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With the rise of electromagnetic radiation-based technologies, considerable attention has been drawn to developing and implementing innovative electromagnetic shielding materials. Carbon nanomaterials and conductive polymers have been appealing to both academia and industry as promising alternatives for the traditionally used metallic materials, owing to their lightness, flexibility, easy processability and resistance to corrosion, which are of special importance for textile applications. In this work, multiwalled carbon nanotubes (MWCNTs) and poly (3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) have been applied to cotton textile substrates by straightforward scalable dyeing and coating processes, respectively. These processes led to uniform and homogeneous coatings with distinct properties: the fabric coated with MWCNT presented higher thickness and lower loading of incorporated material than the textile coated with PEDOT:PSS (thickness: 995 μm vs. 208 μm; material loading: 9.4 wt.% vs. 70.7 wt.%). The electromagnetic shielding properties were outlined for each shielding textile in the frequency range of 5.85–18 GHz: an average shielding effectiveness of ~35.6 dB was obtained for MWCNT@tex, while PEDOT:PSS@tex reached ~38.3 dB. Thus, PEDOT:PSS provided enhanced radiation shielding with lower coating thickness, while the MWCNTs led to improved attenuation with less material usage. Shielding effectiveness values above 30 dB were obtained for both electromagnetic interference shielding textiles, which corresponds to an excellent classification for general use applications, such as casual clothing and maternity wear.

CoFeB-based ultrathin films with perpendicular magnetic anisotropy are promising for different emerging technological applications such as nonvolatile memories with low power consumption and high-speed performance. In this work, the dynamical properties of [CoFeB (tCoFeB)/Pd (10 Å)]5 multilayered ultrathin films (1 Å ≤ tCoFeB ≤ 5 Å) are studied by using two complementary methods: time-resolved magneto-optical Kerr effect and broadband ferromagnetic resonance. The perpendicular magnetization is confirmed for multilayers with tCoFeB ≤ 4 Å. The effective perpendicular magnetic anisotropy reaches a clear maximum at tCoFeB = 3 Å. Further increase of CoFeB layer thickness reduces the perpendicular magnetic anisotropy and the magnetization became in-plane oriented for tCoFeB ≥ 5 Å. This behaviour is explained by considering competing contributions from surface and magnetoelastic anisotropies. It was also found that the effective damping parameter αeff decreases with CoFeB layer thickness and for tCoFeB = 4 Å reaches a value of ~ 0.019 that is suitable for microwave applications., This work was supported by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) and COMPETE 2020 (FEDER) under the projects MIT-EXPL/IRA/0012/2017, POCI-01-0145-FEDER-031302, PTDC/FIS- OTI/32213/2017, EXPL/IF/00541/2015 and UIDB/04968/2020. The authors also acknowledge financial support from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 734801 (MAGNAMED Project). The Portuguese team acknowledges the Network of Extreme Conditions Laboratories-NECL support through the project NORTE-01-0145-FEDER-022096. D.N. acknowledges the Spanish Ministry for Science, Innovation and Universities, for funding through the "Ramón y Cajal" program RYC-2017-22820. C.G. acknowledges the financial support received by ANID FONDECYT/REGULAR 1201102, ANID FONDEQUIP EQM140161, and ANID PIA/APOYO AFB180002.

In memory nanodevices based on phase changes induced thermally, the process of information recording is a reversible transition between the structurally ordered (crystalline) and disordered (amorph...

Magnetic anisotropy critically determines the utility of magnetic nanocrystals (NCs) in new nanomagnetism technologies. Using angular-dependent electron magnetic resonance (EMR), we observe magnetic anisotropy in isotropically arranged NCs of a nonmagnetic material. We show that the shape of the EMR angular variation can be well described by a simple model that considers magnetic dipole–dipole interactions between dipoles randomly located in the NCs, most likely due to surface dangling bonds. The magnetic anisotropy results from the fact that the energy term arising from the magnetic dipole–dipole interactions between all magnetic moments in the system is dominated by only a few dipole pairs, which always have an anisotropic geometric arrangement. Our work shows that magnetic anisotropy may be a general feature of NC systems containing randomly distributed magnetic dipoles., ACS Omega, 5 (20), ISSN:2470-1343

Discontinuous metal-insulator multilayers (DMIMs) are a special type of nanostructures with a layered arrangement of metallic particles sandwiched between continuous insulating layers. DMIMs exhibit moderate tunneling magnetoresistance (TMR) ratio but enhanced low-field sensitivity, which makes them promising candidates for magnetic field sensors. Recently we have grown epitaxial Fe/MgO DMIMs on MgO (0 0 1) single crystal substrates at different deposition conditions. Here, based on the analysis of magnetic isotherms in a broad temperature range, the effect of deposition temperature (TS) on microstructure of DMIMs is being studied and compared with the results of Transmission Electron Microscopy. It is shown that metallic layers consist of flat nanoparticles whose average size decreases, and their crystallinity improves with the increase of TS., Portuguese team acknowledges Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects NORTE-01-0145-FEDER-022096, EXPL/IF/00981/2013 (G.N.K), and grant SFRH/BPD/81710/2011 (A.G.-G.). This work was partially supported by Spanish Ministerio de Economía y Competitividad through project MAT2017-82970-C2, and from regional Gobierno de Aragón through project E26 including FEDER funding.

