Surface and interface anisotropies measured using inductive magnetometry

Kimberly Kennewell

    Research output: ThesisDoctoral Thesis

    431 Downloads (Pure)

    Abstract

    In this thesis, an inductive ferromagnetic resonance (FMR) technique is developed to measure the magnetisation dynamics in thin films across a wide range of frequencies and fields. In particular, this project concentrates on measuring higher order exchange dominated modes to observe surface and interface effects in bilayer films. The experimental technique was first developed as a time domain technique, utilising a fast rise time (~50 ps) step pulse to disturb the equilibrium position of the magnetisation. The subsequent precessional damped decay was measured at different applied fields to observe the resonant modes. The data is Fourier transformed to extract a frequency dependent susceptiblity, and results are presented for the frequency and linewidth dependence of excitations of a permalloy film as a function of applied field. This technique is limited to a frequency range dictated by the rise time of the pulse. The technique was then extended so as to use a continuous wave perturbation, utilising a network analyser as both the excitation source and the measurement device. The scattered wave parameters of both the transmission and reflection from the sample were measured, and a magnetic susceptibility is extracted. This method has a frequency range which is dictated by the bandwidth of the network analyser and the microwave circuit. In this project, results are presented for frequencies up to 15 GHz. The signal to noise ratio was also found to be lower than the pulsed technique. Fundamental resonant mode studies are presented for a Fe/MnPd exchange bias bilayer film. Crystalline and exchange anisotropies are extracted from angular measurements, and the behaviour of the magnetisation is investigated during its reorientation to a hard axis direction. Information about the distribution of the local exchange field strength and direction is predicted. Fundamental mode studies are also presented for a Py/Co exchange spring bilayer film. Two modes are observed, approximating an optical and acoustical excitation. Film systems were also designed with suitable thicknesses to observe in the experimentally available frequency range non-uniform exchange dominated excitations through the thickness of the film. The broadband nature of the experiment allowed the frequency of the modes to be measured as a function of field. Results from a single permalloy layer showed two observable modes, the fundamental and the first exchange mode. Measurements were also taken of bilayer films where permalloy is coupled to cobalt. In this system the effect of the cobalt is seen to shift the single layer Py mode frequencies, as well as introduce new modes. The relative intensities of the modes also change with the addition of cobalt. Results are shown for a Pt/Co multilayer coupled to a permalloy layer through a Cu spacer of varying thickness. The observation of excitations through the thickness of the film motivated the development of a suitable theory. A system of integro-differential equations were derived which account for dipole and exchange coupling in the film as well as the field screening by the metal of the coplanar line. The conductivity of the sample and the finite wavevector excitation of the stripline are also included. Numerical solution of the equations results in a spectrum of acoustical, optical and higher-order modes. Fitting of the model to the experimental results allowed extraction of the film parameters including; the exchange constants in the film; the surface pinning from any surface layer anisotropy; as well as the interlayer exchange coupling across the interface.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2008

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