Antonio Azevedo

Amplification of spin waves in yttrium iron garnet films through the spin Hall effect

We demonstrate that spin waves propagating in a film of yttrium iron garnet (YIG) can be amplified by a dc current in an adjacent Pt layer by means of the spin Hall effect. The experiments are done at room temperature using pulsed currents to avoid sample heating. Amplification occurs only for surface like modes propagating in a direction perpendicular to the applied in-plane field. The results are interpreted with a model for spin-wave propagation in a YIG film with magnetic losses and subject to a spin-transfer torque due to spin currents created by the spin Hall effect in the Pt layer.

Ferromagnetic resonance linewidth and anisotropy dispersions in thin Fe films

In-plane ferromagnetic resonance (FMR) has been used to study the room-temperature linewidth ΔH of single crystal Fe films grown by dc magnetron sputtering onto MgO(100) substrates. Several samples were grown with the film thickness in the range 70 A<tFe<250 A. The measurements were carried out in the frequency range from 7.0 to 12.3 GHz. A phenomenological model for the FMR linewidth was developed that includes simultaneous effects due to intrinsic damping and angular dispersions of the cubic and uniaxial axes of anisotropy. These angular dispersions are found to be responsible for the relatively larger linewidths observed as a function of the in-plane field direction with fixed frequency, and as a function of frequency for the easy and hard directions as well. The behavior of the linewidth with the film thickness can be described by a sum of a constant volume term plus a term proportional to 1/tFe, representing the relaxation due to the misfit dislocations.

Unidirectional anisotropy in the spin pumping voltage in yttrium iron garnet/platinum bilayers

Detailed measurements of the dc voltage generated in a thin Pt layer deposited on films of yttrium iron garnet (YIG) have been carried out to study the spin pumping effect produced by magnetostatic (MS) modes excited by a microwave field. In relatively thick YIG films the modes are far apart so that one can identify clearly the spin pumping voltage in VSP produced by each MS mode. We have discovered that when the sputter deposition of the thin Pt layer is made on the YIG film magnetized by a static magnetic field, VSP exhibits a strong unidirectional anisotropy.

Magnetic properties of Fe films epitaxially grown on Cr/GaAs(100) by dc magnetron sputtering

We report on the growth of single-crystal Fe films by magnetron sputtering onto GaAs substrates. In order to establish the epitaxial orientation with the substrate a 100 A Cr buffer layer was rf sputtered. The crystalline and magnetic properties were studied by x-ray diffraction, ferromagnetic resonance, and Kerr effect magnetometry techniques. The θ-2θ x-ray measurements show that only the Fe (200) peak is present, and the rocking curve shows a full width half maximum of 2°. Ferromagnetic resonance lines exhibit a line width of about 30 Oe, and the in-plane resonance field as a function of the azimuth angle presents a fourfold symmetry with no induced anisotropy. The in-plane hard- and easy-axis magneto-optical hysteresis loop traces are consistent with the ferromagnetic resonance results.

Spin‐wave auto‐oscillations still in need of a good model

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.

Possible Interplay Between Intrinsic and Extrinsic Ferromagnetic Damping Mechanisms

Here we present a ferromagnetic resonance investigation in which we measure the resonance field and linewidth in a series of films that were fabricated by oblique sputtering deposition. From the angular dependence of the resonance field we were able to extract the uniaxial anisotropy field. We show that microstructures that grow in the inclined films, induce very strong uniaxial anisotropy field values that vary from ~0 Oe to around 300 Oe, for deposition angles varying from 0° to 70°. From the angular dependence of the linewidth we were also able to investigate the spin relaxation mechanisms existent in these films. From the measurements it is possible to separate the contributions from intrinsic (Gilbert parameter) and extrinsic (two-magnon) relaxation processes. For films grown at deposition angles between 0° and 40° the Gilbert parameter ¿ value keeps an almost fixed value of 0.011 while the two-magnon contribution is negligible. For films grown at the oblique angle of 40° the Gilbert parameter ¿ abruptly decreases to 0.008 and the two-magnons parameter ¿¿ suddenly jumps to almost 1.0 GHz. For films grown at angles bigger than 40°, ¿ monotonically increases and the two-magnon parameter contribution monotonically decreases.

Substrate dependent ultrafast dynamics in thin NiFe films

We have studied the ultrafast electronic response of thin NiFe films by femtosecond transient reflectivity measurements. The experiments were performed on films with varying thicknesses, substrates, and pump fluences. It has been observed that for high excitation densities the electron cooling time depends strongly on the nature of the underlying substrate and we attribute our results to transport of hot carriers out of the excited region. In particular, we have observed that for NiFe over NiO, carrier transport should be less important than for NiFe over Si.

Homogeneous growth of antidot structures electrodeposited on Si by nanosphere lithography

The characteristics of cobalt antidot structures, electrodeposited on Si by nanosphere lithography, is investigated by analysis of current transients and optical and atomic force microscopy. Polystyrene colloidal masks of 1 ML (monolayer) or a maximum of 2, with sphere diameters ranging between 165 and 600nm were fabricated by spin coating. For masks partially covered with bilayer regions, it is shown that the volume of deposited material in bilayer areas corresponds to only 5% of the cobalt deposited in the monolayer areas. This drastic reduction in the deposition rate affects the quality of the deposit. Therefore, the use of colloidal masks of homogeneous thickness is necessary to guarantee the electrodeposition of nanostructured films of controlled thickness. It will also be shown that the use of high quality colloidal masks yields a reproducible electrodeposition process, enabling the use of the current transient as a reliable tool for assessment of the deposition process.

Magnetic properties of Nd/Fe double layers grown on Si(111) by electron beam evaporation

In-plane ferromagnetic resonance (FMR) has been used to study the room-temperature magnetic and crystalline properties of Nd/Fe bilayers. Several samples were grown by electron beam evaporation onto Si(111) substrates, for various Fe and Nd thicknesses. The symmetry of the resonance field as a function of the azimuthal angle departs from the sixfold one expected for a (111)Fe film. The FMR data were analyzed in the framework of a phenomenological model, which takes into account first- and second-order magnetocrystalline, uniaxial and perpendicular anisotropies. Our analysis reveals that the deviation from the sixfold symmetry is due to a combined effect of an induced uniaxial in-plane anisotropy with a small misorientation of the substrate surface from the (111) plane. We suggest that the in-plane uniaxial anisotropy may originate from an arrangement of (111) terraces separated by well oriented atomic steps formed during the small miscut. The effect of the Nd overlayer is to induce a perpendicular anisotr...

Spin‐wave self‐oscillations: Experimental verification of the two‐mode origin (invited)

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≳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 supp...