G. N. Kakazei

University of Porto, Oporto, Porto, Portugal

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Publications (113)223.78 Total impact

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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.
    Applied Surface Science 05/2015; 336. DOI:10.1016/j.apsusc.2014.12.164 · 2.54 Impact Factor
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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.
    Physical Review B 03/2015; 91(10):104407. DOI:10.1103/PhysRevB.91.104407 · 3.66 Impact Factor
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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.
    Journal of Magnetism and Magnetic Materials 12/2014; 374:663. DOI:10.1016/j.jmmm.2014.09.036 · 2.00 Impact Factor
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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.
    Journal of Applied Physics 09/2014; 116(9):093908-093908-10. DOI:10.1063/1.4894164 · 2.19 Impact Factor
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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.
    New Journal of Physics 06/2014; 16(6):063044. DOI:10.1088/1367-2630/16/6/063044 · 3.67 Impact Factor
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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.
    Applied Physics Letters 05/2014; 104(19):192405-192405-5. DOI:10.1063/1.4878617 · 3.52 Impact Factor
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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.
    Scientific Reports 04/2014; 4:4796. DOI:10.1038/srep04796 · 5.58 Impact Factor
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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.
    Physical Review B 04/2014; 89:144410. DOI:10.1103/PhysRevB.89.144410 · 3.66 Impact Factor
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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.
    Applied Physics Letters 02/2014; 104(8):082408-082408-4. DOI:10.1063/1.4867162 · 3.52 Impact Factor
  • 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.
    Applied Physics Letters 01/2014; 104(4):042403-042403-4. DOI:10.1063/1.4863508 · 3.52 Impact Factor
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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.
    Physical Review B 10/2013; 83(5). DOI:10.1103/PhysRevB.83.054109 · 3.66 Impact Factor
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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.
    Physical Review B 09/2013; · 3.66 Impact Factor
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    ABSTRACT: Magnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120–310 K temperature range. Comparison between longitudinal and transverse (with respect to the film plane) field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic (SPM, consisting of small spherical Co-Fe granules) and superferromagnetic (SFM, by bigger Co-Fe clusters), was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely (up to ∼97%) defined by the SFM contribution and practically independent of temperature (in this range), the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations (suppressed with temperature) between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering (enhanced with temperature) of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field.
    Physical Review B 09/2013; 82(14). DOI:10.1103/PhysRevB.82.144432 · 3.66 Impact Factor
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    ABSTRACT: Amorphous Nd–Co films with perpendicular magnetic anisotropy have been nanostructured in a lateral magnetic multilayer geometry in order to analyse and modify in a controlled way the configuration of its characteristic stripe domains as well as their rotation processes. Magnetic force microscopy measurements reveal that, actually, the artificial thickness modulation results in size effects on the magnetic stripes, so that the stripe domains configuration can be tuned with the lateral multilayer periodicity, and, due to the consequent reduction of the rotatable magnetic anisotropy, it is possible to modify the stripe rotation processes for in-plane magnetization reversal.
    Journal of Physics D Applied Physics 08/2013; 46(34-34):345001. DOI:10.1088/0022-3727/46/34/345001 · 2.52 Impact Factor
  • X. M. Liu, J. Ding, G. N. Kakazei, A. O. Adeyeye
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    ABSTRACT: Modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using multi-level process based on deep ultraviolet lithography at 248 nm exposure wavelength. We observed a drastic change in both the static and dynamic properties of the modulated structures by varying the Ni80Fe20 film thickness t in the range from 5 to 60 nm. It was revealed via comparison between experimental results and micromagnetic simulations that the dots create perturbations of internal fields in the neighbor regions of the film which can be controlled by magnetic field and film thickness.
    Applied Physics Letters 08/2013; 103(6). DOI:10.1063/1.4817798 · 3.52 Impact Factor
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    ABSTRACT: We use the resonant and non-resonant microwave absorption to probe the dynamic and static magnetic parameters of weakly coupled spin valves. The sample series include spin valve structures with varying thickness of the non-magnetic metallic spacer and reference samples comprised only a free or fixed magnetic layer. Beside the common resonance absorption peaks, the observed microwave spectra present step-like features with hysteretic behavior. The latter effect is a direct manifestation of the interlayer coupling between the ferromagnetic layers and provides two static magnetic parameters, the switching field and coercivity of the fixed layer. The analysis of the microwave absorption spectra under in-plane rotation of the applied magnetic field at different spacer thicknesses permits a deeper insight in the magnetic interactions in this system as compared to the conventional magnetometry. We combine the standard Smit-Beljers formalism for the angular dependence of the resonance fields with a Landau-Lifshitz-Gilbert dynamics extended to describe in detail the intensity of microwave absorption in the spin valves. In this way, we extract a set of parameters for each layer including the effective magnetization and anisotropy, exchange bias and interlayer coupling, as well as Gilbert damping. The model reproduces well the experimental findings, both qualitatively and quantitatively, and the estimated parameters are in a reasonable agreement with the values known from the literature. The proposed theoretical treatment can be adopted for other multilayered dynamic systems as, e.g., spin-torque oscillators.
