NanoSQUIDs for Studies on the Magnetization Reversal of Individual Magnetic Nanoparticles

Abstract

The subject of this thesis is the development, characterization and optimization of nanometer-sized superconducting quantum interference devices (nanoSQUIDs) for operation at cryogenic temperatures. This task is motivated by the need for convenient detectors for the investigation of individual magnetic nanoparticles, nanotubes, nanowires or molecular magnets. Two types of devices are considered in this thesis: (a) nanoSQUIDs based on Nb as a superconductor with Josephson junctions having normal metal HfTi barriers and (b) nanoSQUIDs based on Yttrium barium copper oxid (YBCO) as a superconductor with grain boundary Josephson junctions. The nanoSQUIDs have been investigated in terms of sensitivity to magnetic flux in low- and high-field environments. Numerical simulations based on the London and Maxwell equations have been deployed to determine the coupling between the nanoSQUID and a point-like magnetic moment. By using a hybrid magnetometer system consisting of an Nb nanoSQUID and a Si cantilever, individual ferromagnetic nanotubes have been investigated simultaneously by nanoSQUID and torque magnetometry, which yield complementary information of the magnetization reversal processes in the magnetic nanotubes