Abstract:
Measurements on magnetic nanoparticles (MNPs) are very relevant for research as well as for applications, since they can, on the one hand, contribute to a deeper understanding of nanomagnetism and, on the other hand, lots of technical or clinical applications of MNPs require precise knowledge of their properties. For the characterization of single MNPs, tiny signals have to be detected in huge magnetic background fields, a task that can be achieved by using superconducting quantum interference devices that are miniaturized to the submicron range (nanoSQUIDs).
The first part of this thesis deals with the usage of nanoSQUIDs, based on the high critical temperature superconductor YBa2Cu3O7 (YBCO), in magnetization reversal measurements on different MNPs. First, the magnetization reversal of an iron nanowire embedded in a carbon nanotube is investigated. Further measurements are performed on cobalt MNPs that were fabricated by focused electron-beam-induced deposition. For cobalt nanowires, an increase in the cobalt content as well as the saturation magnetization of the wires after thermal annealing is shown. For cobalt particles with radii < 100 nm that are directly grown on the nanoSQUIDs, measurements at temperatures between 0.3 K and 80 K verify a thermally assisted reversal of the magnetization, which partly takes place by formation of a magnetic vortex state.
In the second part of this thesis, different studies for the improvement of future nanoSQUIDs are conducted. By simultaneously measuring the magnetization reversal of a MNP in all 3 spatial directions, the anisotropic magnetic properties of the MNP can be fully captured. The general feasibility of such measurements using nanoSQUIDs is shown in the characterization of a vector nanoSQUID that is realized by combination of three niobium-based nanoSQUIDs.
However, transferring this approach to YBCO is not possible with the current grain boundary-based Josephson junctions (JJs) of YBCO nanoSQUIDs for topological reasons. Therefore, JJs that can be induced in YBCO by irradiation with a focused helium ion beam are fabricated and characterized in this thesis. The position of such junctions can be defined by the location of irradiation, and adjustments of the electronic properties are possible by choice of the irradiation dose. For irradiation with a sufficiently high dose, superconductivity is suppressed completely, which enables defining the geometry of electronic structures without removal of material.
Furthermore, artificial pinning centers for Abrikosov vortices can be created by local suppression of superconductivity. The focused helium ion beam allows for fabrication of pinning arrays with spacings below 100 nm. The characterization of such ultradense pinning arrays in magnetic fields reveals matching effects at temperatures significantly below the critical temperature, which confirms strong pinning of vortices at the pinning centers. Therefore, such pinning centers could be used to improve the low-frequency flux noise of YBCO nanoSQUIDs in magnetic fields.
