Over the past few decades, ferromagnetic spinwave resonance inmagnetic thin films has been used as a tool for studying the propertiesof magnetic materials. A full understanding of the boundary conditionsat the surface of the magnetic material is extremely important. Suchan understanding has been the general objective of this thesis. Theapproach has been to investigate various hypotheses of the surface conditionand to compare the results of these models with experimentaldata. The conclusion is that the boundary conditions are largely dueto thin surface regions with magnetic properties different from the bulk.In the calculations these regions were usually approximated by uniformsurface layers; the spins were otherwise unconstrained except by thesame mechanisms that exist in the bulk (i.e., no special "pinning" atthe surface atomic layer is assumed). The variation of the ferromagneticspinwave resonance spectra in YIG films with frequency, temperature,annealing, and orientation of applied field provided anexcellent experimental basis for the study.
This thesis can be divided into two parts. The first part isferromagnetic resonance theory; the second part is the comparison ofcalculated with experimental data in YIG films. Both are essentialin understanding the conclusion that surface regions with propertiesdifferent from the bulk are responsible for the resonance phenomenaassociated with boundary conditions.
The theoretical calculations have been made by finding the wavevectors characteristic of the magnetic fields inside the magneticmedium, and then combining the fields associated with these wavevectors in superposition to match the specified boundary conditions.In addition to magnetic boundary conditions required for the surfacelayer model, two phenomenological magnetic boundary conditions arediscussed in detail. The wave vectors are easily found by combiningthe Landau-Lifshitz equations with Maxwell's equations. Modepositions are most easily predicted from the magnetic wave vectorsobtained by neglecting damping, conductivity, and the displacementcurrent. For an insulator where the driving field is nearly uniformthroughout the sample, these approximations permit a simple yet accuratecalculation of the mode intensities. For metal films thiscalculation may be inaccurate but the mode positions are still accuratelydescribed. The techniques necessary for calculating the powerabsorbed by the film under a specific excitation including the effectsof conductivity, displacement current and damping are also presented.
In the second part of the thesis the properties of magneticgarnet materials are summarized and the properties believed associatedwith the two surface regions of a YIG film are presented. Finally, theexperimental data and calculated data for the surface layer model andother proposed models are compared. The conclusion of this study isthat the remarkable variety of spinwave spectra that arises fromvarious preparation techniques and subsequent treatments can be explainedby surface regions with magnetic properties different from thebulk.
