The AC Susceptibility for Dilution Refrigerator (ACDR) option enables measurements of the dynamic sus-ceptibility of samples within the low temperature regime of the Dilution Refrigerator option for the PPMS and DynaCool systems (0.05 K - 4 K). To achieve this, the sample is mounted to a sapphire rod which is fixed to the mixing chamber of the dilution refrigerator (DR). This cools down the sample. When the DR is installed, the rod sits concentrically within the ACDR coil set. It is thermally decoupled from the DR and remains at or near the base temperature of the PPMS. The coil set provides the oscillating AC magnetic field excitation at a user-selected frequency (between 10 Hz - 10.000 Hz) and amplitude (up to 4 Oe) and detects the oscillating response of the sample.
A lock-in, amplifier-based technique extracts the amplitudes of the in- and out-of-phase components (as compared to the drive signal) of the induced voltage in the detection coils. The voltage can be related to the dynamic susceptibility through calibration constants and is reported to the user. The calibration constants are provided by Quantum Design upon the installation. They can be verified at any time by using the supplied GGG reference sample.
Though the fundamental ACDR measurement technique is basically the same as that of the AC Measurement System (ACMS-II) option for the PPMS, the mechanical limitations and space constraints of the DR make multi-point measurements impossible. As a result, there is an additional step required (nulling) to remove spurious background signals and ensure the reported signal is as accurate as possible.
The ACDR option is designed as an automated, turn-key system for measuring the dynamic susceptibility of a material at DR temperatures. This means, users need not be experts on the topics of susceptometer construction and lock-in amplifiers to successfully use the system and acquire high-quality data.