Magneto–Optical Kerr Effect Microscopy Investigation on Permalloy Nanostructures

This thesis focuses on the investigation of magnetic domains in ultrasmall permalloy (Ni80Fe20) structures down to nanometre size. Magnetic domains and domain walls in nano objects are often observed using a very high resolution and high power microscope such as magnetic soft x-ray microscope, magnetic force microscopy imaging and photoemission electron microscopy. A reason for this is because the Kerr signal in nanostructures is very weak. However the results from this thesis demonstrate that magnetic domains in permalloy magnetic nanostructures can still be observed with very good contrast using a Magneto-optical Kerr effect (MOKE) microscope. The constructed Kerr microscope is a home-build wide field microscope and is able to produce magnetic domains image of permalloy nanowire as small as 245 nm, although the resolution limit of the microscope is 505 nm. For the first time, a magnetic domain in nanowire with width of 245 nm is observed using a wide-field microscope. The combination of hysteresis loops and magnetic domains observations for studying a magnetic sample provides a three-dimensional understanding of the magnetic characteristic of the sample. This is crucial in investigating nano samples as the theoretical arguments with the experimental results are always constrained by the experimental part. Three kinds of nanostructure sample were observed using the Kerr microscope; a cross nanowire, zigzag nanowire and a nanowire with notch and a nucleation pad at one end. It was found that a cross nanowire can form magnetic domains upon reversal and the junction forms a magnetisation vortex. Findings from zigzag nanowire demonstrate a complex, multiple magnetic domains formation upon magnetisation reversal. A weak domain wall pinning effect was observed in the nanowire, causing a multiple domains formation in the nanowire upon reversal. It can be confirmed that this effect was caused by the high coercivity of the nucleation pad. For the nanowire with notch, it was demonstrated that the coercivities were different at negative and positive field. But for such case, there is a relationship observed between the percentage notch depth and the coercivity at the junction.