G. N. Kakazei

University of Porto, Oporto, Porto, Portugal

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Publications (126)253.4 Total impact

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    ABSTRACT: 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.
    Full-text · Article · Dec 2015 · Scientific Reports
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    ABSTRACT: 2-D modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20film were fabricated using deep ultraviolet lithography. The array pitch for all the samples was fixed at 620 nm, while the dot diameter varied from 300 nm to 550 nm. Four-fold in-plane magnetic anisotropy (FFA) was detected in the samples using ferromagnetic resonance technique. It was found that the deposition of continuous film on top of the array drastically increases the FFA of the system, with a maximum anisotropy field of 120 Oe being observed for the sample with intermediate diameter. This increase is due to the appearance of a field well in the film underneath the dots with a shape that is dependent on magnetic field orientation and to the coupling of magnetization oscillations in dots and film.
    Full-text · Article · Dec 2015 · Applied Physics Letters
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    K Y Guslienko · G N Kakazei · J Ding · X M Liu · A O Adeyeye
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    ABSTRACT: 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.
    Full-text · Article · Sep 2015 · Scientific Reports

  • No preview · Article · Aug 2015 · Physical Review Letters
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    ABSTRACT: Enhancement of Gilbert damping in polycrystalline cobalt thin-film multilayers of various thicknesses, overlayered with copper or iridium, was studied in order to understand the role of local interface structure in spin pumping. X-ray diffraction indicates that cobalt films less than 6 nm thick have strong fcc(111) texture while thicker films are dominated by hcp(0001) structure. The intrinsic damping for cobalt thicknesses above 6 nm is weakly dependent on cobalt thickness for both overlayer materials, and below 6 nm the iridium overlayers show higher damping enhancement compared to copper overlayers, as expected due to spin pumping. The interfacial spin mixing conductance is significantly enhanced in structures where both cobalt and iridium have fcc(111) structure in comparison to those where the cobalt layer has subtly different hcp(0001) texture at the interface.
    Full-text · Article · Jul 2015 · Physical Review Letters
  • G.N. Kakazei · X. Liu · J. Ding · V. Golub · O. Salyuk · S. Bunyaev · A. Adeyeye
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    ABSTRACT: Recently a new type of magnonic crystal - Ni80Fe20 films deposited on top of periodic 2-D arrays of Ni80Fe20 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.
    No preview · Article · Jul 2015
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    ABSTRACT: We investigate the origin of ferromagnetism induced in thin-film (∼20 nm) Fe-V alloys by their irradiation with subpicosecond laser pulses. We find with Rutherford backscattering that the magnetic modifications follow a thermally stimulated process of diffusion decomposition, with formation of a-few-nm-thick Fe enriched layer inside the film. Surprisingly, similar transformations in the samples were also found after their long-time (∼103 s) thermal annealing. However, the laser action provides much higher diffusion coefficients (∼4 orders of magnitude) than those obtained under standard heat treatments. We get a hint that this ultrafast diffusion decomposition occurs in the metallic glassy state achievable in laser-quenched samples. This vitrification is thought to be a prerequisite for the laser-induced onset of ferromagnetism that we observe.
    Full-text · Article · May 2015 · Applied Surface Science
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    ABSTRACT: The ultrafast laser-induced response of spins and charges in CoFe/Al 2 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.
    Full-text · Article · Mar 2015 · Physical Review B

