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1.
Artigo em Inglês | MEDLINE | ID: mdl-32009381

RESUMO

Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, and conversion between charge and spin currents and vice versa. These interfacial phenomena play crucial roles in magnetic data storage and transfer applications, which require the formation of FM nanostructures embedded in non-ferromagnetic matrices. Here, we investigate the possibility of creating such nanostructures by ion irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of nonmagnetic metal oxides (e.g., antiferro- or paramagnetic) to form ferromagnetic metals. Our findings are later exploited to form three-dimensional magnetic interfaces between Co, CoO, and Pt by spatial-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of O atoms plays a crucial role in the reduction from insulating, non-ferromagnetic cobalt oxides to metallic cobalt. Metallic cobalt yields both perpendicular magnetic anisotropy in the generated Co/Pt nanostructures and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely packed, atomic-scale ferromagnetic point-contact spin-torque oscillator (STO) networks or conductive channels for current-confined-path-based current perpendicular-to-plane giant magnetoresistance read heads.

2.
Small ; 15(49): e1904315, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31709700

RESUMO

The magnetization dynamics of individual Fe-filled multiwall carbon-nanotubes (FeCNT), grown by chemical vapor deposition, are investigated by microresonator ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) microscopy and corroborated by micromagnetic simulations. Currently, only static magnetometry measurements are available. They suggest that the FeCNTs consist of a single-crystalline Fe nanowire throughout the length. The number and structure of the FMR lines and the abrupt decay of the spin-wave transport seen in BLS indicate, however, that the Fe filling is not a single straight piece along the length. Therefore, a stepwise cutting procedure is applied in order to investigate the evolution of the ferromagnetic resonance lines as a function of the nanowire length. The results show that the FeCNT is indeed not homogeneous along the full length but is built from 300 to 400 nm long single-crystalline segments. These segments consist of magnetically high quality Fe nanowires with almost the bulk values of Fe and with a similar small damping in relation to thin films, promoting FeCNTs as appealing candidates for spin-wave transport in magnonic applications.

3.
Small ; 15(52): e1904738, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31709733

RESUMO

Nanoscale modifications of strain and magnetic anisotropy can open pathways to engineering magnetic domains for device applications. A periodic magnetic domain structure can be stabilized in sub-200 nm wide linear as well as curved magnets, embedded within a flat non-ferromagnetic thin film. The nanomagnets are produced within a non-ferromagnetic B2-ordered Fe60 Al40 thin film, where local irradiation by a focused ion beam causes the formation of disordered and strongly ferromagnetic regions of A2 Fe60 Al40 . An anisotropic lattice relaxation is observed, such that the in-plane lattice parameter is larger when measured parallel to the magnet short-axis as compared to its length. This in-plane structural anisotropy manifests a magnetic anisotropy contribution, generating an easy-axis parallel to the short axis. The competing effect of the strain and shape anisotropies stabilizes a periodic domain pattern in linear as well as spiral nanomagnets, providing a versatile and geometrically controllable path to engineering the strain and thereby the magnetic anisotropy at the nanoscale.

4.
Sci Rep ; 9(1): 9541, 2019 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-31266999

RESUMO

Spin-transfer torques (STTs) can be exploited in order to manipulate the magnetic moments of nanomagnets, thus allowing for new consumer-oriented devices to be designed. Of particular interest here are tuneable radio-frequency (RF) oscillators for wireless communication. Currently, the structure that maximizes the output power is an Fe/MgO/Fe-type magnetic tunnel junction (MTJ) with a fixed layer magnetized in the plane of the layers and a free layer magnetized perpendicular to the plane. This structure allows for most of the tunnel magnetoresistance (TMR) to be converted into output power. Here, we experimentally and theoretically demonstrate that the main mechanism sustaining steady-state precession in such structures is the angular dependence of the magnetoresistance. The TMR of such devices is known to exhibit a broken-linear dependence versus the applied bias. Our results show that the TMR bias dependence effectively quenches spin-transfer-driven precession and introduces a non-monotonic frequency dependence at high applied currents. This has an impact on devices seeking to work in the 'THz gap' due to their non-trivial TMR bias dependences.

