Low-temperature scanning magnetic probe microscopy of exotic superconductors

Scanning magnetic probe microscopy is one of the many scanning probe microscopy (SPM) techniques that have been developed in the last two decades. The basic idea of the technique is conceptually simple: a micro- or nano-scale magnetic sensor is rastered over a sample and measures the magnetic field locally, giving an image of the magnetic fields at the surface. This thesis details the construction of a scanning magnetic microscope which utilizes SQUID (Superconducting QUantum Interference Device) or Hall probe sensors in a dilution refrigerator, extending the temperature range for this measurement technique down to the millikelvin range; the development of the probes used in the microscope; and the measurements for which it has been used.; The primary experiment which I report is searching for signs of time reversal symmetry breaking in the unconventional superconductor strontium ruthenate (Sr2RuO4). There is strong published evidence that this material is a spin-triplet superconductor. In addition, there is experimental evidence from muSR (muon spin resonance) and small-angle neutron scattering experiments that the wavefunction is a two-component Ginzburg-Landau wavefunction which exhibits time reversal symmetry breaking (TRSB) properties.; A direct consequence of TRSB is that there should be spontaneously generated magnetic fields locally at sample edges. I have searched for this signature of TRSB in single-crystal samples of Sr2RuO4, including samples that have been patterned with an array of dimples in order to generate artificial edges which should enhance the effect. No signatures of TRSB have been found in these experiments. This contradicts theoretical estimates, and the discrepancy indicates that either Sr2RuO4 does not have TRSB properties, or the theoretical estimates are insufficient in that they do not take factors such as domain formation into account.; In a related experiment, I have studied the local susceptibility of the "3 K phase" of Sr2RuO4. This is a phase of the material that contains inclusions of metallic Ru. The measurements reported in this thesis show that the diamagnetism of the inclusions is strongly enhanced at temperatures below the Tc of Ru, indicating that the inclusions are not homogeneously integrated into the surrounding superconducting material.