Haut PDF Magnetic vortex dynamics nanostructures

Magnetic vortex dynamics nanostructures

Magnetic vortex dynamics nanostructures

After a first attempt using a mechanical detection by Evans in the 60’s [41], the idea of performing Magnetic Resonance Force Microscopy (MRFM) was proposed in the early 90’s by John Sidles [120]. He was searching for a reliable and precise technique to determine the molecular structure of biological objects. For this purpose, he wanted a spectroscopic signature to understand the atomic structure of unknown objects. In order to achieve this goal, the atomic resolution is needed with the ability to study individual objects in the three spatial dimensions. The studied objects being fragile and sensible to external conditions, a non-destructive and non-invasive technique was also needed [121]. The development in the 80’s of the scanning surface probe techniques such as Scanning Tunnelling Microscopy (STM), Atomic and Magnetic Force Microscopy (AFM/MFM) gives access to a high spatial resolution. The idea is to couple a highly sensitive force sensor like a micrometre sized cantilever to the longitudinal component of magnetic moments, either nuclear or electronic spins. Following the principles of MRI techniques developed for medical applications, the spins precession excited by a microwave field could be localised spatially in a “resonant slice” of the sample by an appropriate field gradient. The force exerted on the cantilever being also proportional to the field gradient, the spatial resolution could be increased by keeping the same signal-to-noise ratio. The sensitivity then becomes independent of the spatial resolution.
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Vortex-antivortex dynamics and field-polarity-dependent flux creep in hybrid superconductor/ferromagnet nanostructures

Vortex-antivortex dynamics and field-polarity-dependent flux creep in hybrid superconductor/ferromagnet nanostructures

1 nm Ge base layer on a Si substrate with an amorphous SiO 2 top layer. The Pb film is covered by a 10 nm Ge layer, which prevents the influence of the proximity effect with the metallic dot array. In order to avoid inhomogeneities of the current, this continuous Ge/ Pb/ Ge trilayer is patterned into a transport bridge 共width w=200 ␮ m, distance between volt- age contacts d = 630 ␮ m兲 by optical lithography and chemi- cal wet etching. Measurements of the upper critical field of this transport bridge allow an estimation of the coherence length ␰ 共0兲=34 nm and of the penetration depth ␭共0兲 = 49 nm. The transport bridge is covered by ferromagnetic dots using electron-beam evaporation and electron-beam li- thography. The dots consist of a 3.5-nm Pd base layer and a 关Co共0.4 nm兲/Pd共1.4 nm兲兴10 multilayer with perpendicular magnetic anisotropy. 28 The dots are arranged in a square ar- ray with period L = 1.5 ␮ m. They have a square shape with a side length of approximately 0.8 ␮ m with slightly irregular edges.
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Magnetization dynamics in magnetic nanostructures

Magnetization dynamics in magnetic nanostructures

Since 1940, the studies of the magnetization dynamics in different magnetic systems have become of large interest, in particular with respect to the industrial applications such as magnetic memories. One of the first magnetic recording devices based on the magnetization dynamics used ferrite heads to write and read the information. Because the ferrite permeability falls above 10 MHz [Doyle 1998], the read/write process was possible only with a reduced rate. An important improvement (i.e. decreased response time) was obtained using magnetic thin film heads. Upon reduction of the dimensions of the magnetic system (i.e. reduced film thickness compared to the other two dimensions) strong demagnetizing fields will be induced, and thus a high value of the demagnetizing factor (~1) creating a large anisotropy field perpendicular to the film surface. In this way, reasonable permeabilities for applied frequencies larger than 300 MHz were obtained in thin magnetic films devices [Doyle1998].
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Probing vortex dynamics on a single vortex level by scanning ac-susceptibility microscopy

Probing vortex dynamics on a single vortex level by scanning ac-susceptibility microscopy