5.
Sci Rep ; 9(1): 4020, 2019 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-30858481

RESUMO

Due to its negligible spontaneous magnetization, high spin polarization and giant perpendicular magnetic anisotropy, Mn2RuxGa (MRG) is an ideal candidate as an oscillating layer in THz spin-transfer-torque nano-oscillators. Here, the effect of ultrathin Al and Ta diffusion barriers between MRG and MgO in perpendicular magnetic tunnel junctions is investigated and compared to devices with a bare MRG/MgO interface. Both the compensation temperature, Tcomp, of the electrode and the tunneling magnetoresistance (TMR) of the device are highly sensitive to the choice and thickness of the insertion layer used. High-resolution transmission electron microscopy, as well as analysis of the TMR, its bias dependence, and the resistance-area product allow us to compare the devices from a structural and electrical point of view. Al insertion leads to the formation of thicker effective barriers and gives the highest TMR, at the cost of a reduced Tcomp. Ta is the superior diffusion barrier which retains Tcomp, however, it also leads to a much lower TMR on account of the short spin diffusion length which reduces the tunneling spin polarization. The study shows that fine engineering of the Mn2RuxGa/barrier interface to improve the TMR amplitude is feasible.

6.
Nat Nanotechnol ; 14(4): 328-333, 2019 04.
Artigo em Inglês | MEDLINE | ID: mdl-30804478

RESUMO

Spin waves offer intriguing perspectives for computing and signal processing, because their damping can be lower than the ohmic losses in conventional complementary metal-oxide-semiconductor (CMOS) circuits. Magnetic domain walls show considerable potential as magnonic waveguides for on-chip control of the spatial extent and propagation of spin waves. However, low-loss guidance of spin waves with nanoscale wavelengths and around angled tracks remains to be shown. Here, we demonstrate spin wave control using natural anisotropic features of magnetic order in an interlayer exchange-coupled ferromagnetic bilayer. We employ scanning transmission X-ray microscopy to image the generation of spin waves and their propagation across distances exceeding multiples of the wavelength. Spin waves propagate in extended planar geometries as well as along straight or curved one-dimensional domain walls. We observe wavelengths between 1 µm and 150 nm, with excitation frequencies ranging from 250 MHz to 3 GHz. Our results show routes towards the practical implementation of magnonic waveguides in the form of domain walls in future spin wave logic and computational circuits.

7.
Sci Rep ; 8(1): 16040, 2018 Oct 30.
Artigo em Inglês | MEDLINE | ID: mdl-30375413

RESUMO

Spin Hall oscillators (SHO) are promising candidates for the generation, detection and amplification of high frequency signals, that are tunable through a wide range of operating frequencies. They offer to be read out electrically, magnetically and optically in combination with a simple bilayer design. Here, we experimentally study the spatial dependence and spectral properties of auto-oscillations in SHO devices based on Pt(7 nm)/Ni80Fe20(5 nm) tapered nanowires. Using Brillouin light scattering microscopy, we observe two individual self-localized spin-wave bullets that oscillate at two distinct frequencies (5.2 GHz and 5.45 GHz) and are localized at different positions separated by about 750 nm within the SHO. This state of a tapered SHO has been predicted by a Ginzburg-Landau auto-oscillator model, but not yet been directly confirmed experimentally. We demonstrate that the observed bullets can be individually synchronized to external microwave signals, leading to a frequency entrainment, linewidth reduction and increase in oscillation amplitude for the bullet that is selected by the microwave frequency. At the same time, the amplitude of other parasitic modes decreases, which promotes the single-mode operation of the SHO. Finally, the synchronization of the spin-wave bullets is studied as a function of the microwave power. We believe that our findings promote the realization of extended spin Hall oscillators accomodating several distinct spin-wave bullets, that jointly cover an extended range of tunability.