  • No preview · Article · Jan 2015
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    ABSTRACT: Highly ordered CoxNi1-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.
    No preview · Article · Dec 2014 · Journal of Magnetism and Magnetic Materials
  • X.M. Liu · J. Ding · G.N. Kakazei · A.O. Adeyeye
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    ABSTRACT: Magnonic crystals (MCs), conceived as the magnetic microwave analog of photonic crystals, have received considerable attention recently due to their potential in a wide range of applications such as microwave resonators, filters and spin logic devices. Compared with conventional single component MCs, bi-component MCs consisting of arrays of one FM material in the matrix of another FM material shows advantageous due to an additional degree of freedom in tailoring spin wave properties. In this work, we present a systematic investigation of the static and dynamic behaviors of various types of 1-D and 2-D bi-component MCs. All rights reserved -
    No preview · Article · Nov 2014
  • G.R. Aranda · G.N. Kakazei · J. Gonzalez · K.Y. Guslienko
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    ABSTRACT: We present micromagnetic simulations of ferromagnetic resonance in patterned permalloy films and isolated stripes. Films of the total thickness 20 nm or 40 nm are patterned in the form of 1D periodic structures with rectangular profile (rectangular grooves of depth varying from zero up to the film thickness) and in-plane period of 500 nm. The direction of the applied dc magnetic field is varied in the film plane from the direction parallel to the stripes to perpendicular one. The thickness of the patterned elements and direction of the bias field affect essentially the resonance peaks (changing their position, amplitude and number) and the corresponding dynamical magnetization profiles. We simulated from one up to three ferromagnetic resonance peaks and found the areas of microwave magnetization localization for them.
    No preview · Article · Sep 2014 · Journal of Applied Physics
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    ABSTRACT: 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.
    Full-text · Article · Jun 2014 · New Journal of Physics
  • J. Ding · G.N. Kakazei · X.M. Liu · K.Y. Guslienko · A.O. Adeyeye
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    ABSTRACT: Vortex gyrotropic modes in ferromagnetic nanostructures can be described as flexure oscillations of the vortex core line with different number of nodes n along the dot thickness. By conducting broadband ferromagnetic resonance measurements in the absence of external magnetic field on Ni80Fe20 circular nanodots with radius R = 150 nm and thickness 50 ≤ L ≤ 100 nm, we established that above L = 70 nm the intensity of more complicated n = 1 vortex mode is unexpectedly higher than the one of n = 0 mode. The observed behavior is explained on the basis of the inhomogeneous vortex mode phase profiles extracted from micromagnetic simulations. The phase difference of vortex core gyrations at the top and bottom dot faces is essentially different from 0 and π. The difference is increasing with increase in the dot aspect ratio L/R for the 0th order mode, whereas an inverse relationship is observed for the 1st order mode. The analytical theory indicates that this phase difference has magnetostatic origin.
    No preview · Article · May 2014 · Applied Physics Letters
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    ABSTRACT: 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.
    Full-text · Article · Apr 2014 · Scientific Reports
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    ABSTRACT: Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.
    Full-text · Article · Apr 2014 · Physical Review B
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    ABSTRACT: The aim of the present work is to show a simple method that combines conventional laser interferometry and standard thin film deposition techniques to fabricate modulated magnetic nanostructures with lateral periodicity, and to tailor the magnetic properties by varying geometrical parameters. Well defined Ni80Fe20 magnetic thin films with sinusoidal grating profiles were obtained with a periodicity of 1.2 μm and different grating depths. Magnetic studies via ferromagnetic resonance and magneto optical Kerr effect demonstrate the tunability of the induced in-plane magnetic anisotropy with depth profile.
    Full-text · Article · Feb 2014 · Applied Physics Letters
  • G.N. Kakazei · X.M. Liu · J. Ding · A.O. Adeyeye
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    ABSTRACT: Magnonic crystals with periodically modulated thickness were fabricated using deep ultraviolet lithography by covering Ni80Fe20 nanowire arrays with continuous Ni80Fe20 film. It is shown that the magnetization reversal and ferromagnetic resonance fields can be tuned either varying the film thickness (t) or the stripe width (w). For t ≥ 20 nm, two different stable magnetic configurations in zero external field were obtained. These configurations are characterized by noticeably different frequencies of ferromagnetic resonance up to 1 GHz and can be switched by applying magnetic pulses of proper amplitude and polarity.
    No preview · Article · Jan 2014 · Applied Physics Letters
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    ABSTRACT: We report on the discovery of an isothermal structural transition observed in Bi1-xLaxFeO3 (0.17⩽x⩽0.19) ceramics. At room temperature, an initially pure polar rhombohedral phase gradually transforms into a pure antipolar orthorhombic one. The polar phase can be recovered by annealing at T>300 °C. In accordance with neutron powder diffraction data, an inverse isothermal antipolar-polar transition takes place at T>300 °C, where the polar phase becomes more stable. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar phase has been obtained in a mixed antiferromagnet–weak ferromagnet state. The relatively low external pressure induces polar-antipolar transition, but there is no evidence of electric-field-driven antipolar-polar transition. The observed large local piezoelectric response is associated with structural instability of the polar phase, whereas local multistate piezoelectric loops can be related to the domain wall pinning effect.
    Full-text · Article · Oct 2013 · Physical Review B
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    ABSTRACT: The superconducting and magnetic properties of a-Co x Si 1−x / Nb bilayers have been studied as a function of Co content in order to analyze the superconducting/ferromagnetic proximity effect in a system with strong disorder in the magnetic layers. As Co atoms become more diluted, the magnetization of the amorphous a-Co x Si 1−x alloy decreases gradually, whereas their resistivity increases and enters in a weak localization regime. The superconducting transition temperatures of the a-Co x Si 1−x / Nb bilayers follow a decreasing trend as Co content is reduced, reaching the lowest value at the boundary between the ferromagnetic-nonmagnetic amorphous phases. These results can be understood in terms of the increase in interface transparency together with the changes in the spin-flip scattering term as magnetic disorder increases and the amorphous a-Co x Si 1−x layers loose their magnetic character.
    Full-text · Article · Sep 2013 · Physical Review B

Publication Stats

1k Citations
253.40 Total Impact Points


  • 1999-2015
    • University of Porto
      • • Faculty of Sciences
      • • Departamento de Engenharia Física
      Oporto, Porto, Portugal
  • 2013-2014
    • National University of Singapore
      • Department of Electrical & Computer Engineering
      Tumasik, Singapore
  • 2008
    • Spanish National Research Council
      Madrid, Madrid, Spain
  • 2007
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
    • University of Aveiro
      • Department of Physics
      Aveiro, Aveiro, Portugal
  • 2006
    • National Academy of Sciences of Ukraine
      • Institute of Magnetism
      Kiev, Misto Kyyiv, Ukraine
  • 2001-2006
    • The Ohio State University
      • Department of Physics
      Columbus, OH, United States
  • 2002
    • Institute of Magnetism (IMAG) NAS
      Kievo, Kyiv City, Ukraine