Surface magnetic properties of ferromagnetic nickel thin films and multilayer structures

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(Thesis) Ph.D.
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Author: Xu, Tianhong
Devices based on magnetic multilayer structures are widely used in the magnetic storage industry. The interfacial magnetic behaviour in these and related structures determines their spin-dependent electron transport properties. This thesis addresses the effects of the surface quality and magnetic properties of Ni thin films and multilayer structures to understand the relationship between the microscopic physical and magnetic structure and macroscopic transport properties. The effects of surface roughness on the magnetic properties of Ni-based thin films, nanowires and multilayer structures have been investigated using linear and nonlinear optical Kerr effect methods, magnetometry and scanning probe microscopy. The magnetic properties of Ni films possessing different surface roughness have been investigated through magnetic second harmonic generation (MSHG) studies. MSHG studies show significant differences between the films’ magnetic properties, indicating that surface roughness plays an important role in determining the surface magnetic properties. MSHG studies of Ni nanowire arrays show large magnetic contrast associated with large effective surface areas and additional contributions to the susceptibility, not present in thin films. Electrodeposition of Ni/Cu multilayer structures yield films which display giant magneto-resistance (GMR) behaviour. The effects of layer thickness, roughness and deposition method on their macroscopic magnetoresistance are described. In situ scanning tunnelling microscopy (STM) was employed to study the growth kinetics of Ni films on single crystal Au(111) surfaces through electrodeposition. It has been shown that 2-D or 3-D growth of Ni thin films can be controlled by changing the deposition potentials. The nonlinear optical response from the electrodeposited films indicates that Ni films deposited at a low overpotential possess greater magnetic contrasts and coercive fields. This study reveals the important connection between the film growth mechanisms and the magnetic properties in the ferromagnetic materials. Ultrafast magnetization dynamics of electrodeposited nickel surfaces have been studied using time-resolved MSHG with laser pulses of various pump beam fluence. A fast drop of the second-harmonic intensity within 700 fs after optical excitation is observed which is followed by a partial recovery, within a few picoseconds, indicating the surface magnetic property dependence on the magnetization after the perturbation of the pump laser.
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