Study of Electronic and Magnetic Properties of Cobalt Nanoclusters on Graphite Master Thesis Report

We investigated the combination of cobalt (Co) and highly ordered pyrolytic graphite (HOPG) for spintronic applications. We deposited Co on HOPG in UHV to study its growth, and to characterize the interfacial electronic and magnetic properties. In our study, we remained in the nucleation-dominated regime with Co coverage below 5%, as characterized by scanning tunneling microscopy (STM). We observed individual Co ad-atoms initially sitting at the β-sites of the graphite surface lattice, and subsequently filling the other sites to form nanoclusters. The Co nanoclusters growth was found to follow the Volmer-Weber mode, independent of deposition time. The mean diameter and height of the isolated nanoclusters are 3.4 ± 0.2 nm and 0.49 ± 0.045 nm respectively. The aspect ratio of the nanoclusters, defined as height divided by diameter (h/d), is ~ 0.15. I-z scanning tunneling spectroscopy (STS) measurements, recorded with various applied bias voltages and a fixed current set-point of 1 nA, were taken at room temperature. For bare graphite samples, these data measurements later were interpreted as a mapping of the inverse decay length (κ) which represents electronic tunneling properties of the tip-graphite system at low bias range (near Fermi level). From spatial mapping of the inverse decay length, with a maximum value of 2.46 nm at -0.5 V, it was shown that the tunneling current into the graphite was carried by electrons with a much lower parallel wave vector k|| value than k|| = K points = 17 nm (from -0.5 V to 0.5 V). After Co deposition, I-V STS data, using various various applied bias voltages and current set-points, were taken both at room temperature and at 95 K, to study the electronic properties of the Co nanoclusters. It turned out, however, that it is not possible to distinguish the Co nanoclusters from the HOPG substrate, mainly due to their similar conductivity behavior. To probe the magnetic and electronic properties of the nanoclusters, we also performed X-ray magnetic circular dichroism (XMCD) at 77 K. It is known that oxygen contaminants and defects on the graphite surface, as well as minute residual gases even in UHV, have a profound influence on the electronic and magnetic properties of Co nanoclusters. The measurements showed that the adsorbed Co clusters on HOPG were metallic, as evidenced by the lack of multiplet structure at the Co L2,3 edges in the measured spectra. Other magnetic properties characterization was done using a vibrating sample magnetometer (VSM), to investigate the magnetization of the Co nanoclusters. Both measurements indicated paramagnetic behavior of the Co nanoclusters. 1 Both Co and graphite are conductors. Just like any metal, Co is conducting due to overlap electronic bands structure at Fermi level. Graphite is defined as semimetal has electronic bands structure that only crossed at the K points at the Fermi level.