[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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). · 3.77 Impact Factor
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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). · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The dependence of current-in-plane resistance (R) and magnetoresistance (MR) of Al2O3(30 Å)/[Co80Fe20(13 Å)/Al2O3(30 Å)]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 Al2O3 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 Al2O3 layer thickness above 50 Å both R and MR decrease drastically, which is explained by the appearance of more patches at smoother Al2O3 surface. In contrast to the granular layered structures based on SiO2 and HfO2, the annealing in our system lowers MR values, presumably due to the oxidation of CoFe granules already existing in the as-deposited layers.
Journal of Magnetism and Magnetic Materials 09/2013; · 1.83 Impact Factor
[show abstract][hide abstract] 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). · 2.21 Impact Factor
[show abstract][hide abstract] 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). · 2.21 Impact Factor
[show abstract][hide abstract] 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. · 2.21 Impact Factor
[show abstract][hide abstract] 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
[show abstract][hide abstract] 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 01/2013; 46(34):345001. · 2.53 Impact Factor
[show abstract][hide abstract] 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). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Pulsed electrodeposition prepared porous alumina templates with Ni nanowires pore filling ranged from 1 to 100%, depending on the alumina barrier-layer thickness, were probed by continuous wave ferromagnetic resonance at room temperature. For completely filled samples, a single resonance peak was observed in the whole range of angles between the applied magnetic field and normal to the sample plane. Its position was described by Kittel formula that takes into account shape anisotropy of individual Ni wires and dipolar interactions between them. For the samples with lower pore filling the effective anisotropy field decreased and the resonance linewidth in the perpendicular configuration increased. Also a quite intense second peak was observed at lower fields for these samples. These changes are associated with reduction of pore filling percentage that can lead to decrease of dipolar interactions between nanowires and to appearance of magnetic inhomogeneities inside wires.
Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7486-90. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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)] x 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.
Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7505-9. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Magnetization reversal processes have been analyzed by Magnetic Force Microscopy in dense arrays of Co bars with well defined shape anisotropy and strong magnetostatic interactions. Two different geometries have been used: rectangular and rhombic so that the sign of dipolar interactions between adjacent chains of bars is changed from antiferromagnetic (rectangular array) to ferromagnetic (rhombic array), having a profound influence on the shape of a nucleus of inversion at the magnetization reversal.
Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7510-5. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanoparticles of Co10Cu90 alloy have been prepared by sonochemical wet method. According to transmission electron microscopy, bimetallic particles with typical diameter of 50-100 nm consisting of nanocrystallites with average diameter of 15-20 nm were obtained. The samples were annealed at 300 degrees C and 450 degrees C. Zero field cooled and field cooled temperature dependences of magnetization in the temperature range of 5-400 K at 50 Oe, as well as magnetization hysteresis loops at 15, 100 and 305 K were measured by vibrating sample magnetometry. Presence of antiferromagnetic phase, most probably of the oxide Co3O4, was observed in as-prepared sample. The lowest coercivity was found for the CoCu sample annealed at-300 degrees C, whereas for as prepared sample and the one annealed at 450 degrees C it was significantly higher. The samples were additionally probed by continuous wave ferromagnetic resonance at room, temperature using a standard X-band electron spin resonance spectrometer. A good correspondence between evolution of the coercivity and the microwave resonance fields with annealing temperature was observed.
Journal of Nanoscience and Nanotechnology 09/2012; 12(9):7529-34. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: We performed ferromagnetic resonance measurements of square arrays of noninteracting Permalloy circular dots for different orientations of external magnetic field with respect to the patterned film plane (θ). Out-of-plane angular dependence of the main resonance peak was measured in the whole range of the field angles 0° ⩽ θ ⩽ 90°. The main eigenmodespatial distribution is strongly nonuniform due to the dot nonellipsoidal shape. Nevertheless, for dots with small aspect ratio b=L/R⩽0.1 (where R is dot radius and L is dot thickness) Kittel's equation, assuming uniform dynamic magnetization (no pinning at the dot lateral edges), describes the peak position with high accuracy. Analytical calculations and micromagnetic simulations confirmed the gradual evolution of the main mode profile and a smooth transition from the strong to relatively weak pinning conditions with the change of external magnetic field angle.
[show abstract][hide abstract] ABSTRACT: The application of femtosecond laser interferometry to direct patterning of thin-film magnetic alloys is demonstrated. The formation of stripe gratings with submicron periodicities is achieved in Fe1−xVx (x = 18–34 wt. %) layers, with a difference in magnetic moments up to Δμ/μ ∼ 20 between adjacent stripes but without any significant development of the topographical relief (<1% of the film thickness). The produced gratings exhibit a robust effect of their anisotropy shape on magnetization curves in the film plane. The obtained data witness ultrafast diffusive transformations associated with the process of spinodal decomposition and demonstrate an opportunity for producing magnetic nanostructures with engineered properties upon this basis.
[show abstract][hide abstract] ABSTRACT: Rectangular and rhombic patterned superlattices of magnetic bar elements have been experimentally studied and theoretically modeled in order to analyze the role of the array geometry in their magnetization reversal and coercivity. The results show that a dominating part of the coercive field (≈250 Oe) is due to the reversal processes within a single bar element (independent of the array geometry) which is well described by the standard micromagnetic calculation. Otherwise, a smaller (≈60 Oe) but significant difference between the magnetization loops in the two geometries is related to the magnetostatic coupling effects between the bars and it is reasonably accounted for with a simple model of Coulomb-like interaction between terminal magnetic charges. The possibility to use this geometry effect to control the performance of artificial magnetic media is discussed.