    Journal of Applied Physics 07/2013; 114(2). DOI:10.1063/1.4813449 · 2.19 Impact Factor
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    ABSTRACT: Spin wave spectra of 40-nm thick perpendicularly magnetized circular Permalloy dots of 250 nm radius were measured using ferromagnetic resonance technique in 70-80 GHz range at 4.2 K and in 10 GHz at room temperature. The five sharp resonance peaks were observed for both frequency ranges. The resonance fields can be well described by a magneto-exchange dispersion relation, implying that the observed resonances correspond to circular “drumhead” modes with Bessel-function profiles. The relative distances between neighbor peaks for different frequency ranges were almost the same, while the absolute interpeak distances in millimeter range were ∼30% bigger than at 10 GHz, as predicted by the theory.
    Journal of Applied Physics 04/2013; 113(17). DOI:10.1063/1.4799528 · 2.19 Impact Factor
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    ABSTRACT: Ion-beam deposited (Glass/Ta/NiFe/CoFe/Cu/CoFe/MnIr/Ta) spin valves (SVs) with a Cu-spacer thickness (tCu) varying from 14 to 28 Å have been studied by ferromagnetic resonance (FMR) and magnetoresistance (MR) measurements. With respect to the interlayer coupling strength between the free and fixed ferromagnetic layers, the samples have been divided in those with a weak coupling (for tCu > 16 Å) and a strong coupling regimes (for tCu ≤ 16 Å). The FMR behavior in these two regimes is quite different. For the weakly coupled series, there are two well-defined FMR peaks stemming from the free and fixed layers. Their in-plane angular dependences exhibit 180° and 360° symmetries, respectively. For the strongly coupled SVs, the resonance modes are hybridized and possess features of both layers simultaneously. The main coupling mechanism between the two layers, as concluded from the FMR and MR measurements, is the Néel “orange-peel” magnetostatic interaction, accompanied by a direct exchange due to pinholes in the Cu spacer for tCu < 17 Å.
    Journal of Applied Physics 04/2013; 113(17):17D713. DOI:10.1063/1.4798615 · 2.19 Impact Factor
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    ABSTRACT: Ordered hexagonal arrays of Co(x)Ni(1-x) nanowires (NWs) with different content of cobalt (0.35 < x < 0.75), were successfully fabricated by a potentiostatic electrodeposition technique inside nanoporous anodic alumina templates. The CoNi NWs were electrodeposited using only one electrolyte bath and tuning the Co% by the applied deposition potential. The systematic study of the morphology, chemical composition and magnetic properties of the CoNi NW arrays was performed. The obtained results unambiguously demonstrated the influence of the Co content on the magnetic properties of CoNi NWs. The Co% was found to decrease when increasing the applied cathodic potential and the coercivity of the CoNi NWs linearly increased with the Co%.
    2013 International Symposium on Electrodynamic and Mechatronic System (SELM), Opole-Zawiercie; 01/2013
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    ABSTRACT: Highly dispersed powders of Ni50,3Mn30Ga19,7 were prepared by spark-erosion in ethanol, water, and kerosene. Powder particles have mostly spherical shape and broad size distribution, with a maximum around 1 μm. In the as-prepared state, two series of peaks are observed by x-ray diffraction. They are associated with the disordering of two ordered phases, existed in bulk Ni-Mn-Ga: the high-temperature L21 austenitic phase and the low-temperature L10 martensitic one. Annealing decreases the half-width of the peaks and increases the L10/L21 intensity ratio. Magnetically as-prepared powders demonstrate a clear superparamagnetic behavior that changes to ferromagnetic one after annealing. These properties are quite different from the ones of mechanically dispersed powders and rapidly quenched ribbons of the same composition. The spark-erosion conditions, particularly cooling rates up to 109 K/s, lead to the formation of the mixed phase state, much higher atomic disorder, and to the appearance of different types of structural inhomogeneities.
    Journal of Applied Physics 11/2012; 112(9). DOI:10.1063/1.4764017 · 2.19 Impact Factor

Publication Stats

876 Citations
223.78 Total Impact Points

Institutions

  • 1999–2015
    • University of Porto
      • • Faculdade de Ciências
      • • Departamento de Engenharia Física
      Oporto, Porto, Portugal
  • 2014
    • Ikerbasque - Basque Foundation for Science
      Bilbo, Basque Country, Spain
  • 2013–2014
    • National University of Singapore
      • Department of Electrical & Computer Engineering
      Tumasik, Singapore
  • 2010
    • University of Oviedo
      • Department of Physics
      Oviedo, Asturias, Spain
  • 2008
    • Spanish National Research Council
      • Institute of Marine Sciences
      Madrid, Madrid, Spain
    • Universidad Autónoma de Madrid
      • Department of Condensed Matter Physics
      Madrid, Madrid, Spain
  • 2006–2007
    • National Academy of Sciences of Ukraine
      • Institute of Magnetism
      Kievo, Kyiv City, Ukraine
  • 1999–2007
    • University of Aveiro
      • Department of Physics
      Aveiro, Aveiro, Portugal
  • 2003–2006
    • The Ohio State University
      • Department of Physics
      Columbus, OH, United States
  • 2002
    • Institute of Magnetism (IMAG) NAS
      Kievo, Kyiv City, Ukraine