RSO Measurements

The data were taken using the Reciprocating Sample Option (RSO). Unlike DC measurements where the sample is moved through the coils in discrete steps the RSO measurements are performed using a servo motor which rapidly oscillates the sample, see Figure 4.4. These measurements have a sensitivity of $5\times10^{-9}\ensuremath{\unskip\,\mathrm{EMU}}$.[17]

Figure 4.4: Illustration of an RSO measurement with a small amplitude. (a) shows the ideal SQUID response for a dipole and (b) shows the movement of the sample within the SQUID pickup coils.

A shaft encoder on the servo motor records the position of the sample synchronous with the SQUID signal. The data received is fitted to an ideal dipole moment response. To ensure this assumption is applicable samples need to be small: the calibration sample is a cylinder of $3\ensuremath{\unskip\,\mathrm{mm}}$ diameter and $3\ensuremath{\unskip\,\mathrm{mm}}$ height. Samples of this size or smaller match an ideal point dipole to an accuracy of approximately $0.1\%$.[17]

RSO measurements can be made in one of two configurations: Center or Maximum slope. Center scans use large oscillations (2 to $3\ensuremath{\unskip\,\mathrm{cm}}$) around the center point of the pickup coils. These scans take a long time but the sample always remains properly located and a large number of measurements are recorded. These give the most accurate readings.

The Maximum Slope method oscillates the sample over a small region ( $2\ensuremath{\unskip\,\mathrm{mm}}$) at the most linear part of the SQUID response (as shown in Figure 4.4). The smaller amplitude makes measurements quicker and prevents the sample being subjected to significant magnetic field variation, however it also makes the measurement less accurate and susceptible to drift in the sample position.

All measurements taken in this thesis using the MPMS XL SQUID were performed in Center mode with an amplitude of $3\ensuremath{\unskip\,\mathrm{cm}}$.