We have constructed a scanning probe microscope for magnetic imaging, which can function as a scanning Hall probe microscope (SHPM) and as a scanning SQUID microscope (SSM). The scanning scheme, applicable to SHPM and SSM, consists of a mechanical positioning (sub) micron-XY stage and a flexible direct contact to the sample without a feedback control system for the Z-axis. With the interchangeable capability of operating two distinct scanning modes, our microscope can incorporate the advantageous functionalities of the SHPM and SSM with large scan range up to millimeter, high spatial resolution (⩽4 μm), and high field sensitivity in a wide range of temperature (4.2 K-300 K) and magnetic field (10(-7) T-1 T). To demonstrate the capabilities of the system, we present magnetic images scanned with SHPM and SSM, including a RbFeB magnet and a nickel grid pattern at room temperature, surface magnetic domain structures of a La(2/3)Ca(1/3)MnO3 thin film at 77 K, and superconducting vortices in a striped niobium film at 4.2 K.

We use the scanning SQUID microscope (SSM) to image the vortices in superconducting FeSe0.3Te0.7 (FST) thin films. The observed peak flux value of FST is nearly a quarter of that of an accompanying Nb film. We developed a method for quantitatively determining the London penetration depth of the FST film from the known value of Nb. The obtained value, 0.88 μm, is significantly larger than those obtained from single crystals of similar compositions by using other methods. The methodology developed for this study is useful for measuring London penetration depths of thin-film superconductors in general.

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The Nb-based superconductor-insulator-superconductor (SIS) tunnel junctions have been broadly used in many applications. The critical current density (), one of the most important parameters of SIS tunnel junction, is usually controlled by the oxygen exposure (2) of the Al oxidation process. R. E. Miller et al. demonstrated the relation between and oxygen exposure using the SNEP process. However, the value of still varies with Nb/AlO Al/Nb deposition system, even run-to-run process. A new AuAl 2 /Al composite, instead of pure Al, has been used in the oxidation process. From the-2 relation, we have demonstrated the oxidation rate of AuAl 2 is about 400 times lower than that of Al. Using AuAl 2 layer, two advantages are observed. 1) For low tunnel junctions, the thickness of AlO , or , can be controlled easily by inserting AuAl 2 layer as a blocking layer in oxidation process. 2) High quality factor tunnel junctions with 100 kA/cm 2 are achieved by oxidation of AuAl 2 layer directly.