This article belongs to the Special Issue Magnetic and Magnetoelectric Nanomaterials: Synthesis, Characterization and Applications., Thin polycrystalline Co2FeGe films with composition close to stoichiometry have been fabricated using magnetron co-sputtering technique. Effects of substrate temperature (TS) and post-deposition annealing (Ta) on structure, static and dynamic magnetic properties were systematically studied. It is shown that elevated TS (Ta) promote formation of ordered L21 crystal structure. Variation of TS (Ta) allow modification of magnetic properties in a broad range. Saturation magnetization ~920 emu/cm3 and low magnetization damping parameter α ~ 0.004 were achieved for TS = 573 K. This in combination with soft ferromagnetic properties (coercivity below 6 Oe) makes the films attractive candidates for spin-transfer torque and magnonic devices., Portuguese team acknowledges Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects PTDC/FIS-MAC/31302/2017, NORTE-01-0145-FEDER-022096, EXPL/IF/00541/2015 (S.A.B.), SFRH/BPD/84948/2012 (A.V.) and SFRH/BPD/87430/2012 (A.A.). This work was partially supported by Spanish Ministerio de Economía y Competitividad through project MAT2017-82970-C2 and from regional Gobierno de Aragón through project E28 20R including FEDER funding.

Extending nanostructures into the third dimension has become a major research avenue in modern magnetism, superconductivity and spintronics, because of geometry-, curvature- and topology-induced phenomena. Here, we introduce Co-Fe nanovolcanoes-nanodisks overlaid by nanorings-as purpose-engineered 3D architectures for nanomagnonics, fabricated by focused electron beam induced deposition. We use both perpendicular spin-wave resonance measurements and micromagnetic simulations to demonstrate that the rings encircling the volcano craters harbor the highest-frequency eigenmodes, while the lower-frequency eigenmodes are concentrated within the volcano crater, due to the non-uniformity of the internal magnetic field. By varying the crater diameter, we demonstrate the deliberate tuning of higher-frequency eigenmodes without affecting the lowest-frequency mode. Thereby, the extension of 2D nanodisks into the third dimension allows one to engineer their lowest eigenfrequency by using 3D nanovolcanoes with 30% smaller footprints. The presented nanovolcanoes can be viewed as multi-mode microwave resonators and 3D building blocks for nanomagnonics., Comment: 5 pages, 4 figures

Media with engineered magnetization are essential building blocks in superconductivity, magnetism and magnon spintronics. However, the established thin-film and lithographic techniques insufficiently suit the realization of planar components with on-demand-tailored magnetization in the lateral dimension. Here, we demonstrate the engineering of the magnetic properties of CoFe-based nanodisks fabricated by the mask-less technique of focused electron beam induced deposition (FEBID). The material composition in the nanodisks is tuned \emph{in-situ} via the e-beam waiting time in the FEBID process and their post-growth irradiation with Ga ions. The magnetization $M_s$ and exchange stiffness $A$ of the disks are deduced from perpendicular ferromagnetic resonance measurements. The achieved $M_s$ variation in the broad range from $720$ emu/cm$^3$ to $1430$ emu/cm$^3$ continuously bridges the gap between the $M_s$ values of such widely used magnonic materials as permalloy and CoFeB. The presented approach paves a way towards nanoscale 2D and 3D systems with controllable and space-varied magnetic properties., Comment: 5 pages, 4 figures

The increasing demand for nanoscale magnetic devices requires development of 3D magnetic nanostructures. In this regard, focused electron beam induced deposition (FEBID) is a technique of choice for direct-writing of complex nano-architectures with applications in nanomagnetism, magnon spintronics, and superconducting electronics. However, intrinsic properties of nanomagnets are often poorly known and can hardly be assessed by local optical probe techniques. Here, an original spatially resolved approach is demonstrated for spin-wave spectroscopy of individual circular magnetic elements with sample volumes down to about 10−3 μm3. The key component of the setup is a coplanar waveguide whose microsized central part is placed over a movable substrate with well-separated CoFe-FEBID nanodisks which exhibit standing spin-wave resonances. The circular symmetry of the disks allows for the deduction of the saturation magnetization and the exchange stiffness of the material using an analytical theory. A good correspondence between the results of analytical calculations and micromagnetic simulations is revealed, indicating a validity of the used analytical model going beyond the initial thin-disk approximation used in the theoretical derivation. The presented approach is especially valuable for the characterization of direct-write magnetic elements opening new horizons for 3D nanomagnetism and magnonics.

