Co/Pt]₅ multilayers on SiO₂/Si substrates without the prior application of electric current or magnetic field. While decreasing the Co thickness, a transition of the magnetic domain patterns from worm-like state to separated stripes is observed. The skyrmions are clearly imaged in both states using magnetic force microscopy. The density of skyrmions can be significantly enhanced after applying the "in-plane field procedure". Our results provide means to manipulate magnetic skyrmion density, further allowing for the optimised engineering of skyrmion-based devices.

Tailoring interfacial effect in multilayers with Dzyaloshinskii-Moriya interaction by helium ion irradiation

We show a method to control magnetic interfacial effects in multilayers with Dzyaloshinskii-Moriya interaction (DMI) using helium (He[Formula: see text]) ion irradiation. We report results from SQUID magnetometry, ferromagnetic resonance as well as Brillouin light scattering results on multilayers with DMI as a function of irradiation fluence to study the effect of irradiation on the magnetic properties of the multilayers. Our results show clear evidence of the He[Formula: see text] irradiation effects on the magnetic properties which is consistent with interface modification due to the effects of the He[Formula: see text] irradiation. This external degree of freedom offers promising perspectives to further improve the control of magnetic skyrmions in multilayers, that could push them towards integration in future technologies.

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.

Magnetization dynamics of weak stripe domains in Fe-N thin films: a multi-technique complementary approach

The resonant eigenmodes of an α'-FeN thin film characterized by weak stripe domains are investigated by Brillouin light scattering and broadband ferromagnetic resonance experiments, assisted by micromagnetic simulations. The spectrum of the dynamic eigenmodes in the presence of the weak stripes is very rich and two different families of modes can be selectively detected using different techniques or different experimental configurations. Attention is paid to the evolution of the mode frequencies and spatial profiles under the application of an external magnetic field, of variable intensity, in the direction parallel or transverse to the stripes. The different evolution of the modes with the external magnetic field is accompanied by a distinctive spatial localization in specific regions, such as the closure domains at the surface of the stripes and the bulk domains localized in the inner part of the stripes. The complementarity of BLS and FMR techniques, based on different selection rules, is found to be a fruitful tool for the study of the wealth of localized magnetic excitations generally found in nanostructures.

Brillouin light scattering studies of 2D magnonic crystals

Magnonic crystals, materials with periodic modulation of their magnetic properties, represent the magnetic counterpart of photonic, phononic and plasmonic crystals, and have been largely investigated in recent years because of the possibility of using spin waves as a new means for carrying and processing information over a very large frequency bandwidth. Here, we review recent Brillouin light scattering studies of 2D magnonic crystals consisting of single- and bi-component arrays of interacting magnetic dots or antidot lattices. In particular, we discuss the principal properties of the magnonic band diagram of such systems, with emphasis given to its dependence on both magnetic and the geometrical parameters. Thanks to the possibility of tailoring their band structure by means of several degrees of freedom, planar magnonic crystals offer a good opportunity to design an innovative class of nanoscale microwave devices.

Towards zero-power ICT

Is it possible to operate a computing device with zero energy expenditure? This question, once considered just an academic dilemma, has recently become strategic for the future of information and communication technology. In fact, in the last forty years the semiconductor industry has been driven by its ability to scale down the size of the complementary metal-oxide semiconductor-field-effect transistor, the building block of present computing devices, and to increase computing capability density up to a point where the power dissipated in heat during computation has become a serious limitation. To overcome such a limitation, since 2004 the Nanoelectronics Research Initiative has launched a grand challenge to address the fundamental limits of the physics of switches. In Europe, the European Commission has recently funded a set of projects with the aim of minimizing the energy consumption of computing. In this article we briefly review state-of-the-art zero-power computing, with special attention paid to the aspects of energy dissipation at the micro- and nanoscales.

Universal dependence of the spin wave band structure on the geometrical characteristics of two-dimensional magnonic crystals

In the emerging field of magnon-spintronics, spin waves are exploited to encode, carry and process information in materials with periodic modulation of their magnetic properties, named magnonic crystals. These enable the redesign of the spin wave dispersion, thanks to its dependence on the geometric and magnetic parameters, resulting in the appearance of allowed and forbidden band gaps for specific propagation directions. In this work, we analyze the spin waves band structure of two-dimensional magnonic crystals consisting of permalloy square antidot lattices with different geometrical parameters. We show that the frequency of the most intense spin-wave modes, measured by Brillouin light scattering, exhibits a universal dependence on the aspect ratio (thickness over width) of the effective nanowire enclosed between adjacent rows of holes. A similar dependence also applies to both the frequency position and the width of the main band gap of the fundamental (dispersive) mode at the edge of the first Brillouin zone. These experimental findings are successfully explained by calculations based on the plane-wave method. Therefore, a unified vision of the spin-waves characteristics in two-dimensional antidot lattices is provided, paving the way to the design of tailored nanoscale devices, such as tunable magnonic filters and phase-shifters, with predicted functionalities.