8.
Nano Lett ; 18(5): 2828-2834, 2018 05 09.
Artigo em Inglês | MEDLINE | ID: mdl-29620910

RESUMO

We present a detailed study on the static magnetic properties of individual permalloy nanotubes (NTs) with hexagonal cross-sections. Anisotropic magnetoresistance (AMR) measurements and scanning transmission X-ray microscopy (STXM) are used to investigate their magnetic ground states and its stability. We find that the magnetization in zero applied magnetic field is in a very stable vortex state. Its origin is attributed to a strong growth-induced anisotropy with easy axis perpendicular to the long axis of the tubes. AMR measurements of individual NTs in combination with micromagnetic simulations allow the determination of the magnitude of the growth-induced anisotropy for different types of NT coatings. We show that the strength of the anisotropy can be controlled by introducing a buffer layer underneath the magnetic layer. The magnetic ground states depend on the external magnetic field history and are directly imaged using STXM. Stable vortex domains can be introduced by external magnetic fields and can be erased by radio-frequency magnetic fields applied at the center of the tubes via a strip line antenna.

9.
ACS Appl Mater Interfaces ; 10(17): 15232-15239, 2018 May 02.
Artigo em Inglês | MEDLINE | ID: mdl-29665332

RESUMO

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in Fe60Al40, from the B2 to the A2 structure and vice versa. A single laser pulse above a threshold fluence causes nonferromagnetic B2 Fe60Al40 to disorder and form the ferromagnetic A2 structure. Subsequent laser pulsing below the threshold reverses the surface to B2 Fe60Al40, erasing the laser-induced ferromagnetism. Simulations reveal that the order-disorder transition is regulated by the extent of surface supercooling; above the threshold for complete melting throughout the film thickness, the liquid phase can be deeply undercooled before solidification. As a result, the vacancy diffusion in the resolidified region is limited and the region is trapped in the metastable chemically disordered state. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling diffusion-assisted reordering. This demonstrates that ultrafast lasers can achieve subtle atomic rearrangements in bimetallic alloys in a reversible and nonvolatile fashion.

10.
Sci Rep ; 7: 41157, 2017 02 01.
Artigo em Inglês | MEDLINE | ID: mdl-28145463

RESUMO

Programmability of stable magnetization configurations in a magnetic device is a highly desirable feature for a variety of applications, such as in magneto-transport and spin-wave logic. Periodic systems such as antidot lattices may exhibit programmability; however, to achieve multiple stable magnetization configurations the lattice geometry must be optimized. We consider the magnetization states in Co-antidot lattices of ≈50 nm thickness and ≈150 nm inter-antidot distance. Micromagnetic simulations were applied to investigate the magnetization states around individual antidots during the reversal process. The reversal processes predicted by micromagnetics were confirmed by experimental observations. Magnetization reversal in these antidots occurs via field driven transition between 3 elementary magnetization states - termed G, C and Q. These magnetization states can be described by vectors, and the reversal process proceeds via step-wise linear operations on these vector states. Rules governing the co-existence of the three magnetization states were empirically observed. It is shown that in an n × n antidot lattice, a variety of field switchable combinations of G, C and Q can occur, indicating programmability of the antidot lattices.

11.
Nat Nanotechnol ; 11(11): 948-953, 2016 11.
Artigo em Inglês | MEDLINE | ID: mdl-27428277

RESUMO

The use of spin waves as information carriers in spintronic devices can substantially reduce energy losses by eliminating the ohmic heating associated with electron transport. Yet, the excitation of short-wavelength spin waves in nanoscale magnetic systems remains a significant challenge. Here, we propose a method for their coherent generation in a heterostructure composed of antiferromagnetically coupled magnetic layers. The driven dynamics of naturally formed nanosized stacked pairs of magnetic vortex cores is used to achieve this aim. The resulting spin-wave propagation is directly imaged by time-resolved scanning transmission X-ray microscopy. We show that the dipole-exchange spin waves excited in this system have a linear, non-reciprocal dispersion and that their wavelength can be tuned by changing the driving frequency.

