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Title: Magnetisation dynamics of nanostructured spintronic devices
Author: Durrant, Christopher John
ISNI:       0000 0004 6060 4324
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2016
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In this thesis investigations of the static and dynamic properties of magnetic thin films and thin magnetic multilayers with spintronic properties are presented. A selective area chemical vapour deposition technique has been used to fabricate continuous and patterned epitaxial CrO$_2$ thin films grown on (100)-oriented TiO$_2$ substrates. Precessional magnetization dynamics were stimulated both electrically and optically, and probed by means of time-resolved Kerr microscopy (TRSKM) and vector network analyser ferromagnetic resonance (VNA-FMR) techniques. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. All films exhibited a large in-plane uniaxial anisotropy. The effective damping parameter was found to exhibit strong field dependence in the vicinity of the hard axis saturation field. However, continuous and patterned films were found to generally possess similar dynamic properties, confirming the suitability of the deposition technique for fabrication of future spintronic devices. Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy were fabricated by magnetron sputtering and patterned into Hall bars by photolithography. Scanning Kerr microscopy (SKM) was combined with electrical transport measurements to gain insight into the underlying mechanisms of current-induced spin-orbit torque (SOT) switching within such devices. Switching was found to be a stochastic, domain wall driven process, the speed of which is strongly dependent on the switching current. Kerr imaging shows domain nucleation at one edge of the device which modelling reveals is likely assisted by the out-of-plane component of the Oersted field. Further domain growth, leading to magnetisation reversal, may still be dominated by spin torques, but the Oersted field provides an additional mechanism by which to control the switching process. Pulsed current TRMOKE experiments were performed on both Ta/CoFeB/MgO trilayers patterned into large Hall bars, and on patterned Ta/CoFeB/MgO trilayers formed upon planar waveguides. Magnetisation dynamics in these structures were found to be complex and to have strong dependence on bias field direction, bias field polarity, pulsed current direction, current density and device size. Two components of the dynamic response were observed in Hall bars, a fast oscillatory component and a slow unipolar deflection. A strong spatial dependence of the dynamic response was observed for patterned CoFeB/MgO devices formed upon planar waveguides with a particularly large in-plane rotation observed at the edges of a square element. These studies highlight the complexity of the SOTs generated by the spin Hall and Rashba effects. Spin pumping within Ta/Ag/Co2MnGe(5 XFMR was used to directly detect the motion of the sink layer magnetization at the source layer resonance. nm)/Ag(6 nm)/Ni81Fe19(0-5 nm)/Ag/Ta large area spin valve structures was studied by vector network analyser ferromagnetic resonance (VNA-FMR), and element-specific phase-resolved X-ray ferromagnetic resonance (XFMR). Spin current absorption as a function of Ni81Fe19 sink layer thickness was explored indirectly by VNA-FMR as a modification of the Co2MnGe source layer damping. XFMR was used to directly detect the motion of the sink layer magnetization at the source layer resonance. The bipolar form of the sink layer signal clearly indicates the action of spin transfer torque (STT) resulting from spin pumping, while comparison with a macrospin model allows the real part of the spin mixing conductance to be directly determined. The dependence of the source layer damping upon the sink layer thickness was observed to be different to previous studies, due to both spin current absorption in the outer Ta layers, and superparamagnetic relaxation in sink layers with thickness less than or equal to 0.6 nm. The XFMR measurements show that the absorption of spin current within the sink layer continues to increase up to the largest sink layer thickness of 5 nm, presumably due to improving interface quality.
Supervisor: Hicken, Robert J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available