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Title: Spin polarised radiation studies of ultrathin magnetic films
Author: Hope, S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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Two distinct spin polarised radiation techniques have been employed to determine the magnetic properties of ultrathin magnetic films in-situ and ex-situ. The magneto-optical Kerr effect has been used to study the evolution of the magnetic properties during room temperature growth of Co/Cu(110) in -situ. The thickness dependence of the magnetic moment per atom in the Cu/Ni/Cu/Si(001) system has been investigated ex-situ using polarised neutron reflection. The Co/Cu(110) system is found to exhibit a 3D growth mode, becoming ferromagnetic at a critical thickness of dc=4.6±1.1. ML. Remarkably, the magnetic susceptibility χ follows a power law near dc with a critical exponent γ=2.39±0.08, which is in excellent agreement with the theoretical value of γ=2.43 for a 2D percolation phase transition. STM measurements on the same crystal indicate that the percolation phase transition is related to the coalescence of Co island clusters across the entire sample area. For a given Co thickness in the range 5MLCo<40ML the magnetic easy axis is found to switch through 90° over a repeatable duration (of the order of one hour) dependent on the thickness of the Co film. The behaviour is attributed to the reversal in sign of the effective uniaxial anisotropy constant, due to the adsorption of submonolayer quantities of residual CO gas in the UHV chamber. The effect of the adsorbed CO gas can be reversed by the adsorption of submonolayer coverages of Cu overlayer thereby switching the easy axis back to its original direction. For thin Co films (dCo<15ML) the easy axis switches abruptly between the two directions. For thicker Co films (dCo>15ML) the magnetic easy axis can take up intermediate directions and allows us to controllably engineer the direction of easy magnetisation at a constant Co thickness. A phenomenological model is developed to explain the switching behaviour based on competing uniaxial and cubic anisotropies. Depositing Co, or annealing the sample to 400K will produce similar behaviour. The nature of the switching for each mechanism is discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available