Quantum spin systems and magnetic frustration investigated with high frequency AC susceptibility and muons

An ac susceptometer is described that can measure up to 3.5 MHz at temperatures down to 2 K and in dc magnetic fields up to 1 T by integration with a Quantum Design PPMS. The susceptometer is described with a detailed explanation of the calibration procedure. ac susceptibility data on MgB2 and Dy2Ti2O7 measured by the high frequency susceptometer is presented and compared with data recorded using a commercial susceptometer which shows good agreement. FeraSpin XS magnetic nano particles are measured and the ac susceptibility is used to deter- mine the mean particle radius, which shows good agreement with the literature. The quantum tunnelling plateau is measured in powder CdEr2Se4 and CdEr2S4 using the high frequency susceptometer. Ac susceptibility measurements in dc magnetic fields reveal field induced coexisting relaxation mechanisms, a thermally activated and a quantum process. CdEr2Se4 is measured at down to 70 mK and the two mechanisms are shown to persist to the spin freezing temperature which is also measured. Single crystal samples of CdEr2Se4 do not show any sign of the field induced relaxation mechanism at any temperature or field. The single crystal spin freezing temperature is found to be lower than in the powder. A muon spin rotation experiment in powder CdEr2Se4 finds that the spin fluctuation rates are orders of magnitude faster than those seen with ac susceptibility. Muon spin rotation experiments in PVDF and LiY0.95Ho0.05F4 are performed where the F-μ and F-μ-F quantum states form respectively. Measurements are performed in zero field and in the presence of an RF photon field. Multiple approaches to fitting the zero field data are discussed, including by considering the state as an open quantum system. A close fit is achieved in LiY0.95Ho0.05F4 but not in PVDF which is attributed to difficulty in accounting for amorphous regions. The application of the RF field is shown to alter the measured muon asymmetry. It is shown that the effect of the RF field cannot be accounted by a rotating magnetic field and therefore the RF field must be manipulating the occupation probabilities of the state