Forbidden band gaps in the spin-wave spectrum of a two-dimensional bicomponent magnonic crystal

The spin-wave band structure of a two-dimensional bicomponent magnonic crystal, consisting of Co nanodisks partially embedded in a Permalloy thin film, is experimentally investigated along a high-symmetry direction by Brillouin light scattering. The eigenfrequencies and scattering cross sections are interpreted using plane wave method calculations and micromagnetic simulations. At the boundary of both the first and the second Brillouin zones, we measure a forbidden frequency gap whose width depends on the magnetic contrast between the constituent materials. The modes above and below the gap exhibit resonant spin-precession amplitudes in the complementary regions of periodically varying magnetic parameters. Our findings are key to advance both the physics and the technology of band gap engineering in magnonics.

Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy

It is well known that Fe films deposited on a c(2 × 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 × 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 × 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole-dipole interactions. For both sharp and intermixed c(2 × 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 × 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings.

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.

Direct observation of a propagating spin wave induced by spin-transfer torque

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Brillouin Light Scattering Investigation of the Elastic Properties of Ta/Al Metallic Superlattices

The Brillouin light scattering technique has been exploited for investigating the elastic properties of periodic superlattices made by alternating layers of Ta and Al. These multilayers, deposited by d.c. sputtering on glass and Si substrates, present a polycrystalline structure with (110) and (111) texture for the Ta and Al layers, respectively. They have total thicknesses of about 0.5 μm and periods ranging from 4 to 10 nm. Measurement of the phase velocities of the Rayleigh and Sezawa acoustic modes from the frequency position of the corresponding Brillouin peaks, yielded informaton on the effective elastic constants of the superlattices. for large periods (8-10 nm) the values determined experimentally are in good agreement with those calculated from the elastic constants of the bulk materials, while for lower periods (4-6 nm) the estimated elastic constants exhibit a marked increase. This anomalous behavior has been attributed to the presence of a transition layer at each interface, where Ta and Al interdiffuse, as observed by x-ray and electron microscopy experiments.

Elastic Properties of FeNi/Cu and FeNi/Nb Metallic Superlattices Investigated by Brillouin Light Scattering

The Brillouin light scattering technique has been exploited in order to investigate the elastic properties of periodic superlattices consisting of alternating layers of Fe20Ni80 (permalloy) and either Cu or Nb. These multilayers, with total thicknesses ranging between 0.2 and 0.7 mm and with periods of typically 3-5 nm, present a polycrystalline structure with (110) texture for the Nb layers and (111) texture for both the FeNi and Cu layers. Measurement of the frequency position of the Brillouin peaks corresponding to the Rayleigh and Sezawa acoustic modes allowed the effective elastic constants of these structures to be determined. The values obtained are compared with those calculated from the elastic constants of the bulk materials, taking into account the polycrystalline nature of the superlattices and the crystallographic orientation of the layers.

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.

Discrete modes of a ferromagnetic stripe dipolarly coupled to a ferromagnetic film: a Brillouin light scattering study

Spin wave excitations in a magnetic structure consisting of a series of long permalloy stripes of a rectangular cross section magnetized along the stripe length and situated above a continuous permalloy film are studied both experimentally and theoretically. Stripes and continuous film are coupled by dipole-dipole interaction across 10 nm thick Cu spacers. Experimental measurements made using the Brillouin light scattering technique (with the light wavevector oriented along the stripe width) provide evidence for one dispersive spin wave mode associated with the continuous film and several discrete non-dispersive modes resonating within the finite width of the stripes.To interpret the experimental spectra, an analytic theory based on the spin wave formalism for finite-width magnetic stripes has been developed, achieving a good qualitative and partly quantitative description of the experimentally observed spin wave spectrum of the system. In particular, it is explained why the presence of a continuous magnetic film near the magnetic stripe leads to a substantial decrease of the frequencies of the discrete dipolar spin wave modes localized within the stripes. A more quantitative description of the measured frequencies and of the spatial profiles of the spin wave eigenmodes has been obtained by numerical calculations performed using a finite element method.

Splitting of spin excitations in nanometric rings induced by a magnetic field

We present a Brillouin light scattering investigation of the eigenmode spectrum of nanometric permalloy rings as a function of the applied magnetic field. In particular, different splitting effects induced by the applied magnetic field on the radial and azimuthal excitations have been observed and explained in terms of either mode localization or symmetry. The dynamical matrix approach has been used to calculate the whole set of eigenvectors and eigenvalues of the system, in both the vortex and saturated states.

[A rare case of a giant intra-abdominal mesocolic liposarcoma]

Intra-abdominal liposarcomas (IALS) represent a rare localization compared to other liposarcomatous (LS) sites such as the lower extremities and the retroperitoneum. The authors report their experience in a case of giant liposarcoma (weight: Kg 8.2) presenting a massive intra-abdominal extension. Diagnostic and therapeutic problems related to this type of neoplasm as well as a literature review are reported.

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO(2) substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.