The above-stated limitations of the conventional ac-susceptibility technique, namely its inability to resolve the ac response of a single vortex and the indirect rela- tion between the vortex dynamics and the integrated response, has provided a drive to develop alternative methods aiming to directly probe the ac properties of a supercon- ductor with single vortex resolution. In this chapter we discuss a recently introduced scanning probe technique, scanning ac-susceptibility microscopy (SSM), which re- veals, with unprecedented resolution, the motion and dissipation of individual units of flux quanta driven by an applied ac magnetic field or current [2]. The local dissi- pation can be inferred from the phase lag between the vortex motion and the driving force induced by an oscillatory magnetic field, whereas the amplitude of the oscilla- tory vortex motion provides us with information about the shape of the local potential that each fluxon experiences. This method has permitted us to reveal the contribution of pinning-driven (thermally activated) dissipative vortex motion [3], to demonstrate the nondissipative nature of the Meissner as well as the dissipative vortex state at microscopic scale [3] and finally, to obtain a detailed cartography of the distribution and intensity of the pinning landscape [2, 4]. This technique not only shed new light on unraveling the basic mechanisms of vortex dissipation with unmatched resolu- tion, but it permitted one to validate the theoretical models introduced to explain the measured integrated ac vortex responses in ac-susceptibility experiments [5]. We show that the technique can be readily implemented in a scanning Hall probe mi- croscopy set-up suited for low magnetic field experiments [2–5] and also extended to a scanning tunneling microscopy [6] or a scanning SQUID microscopy apparatus [7] thus achieving the utmost resolution.
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Synthesis and magnetic reversal of bi-conical Ni nanostructures Synthesis and magnetic reversal of bi-conical Ni nanostructures

Synthesis and magnetic reversal of bi-conical Ni nanostructures Synthesis and magnetic reversal of bi-conical Ni nanostructures

studies since one can trap a head to head or tail to tail vortex domain wall in a constriction with cylindrical symmetry. V. CONCLUSIONS We have developed an original method based upon tem- plate synthesis in a PET matrix to grow bi-conical Ni nano- wires with a constriction of a few tens of nanometers. The micromagnetic states of a single nanowire have been studied through AMR and micromagnetic modeling during magnet- ization reversal. Simulations have revealed a complex vortex like state whose propagation through the constriction depends on the relative angle between the applied field and the cone axis. While simulations predict no pinning of the vortex in the constriction, our magnetoresistance measure- ments provide evidence that small inhomogeneities allow the trapping of head-to-head or tail-to-tail domain walls in the vicinity of the constriction.
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Decay rate of magnetic dipoles near nonmagnetic nanostructures

Decay rate of magnetic dipoles near nonmagnetic nanostructures

Peter R. Wiecha, ∗ Arnaud Arbouet, Aurélien Cuche, Vincent Paillard, and Christian Girard CEMES-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France In this article, we propose a concise theoretical framework based on mixed field-susceptibilities to describe the decay of magnetic dipoles induced by non–magnetic nanostructures. This approach is first illustrated in simple cases in which analytical expressions of the decay rate can be obtained. We then show that a more refined numerical implementation of this formalism involving a volume discretization and the computation of a generalized propagator can predict the dynamics of mag- netic dipoles in the vicinity of nanostructures of arbitrary geometries. We finally demonstrate the versatility of this numerical method by coupling it to an evolutionary optimization algorithm. In this way we predict a structure geometry which maximally promotes the decay of magnetic transitions with respect to electric emitters.
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In situ Lorentz microscopy and electron holography in magnetic nanostructures

In situ Lorentz microscopy and electron holography in magnetic nanostructures

vortex is not located at the center of the DW. Thus, in the 19-nm-thick NW the two different kinds of LM contrast observed are associated to the nucleation of an AVW with the vortex placed either in the upper or in the lower part of the DW. In addition, the contrast inversion observed in the defocused LM image indicates that the chirality is different in each case. Finally, the complex structure observed in the defocused LM images of the thickest NWs (t ³ 22 nm), is produced by two magnetic vortices (2VW) with antiparallel chirality. It has been found in Ref [12] that this exotic DW structure is more favorable in thicker and wider wires. A similar change of the DW configuration as a function of their dimension has been predicted in Permalloy NWs [42]. Micromagnetic simulations of the DW nucleation processes using the real size of the L-shape nanowires have been performed to support our experimental observations. Perfect defect-free NWs with a rectangular profile have been assumed to make these simulations, which were carried out following the experimental procedure to nucleate the DW in the corner of the NWs schematized in Figure 4.4. Simulated images of the DW just after its nucleation in the corner are depicted in the right column of Figure 4.8. Despite the small differences in the positions of the DW, most likely due to morphological differences between the real and the simulated defect-free nanostructure (roughness, bell-shape transversal profile, irregularities in the edges), the micromagnetic simulations are in very good agreement with the TIE reconstructions. They indicate the same evolution of the DW configuration (TW  ATW  AVW  2VW) in the 500-nm wide NWs as a function of the thickness. Comparing these DW configurations with the nucleation field measurements, we find that the maximum value of H N corresponds to the nucleation of an AVW. We also
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Heterogeneous vortex dynamics in high temperature superconductors