Because of their record-low intrinsic magnetic damping properties, single-crystal yttrium-iron-garnet (YIG) films serve as an excellent model medium for studying magnon-induced spintronic phenomena such as spin pumping and the spin-orbit torque effect. For this purpose, YIG films are covered with sub-skin-depth layers of nonmagnetic heavy metals with strong spin-orbit coupling. In the present work, we show experimentally and theoretically that ohmic losses of spin-wave-induced microwave eddy currents in the heavy-metal layer deliver a strong contribution to spin-wave damping in these hybrid structures. We demonstrate that this adverse effect can be controlled and largely eliminated by placing a highly conducting metal plate near to the surface of the YIG/$\mathrm{Pt}$ structures. These findings are of value for a proper interpretation of experiments on the magnon spintronic effects and for the design of future magnon spintronic devices.

We summarize our experimental findings in the arrays of Ni80Fe20 circular nanodots with diameter 300 nm and thickness 20 nm ≤ L ≤ 100 nm, probed by broadband ferromagnetic resonance spectroscopy in the absence of external magnetic field. Spin excitation modes related to the vortex core gyrotropic dynamics were observed in the gigahertz frequency range. Micromagnetic simulations revealed that they are flexure oscillations of the vortex core string with n = 0, 1, 2 nodes along the dot thickness. It was found that for L > 70 nm the intensity of more complicated n = 1 vortex gyrotropic mode is unexpectedly higher than the one of the lowest n = 0 gyrotropic mode. This behavior was clarified on the basis of the inhomogeneous vortex mode phase profiles extracted from micromagnetic simulations and calculated analytically. Precise measurements of the dependence of resonance frequency of the vortex n = 0 mode on the dot thickness demonstrated a clear maximum around L = 70 nm, that was theoretically explained by introducing a vortex mass, which is a result of the vortex distortion due to interaction with spin waves having azimuthal indices m = ±1. Finally, several azimuthal spin-wave modes having curled structure at the dot top and bottom faces were found in the spectrum of the dots with thicknesses L ≥ 40 nm.

During the last two decades, the rapid development of the template-assisted fabrication techniques has permitted the preparation of several kinds of ferromagnetic nanowire arrays with well-controlled geometric and morphologic properties. It was demonstrated that the related magnetic behavior, both static and dynamical responses, depends on the wire array geometrical parameters, such as the interwire distance, lattice array, and wire diameter and length, and these responses can be easily controlled and tuned. Due to this versatility, they are ideal candidates for understanding the magnetostatic interaction and dynamical effects. Moreover and regarding the high-frequency studies, nanowire arrays behave like an effective homogeneous medium with well-defined macroscopic constitutive parameters as the lattice periodicity is smaller than the guided wavelength at microwave frequencies. Therefore these properties have promoted the development of several nanowire-based systems for magnonic applications and in different microwave devices. This chapter presents an overview of the published experimental and theoretical works focused on the dynamical behavior of ferromagnetic nanowire arrays from one-dimensional to three-dimensional configurations.

The interest in artificial magnetic media such as magnonic crystals increased substantially in recent years due to their potential applications in information processing at microwave frequencies. The main features of these crystals are the presence of band gaps in the spin-wave spectra, usually formed due to Bragg reflections of spin-waves on the artificially created periodic structures. Here, we study spin-wave propagation in longitudinally magnetized width- and thickness-modulated yttrium iron garnet waveguides by means of Brillouin light scattering and microwave spectroscopy techniques. It is found that the width modulated crystal does not manifest noticeable Bragg reflections, but still demonstrates a pronounced band gap in its transmission characteristic. The phenomenon can be explained by the destructive interference between different frequency-degenerated spin-wave modes excited by the crystal. Such a reflection-less crystal is promising for future design of multi-element magnonic devices. By modulating the dimensions of magnonic crystals, their transmission properties in the microwave regime may be tuned and so applications including information processing are anticipated. The authors here show reflection-less spin-wave propagation may be realised in a width-modulated waveguide.

