Daniel Mercier

Forbidden frequency gaps in magnonic spectra of ferromagnetic layered composites

Abstract Spin wave spectra in layered composite materials are computed by means of a transfer matrix method with evidence for existence of numerous frequency gaps. Essential properties of these spectra are successfully explained within the framework of approximate models of independent films or an effective medium, according to the strength (strong and weak, respectively) of the magnetization and exchange contrasts; this includes an evidence of the existence of resonance states, for which both materials are simultaneously excited.

Theoretical study of spin wave resonance filling fraction effect in composite ferromagnetic [A|B|A] trilayer

Abstract Spin wave resonance (SWR) spectra in composite ferromagnetic trilayer [ A | B | A ] are calculated by means of a transfer matrix method. Both the resonance spectra and the respective spin wave mode profiles are found to be sensitive to exchange and magnetization contrasts between the constituents A and B ; thorough calculations are performed for two specific trilayer systems, [iron|nickel|iron] and [permalloy|nickel|permalloy] systems. We found that some remote lines of the trilayer SWR spectra may have higher intensity than the first ones. This high intensity of remote resonance lines is explained as an effect originating in the spin wave mode “localization” in the outer sublayers A ; this is why the intensity of these “amplified” resonance lines are found to vary with the filling fraction (the width ratio of sublayers A and B ). Depending on the surface spin pinning conditions this filling fraction effect may involve (for surface spins unpinned) resonance intensity changes alone or (for surface spins pinned) with an additional shift of the corresponding resonance lines. This property offers a possibility of a simple qualitative SWR determination of the surface pinning conditions in ferromagnetic trilayers.

Roughness effects on spin waves

Abstract A simple modelling of surface roughness in magnetic thin films shows that roughness tends to unpin spin waves at external surfaces. This unpinning effect increases with the roughness height as long as this height remains small. In spin wave resonance, the spin wave intensity spectrum is shown to be quite sensitive to roughness, and new surface spin waves appear because of this roughness.

Spin waves in multilayers: Geometric effects of antiferromagnetic coupling

Abstract Ferromagnetic and antiferromagnetic couplings through the non-magnetic multilayers in magnetic-non-magnetic multilayers act as effective local couplings for spin wave resonance in saturating applied fields. The calculated spin wave resonance modes have singularities in the limit of weak positive or negative couplings. These effects are due to an effective pinning of spin waves at weakly coupled interfaces. Moreover the spin wave propagation is shown to strongly depend upon the relative thickness of the successive magnetic films. When these thicknesses do not define simple rational ratios, all spin waves are predicted to badly propagate for weak ferromagnetic or antiferromagnetic couplings.

Magnetic and theoretical studies of NiFe layers coupled through a chromium interlayer

Abstract MBE prepared epitaxial Permalloy/Cr/Permalloy trilayers have been investigated by FMR and BH hysteresigraph. The film growth was monitored by Reflection High Energy Electron Diffraction (RHEED) and Auger Electron Spectroscopy (AES) which indicate Frank-Van der Merve growth of the Cr. The dependence of resonant field position and coercivity on the thickness of the Cr interfilm indicates the existence of an oscillatory long range exchange coupling of the Permalloy films. A transfer matrix theory previously used to analyse ferromagnetic coupling between magnetic films has been extended to include antiferromagnetic coupling and is used to interpret the FMR results.

Spin waves in multilayers: effects of complex coupling

Abstract Long ranged couplings in magnetic-non-magnetic multilayers can be accounted for as complex couplings for spin wave resonance in saturating applied fields. As a result large linewidths are found for some modes and the transition from coupling to decoupling of the magnetic films appears to depend upon the spin wave wavelength.

Spin wave propagation in magnetic sandwiches

Abstract A calculation of the spin wave propagation through a non-magnetic interlayer is given as a function of its thickness and of a parameter which describes either the roughness of the interface, or the superexchange through this layer or both of them.

SOME PROPERTIES OF NORMAL MODES IN A FINITE LINEAR CHAIN WITH NEXT-NEAREST-NEIGHBOUR INTERACTIONS

Abstract We analyze the energies and amplitude profiles of normal modes of a finite monatomic chain regarding their dependence on the competition between next-nearest- and nearest-neighbour interactions.

Roughness effects on spin wave resonance in thin films

Two models of surface roughness are introduced in a transfer matrix approach of spin wave resonance. A one-layer surface roughness is shown to lead in the case of a weak antiferromagnetic coupling to surface modes while an extended roughness leads to an effective unpinning with nonlinear contributions. Applications to multilayers are deduced.

Numerical investigations on the interface mode in spin-wave resonance

Abstract The numerical approach of the transfer matrix is used to investigate the effects of the interlayer exchange interaction between magnetic layers separated by a nonmagnetic interlayer, on the spin-wave spectra. The results show that the interface mode is only observed under the conditions of antiferromagnetic coupling and asymmetric pinning conditions on the external surfaces.