Tunneling stabilized magnetic force microscopy: prospects for low temperature applications to superconductors

A low-temperature STM (scanning tunneling microscope) was built with the aim of getting TSMFM (tunneling stabilized magnetic force microscopy) images of the flux lattice in superconductors. The STM has been operated in a cryogenic bathysphere cryostat. The bathysphere cryostat will allow measurements from 4 K to 300 K in the bore of a high-field magnet. The STM has been operated in two modes at low temperatures: the STM mode with a rigid tunneling tip, and the TSMFM mode with a flexible magnetic tunneling tip. Magnetic bit racks were imaged on a hard disk with submicrometer resolution in the TSMFM mode at room temperature with an Au-coated Ni-film flexible tip. The TSMFM contrast improved markedly with this type of tip versus thicker Fe-film tips used previously. Preliminary TSMFM images of a YBa₂Cu ₃O_x (YBCO) film (T_c=88 K) in a 50-mT field show that relatively large magnetic forces are acting on the flexible tip while scanning at 48 K