Electromagnetic and Cryogenic Design of a Liquid-Helium-Free 9-T NbTi Magnet for Scanning Tunneling Microscopy

The strong magnetic field generated by the superconducting magnet can significantly enhance the capabilities of scanning tunneling microscopy (STM) in exploring novel quantum materials, nano-magnetic systems, and topological materials. A liquid-helium-free 9-T NbTi magnet for STM has been developed at the Wuhan National High Magnetic Field Center and is entirely cooled with a Gifford-McMahon (GM) cryocooler. This magnet features five independent coils, a cold bore diameter of 95 mm, a rated operating current of 80.7 A, and a rated central magnetic field of 9.0 T. The magnetic field uniformity over a 1-cm diameter spherical volume is 0.1%. This article presents the electromagnetic and cryogenic design of the magnet, along with a thermal and mechanical analysis and optimization of the current leads and cooling paths. The optimized magnet system successfully reaches a temperature below 4 K. After six training quenches, the magnet was successfully ramped to and parked at 9.2 T.