Time-Resolved Magneto-Optical Investigations of Picosecond Magnetisation Dynamics in Arrays of Non-Ellipsoidal Ferromagnetic Nano-Elements

In this thesis the results of magneto-optical experiments will be presented. The experiments were performed on micro-arrays of square nanomagnets in order to characterise the static and time-dependent behaviour of the nanomagnets. The static behaviour was investigated in vector-resolved scanning Kerr microscopy experiments, while the time-dependent behaviour was investigated in time-resolved scanning Kerr microscopy experiments. In the latter so-called pump-probe experiments, magnetisation dynamics were induced by exciting the sample magnetisation with a pulsed magnetic field (pump). The magnetisation dynamics were then detected using the magnetooptical polar Kerr effect (probe). The longitudinal Kerr effect was utilised in the vectorresolved scanning Kerr microscope in order to measure the in-plane components of the static magnetisation. The experimental set-up and methodology of the vector- and timeresolved scanning Kerr microscopy experiments will be discussed in detail, in particular, the detection technique that allows three components of the vector magnetisation to be measured simultaneously. Since the spatial resolution of the magneto-optical probe was insufficient to resolve the spatial character of the magnetisation dynamics within individual nanomagnets, micromagnetic simulations were used to gain insight into the character of the excited modes. Extensive testing of different micromagnetic models was carried out in order to investigate the effect of the different models on the simulated dynamics. The results of measurements carried out on the arrays of square nanomagnets revealed that the static and time-dependent behaviour of the magnetisation became more complicated as the size of the nanomagnets was reduced. In particular, similar hysteresis loops were acquired when the elements were magnetised along the uniaxial anisotropy easy and hard axes, while fast Fourier transform spectra of time-resolved signals revealed that the character of the magnetisation dynamics changed significantly as the element size and/or applied magnetic field were reduced. Interpretation of the experimental results using micromagnetic simulations revealed that the elements had a non-uniform single domain ground state magnetisation. When the field was applied along either edge of the square elements and reversed, the magnetisation was found to switch via a series of metastable non-uniform single domain states. Furthermore, the increasing non-uniformity of the single domain ground state as the element size and/or applied field were reduced lead to significant changes in the mode character excited within the elements. Comparison of 2 experimental spectra with simulated spectra and Fourier images of the dynamic magnetisation revealed that as the element size and/or applied field were reduced, the mode character changed from one that occupied the majority of the volume of the element, to several modes that were localised near to the edges of the element that were perpendicular to the applied field. Furthermore, deviation of the direction of the wavevector of the dynamic magnetisation from the direction of the static magnetisation was found to lead to a dynamic configurational anisotropy within nanomagnets. Following the presentation of the experimental results, the recent developments for future experimental work are presented with the aim to study precessional switching in an isolated nanomagnet. The results obtained in the experiments presented in this thesis are expected to lead to a better understanding of the non-uniform magnetisation dynamics in square nanomagnets, which have application in future magnetic data storage technologies

Ultrafast magnetization dynamics of spintronic nanostructures

Copyright © 2011 The Royal SocietyThe ultrafast (sub-nanosecond) magnetization dynamics of ferromagnetic thin films and elements that find application in spintronic devices is reviewed. The major advances in the understanding of magnetization dynamics in the two decades since the discovery of giant magnetoresistance and the prediction of spin-transfer torque are discussed, along with the plethora of new experimental techniques developed to make measurements on shorter length and time scales. Particular consideration is given to time-resolved measurements of the magneto-optical Kerr effect, and it is shown how a succession of studies performed with this technique has led to an improved understanding of the dynamics of nanoscale magnets. The dynamics can be surprisingly rich and complicated, with the latest studies of individual nanoscale elements showing that the dependence of the resonant mode spectrum upon the physical structure is still not well understood. Finally, the article surveys the prospects for development of high-frequency spintronic devices and highlights areas in which further study of fundamental properties will be required within the coming decade

Excitation and Imaging of Precessional Modes in Soft-Magnetic Squares

Copyright © 2008 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.We investigated the high-frequency modes of 40 mum wide, 160 nm thick Fe 70Co8B12Si10 squares using time-resolved scanning Kerr microscopy. Two modes were identified using pulsed field excitation while the spatial character of the out-of-plane and in-plane magnetization component was investigated using harmonic field excitation. The field dependence of the two modes has been fitted using the Damon-Eshbach model

Time resolved studies of edge modes in magnetic nanoelements (invited)

Copyright © 2006 American Institute of PhysicsMicromagnetic simulations have been performed to investigate the frequencies and relative amplitudes of resonant magnetic modes within nanomagnetic elements of varying size that have been previously studied by time resolved Kerr magnetometry. The magnetic response of a nanoscale element generally consists of the edge and center localized modes. For 2.5 nm thick elements, a crossover from center to edge mode excitation occurs as the element size is reduced to less than 220 nm. Additional modes appear in the spin wave spectrum as the thickness of the element is increased. The frequency of the edge mode is particularly sensitive to the strength of the exchange interaction, dipolar interactions with nearest neighbor elements, and rounding of the corners of the element. Simulations with in-plane pulsed fields show that the edge mode becomes dominant in elements of somewhat larger size, emphasizing the importance of the edge mode in technological applications. (C) 2006 American Institute of Physics

