S. M. Rezende

A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft‐ferromagnetic alloys

Giant magnetoimpedance (GMI) measured in ribbons of the soft ferromagnet Co75−xFexSi15B10 annealed in the presence of a transverse magnetic field exhibits peaks in its field dependence. The GMI is strongly dependent on the magnitude of the longitudinal field and on the frequency of the applied current. We present a theoretical model which explains the existence of the peaks and its frequency dependence. The model is based on the skin depth effect and on the domain‐wall motion due to the magnetic field and the ac current.

dc effect in ferromagnetic resonance: Evidence of the spin-pumping effect?

Direct current voltage appears across and in plane of a ferromagnetic multilayer experiencing ferromagnetic resonance. We have investigated the dc voltage simultaneously generated with the excitation of the uniform mode of magnetization precession in ferromagnetic∕normal-metal∕ferromagnetic trilayers with different spacer-layer materials. The generated voltage strongly depends on the chemical nature and the thickness of the normal-metal spacer as well as on the microwave incident power. This dc voltage might be correlated with the spin-pumping effect recently predicted.

Studies of coupled metallic magnetic thin-film trilayers

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 A)Ni81Fe19(d), with d=200 and 300 A, shows that it is essential to treat the dipolar interaction adequately in moderately thick systems. The results are also applied...

Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe4.6Si15B10

ac magnetoresistance (MR) measurements performed in thin ribbons of the amorphous soft ferromagnet Co70.4Fe4.6Si15B10 are reported with a longitudinal dc applied magnetic field H varying from 0 to ±15 Oe. The dependence of the MR with the frequency f of the measuring ac current was investigated for 0.4≤f≤100 kHz. No significant frequency dependence in the resistivity ρ for H=0 and H=±15 Oe was found. For intermediate values of H, ρ(H) presents a peak in H which increases linearly in frequency. The MR peak in H at room temperature varies from typical values of 0.13% in the low‐frequency range to a giant value of 27% at 100 kHz. Two possible sources for the frequency dependence of the magnetoresistance in amorphous alloys are discussed.

Spin-wave instabilities, auto-oscillations, and chaos in yttrium-iron-garnet

A review is presented of recent experimental results in yttrium-iron-garnet subject to three different spin-wave pumping mechanisms: parallel pumping, subsidiary resonance (first-order Suhl process), and premature saturation of the main resonance (second-order Suhl process). A theoretical model derived from first principles and leading to coupled nonlinear spin-wave equations, is used to interpret the observed spin-wave instabilities, auto-oscillations, and chaotic dynamics. It is shown that a two-mode model explains the main features of the experimental observations, both in parallel and perpendicular pumping. Improvements needed in the model are indicated. >

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.

Measurements of exchange anisotropy in NiFe/NiO films with different techniques

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.

Evidence for a spin‐glass behavior in the diluted antiferromagnet FexZn1−xF2

We report dc susceptibility ( χ) measurements in the diluted antiferromagnet Fex Zn1−x F2 with concentrations x=0.10, 0.25, 0.31, and 0.46. The field and temperature dependencies of χ in the x=0.10 and 0.25 samples reveal that a spin‐glass (SG) phase appears at low temperatures. For x=0.31, which is slightly above the percolation limit (xp=0.24), the SG phase is also present at low temperatures, but there is a clear indication of long‐range antiferromagnetic (AF) ordering at intermediate temperatures. Finally, in the sample with x=0.46, AF ordering with random‐field effects determines the characteristic behavior. These studies will help in the understanding of the crossover from random field to SG behavior in dilute antiferromagnets.

Spin Damping in Ultrathin Magnetic Films

This chapter reviews the origin of the damping of spin motions in ultra-thin ferromagnetic films and multilayer structures, with focus on the linear response regime probed by ferromagnetic resonance or Brillouin light scattering. We begin with a description of the spin response provided by the Landau-Lifshitz equation, which ascribes damping to dissipative processes of intrinsic origin. It is noted that the form of the damping term should be modified in anisotropic materials, and explicit expressions are provided for the form of a generalized damping term in bulk matter. We then turn to an extrinsic damping mechanism, the two-magnon process, which, recent experiments illustrate, plays a major role in spin damping in ultrathin films and multilayer structures. The history of this mechanism in ferromagnetic resonance studies is reviewed, the physical reasons for it to be active in ultrathin ferromagnetic films are discussed, and we the review recent experimental studies that have verified central predictions of the theory.