Static magnetization configurations of thin soft ferromagnetic films and nanodots, coupled to a hard antidot matrix with out-of-plane magnetization, are studied by micromagnetic simulations and analytical calculations. When the antidot matrix produces sufficient stray fields, having radial symmetry, these nanostructures support the formation of topologically nontrivial magnetic configurations-vortices and skyrmions in nanodots and films, respectively. It is demonstrated that the studied nanostructure reveals an additional degree of freedom-the helicity of the vortex or skyrmion-which can be tuned on demand by a variation of the material parameters and geometry. The variation of helicity γ is not abrupt. In addition to Neel-like (radial) vortices and skyrmions (γ=0,π), it is possible to achieve unconventional configurations with an intermediate helicity γ≠0,±π/2,π, which transform to common Bloch-like configurations (γ=±π/2) in the limit of negligible stray fields from the matrix. We present an analytical model, which allows us to calculate the stability region of pure Neel-like states, outside which unconventional magnetization states with intermediate helicity are realized., The Portuguese team acknowledges the Network of Extreme Conditions Laboratories and Portuguese Foundation of Science and Technology for support through Projects No. NORTE-01-0145-FEDER-022096, No. MIT-EXPL/IRA/0012/2017, No. PTDC/FISMAC/31302/2017, No. POCI-0145-FEDER-030085(NOVAMAG), No. PTDC/FIS-MAC/31302/2017, andNo. EXPL/IF/00541/2015 (S.A.B.) and Grant No.SFRH/BPD/90471/2012 (A.H.-R.). Work was also supported by the Ministry of Education and Science of Ukraine, Project No. 0118U004007 (R.V.V., B.A.I.); IKERBASQUE (the Basque Foundation for Science) (K.Y.G.); the Spanish Ministry of Economy, Industry and Competitiveness, Grant No. FIS2016-78591-C3-3-R (K.Y.G.); the European Union Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie Grants No. 644348 (R.V.V., B.A.I., K.Y.G.), No. H2020-MSCA-RISE-2016-734801 (D.N., G.N.K.), and No. H2020-MSCA-IF-2016-74695 (A.H.-R.); the Program of NUST “MISiS,” Grant No. K2-2019-006, implemented by Russian Federation government Decree No. 211 dated 16 March 2013 (B.A.I.); the Spanish Ministry for Science, Innovation and Universities, for funding through the “Ramón y Cajal” Program No. RYC-2017-22820 (D.N.); and European Cooperation in Science and Technology, Projects No. CA16218 “NANOCOHYBRI” (G.N.K.) and No. CA17123 “MAGNETOFON” (B.A.I).

The stability of the skyrmion magnetization configurations in ultrathin circular magnetic nanodots is calculated analytically and simulated micromagnetically considering realistic magnetic and geometrical dot parameters taken from recent experiments. The chiral Dzyaloshinskii-Moriya exchange interaction (DMI) is taken into account as an interfacial energy term. The diagram of single Neel skyrmion stability/metastability, skyrmion sizes and magnetization profiles are found as a function of the magnetic and geometrical dot parameters. It is shown that most of skyrmions are unstable or metastable and the stability region where the Neel skyrmion is the ground state is small. The Dzyaloshinskii criterion for the instability of ferromagnetic state in bulk ferromagnets with respect to the increasing DMI strength should be essentially modified to describe the stability of magnetic skyrmions in 2D systems like ultrathin ferromagnetic films and nanodots.

The interference laser lithography and ion-beam sputtering have been reported. Magneto-optical Kerr effect magnetometry indicated that the sample exhibits four-fold anisotropic behaviour, i.e. different magnetization loops were observed when the external magnetic field was applied along either x-or y-axis, or along the array diagonal. Broadband ferromagnetic resonance measurements revealed a rich variety of different magnetization configurations in the unsaturated state that can be controlled by the orientation of the external magnetic field. Micromagnetic simulations have been performed to explain the observed results. On the contrary, in the saturated regime the system demonstrated almost isotropic magnetic behaviour that improves with external field increase. The obtained results show the potential of interference lithography for the fabrication of large area antidot arrays. (C) 2019 Author(s). The Portuguese team acknowledges the Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects NORTE-01-0145-FEDER-022096, MIT-EXPL/IRA/0012/2017, POCI-01-0145-FEDER-031302, EXPL/IF/01191/2013 (D.N.), EXPL/IF/00541/2015 (S.A.B.), EXPL/IF/00981/2013 (G.N.K). D.N., G.N.K., C.R and R.M. acknowledge the support by the European Union Horizon 2020 Research and Innovation Programme under Marie Sklodowska-Curie Grant Agreement EU H2020-MSCA-RISE-2016 (No 734801). The Spanish team acknowledges the support from Spanish MINECO through the grant FIS2016-76058 (AEI/FEDER, UE). A.H.-R. acknowledges the support from European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Action (reference H2020-MSCA-IF-2016-746958). G.N.K. and O.V.D. acknowledge the support from European Cooperation in Science and Technology (COST) project CA16218 "NANOCOHYBRI."