Large amplitude magnetization dynamics and the suppression of edge modes in a single nanomagnet

Copyright © 2011 American Institute of PhysicsLarge amplitude magnetization dynamics of a single square nanomagnet have been studied by time-resolved Kerr microscopy. Experimental spectra revealed that only a single mode was excited for all bias field values. Micromagnetic simulations demonstrate that at larger pulsed field amplitudes the center mode dominates the dynamic response while the edge mode is almost completely suppressed. Controlled suppression of edge modes in a single nanomagnet has potential applications in the operation of nanoscale spin transfer torque oscillators and bistable switching devices for which the amplitude of the magnetization trajectory is often large and a more uniform dynamic response is desirable

Imaging collective magnonic modes in 2D arrays of magnetic nanoelements

Copyright © 2010 The American Physical SocietyWe have used time resolved scanning Kerr microscopy to image collective spin wave modes within a 2D array of magnetic nanoelements. Long wavelength spin waves are confined within the array as if it was a continuous element of the same size but with effective material properties determined by the structure of the array and its constituent nanoelements. The array is an example of a magnonic metamaterial, the demonstration of which provides new opportunities within the emerging field of magnonics

Ferrite-filled cavities for compact planar resonators

Copyright © 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters, Volume 104 (2), article 022405, and may be found at http://dx.doi.org/10.1063/1.4811521Sub-wavelength metallic planar cavities, closed at one end, have been constructed by wrapping aluminium foil around teflon or ferrite slabs. Finite cavity width perturbs the fundamental cavity mode frequency of ferrite-filled cavities due to different permeability inside and outside of the cavity, in contrast to teflon-filled cavities, while the cavity length required to achieve a specific resonance frequency is significantly reduced for a ferrite-filled cavity. Ferrite-filled cavities may be excited by an in-plane alternating magnetic field and may be advantageous for high-frequency (HF) and ultra HF tagging and radio frequency identification of metallic objects within security, manufacturing, and shipping environments.Engineering and Physical Sciences Research Council (EPSRC)University of Exeter Open Innovation FundCrown Packaging UK PL

Time- and vector-resolved Kerr microscopy of hard disk writers

Copyright © 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 99 (2011) and may be found at http://dx.doi.org/10.1063/1.3665957Time-resolved scanning Kerr microscopy has been used to make wafer level measurements of magnetization dynamics within the yoke and pole piece of partially built hard disk writer structures. Three Cartesian components of the vector magnetization were recorded simultaneously using a quadrant photodiode polarization bridge detector. The rise time, relaxation time, and amplitude of each component has been related to the magnetic ground state, the initial torque, and flux propagation through the yoke and pole piece. Dynamic images reveal “flux-beaming” in which the magnetization component parallel to the symmetry axis of the yoke is largest along that axis

Time- and vector-resolved magneto-optical Kerr effect measurements of large angle precessional reorientation in a 2×2 μ m2 ferromagnet

Copyright © 2009 American Institute of PhysicsThe precessional dynamics of a 2×2 μm2 CoFe/NiFe (4.6 nm) element stimulated by an in-plane pulsed magnetic field have been investigated using time- and vector-resolved Kerr microscopy measurements and micromagnetic simulations. The time-resolved signals were normalized to in-plane hysteresis loops obtained from the patterned material, and suggest that the magnetization reorients through an angle of 100°±10°. The simulations reveal that only the magnetization of the center region undergoes large angle reorientation, while the canted magnetization at the edges of the element remains pinned. An enhanced Gilbert damping parameter of 0.1 was required to reproduce the experimentally observed Kerr signals

Interfacial structure and half-metallic ferromagnetism in Co2MnSi-based magnetic tunnel junctions

Copyright © 2006 The American Physical SocietyX-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) techniques are utilized to explore the ferromagnetic/barrier interface in Co2MnSi full Heusler alloy magnetic tunnel junctions. Structural and magnetic properties of the interface region are studied as a function of the degree of site disorder in the alloy and for different degrees of barrier oxidation. Photoelectron scattering features that depend upon the degree of L2(1) ordering are observed in the XAS spectra. Additionally, the moments per 3d hole for Co and Mn atoms are found to be a sensitive function of both the degree of L2(1) ordering and the barrier oxidation state. Significantly, a multiplet structure is observed in the XMCD spectra that indicates a degree of localization of the moments and may result from the half-metallic ferromagnetism (HMF) in the alloy. The magnitude of this multiplet structure appears to vary with preparation conditions and could be utilized to ascertain the role of the constituent atoms in producing the HMF, and to examine methods for preserving the half-metallic state after barrier preparation. The changes in the magnetic structure caused by barrier oxidation could be reversed by inserting a thin Mg interface layer in order to suppress the oxidation of Mn in the Co2MnSi layer