Vortex dynamics in confined stratified conditions

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions

Magnetic properties of permalloy antidot arrayfabricated by interference lithography

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.

Dominant higher-order vortex gyromodes in circular magnetic nanodots

The transition to the third dimension enables the creation of spintronic nanodevices with significantly enhanced functionality compared to traditional 2D magnetic applications. In this study, we extend common two-dimensional magnetic vortex configurations, which are known for their efficient dynamical response to external stimuli without a bias magnetic field, into the third dimension. This extension results in a substantial increase in vortex frequency, reaching up to 5 GHz, compared to the typical sub-GHz range observed in planar vortex oscillators. A systematic study reveals a complex pattern of vortex excitation modes, explaining the decrease in the lowest gyrotropic mode frequency, the inversion of vortex mode intensities, and the nontrivial spatial distribution of vortex dynamical magnetization noted in previous research. These phenomena enable the optimization of both oscillation frequency and frequency reproducibility, minimizing the impact of uncontrolled size variations in those magnetic nanodevices

Higher order vortex gyrotropic modes in circular ferromagnetic nanodots

Magnetic vortex that consists of an in-plane curling magnetization configuration and a needle-like core region with out-of-plane magnetization is known to be the ground state of geometrically confined submicron soft magnetic elements. Here magnetodynamics of relatively thick (50-100 nm) circular Ni80Fe20 dots were probed by broadband ferromagnetic resonance in the absence of external magnetic field. Spin excitation modes related to the thickness dependent vortex core gyrotropic dynamics were detected experimentally in the gigahertz frequency range. Both analytical theory and micromagnetic simulations revealed that these exchange dominated modes are flexure oscillations of the vortex core string with n = 0,1,2 nodes along the dot thickness. The intensity of the mode with n = 1 depends significantly on both dot thickness and diameter and in some cases is higher than the one of the uniform mode with n = 0. This opens promising perspectives in the area of spin transfer torque oscillators.This work was supported by the SMF-NUS New Horizon Awards, National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP 10-2012-03) and Ministry of Education of Singapore. K.Y.G. acknowledges support by IKERBASQUE (the Basque Science Foundation) and the Spanish MINECO grants PIB2010US-00153, FIS2010-20979-C02-01. The authors would like to thank Dr. N. Singh from A*Star Institute of Microelectronics, Singapore for his help with template fabrication

Microwave absorption properties of permalloy nanodots in the vortex and quasi-uniform magnetization states

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1-8 GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials.G M and Y K are grateful to the Rohde & Schwarz Oestereich GmbH Representative Office for the opportunity to use the R&S® ZVA8 Vector Network Analyzer. This work was supported in part by grant No. UU34/008 from the State Fund for Fundamental Research of Ukraine, by the National Science Foundation of USA (grant DMR-1015175), by DARPA, by the contract from the US Army TARDEC, by the US DOE Office of Science (contract DEAC02-06CH11357), and by the Spanish MINECO grants PIB2010US-00153 and FIS201020979- C02-01. K G acknowledges support by IKERBASQUE (the Basque Science Foundation). G N K is supported by Foundation of Science & Technology of Portugal through the program 'Investigador FCT (grant IF/00981/2013)'

Dynamical behaviour of ultrathin [CoFeB (tCoFeB)/Pd] films with perpendicular magnetic anisotropy

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

Activation parameters of conjugated polyaniline electrolyte via dielectric relaxation technique

830-836The synthesis and relaxation mechanism of polyaniline electrolyte in form of thin film by oxidative polymerization in the presence of inorganic acid have been reported. The films have been characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, 2-point probe and UV-Vis absorption spectroscopy. The mechanism of electrical conductivity for the disordered system has been explained in terms of Mott’s variable-range hopping for the intrinsic conduction of the protonated long chain. The dielectric parameters including real part, imaginary part and loss tangent have been measured between the wavelength range 400 – 1100 nm. These have been employed in the determination of dielectric relaxation time τ which exhibits Arrhenius-like behaviour. Finally, the temperature dependent relaxation time which is a function of activation energy of dipole orientation otherwise known as Gibbs free energy ∆G#, has been utilized in estimation of some thermodynamic parameters such as enthalpy ∆H# and entropy ∆S#. The SEM shows an aggregation of randomly oriented fibrous network of polyaniline on the surface of substrate. Fourier transform infrared confirms the acoustic vibrational modes of the long chain polymer

Anisotropic Magnetic Resonance in Random Nanocrystal Quantum Dot Ensembles

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

Giant four-fold magnetic anisotropy in nanotwinned NiMnGa epitaxial films

A giant four-fold magnetic anisotropy (with an anisotropy field up to 4 kOe) was observed in the twinned NiMnGa epitaxial film. Its appearance is explained in terms of moderate coupling between twin variants having strong uniaxial magnetocrystalline anisotropies directed orthogonally when the intertwin exchange field is comparable with the anisotropy field. This finding paves the way to increase the order of magnetic anisotropy in a many-component system while keeping the value of the anisotropy field by tuning the intercomponent exchange strength and can be extended to exchange-coupled multilayers and arrays of nanoelements

Control of structural and magnetic properties of polycrystalline Co2FeGe films via deposition and annealing temperatures

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.Peer reviewe