Engineering of demagnetisation fields in exchange biased antidots studied using ferromagnetic resonance and Lorentz microscopy

Trindade Gonçalves, Francisco José (2015) Engineering of demagnetisation fields in exchange biased antidots studied using ferromagnetic resonance and Lorentz microscopy. PhD thesis, University of Glasgow.

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Nanostructured ferromagnetic materials have gained considerable attention recently for use in novel devices in the areas of data storage, microwave signal processing and propagation of electromagnetic waves. Structural modifications allow a control over the internal magnetic properties of the system down to the nanoscale. Lateral confinement gives rise to a distribution of demagnetisation fields in the plane of the thin film which can create interesting effects on static and dynamic magnetic properties.
In the context of this dissertation, nanostructuring is used as a means to engineer the static and dynamic response of magnetic systems. In particular, periodic arrays of holes embedded in continuous films are studied. With patterning one observes the emergence of an anisotropic field dependence as well as the emergence of non-uniform spin precession modes due to the dipole distribution at the film interfaces. These properties were probed primarily using a technique called broadband ferromagnetic resonance, whereby a microwave field drives the local spin dynamics of the magnetic system.
A way to further modify the magnetic properties induced due to the symmetry of the periodic structuring is by introducing an unidirectional field contribution which is characteristic of an exchange biased system. It was found that the magnetic pole distribution in antidots is particularly sensitive to the effects of exchange bias, giving rise to an asymmetric frequency response with respect to the applied field. The asymmetric microwave properties of an exchange biased FeMn/NiFe system with antidot structuring, obtained using electron beam lithography, are investigated. The ferromagnetic resonance (FMR) spectra exhibited several magnetostatic spin wave modes with 8-fold and 4-fold anisotropy components. Moreover, it was observed that large frequency asymmetries are obtained along the directions 10 degrees off the main antidot lattice axes, as result of the competing anisotropies.
Brillouin light scattering measurements showed the presence of a magnonic band gap as expected for this type of structure. To interpret these observations, the spin precessional modes obtained experimentally are correlated with localised mode profiles obtained by micromagnetic simulations.
This hybrid structure is a good candidate for applications such as selective microwave filtering and for use in multi-state magnetic logic.
The prospects of using patterned exchange biased systems to engineer microwave properties is greatly increased if one goes beyond the 2-dimensional perspective. In multilayered structures, one can modify the magnetic properties layer-by-layer to achieve the desired response. This concept is demonstrated here by using a three dimensional structure in which an exchange biased and a free magnetic layer are stacked upon one another and patterned with an antidot configuration. The exchange bias acts as a pinning field for one layer, while the free layer reverses, promoting a zero net moment state. Interlayer dipole interactions are found to result in the partial cancellation of the microwave response. Micromagnetic simulations support the existence of a diminished microwave response which was confirmed by FMR measurements of an equivalent structure. The net moment cancellation, indicative of the antiparallel alignment, was observed on a Lorentz differential phase contrast scanning transmission electron microscope equipped with an FMR probe, which was designed and built for the purpose. This unique tool allows access to complex microwave response while the ground state of a nanostructured film is imaged via Lorentz microscopy. From the magnetostatic viewpoint, our results differ greatly from previous studies in a way that this sample shows distinct magnetic history and the near remanence states exhibit unique magnetic textures: magnetic vortices.
The applicability of the TEM in-situ FMR probe was extended to the mapping of radio frequency electromagnetic (EM) fields using low angle diffraction (LAD) imaging. The electron beam, propagating in a sample free environment, experiences the field distribution generated by the microstrip waveguide, which alters the electron amplitude and phase, as described by the Aharonov-Bohm effect, and results in different intensity profiles at the detector. As the microwave frequencies were varied, the different polarisation states are imaged directly. Microwave simulations allowed the EM field distribution to be calculated, which was used to reproduce the LAD results. A knowledge of the near field distribution in antennas is often a challenging task so this technique opens up new opportunities for planar devices operating in high vacuum conditions.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Nanomagnetism, ferromagnetic resonance, Lorentz microscopy, electron beam lithography, antidots, spin waves, exchange bias, anisotropy, spin wave dispersion
Subjects: Q Science > Q Science (General)
Q Science > QC Physics
T Technology > T Technology (General)
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: Stamps, Professor Robert
Date of Award: 2015
Depositing User: Mr. Francisco Jose Trindade Goncalves
Unique ID: glathesis:2015-6875
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 16 Nov 2015 15:34
Last Modified: 11 Dec 2015 10:09

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