12.
Sci Rep ; 5: 16786, 2015 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-26584789

RESUMO

Nanomagnets form the building blocks for a variety of spin-transport, spin-wave and data storage devices. In this work we generated nanoscale magnets by exploiting the phenomenon of disorder-induced ferromagnetism; disorder was induced locally on a chemically ordered, initially non-ferromagnetic, Fe60Al40 precursor film using nm diameter beam of Ne(+) ions at 25 keV energy. The beam of energetic ions randomized the atomic arrangement locally, leading to the formation of ferromagnetism in the ion-affected regime. The interaction of a penetrating ion with host atoms is known to be spatially inhomogeneous, raising questions on the magnetic homogeneity of nanostructures caused by ion-induced collision cascades. Direct holographic observations of the flux-lines emergent from the disorder-induced magnetic nanostructures were made in order to measure the depth- and lateral- magnetization variation at ferromagnetic/non-ferromagnetic interfaces. Our results suggest that high-resolution nanomagnets of practically any desired 2-dimensional geometry can be directly written onto selected alloy thin films using a nano-focussed ion-beam stylus, thus enabling the rapid prototyping and testing of novel magnetization configurations for their magneto-coupling and spin-wave properties.

13.
Sci Rep ; 5: 16942, 2015 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-26592432

RESUMO

Spin torque oscillators (STOs) are compact, tunable sources of microwave radiation that serve as a test bed for studies of nonlinear magnetization dynamics at the nanometer length scale. The spin torque in an STO can be created by spin-orbit interaction, but low spectral purity of the microwave signals generated by spin orbit torque oscillators hinders practical applications of these magnetic nanodevices. Here we demonstrate a method for decreasing the phase noise of spin orbit torque oscillators based on Pt/Ni80Fe20 nanowires. We experimentally demonstrate that tapering of the nanowire, which serves as the STO active region, significantly decreases the spectral linewidth of the generated signal. We explain the observed linewidth narrowing in the framework of the Ginzburg-Landau auto-oscillator model. The model reveals that spatial non-uniformity of the spin current density in the tapered nanowire geometry hinders the excitation of higher order spin-wave modes, thus stabilizing the single-mode generation regime. This non-uniformity also generates a restoring force acting on the excited self-oscillatory mode, which reduces thermal fluctuations of the mode spatial position along the wire. Both these effects improve the STO spectral purity.

14.
Opt Express ; 23(13): 16575-81, 2015 Jun 29.
Artigo em Inglês | MEDLINE | ID: mdl-26191669

RESUMO

Magnetic domains and magnetization reversal in 40 nm thick films of Fe0.6Al0.4, have been studied by longitudinal magneto-optical Kerr effect. By varying the Ne(+) ion-energy E between 2 and 30 keV (keeping a constant fluence), we varied the depth-penetration of the ions, and thereby influenced the homogeneity of the induced saturation magnetization M(s). The dependence of coercivity on ion energy shows maximum for 5 keV Ne(+). Considerable differences in the magnetic domain formation and magnetization reversal processes were observed: at low E (≤ 5keV), the reversal process is dominated by domain nucleation mechanism (high density of domain nucleation sites), consistent with the occurrence of an inhomogeneous M(s). Films irradiated with E > 5keV ions exhibit significantly low domain nucleation density, and the reversal is dominated by domain propagation mechanism, suggesting homogeneity in induced M(s). These results demonstrate the tunability of magnetization reversal behavior in materials possessing disorder induced magnetic phase transitions.

15.
Nat Commun ; 5: 5616, 2014 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-25476073

RESUMO

Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality--a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization self-oscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometre length scale.