The influence of film thickness on the formation of twinning structure, martensitic transformation and magnetoelastic properties of epitaxial films of Ni(Co)MnSn magnetic shape memory alloy is investigated by means of ferromagnetic resonance spectroscopy, synchrotron X-ray diffraction and standard magnetic measurements. It is found that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the film thickness results in a progressive stress relaxation and, as a result, the martensitic transformation becomes possible starting from 50 nm. Twinning of the films is required to conserve the films surface area. The elastic energy balance between the film/substrate interface and the twin boundaries leads to the formation of a submicron wide, stripe like, periodical structure of twins, which is of interest for spintronic or magnonic applications. The width of the twin variants increases with the film thickness growth, resulting in the dramatic modification of magnetic properties.

It is well established that electrical methods, such as the application of a high-density (10(10)-10(11) A m(-2)) current, can be employed for the switching of magnetization in thin ferromagnetic films and hetero-structures after their patterning at the nanoscale. We find that magnetization is robustly switchable, i.e., at macroscopic scales (up to a few millimeters in lateral directions), in a single similar to 30 nm-thick layer of alpha-Fe upon discharging a capacitor through the sample. Interestingly, samples sputtered at enhanced base pressures (similar to 10(-6) mbar) exhibited much lower switching thresholds with no bias field, while their electrical conductivity is anomalously high ( higher than that in bulk of alpha-Fe). We argue that the observed enhancement of conduction relates to the mixed oxide/oxyhydroxide (FeO/FeOOH) layer formed naturally on the top of our alpha-Fe films. This provides an explanation for the magnetization switching upon discharging a capacitor in terms of the Oersted field generated by the discharge current. We analyze the ratio needed between the conductions of the FeO/FeOOH layer and the underlying alpha-Fe for the effective action of the Oersted field.

Magnetic shape memory alloys are under intensive investigation due to their unusual physical properties, such as magnetic shape memory effect, magnetic field induced superelasticity, direct and inverse magnetocaloric effect etc., promising for novel applications. One of the intriguing properties of these materials in a single phase state is a giant magnetoresistance. Here we report the remarkable results about the magnetoresistive properties of epitaxial films of Ni52.3Mn26.8Ga20.9 magnetic shape memory alloy in the temperature range of 100–370 K, well below the martensitic transformation temperature. It was found that the formation of non-collinear magnetic structure due to a nanotwinning of the film results in electron scattering on such a structure and noticeable negative magnetoresistance in the entire investigated temperature range.

Static magnetic configurations of thin circular soft (permalloy) magnetic nanodots, coupled to a hard antidot matrix with perpendicular magnetization, are studied by micromagnetic simulations. It is demonstrated, that dipolar fields of the antidot matrix promotes the formation of a magnetic vortex state in nanodots. The vortex is the dot ground state at zero external field in ultrathin nanodots with diameters as low as 60 nm, that is far beyond the vortex stability range in an isolated permalloy nanodot. Depending on the geometry and antidot matrix material it is possible to stabilize either radial vortex state or unconventional vortices with the angle between in-plane magnetization and radial direction ψ ≠ 0 , π / 2 .

Highly ordered Co x Ni 1− x nanowire (NW) arrays were electrodeposited inside nanoporous anodic aluminum oxide membranes. The control of the applied potential during the electrodeposition process allowed us to easily tune the Co% in the alloy. Systematic studies on the morphological and crystallographic properties together with static and dynamic magnetic characterizations were performed. In this work we focus on the study of the dynamic magnetic properties of CoNi NW arrays using the ferromagnetic resonance method at both room (RT) and low (LT) temperatures. The careful comparison analysis performed between the magnetic anisotropy fields obtained at RT and LT, allowed us to extract for the first time the magnetocrystalline anisotropy effect of the Co component and, most importantly, the magnetoelastic anisotropy effect of Ni.

We investigated the spin wave modes in flat circular permalloy rings with a canted external bias field using ferromagnetic resonance spectroscopy. The external magnetic field $H$ was large enough to saturate the samples. For $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ (perpendicular geometry), three distinct resonance peaks were observed experimentally. In the case of the cylindrical symmetry violation due to $H$ inclination from normal to the ring plane (the angle $\ensuremath{\theta}$ of $H$ inclination was varied in the ${0}^{\ensuremath{\circ}}\ensuremath{-}{6}^{\ensuremath{\circ}}$ range), the splitting of all initial peaks appeared. The distance between neighbor split peaks increased with the $\ensuremath{\theta}$ increment. Unexpectedly, the biggest splitting was observed for the mode with the smallest radial wave vector. This special feature of splitting behavior is determined by the topology of the ring shape. Developed analytical theory revealed that in perpendicular geometry, each observed peak is a combination of signals from the set of radially quantized spin wave excitation with almost the same radial wave vectors, radial profiles, and frequencies, but with different azimuthal dependencies. This degeneracy is a consequence of circular symmetry of the system and can be removed by $H$ inclination from the normal. Our findings were further supported by micromagnetic simulations.

