Sub-Kelvin ac magnetic susceptometry.

Designing an apparatus for experiments that operate deep into the milliKelvin regime requires a careful selection of materials and construction techniques, and often entails balancing mutually contradictory requirements. In the case of magnetic susceptibility measurements, this involves choosing materials that realize both high thermal conductivity for good heat sinking and low electrical conductivity for minimal eddy-current heating. A coil mounting system with a completely nonmetallic construction, achieved through the use of a sapphire skeleton and a machined carbon-fiber body, resolves this dilemma. This system permits the use of large ac magnetic field amplitudes at kHz frequencies and above, opening the door to studies of the nonlinear dynamics of a wide variety of magnetic systems that were previously inaccessible.