Search Results - (Author, Cooperation:A. Azevedo)

2018-01-23

BMJ Publishing Group

0021-9746

1472-4146

Medicine

2018-10-24

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2014-06-07

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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Animals ; Brazil ; Cattle ; Conservation of Natural Resources/*trends ; Humans ; Meat/*supply & distribution ; *Public Policy ; Soybeans/*supply & distribution

2018-01-05

BMJ Publishing

2044-6055

Medicine

Open access, Cardiovascular medicine

2018-01-20

Institute of Physics Publishing (IOP)

1748-0221

Physics

2018-02-24

BMJ Publishing

2044-6055

Medicine

Open access, Cardiovascular medicine

2018-07-27

American Society of Hematology (ASH)

0006-4971

1528-0020

Biology

Medicine

Myeloid Neoplasia, Clinical Trials and Observations

2018-08-25

American Physical Society (APS)

0556-2821

1089-4918

Physics

Electroweak interactions

1089-7550

AIP Digital Archive

Physics

Most theoretical models used to explain self-oscillations and chaotic phenomena observed in microwave pumped spin-wave instabilities are based on the assumption that the dynamics is governed by a few interacting modes. This assumption is somewhat disturbing because it is known that in principle an extremely large number of modes can be excited when the microwave field h exceeds the Suhl instability threshold hc. In this paper we describe pulsed microwave (8.2 GHz) spin-wave pumping experiments in spheres and films of yttrium–iron–garnet which clearly demonstrate that, with increasing power, one mode is excited at h=hc, a second mode appears at h(approximately-greater-than)hc, and subsequently the self-oscillation sets in. The observations agree very well with computer results for the transient behavior of the spin-wave equations, showing that the two-mode assumption for the onset of self-oscillation is correct. We also present data for films which add evidence to the sample-size dependence of the self-oscillation frequency and support recent improvements proposed for the two-mode model.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We present an experimental and theoretical study of bifurcations leading to self-oscillations in spin-wave instabilities pumped by microwave fields. Experiments done with bulk samples of yttrium iron garnet pumped by X-band microwave radiation reveal a variety of bifurcations and routes to chaos, depending on the experimental parameters. Generally, the frequency of the self-oscillation shows a definite dependence on sample size not predicted by the existing models. On the theoretical side we show that the usual equations describing two interacting spin-wave modes predict various types of bifurcations, depending on the set of parameters. In particular, we investigate Hopf bifurcations and homoclinic phenomena. We also indicate how the spin-wave equations can be modified to predict a sample-size dependence for the self-oscillation frequency.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The onset of spin-wave auto-oscillations has been studied in spheres of yttrium iron garnet pumped by a microwave field parallel or perpendicular to the dc field. Data were taken for the auto-oscillation threshold field, amplitude, and frequency as a function of sample dimension, shape, and the spin-wave relaxation rate and wave vector. The behavior near the onset confirms the theoretical prediction that the auto-oscillation results from a Hopf bifurcation. However, data for the auto-oscillation frequency and threshold field are in clear disagreement with predictions. These results indicate that although the area of nonlinear spin-wave phenomena has been under investigation for over 30 years, a good model is still needed for the auto-oscillations.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A description is given of pulsed high-power ferromagnetic resonance experiments in a YIG sphere at room temperature. Beyond the Suhl instability threshold, evidences were found for the alternating periodic-chaotic sequence. The chaotic spin-wave self-oscillations were characterized through return maps obtained from the local maxima in the observed transient-free time series. As a result, a simple one-dimensional mapping scheme that exhibits the basic features of the observed transition is presented. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

High-power ferromagnetic resonance experiments in a 1-mm-diam yttrium iron garnet sphere, driven at 8.9 GHz at room temperature, reveal evidence for a universal scenario not yet observed in this system, namely, the periodic–chaotic sequence. Well above the first-order Suhl instability threshold (P/PC∼6 dB), low-frequency (f∼200 kHz) auto-oscillations consisting of large-amplitude peaks followed by n small undulations (mixed-mode oscillations) have been observed. Both periodic and aperiodic trajectories were detected with n=1, 2, 3, and 4, which are reminiscent of the so-called homoclinic chaos. A discussion is made on the basis of a standard two-mode model. © 1999 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

One of the puzzles of the recent investigations on the exchange anisotropy in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is the fact that different techniques yield different values for the exchange field (HE) between the layers. We report an investigation on sputtered NiFe/NiO carried out with three different techniques, namely, magneto-optical Kerr effect magnetometry (MOKE), Brillouin light scattering (BLS), and ferromagnetic resonance (FMR). In an attempt to reconcile the measurements obtained with the various techniques, we interpret the data with a model that includes the formation of a planar domain wall in the AF layer, giving rise to a torque on the FM moment represented by an effective domain wall field (HW). We find out that while the same pair of values of HE and HW provide equally good fits to the reversible FMR and BLS measurements, different pairs are necessary to fit the irreversible magnetometry data. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

