Scanning probe microscopy at ultra-high magnetic fields

Up to now, low temperature scanning probe microscopy (SPM) has been limited to a magnetic field strength of around 20 T, as most designs have been based on superconductor magnets. For some experimental applications – for example fractal spectra in graphene superlattices, the room temperature quantum Hall effect and some metamagnetic transitions – higher fields are required. Static fields of more than 30 T can be generated in dedicated high-field facilities by water-cooled, resistive Bitter magnets or hybrid resistive-superconducting magnets. However, implementing SPM in a Bitter magnet is a major challenge, due to the high level of vibrational noise produced by the turbulent cooling water, in addition to the space constraints resulting from the small magnet bore.

We present a novel cryogenic scanning tunnelling microscope (STM) designed to operate inside a 38 T water-cooled Bitter magnet. The performance of the STM is demonstrated through Landau level tunnelling spectroscopy of graphite, at 4.2 K in magnetic fields up to 34 T. Additionally we show the design of a highly compact atomic force microscope (AFM) for operation at cryogenic temperatures in an extremely high magnetic field. We show preliminary imaging data on the frustrated spinel CdCr₂O₄ at up to 30 T.