NanoSQUIDs for Studies on the Magnetization Reversal of Individual Magnetic Nanoparticles

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 nanotube

Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs -- towards detection of small spin systems with single-spin resolution

We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the SQUID loop. The separation of the location of optimum coupling from the junctions allows for an independent optimization of the coupling factor $\phi_\mu$ and junction properties. We present different methods for calculating $\phi_\mu$ (for a magnetic nanoparticle placed 10\,nm above the constriction) as a function of device geometry and show that those yield consistent results. Furthermore, by numerical simulations we obtain a general expression for the dependence of the SQUID inductance on geometrical parameters of our devices, which allows to estimate their impact on the spectral density of flux noise $S_\Phi$ of the SQUIDs in the thermal white noise regime. Our analysis of the dependence of $S_\Phi$ and $\phi_\mu$ on the geometric parameters of the SQUID layout yields a spin sensitivity $S_\mu^{1/2}=S_\Phi^{1/2}/\phi_\mu$ of a few $\mu_{\rm{B}}/\rm{Hz^{1/2}}$ ($\mu_B$ is the Bohr magneton) for optimized parameters, respecting technological constraints. However, by comparison with experimentally realized devices we find significantly larger values for the measured white flux noise, as compared to our theoretical predictions. Still, a spin sensitivity on the order of $10\,\mu_{\rm B}/\rm{Hz^{1/2}}$ for optimized devices seems to be realistic.Comment: 10 pages, 5 figures, Superconductor Science and Technology (submitted

Quantum interference in an interfacial superconductor

The two-dimensional superconductor formed at the interface between the complex oxides, lanthanum aluminate (LAO) and strontium titanate (STO) [1] has several intriguing properties [2–6] that set it apart from conventional superconductors. Most notably, an electric field can be used to tune its critical temperature (Tc) [7], revealing a dome-shaped phase diagram reminiscent of high Tc superconductors [8]. So far, experiments with oxide interfaces have measured quantities which probe only the magnitude of the superconducting order parameter and are not sensitive to its phase. Here, we perform phase-sensitive measurements by realizing the first superconducting quantum interference devices (SQUIDs) at the LAO/STO interface. Furthermore, we develop a new paradigm for the creation of superconducting circuit elements, where local gates enable insitu creation and control of Josephson junctions. These gate-defined SQUIDs are unique in that the entire device is made from a single superconductor with purely electrostatic interfaces between the superconducting reservoir and the weak link. We complement our experiments with numerical simulations and show that the low superfluid density of this interfacial superconductor results in a large, gate-controllable kinetic inductance of the SQUID. Our observation of robust quantum interference opens up a new pathway to understand the nature of superconductivity at oxide interfaces