In this work we report on the magnetic and low-field ferromagnetic resonance properties of iron garnet films of general composition Y3−x−yPrxLuyFe5O12 for 0≤x≤0.3 and 0≤y≤0.7. The films were grown onto [111] gadolinium gallium garnet substrates by liquid phase epitaxy. Both growth-induced and cubic anisotropies were investigated by means of torque magnetometry and ferromagnetic resonance at room temperature. The growth-induced anisotropy constant Ku and the first order magnetocrystalline cubic constant K1 exhibit a linear dependence with the praseodymium content. The addition of Lu with Pr enhances both Ku and K1 dependence on the doping, yielding ΔKu/x∼−2.4×105 erg/cm3 and ΔK1/x∼−1.1×105 erg/cm3. The addition of praseodymium in the garnet films significantly improves the soft magnetic properties. In-plane coercivity is about 0.1 Oe and there is a negligible anisotropy in the plane of the film. The effect of the doping by praseodymium and lutetium on the saturation magnetization is insignificant at room temperature.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Microwave pumped spin-wave instabilities in YIG spheres were one of the first experimental systems used to demonstrate that chaos can be suppressed by small periodic modulations in an accessible system parameter. Here we show that the equations describing two interacting spin-wave modes account satisfactorily for the experimental results, as long as the field modulation is appropriately introduced in the model. The finite detuning parameters expressing the boundary conditions of the sample provide a natural way for introducing the field modulation. Thus the present results constitute additional evidence of the validity of the two-mode model with momentum-nonconserving driving Hamiltonian used to explain the sample size dependence of the self-oscillations.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have used magnetoresistance, magneto-optical Kerr effect, ferromagnetic resonance (FMR), and Brillouin light scattering to investigate interlayer exchange coupling in Fe(40 Å)/Cr(t)/Fe(40 Å) trilayers. At the right-hand side of the first antiferromagnetic peak (13 Å〈t〈16 Å), a common feature in both transport and magnetic measurements is the presence of sudden discontinuous jumps as the magnetic field is varied. By treating both transport and magnetic data on equal footing, we demonstrate that all measurements can be described by model calculations which elegantly take into account the same set of phenomenological parameters, characterizing the magnetic anisotropy, Zeeman, and bilinear and biquadratic exchange energies. Temperature-dependent FMR measurements reveal intriguing similarities between the bilinear and biquadratic coupling behaviors. © 1997 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The spin-wave spectra of Ni81Fe19(t)/Cu(25 Å)/Ni81Fe19(t) with t=200 and 300 Å are investigated by means of Brillouin light scattering and ferromagnetic resonance. Samples grown on three different substrates, namely, MgO(100), MgO(110), and sapphire(0001), exhibit clear-cut volume and surface modes. Sharp linewidths are signatures of the excellent quality of the samples. Angle-dependent measurements reveal that, while the samples grown on MgO(100) and sapphire(0001) are nearly isotropic, a surprisingly large in-plane uniaxial anisotropy is a characteristic feature of the samples grown on MgO(110). Quantitative agreement between theoretical and experimental results is reported for a realistic set of the samples' parameters. © 1997 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Results are reported of a detailed study of static and dynamic responses in symmetric systems consisting of two ferromagnetic films separated by a nonferromagnetic spacer layer. A comparison is made with experimental results for two systems grown by sputter deposition in an UHV chamber, namely, NiFe/Cu/NiFe and Fe/Cr/Fe. First, we present model calculations where the coupling between the magnetic films through magnetic dipolar, bilinear, and biquadratic exchange interactions are fully taken into account, together with surface, in-plane uniaxial, and cubic anisotropies. An analytical expression is given that can readily be used to consistently interpret magnetoresistance, magneto-optical Kerr effect, ferromagnetic resonance, and Brillouin light scattering (BLS) data in such trilayers. Application of the results to BLS data in Ni81Fe19(d)/Cu(25 Å)Ni81Fe19(d), with d=200 and 300 Å, shows that it is essential to treat the dipolar interaction adequately in moderately thick systems. The results are also applied to interpret very interesting data in Fe(40 Å)/Cr(s)/Fe(40 Å), with 5 Å〈s〈35 Å, investigated by the four techniques mentioned above, at room temperature. It is shown that consistent values for all magnetic parameters can be extracted from the data with a theory that treats both static and dynamic responses on equal footing. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The bilinear (J1) and biquadratic (J2) exchange coupling constants were measured in sputtered trilayers of (100) Fe(40 Å)/Cr(s)/Fe(40 Å) for several Cr spacer layer thicknesses in the range s=8–35 Å and as a function of temperature T, using magneto-optical Kerr effect magnetometry, Brillouin light scattering, and ferromagnetic resonance. In the samples in the range s=8–13 Å, corresponding to the first antiferromagnetic peak of J1, J2 follows J1 with a room temperature ratio J2/J1≅0.1, while in the range 25–35 Å, corresponding to the second antiferromagnetic peak, J2 also follows J1 but with a much larger ratio J2/J1≅1. This result, as well as the temperature dependence of J2 in all samples but the one with s=15 Å, cannot be explained by any of the intrinsic or extrinsic mechanisms that have been proposed for the origin of the biquadratic exchange coupling in Fe/Cr/Fe. © 1999 American Institute of Physics.

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