Heterogeneous vortex dynamics in high temperature superconductors

otherwise free of any macroscopic defects. Strongly pinning walls, introduced by heavy ion irradiation through a 30 µm thick Ni mask, define weakly pinning channels through which the vortex ensemble is forced to flow. An additional, essential ingredient of the experiment is the inclusion of heavy-ion irradiated contact pads directly adjacent to the channel structure, and far removed from the crystal boundaries. By injecting the transport current through these irradiated pads, one forces the transport current to flow through the bulk; shear flow of the vortex ensemble then takes place through the channels. The shear viscosity of the vortices in the bulk can then be probed by a standard resistivity measurement. The signatures of shear flow are clearly noticeable in the resistance curves. The nature of the various features of the resistivity curves can be unambiguously identified by comparing them to magneto-optical images of the magnetic field distribution created by a transport current.
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In situ Lorentz microscopy and electron holography in magnetic nanostructures

In situ Lorentz microscopy and electron holography in magnetic nanostructures

vortex is not located at the center of the DW. Thus, in the 19-nm-thick NW the two different kinds of LM contrast observed are associated to the nucleation of an AVW with the vortex placed either in the upper or in the lower part of the DW. In addition, the contrast inversion observed in the defocused LM image indicates that the chirality is different in each case. Finally, the complex structure observed in the defocused LM images of the thickest NWs (t ³ 22 nm), is produced by two magnetic vortices (2VW) with antiparallel chirality. It has been found in Ref [12] that this exotic DW structure is more favorable in thicker and wider wires. A similar change of the DW configuration as a function of their dimension has been predicted in Permalloy NWs [42]. Micromagnetic simulations of the DW nucleation processes using the real size of the L-shape nanowires have been performed to support our experimental observations. Perfect defect-free NWs with a rectangular profile have been assumed to make these simulations, which were carried out following the experimental procedure to nucleate the DW in the corner of the NWs schematized in Figure 4.4. Simulated images of the DW just after its nucleation in the corner are depicted in the right column of Figure 4.8. Despite the small differences in the positions of the DW, most likely due to morphological differences between the real and the simulated defect-free nanostructure (roughness, bell-shape transversal profile, irregularities in the edges), the micromagnetic simulations are in very good agreement with the TIE reconstructions. They indicate the same evolution of the DW configuration (TW  ATW  AVW  2VW) in the 500-nm wide NWs as a function of the thickness. Comparing these DW configurations with the nucleation field measurements, we find that the maximum value of H N corresponds to the nucleation of an AVW. We also
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Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires

Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires

2 One-dimensional magnetic nanostructures have for the past two decades been of increased interest for the develop- ment of future spintronic devices 1–4 motivated by concepts like Magnetic Race Track Memory 5 and the wish to manipulate magnetic domain walls (DW). Cylindrical nanowires (NWs) are particularly interesting candidates to reach this goal, much due to the fast DW motion induced by an external magnetic field or electric current, where theoretical studies have anticipated the absence of a Walker breakdown. 6,7 In addition, curvature effects have re- cently been proved to induce effects related to topology, chirality, and symmetry, 8 and unidirectional reversal pro- cess has been reported by engineering the geometry in multi-segmented nanowires. 9 However, to further technical developments in spintronics and a better control of DW motion, a thorough understanding of the fine structures of DWs in magnetic NWs, in which shape and crystal structure are contributing factors to the minimization of the system’s magnetic energy, 10–13 is required.
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Band structure of magnetic excitations in the vortex phase of a ferromagnetic superconductor

Band structure of magnetic excitations in the vortex phase of a ferromagnetic superconductor

The outline of the paper is as follows. In Sec. II A we present a model of the ferromagnetic superconductor and review the result for the magnetic excitations spectrum in the Meissner state (see Ref. 8 ). In Sec. II B we derive the basic equations for the collective vortex-magnetization dynamics. In Secs. II C and II D the weak-binding approximation is developed and the frequency gaps between adjacent bands of the magnon spectrum are determined analytically. The numerical spectra, obtained using realistic parameters, are presented in Sec. II E . In Sec. II F the role of dissipation connected with viscous vortex motion is discussed. Finally, in Sec. III we consider a boundary problem for an electromag- netic wave incident at a ferromagnetic superconductor. The frequency-dependent reflectivity coefficient is examined for frequencies lying within and close to the gaps of the magnon scpectrum.
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DYNAMICS OF ELECTRONS IN GRADIENT NANOSTRUCTURES ( EXACTLY SOLVABLE MODEL) .

