The growth, formation and magnetic behaviour of noble metal capped atomic wires

Abstract

Transition metal nanostructures have been observed to have several interesting magnetic properties. Nanowire of atomic widths on a vicinal surfaces show these interesting properties clearly, which also vary with the width of the wires. If these nanostructures are to be of future use in devices they will have to be able to survive outside of a ultra high vacuum environment. The most obvious means to prevent the nanowires from oxidising is to cap noble metal or inert overlayer. It is a system of this type that is the subject of the study, specifically cobalt atomic wires on a vicinal platinum surface, which have been capped with four to eight monolayers of gold. During this work the growth of the wires, and the capping layer was investigated using low energy electron diffraction, scanning tunnelling microscopy and Auger electron spectroscopy. Along with their lifetime before oxidisation outside of a ultra high vacuum environments by x-ray absorption and photoemission spectroscopy. The magnetic properties of this system was investigated by both x-ray magnetic circular dichroism and the magneto optical Kerr effect. It was found that both the cobalt and gold grow pseudomorphically on the platinum surface; with row by row growth being achieved for the nanowires. With the presence of the pseudomorphic gold capping layer increasing the length of time the nanowires remain unoxidised outside of a ultra high vacuum environment to a time frame that is measured in months instead of nanoseconds for the uncapped wires. The x-ray photoemission spectroscopy spectra from the cobalt nanowires was observed to differ from that of bulk cobalt due to its different local environment and by studying the gold and platinum spectra the coverage are found to be the intended coverage for the different samples grown. The magnetic properties of capped nanowires differ significantly from the bare wires with an altered orbital magnetic moment which is decreased with respect to the bare wires but still increased when compared to bulk cobalt, the easy axis has moved to being out of plane, and an increased Curie temperature. This enhanced Curie temperature allowed for the observation for a temperature dependent hysteresis which can be described by the Gaunt strong pinning model. These magnetic properties were also found to be very sensitive to disorder and the exact coverage of the sample, with both being needed to be reproduced in a sample to obtain the same magnetic properties.