Phase-Resolved Ferromagnetic Resonance Studies of Thin Film Ferromagnets

Precessional dynamics are exploited in the operation of high frequency magnetic devices such as magnetic disk drives, non reciprocal microwave devices and spin transfer oscillators. The trajectory of the precession and its damping are of crucial importance. This thesis presents the characterisation of a variety of magnetic thin film structures performed with a range of phase sensitive techniques. It is possible to obtain new insight by utilising the chemical and site specificity of X-ray Magnetic Circular Dichroism (XMCD) to isolate the precession in different chemical species or at distinct sites in the crystal structure of a chosen material. X-ray Ferromagnetic Resonance (XFMR) combines XMCD and Ferromagnetic Resonance (FMR) phenomena in a technique capable of measuring the FMR response of an alloy or multilayer with both chemical and site specificity. To complement the XFMR technique a low temperature Time-Resolved Magneto Optical Kerr Effect (TR-MOKE) setup has been developed. This allowed for the characterisation of samples at temperatures in the range 4 K to room temperature. A frequency swept Vector Network Analyser FMR (VNA-FMR) setup was developed to allow for a fast method for determining the resonance condition and damping of a range of ferromagnetic thin film samples. In addition a TR-X-ray Photoemission Electron Microscopy (TR-XPEEM) setup has been established which allows images to be obtained with magnetic contrast. The combination of the above techniques has lead to studies on rare earth capped spin valve free layers and the measurement of spin pumping in industrially relevant spin valves.EPSR

Observation of vortex dynamics in arrays of nanomagnets

Vortex dynamics within arrays of square ferromagnetic nanoelements have been studied by time-resolved scanningKerr microscopy (TRSKM),while x-ray photoemission electronmicroscopy has been used to investigate the equilibrium magnetic state of the arrays. An alternating field demagnetization process was found to initialize a distribution of equilibrium states within the individual elements of the array, including quasiuniform states and vortex states of different chirality and core polarization. Repeated initialization revealed some evidence of stochastic behavior during the formation of the equilibrium state. TRSKM with a spatial resolution of ∼300 nm was used to detect vortex gyration within arrays of square nanoelements of 250-nm lateral size. Two arrays were studied consisting of a 9 × 9 and 5 × 5 arrangement of nanoelements with 50- and 500-nm element edge-to-edge separation to encourage strong and negligible dipolar interactions, respectively. In the 5 × 5 element array, TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In the 9 × 9 array, strong magneto-optical response due to vortex gyration was observed across regions with length equal to either one or two elements. Micromagnetic simulations performed for 2 × 2 arrays of elements suggest that particular combinations of vortex chirality and polarization in neighboring elements are required to generate the observed magneto-optical contrast.Engineering and Physical Sciences Research Council (EPSRC

Phase-resolved x-ray ferromagnetic resonance measurements in fluorescence yield

Copyright © 2011 American Institute of PhysicsPhase-resolved x-ray ferromagnetic resonance (XFMR) has been measured in fluorescence yield, extending the application of XFMR to opaque samples on opaque substrates. Magnetization dynamics were excited in a Co50Fe50(0.7)/Ni90Fe10(5) bilayer by means of a continuous wave microwave excitation, while x-ray magnetic circular dichroism (XMCD) spectra were measured stroboscopically at different points in the precession cycle. By tuning the x-ray energy to the L-3 edges of Ni and Fe, the dependence of the real and imaginary components of the element specific magnetic susceptibility on the strength of an externally applied static bias field was determined. First results from measurements on a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1) sample confirm that enhanced damping results from the addition of the Dy cap