DYNAMICS OF ELECTRONS IN GRADIENT NANOSTRUCTURES ( EXACTLY SOLVABLE MODEL) .

I - Introduction. The ability to tailor the potential of electrons on the scale of their de Broglie wavelength has opened the new horizons in nanoelectronics. Dynamics of quantum particles in these heterogeneous fields, shaped by continuous spatial variations of potential as well as its gradient, attracts a growing attention in several fields of atomic, optical and solid state physics. Namely, engineering of complicated potential barriers for controlled transport of electrons in semiconductor superlattices and heterostructures /1- 3/, is widely used in microelectronic systems. This approach, generalized for traveling and tunneling regimes in motion of quasiparticles, proves to be the useful tool for analysis of the dynamics of polaritons in molecular crystals /4/ as well as quantum defects /5/ and magnetic moments /6/ in solids. A special attention was brought to periodical potentials, particularly to the dynamics of atom wavepackets in magnetic potentials, supported by current – carrying wires /7/ and, in particular, to the control of atomic ensembles and matter waves in optical lattices, arising from a set of interfering laser beams /8-10/. A wealth of literature has been devoted to transport and trapping of quantum objects in the double-well (DW) potentials of both natural and technological origin /11-13/.
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Strain Induced Vortex Core Switching in Planar Magnetostrictive Nanostructures

Strain Induced Vortex Core Switching in Planar Magnetostrictive Nanostructures

DOI: 10.1103/PhysRevLett.115.067202 PACS numbers: 75.78.-n, 75.60.Jk, 77.55.Nv, 85.80.Jm Understanding and controlling magnetism in laterally confined nanostructures has been an area of high interest in the past decade. In planar structures the lateral confinement often leads to a ground state where a vortex core is present, the Landau flux closure and vortex domain configurations being the most obvious examples. Apart from fundamental interest in these topological entities, magnetic structures containing vortex cores have been recognized for applica- tions in magnetic random access memory [1] and micro- wave oscillators including spin torque vortex oscillators [2] . Excitation of vortex core motion [3 –5] is not only essential for microwave emission in spin torque vortex oscillators, but has recently been shown to provide a means to flip the direction of the vortex core itself [3] , allowing for data writing in magnetic memory applications.
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Chaos in Magnetic Nanocontact Vortex Oscillators

Chaos in Magnetic Nanocontact Vortex Oscillators

The capacity to identify chaotic behavior from the time series data from the nanocontact vortex oscillator opens up a number of perspectives for both fundamental and applied studies. The magnetoresistance signal repre- sents an indirect measurement of the vortex core polarity, whose dynamics is challenging to probe electrically. Our study may provide a way of studying the inertial effects and transient dynamics related to core reversal in nan- odevices. The chaotic dynamics measured in the mag- netoresistance signal is also associated with the erratic generation of regular patterns (as shown in the insert of Fig. 3 ), which could lead to the determination of sym- bolic dynamics for the system and hence open the way towards controlling the chaotic properties of the oscillator at the nanoscale. Finally, the use of chaotic dynamics in spintronics could lead to the development of novel appli- cations in information processing, such as physical-layer encryption and random number generation [ 41 ].
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Pattern generation and symbolic dynamics in a nanocontact vortex oscillator

Pattern generation and symbolic dynamics in a nanocontact vortex oscillator

Here, we demonstrate experimentally that the chaotic regime of the NCVO involves simple aperiodic waveform patterns. These can be encoded into bit sequences, which are correlated with the core-polarity state of the magnetic vortex. First, we describe time-resolved signals from the NCVO at 77 K and validate their chaotic characteristics from sensitivity to initial conditions and correlation dimension analysis. Then, we show that the time traces are in fact only composed by a few waveform patterns which are ordered aperiodically in the chaotic regime. By reconstructing attractor geometries from the measured time ser- ies, we reveal the symbolic dynamics of chaotic NCVOs, which is in good agreement with the patterns observed in simulation. We extract bit sequences based on this symbolic analysis and show that the generated bits can achieve maximal values of the Shan- non block entropy and Lempel–Ziv complexity.
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Comprehensive experimental studies on vortex dynamics over military wing configurations in IAR

Comprehensive experimental studies on vortex dynamics over military wing configurations in IAR

