Magnetoelectric coupling and magnetic correlations are shown to affect coupled order parameters, magnetisation M and electric polarisation P, in a multiferroic material. A ferromagnetic ferroelectric system is considered with two types of magnetoelectric coupling (anisotropic and isotropic) which couple P to different magnetic correlations. Using a mean field calculation for anisotropic coupling, it is shown that magnetic fluctuations h(Sz i )2i alter P even for temperatures above the magnetic Curie temperature. For isotropic coupling, a Green's function technique with a Random Phase Approximation is used to calculate how transverse correlations between neighbouring magnetic spins at sites i and j, such as hSx i Sx j i, affect P. The ferroelectric transition temperature and even the order of the transition is altered by the inclusion of magnetic correlations. Thermal hysteresis of the electric polarisation is also shown to exist in this model system. The methods presented can be extended to treat multiferroic systems with more complicated magnetic orderings. Another consequence of magnetoelectric coupling is the hybridisation of magnetic/electric excitations, known as "electromagnons." We show that for a ferroelectric canted antiferromagnet (with a weak ferromagnetic moment) with isotropic magnetoelectric coupling an electromagnon mode exists. When such a material, together with a ferromagnet, form a thin film heterostructure, dipolar coupling between the films allows for the electromagnon frequency to be tuned in the microwave regime. We use an effective medium method that takes into account electromagnetic boundary conditions to find the high frequency susceptibility of such composites and identify the type of resonant modes. However, the magnetoelectric coupling is usually weak through dipolar coupling. Magnetostrictive/piezoelectric composites have been shown to have a much larger strain-mediated magnetoelectric coupling so we detail how the effective medium method can be extended to treat such systems. It has been shown that in magnetostrictive/piezoelectrive composites, electric field can drive a magnetisation reorientation transition. Such electric field control of magnetisation is one of the chief aims in the study of multiferroic materials. However, the dynamics of such an electric field-driven magnetisation reorientation have not been studied as yet, either experimentally or theoretically. As a first step, a study of how nonlinear spin wave processes (or correlations) may alter the dynamics of magnetic field-driven magnetisation reorientation in thin magnetic films is presented here. A classical Hamiltonian formalism is developed to calculate the threshold magnetisation precession angle above which threeand four-wave decay of uniform precession may occur. The analytic results for fourwave scattering are used to qualitatively explain experimental results of the magnetisation coherency during reorientation in 15 nm Ni81Fe19 films. Possible extensions to multiferroic systems are discussed briefly.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2009|