No abstract

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.

A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is traversed by spin-polarized carriers at high direct-current densities. It is shown that nonlinear effects due to four-magnon interactions arising from dipolar and surface anisotropy energies limit the growth of the driven spin wave and produce shifts in the microwave frequency oscillations. The theory explains quantitatively recent experimental results in nanometric point contacts onto magnetic multilayers showing downward frequency shifts (redshifts) with increasing current, if the external field is on the film plane, and upward shifts (blueshifts), if the field is perpendicular to the film.

Ferromagnetic resonance has been used to study the room-temperature linewidth and frequency shift of the qϭ0 spin-wave mode in thin films of NiFe sputtered on Si͑100͒ substrates. The data on the variation of the linewidth and resonance field with film thickness are completely consistent with the extrinsic mechanism recently proposed by Arias and Mills based on momentum nonconserving two-magnon scattering off defects on the film surfaces.

The dynamics of the magnetization in a thin ferromagnetic film traversed by a spin-polarized direct current is studied. In such a system, spin waves ͑magnons͒ may be critically driven out of equilibrium by an effective spin-injection field that is proportional to the current density. A direct comparison between the predicted critical current and previous experimental results sheds light on the nature of the excited mode. Beyond the threshold, it is assumed that the spin waves are coupled through nonlinear interactions arising from dipolar and surface anisotropy energies. It is shown that the magnon-magnon interactions play two major roles in the dynamics: ͑i͒ They govern and put a limit to the growth in the population of the unstable mode from the thermal level, and ͑ii͒ directly contribute to the renormalization of the magnon energy, which manifests itself through a shift in the precession frequency of the magnetic moments with varying current intensity. Numerical results are presented in remarkable quantitative agreement with recent experiments in nanometric magnetic multilayers, where microwave oscillations generated by direct currents have been observed in the postthreshold regime.