16.
Small ; 10(24): 5161-9, 2014 Dec 29.
Artigo em Inglês | MEDLINE | ID: mdl-25066641

RESUMO

By means of off-axis electron holography the local distribution of the magnetic induction within and around a poly-crystalline Permalloy (Ni81Fe19) thin film is studied. In addition the stray field above the sample is measured by magnetic force microscopy on a larger area. The film is deposited on a periodically nanostructured (rippled) Si substrate, which was formed by Xe(+) ion beam erosion. This introduces the periodical ripple shape to the Permalloy film. The created ripple morphology is expected to modify the magnetization distribution within the Permalloy and to induce dipolar stray fields. These stray fields play an important role in spinwave dynamics of periodic nanostructures like magnonic crystals. Micromagnetic simulations estimate those stray fields in the order of only 10 mT. Consequently, their experimental determination at nanometer spatial resolution is highly demanding and requires advanced acquisition and reconstruction techniques such as electron holography. The reconstructed magnetic phase images show the magnetized thin film, in which the magnetization direction follows mainly the given morphology. Furthermore, a closer look to the Permalloy/carbon interface reveals stray fields at the detection limit of the method in the order of 10 mT, which is in qualitative agreement with the micromagnetic simulations.

17.
IEEE Trans Neural Netw Learn Syst ; 25(4): 830-6, 2014 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-24807960

RESUMO

Recurrent neural networks (RNNs) are well known for their capability to minimize suitable cost functions without the need for a training phase. This is possible because they can be Lyapunov stable. Although the global stability analysis has attracted a lot of interest, local stability is desirable for specific applications. In this brief, we investigate the local asymptotical stability of two classes of discrete-time, continuous-state, complex-valued RNNs with parallel update and inner state feedback. We show that many already known results are special cases of the results obtained here. We also generalize some known results from the real-valued case to the complex-valued one. Finally, we investigate the stability in the presence of time-variant activation functions. Complex-valued activation functions in this brief are separable with respect to the real and imaginary parts.


Assuntos
Algoritmos , Retroalimentação , Modelos Estatísticos , Redes Neurais de Computação , Dinâmica não Linear , Processamento de Sinais Assistido por Computador , Simulação por Computador , Reconhecimento Automatizado de Padrão/métodos
18.
Nano Lett ; 14(2): 435-41, 2014 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-24377706

RESUMO

Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼ 40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools.

19.
Rev Sci Instrum ; 85(12): 123701, 2014 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-25554295

RESUMO

We report on a new instrument, which consists of a Kerr microscope combined with resistance measurements. This setup allows for the recording of magnetic domains while measuring the anisotropic magnetoresistance (AMR). For this purpose the development of a special sample holder and the extension of the measurement software was required. The sample holder is equipped with electrical contacts in such a way to apply a current, measure the voltage, and use it in the Kerr microscope. The extension of the measurement software enables the recording of resistance and Kerr images simultaneous. The new setup allows for a better microscopic understanding of the AMR behavior.

20.
Transfus Apher Sci ; 29(3): 247-54, 2003 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-14572817

RESUMO

BACKGROUND: Hereditary angioedema caused by C1 inhibitor deficiency can be life threatening. Acute exacerbations are treated with intravenous purified, pasteurized C1 esterase inhibitor concentrate at doses of 500-1000 IU. METHODS: We reviewed the literature about safety and efficacy of the C1 inhibitor concentrate used in Canada (Berinert P) or relevant to it. RESULTS: Post-marketing experience since 1985 is reassuring. There were few adverse events and no transmission of infection with the pasteurized product. A number of case reports, retrospective studies and few randomized or non-randomized prospective studies have shown good efficacy measured as clinical resolution of symptoms or in time to resolution, time to relief or time to improvement. CONCLUSIONS: There are numerous observational studies showing good efficacy and safety and a long post-marketing experience although few randomized placebo-controlled trials.


Assuntos
Angioedema/tratamento farmacológico , Angioedema/genética , Proteínas Inativadoras do Complemento 1/uso terapêutico , Plasma , Angioedema/prevenção & controle , Canadá , Ensaios Clínicos como Assunto , Proteínas Inativadoras do Complemento 1/administração & dosagem , Relação Dose-Resposta a Droga , Humanos
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