Magnetisation dynamics in magnetostrictive nanostructures

Spin torque oscillator devices have presented themselves as an energy efficient method of generating microwave frequencies in recent years. These are devices which rely on giant magnetoresistance to create a device resistance which oscillates at microwave frequencies due to the microwave oscillation of a magnetically free layer in the device. A spin torque oscillator can be improved by using a vortex core oscillator as the magnetically free layer offering a narrower linewidth and greater synchronisation possibilities, at the expense of a lower power output. The desire to tune the frequency of oscillation of these devices has been the focus of a great deal of research in recent years and one promising avenue of investigation is to alter the frequency of oscillation by inducing a strain anisotropy in such a device by the use of a piezoelectric transducer. In order for the induced anisotropy to be large a material must be used which exhibits a strong magnetostrictive effect such as Fe1−xGax, a material which exhibits strong magnetostrictive properties without the need for rare earth elements. This thesis describes investigations into the magnetisation dynamics of nanostructures fabricated from magnetostrictive thin films of Fe1−xGax under conditions of in-plane uniaxial anisotropy induced by an applied stress. Chapter 4 describes investigations into the effects of altering the thickness of sputter grown Fe1-xGax films on the crystalline anisotropy of the films. It was found that the intrinsic magnetocrystalline uniaxial anisotropy within the films increased with film thickness. The cubic anisotropy was shown to be roughly constant with respect to film thickness except when the film was 20nm thick when the cubic anisotropy of the sample was anomalously high. Investigations of the magnetostrictive properties of these materials revealed sputter grown thin films to exhibit similar magnetostrictive properties as bulk material and thin films grown by molecular beam epitaxy. X-ray analysis performed by Dr. P. Wadley and Prof.V. Holy failed to explain the relationships between film thickness and magnetocrystaline anisotropy observed in the samples, but suggested that the average grain size increases as the thickness of the film increases. Chapter 5 describes the results of time resolved XMCD PEEM measurements performed at the Diamond Light Source synchrotron facility performed in order to investigate the magnetisation dynamics within a series of Fe1−xGax squares. It was found that these squares demonstrated no significant response to an applied stress, probably due to strong shape anisotropy. Preliminary work to investigate Ni squares revealed that they do exhibit a strong response to stress. The dynamic response of the Ni squares was notsuccessfully measured however. Chapter 6 presents results of micromagnetic simulations performed to predict the effects of strain-induced anisotropy on magnetic square nanostructures fabricated from Fe1−xGax. Time resolved simulations demonstrated the ability of a strain induced anisotropy to modify the frequency of oscillation of the vortex core oscillations and the confined spin wave modes as well as the amplitude of the magnetic field pulse required to induce switching of the polarisation of the vortex core. The effects of size and uniaxial anisotropy on the spin wave modes within square devices was studied and an s shaped spin wave mode was shown to form in the presence of a uniaxial anisotropy.