Magnetic properties of $\mathrm{N}{\mathrm{i}}_{46.0}\mathrm{M}{\mathrm{n}}_{36.8}\mathrm{S}{\mathrm{n}}_{11.4}\mathrm{C}{\mathrm{o}}_{5.8}/\mathrm{MgO}(001)$ epitaxial thin film, which undergo a martensitic phase transformation from cubic austenitic phase to a twinned orthorhombic martensitic phase at 270 K, were studied by the magnetic resonance at the microwave frequency of 9.45 GHz. It was found that the single resonance line observed in the austenite splits into three lines in the martensitic phase. A theoretical approach was developed to show that the additional resonance lines are caused by the weak antiferromagnetic coupling of the ferromagnetic twin components across twin boundaries. Fitting of the experimental resonance lines to model gives an effective field of antiferromagnetic coupling of about 1.5 kOe, which is two or three orders of magnitude lower than in the conventional antiferromagnetic solids because the number of magnetic ions interacting antiferromagnetically through the twin boundary is much less than the total number of magnetic ions in the twin. This feature shows a strong resemblance between the submicron twinned martensite and artificial antiferromagnetic superlattices, whereby providing a distinctive insight into magnetism of the studied magnetic shape memory material.

We report magnetic, dynamic and transport properties of discontinuous metal-insulator multilayers Fe/MgO grown on amorphous Corning® glass and single-crystalline MgO (001) substrates. The films of structure Substrate/MgO (3 nm)/[Fe (0.6 nm)/MgO (3.0 nm)]×10 were prepared in ultra-high vacuum conditions using Pulsed Laser Deposition. It was shown that conditions of epitaxial growth are favorable for MgO substrates. As a result a substantial increase of tunneling magnetoresistance caused by spin-filtering effect was observed and reasonably theoretically explained. The value of TMR ̃ 9.2% at room temperature in 18 kOe magnetic field is three times higher comparing to that for the samples grown on Corning® glass substrates. Copyright © 2012 American Scientific Publishers All rights reserved.

Magnetodynamical properties of systems of magnetic nanodisks at the microwave range and low temperatures were studied experimentally. Several periodic square arrays of Permalloy nanodisks deposited onto a glass substrate have been studied. The interdisk center-to-center distance in each array was equal to double disk diameter . The magnetoresonance studies were performed at 70-80 GHz frequency band and at the temperature of 4.2 K. The static external magnetic field was applied perpendicularly to the disks array. As a result the magnetoresonance absorption spectra have been obtained, containing both fundamental mode and spin wave modes. A comparative analysis of the results is performed with the X-band results obtained at T = 300K.

The properties of a magnonic crystal are expected to be strongly influenced by the presence of a thermal gradient. We investigated the propagation of backward volume and surface magnetostatic spin-waves in a 1D magnonic crystal (MC) exposed to a continuous spatial temperature gradient. It is shown that the thermal gradient applied along the propagation direction leads to a frequency shift and a modification of the transmission characteristics of the spin-waves. The frequency shift is caused by a variation in saturation magnetization due to the change in absolute temperature. The altered transmission manifests itself in a broadening of MC band gaps and the corresponding narrowing of the MC passbands and is understood to be a result of a spatial transformation of the spin-waves wavelengths in a thermal gradient. Furthermore, the transmission characteristics of spin-waves in a thermal gradient have been verified by numerical calculations based on the approach of the transmission matrix. The results of the calculations demonstrate a good agreement with the experimentally measured data.

We present a study of magnetization dynamics at room temperature in periodic arrays of 50 nm thick FeNi (Py) circular magnetic dots of 500 nm radius and different center to center distance (1200 and 2500 nm), by using a broadband magnetometer based on vector network analyzer which works between 300 kHz and 8.5 GHz. We also present a comparison between the dynamic response, ferromagnetic resonance (FMR) and its linewidth, with static magnetic characteristics such as magnetization curves. The FMR peak appears just above the nucleation field and is perfectly described by Kittel formula taking into account the demagnetizing factor of an individual magnetic dot. In addition to FMR we observed a spin wave resonance below the uniform mode, which could be attributed to spin waves in confined systems. The FMR linewidth shows a significant broadening close to the field region corresponding to nucleation of magnetic vortex.

