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Title: The thermodynamics of 2D magnetic films
Author: Buckley, M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1996
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The magnetic properties of ultrathin cobalt films grown epitaxially on copper (001) and (110) substrates have been investigated in-situ using the magneto-optical Kerr effect. The evolution of the magnetic properties of Co/Cu(110) during growth have been studied in the few monolayer (ML) regime, and strong differences in the magnetic behaviour of films grown at 80 K and 300 K are seen. For Co growth at 80 K, a magnetic signal is first recorded at ≈3 ML thickness. In the range 2.7 - 4.1 ML, a variation in the M-H loop squareness S is seen - this is attributed to a morphology-driven oscillation in the surface anisotropy of the sample. For Co growth at 300 K, a magnetic response characteristic of superparamagnetic islands is detectable at 5.2±0.6 ML. At 5.65±0.15 ML, a dynamic evolution of the M-H loop is observed at constant Co thickness, from a linear response to a ferromagnetic response characteristic of reversal along an easy axis. This is attributed to lateral diffusion processes in the Co film linking the islands and producing a continuous ferromagnetic film. The effects of a Cu overlayer on the Co/Cu(001) system are reported for Co films with a range of thicknesses. Strong variations in the magnetic properties are observed for submonolayer Cu coverages- in particular, at 0.2 ML a maximum in S, a peak in the M-H loop amplitude and a minimum in the coercivity are seen. The data is attributed to the presence of steps on the substrate inducing a uniaxial contribution to the magnetic anisotropy. A model to describe the behaviour is developed in which the Cu adsorbs preferentially at the steps, compensating the uniaxial anisotropy and simultaneously enhancing the magneto-optical response. The temperature dependence of the magnetisation and the coercive field of a Co/Cu(001) film is reported, for a sample in which the Curie temperature is below 300 K. It is shown that the phase transition is compatible with the 2D Ising model, within errors. The decrease of the magnetisation with increasing temperature is compared to a spin-wave model.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available