5.3.7 FOREBODY/WING/TAIL RESULTS As expected, the addition of a forebody has an important impact on the flow, specifically, the presence of forebody vortices and their interaction with the leading-edge ones result in very substantial changes of the airloads at moderate to high angles of attack. As no attempt was made to fix transition on the forebody, an erratic behavior of the static loads was observed which is contrary to the case of the delta wing that separation is well defined at the sharp leading edge. Fig. 46 depicts the loads observed at several runs for σ = 30°. Most of the scatter occurs at small roll angles although it is present everywhere to a lesser extent. The loads are not symmetrical (or anti-symmetrical) about the origin due to the asymmetric forebody vortex shedding, which as suggested by the load discontinuities switch position at φ ~ -8°. Tests could not be performed near the above roll angle as potentially damaging, rapidly diverging lateral oscillations of the sting were induced. The large oscillation amplitude suggests that a positive feedback results from the coupling between the forebody vortex switching and model motion.
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Probing the low-frequency vortex dynamics in a nanostructured superconducting strip

Probing the low-frequency vortex dynamics in a nanostructured superconducting strip

vortices. However, at a slightly lower temperature, T /T c = 0.93, the interstitial vortices freeze up leading to a strong reduction of the ac screening length. We propose a simple model for the vortex response in this system which fits well to the experimental data. Our analysis suggests that the observed switching to the high mobility regime stems from a resonant effect, where the period of the ac excitation is just large enough to allow interstitial vortices to thermally hop through the weak pinning landscape produced by random material defects. This argument is further supported by the observation of a pronounced enhancement of the out-of-phase response at the crossover between both dynamical regimes.
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Anisotropic magnetic molecular dynamics of cobalt nanowires

Anisotropic magnetic molecular dynamics of cobalt nanowires

From the Bohr-van Leeuwen theorem and without internal degrees of freedom, the magnetization of a classical system is always null. 7 Because magnetism is a quantum effect, the magnetic molecular dynamics (MMD) cannot mimic the magnetization dynamics without introducing the spins as supplemental dynamical variables. In a classical fluid theory, the first formulation of the dynamics of classical particles with spin degrees of freedom was given early by Turski. 8 More recently, another coupling of the molecular dynamics (MD) and atomic spin dynamics was presented by Antropov et al. 9 , 10 This approach is based simultaneously on a quantum mechanical derivation of localized moment and atomic equations of motion but is a colossal consumption of computational resources. However, in simplifying the full set of first-principles equations of motion, Akbar et al. 11 have demonstrated a few-hundred-atom simulation in which com- plex antiferromagnetic order and helical structures are found for γ -Fe. Moreover, explicit calculations of spin damping have been reported on bulk, monolayers, and atomic wires of Fe, Co, and Ni. 12
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Unexpected Magnetic Properties of Gas-Stabilized Platinum Nanostructures in the Tunneling Regime

Unexpected Magnetic Properties of Gas-Stabilized Platinum Nanostructures in the Tunneling Regime

KEYWORDS: Magnetoresistance, platinum oxide, nanocontact, spin-dependent tunneling C alculations suggest that platinum atomic chains of 1 nm or longer should exhibit spontaneous Hund ’s rule superparamagnetism at low temperature. 1 −4 This would imply that the spins can be frozen in a direction determined by an external magnetic field. Moreover, the conductance of a single Pt atomic contact is dominated by conducting channels de fined by the 8 valence electrons, and it is expected to vary as a function of the chain length and interatomic distance. 1,3,5 In ideal magnetic atomic structures, only one spin channel may be available, so that the conductance can vary in multiples of e 2 /h
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Fluctuation dynamics of a single magnetic chain

Fluctuation dynamics of a single magnetic chain

Our simulation scheme differs from previous works @32,33# in that the initial state corresponds to a set of N particles connected to each other in a rigid-rod-like configu- ration and are subsequently subjected to a magnetic field of desired strength, which allows the chain to relax and fluctu- ate. Previous works @32,33# focused on the kinetics of chain formation for which the appropriate initial state is a random configuration of the set of N particles. We simulate the ther- mal fluctuations of a single chain of variable length ~N532 particles to N5108 particles!, where all particles of the chain are moved in each MC step; the motion of one particle at a time is not sufficient to generate independent configurations and therefore calculate the ensemble averages. The dimen- sionless parameter l ~which is the ratio between the induced dipole-dipole interactions and the thermal energy! and the number of particles in the chain are the only parameters in the MC simulation. The fluctuation spectra of the chains were simulated using different magnetic field strengths and chain lengths. The configurations generated by the MC simu- lations were studied by calculating the same correlation functions that were used to treat the experimental data @Eqs. ~5! and ~6!#. The results are presented in Figs. 8–11.
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