X-band ferromagnetic resonance (FMR) was used to characterize in-plane magnetic anisotropies in rectangular and square arrays of circular nickel and Permalloy microdots. In the case of a rectangular lattice, as interdot distances in one direction decrease, the in-plane uniaxial anisotropy field increases, in good agreement with a simple theory of magnetostatically interacting uniformly magnetized dots. In the case of a square lattice a four-fold anisotropy of the in-plane FMR field Hr was found when the interdot distance a gets comparable to the dot diameter D. This anisotropy, not expected in the case of uniformly magnetized dots, was explained by a non-uniform magnetization m(r) in a dot in response to dipolar forces in the patternedmagnetic structure. It is well described by an iterative solution of a continuous variation procedure. In the case of perpendicular magnetization multiple sharp resonance peaks were observed below the main FMR peak in all the samples, and the relative positions of these peaks were independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra was given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account. It was demonstrated that ferromagnetic resonance force microscopy (FMRFM) can be used to determine both local and global properties of patterned submicron ferromagnetic samples. Local spectroscopy together with the possibility to vary the tip-sample spacing enables the separation of those two contributions to a FMRFM spectrum. The global FMR properties of circular submicron dots determined using magnetic resonance force microscopy are in a good agreement with results obtained using conventional FMR and with theoretical descriptions.

The spin wave dynamics in patterned magnetic nanostructures is under intensive study during the last two decades. On the one hand, this interest is generated by new physics that can be explored in such structures. On the other hand, with the development of nanolithography, patterned nanoelements and their arrays can be used in many practical applications (magnetic recording systems both as media and read-write heads, magnetic random access memory and spin-torque oscillators just to name a few). In the present work the evolution of spin wave spectra of an array of non-interacting Permalloy submicron circular dots for the case of magnetic field deviation from the normal to the array plane have been studied by ferromagnetic resonance technique. It is shown that such symmetry violation leads to a splitting of spin-wave modes and that the number of the split peaks depends on the mode number. A quantitative description of the observed spectra is given using a perturbation theory for small angles of field inclination from the symmetry direction. The obtained results give possibility to predict transformation of spin wave spectra depending on direction of the external magnetic field that can be important for spintronic and nanomagnetic applications.

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.

Recently a new type of magnonic crystal — Ni 80 Fe 20 films deposited on top of periodic 2-D arrays of Ni 80 Fe 20 submicron circular disks — was proposed [1]. In such structures the continuous film can be considered as a magnonic waveguide, when the role of dot array is to create periodic perturbations of internal fields in the neighbor regions of the film. Here we study how the change of two parameters — inter-disk distances and the thickness of the film on top of the array will affect the in-plane anisotropy of the system.

The ultrafast laser-induced response of spins and charges in $\mathrm{CoFe}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ multilayers are studied using THz and optical pump-probe spectroscopies. We demonstrate the possibility of ultrafast manipulation of the transport and magnetic properties of the multilayers with femtosecond laser excitation. In particular, using time-resolved THz transmission experiments we found that such an excitation leads to a rapid increase of the THz transmission (i.e., electric resistivity). Our experiments also reveal that femtosecond laser excitation results in the emission of broadband THz radiation. To reveal the origin of the emitted THz radiation, we performed magnetic-dependent measurements of the THz emission. We also compared the observed electric field of the THz radiation to calculations performed using subpicosecond laser-induced demagnetization measurements. The good agreement between the experimentally obtained spectra and the calculations corroborates that the measured THz emission originates from the demagnetization process.

Discontinuous magnetic multilayers provide a simple method of producing magnetic nanoparticles. Such systems can be viewed as two-dimensional nanosystems, depending on separation between the discontinuous magnetic layers via a non-magnetic spacer layer. We have studied the system whereby discontinuous layers of CoFe are separated by Al 2 O 3 interlayers of 40 A (nominal). A set of samples with nominal CoFe thicknesses between 7 and 13 A have been studied using ferromagnetic resonance (FMR). We have made measurements as a function of sample temperature in the range from 5 K to room temperature. The spectra display some strong enhancements of the resonance field at temperatures below 90 K, and are inversely proportional to the effective thickness of the CoFe. This indicates a strong surface contribution to the effective field and is interpreted as surface anisotropy. Variations in the linewidth indicate a change of behaviour which we equate to a transition of coupling regime from superferromagnetic in the thicker samples to superparamagnetic for the thinner samples.

We study CoFe∕Al2O3 multilayers, varying from granular to continuous structure with CoFe nominal thickness (10A⩽t⩽18A). Structural percolation takes place at t=tc≈18A, changing from activated to metallic conductance. A sharp anomaly in ac coercivity of magneto-optical Kerr effect and in magnetization M by superconducting quantum interferometer device is found at t=t*≈17A, interpreted as transition from superferromagnetism (by dipolar coupled CoFe clusters) to exchange ferromagnetism. Onset at t=t* of anisotropic magnetoresistance (AMR) is attributed to skew scattering of bandlike electrons within giant fractal clusters. These structures may cause the observed Barkhausen noises in M and AMR at t≈t*, disappearing at t>t*.

Granular cermet films (Fe50Co50)x–(Al2O3)1−x fabricated using the electron-beam coevaporation technique at oblique incidence of FeCo and alumina atom fluxes have been found to exhibit both oblique and in-plane uniaxial magnetic anisotropy. This anisotropy first appears just below the percolation threshold due to a magnetic coupling of particles taking place at a certain stage of their growth and coalescence. The FeCo content x varied from 0.07 to 0.49. A simple model of the film microstructure is presented based on the results of magnetization measurements and ferromagnetic resonance at intermediate (9.4 GHz) and high (94 GHz) frequencies. At 94 GHz the concentration dependence of the effective anisotropy field follows the solid solution law, since then the magnetic field is sufficient to magnetize the films close to saturation. The 9.4 GHz data points deviate from the solid solution line below the percolation threshold due to both modification of the resonance fields by intergranular interactions in nons...

The dependence of current-in-plane resistance ( R ) and magnetoresistance (MR) of Al 2 O 3 (30 A)/[Co 80 Fe 20 (13 A)/Al 2 O 3 (30 A)] n granular layered films on the number of bilayers n was studied. It was found experimentally that R is proportional to 1/ n , whereas MR is practically independent of n . According to simple model calculations, significant deviations from R ∝1/ n should be expected for thick interlayer Al 2 O 3 spacers s ⊥ , combined with thin intergrain bridges within the layers, s ∥ s ⊥ . The discrepancy between the model and the experiment is attributed to the effects of inhomogeneities in s ∥ and s ⊥ and to the presence in each layer of a small amount of large metallic patches, which can effectively shortcut the neighboring layers. For buffer Al 2 O 3 layer thickness above 50 A both R and MR decrease drastically, which is explained by the appearance of more patches at smoother Al 2 O 3 surface. In contrast to the granular layered structures based on SiO 2 and HfO 2 , the annealing in our system lowers MR values, presumably due to the oxidation of CoFe granules already existing in the as-deposited layers.

Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were recorded for a series of ion-beam-deposited metal–insulator CoFe( t )/Al 2 O 3 (30 A) multilayers, ranging from discontinuous insulating ( t =9 A) to continuous metallic ( t =25 A) behavior, for temperatures T from 8 to 300 K and in applied magnetic fields H from 10 to 100 Oe. Data allow the analysis of the different magnetic phases coexisting in this system. The superferromagnetic (SFM) fraction is characterized by the Curie temperature, T C , and ZFC remanence at T →0, M SFM , both being functions of the CoFe nominal thickness t . The remaining superparamagnetic (SPM) fraction starts getting blocked below a certain blocking temperature T b , which depends both on t and H . The SFM phase develops above some critical thickness t cr ≈11 A, so that M SFM ( t ) is almost linear: M SFM ∝( t − t cr ), at t cr t t p , and attains the maximum (bulk) value at t → t p ≈18 A, which is the metallic percolation threshold. In the SFM–SPM coexistence range t cr t t p , the blocking temperature T b shows a monotonic increase with thickness t but a pronounced decrease with field H . This latter effect is attributed to field-induced suppression of the energy barriers for alignment of pre-formed domains in the SFM fraction.

The effect of oblique co-deposition on magnetic properties of cermet films is studied on a series of (Fe 50 Co 50 ) x -(Al 2 O 3 ) 1-x granular films with nominal FeCo content x varied at 0.07 < x < 0.49, using FMR. Three observed signals are attributed to local regions differing of compositions, each varying from the nominal x. A microstructure-induced oblique anisotropy has been found above percolation. Its origin may be due to some elongation of grains along a growth direction tilted from the film normal.

Current-in-plane (CIP) transport properties are reported in discontinuous granular layers of CoFe/Al 2 O 3 across a 10 μm gap between lithographed Al contacts. CIP magnetoresistance is found to degrade sensibly with bias voltage V , e.g., from 7% to 3.5% (at room temperature) with V growing from 100 mV to 14 V. The I – V characteristics display a strong non-linear dependence, similar to the high-field tunneling regime known for current-perpendicular-to-plane (CPP) transport, but here it occurs under much weaker electric field and even at room temperature when Coulomb blockade is ruled out. We explain these anomalies by the processes of dynamic charge accumulation under the macroscopic contacts.