Applied Superconductivity - Walther Meißner Institut - Bayerische ...

Section 4.5 APPLIED SUPERCONDUCTIVITY 207SQUID chipatmosphereSi x N y windowwindowadjustmentSi basevacuumcoldfingerFigure 4.32: Cross-sectional view of a scanning SQUID microscope. The distance between the SQUID sensorand the Si x N y window can be adjusted by adjustment screws.on a Si chip. In the first case, the SQUID-to-sample separation is typically 150 µm, whereas in the latter,the separation can be as low as 15 µm. Note that the smaller the SQUID-to-sample distance the betterthe spatial resolution. The entire system is surrounded by a µ-metal shield to exclude spurious magneticfield fluctuations.For low-frequency operation (typically less than 1 kHz) the SQUID is operated in the flux lock loop.Here, a typical application is the study of the magnetic properties of superconducting thin-film structuresas well as the diagnostics of superconducting integrated circuits or highly sensitive bolometers. In suchapplication the sample is usually at the same temperature as the SQUID sensor. Two typical examplesare shown in Fig. 4.33. Furthermore, SSM can be applied to study magnetic properties of the ultrathinmagnetic films. The distribution of stray magnetic fields produced by the remnant magnetizationor the induced magnetization of the film can be visualized. The domain structures, the orientation ofthe magnetic moment and the value of the magnetization can be determined. Moreover, SSM has thepotential to image the magnitude of a magnetic field normal component over the surface of magneticrecording media. It also can be applied to the analysis of chips or wafers in microelectronics 126,127 or thestudy of magnetically active bacteria. 128 For higher-frequency operation (typically 1 kHz to 1 MHz) theSQUID is operated open loop and a drive coil is used to apply a sinusoidal magnetic field to induce eddycurrents in the sample and modulate the flux in the SQUID. 129 The magnetic response of the sampleis determined by measuring the amplitude and phase of the output from the SQUID: the out-of-phasecomponent corresponds to the eddy current in the sample. The imaging frequency has further beenextended from 1 MHz to 1 GHz by applying a rf-field to the sample, which, in turn, couples an rf-fluxinto the SQUID. 130There are many other techniques for imaging magnetic fields at surfaces: decoration techniques, 131magneto-optical imaging, 132 magnetic force microscopy, 133 scanning Hall probe microscopy, 134 scan-126 S. Chatraphorn, E.F. Fleet, F.C. Wellstodd, J. Appl. Phys. Lett. 92, 4731 (2002).127 J. Beyer, H. Matz, D. Drung, Th. Schurig, Appl. Phys. Lett. 74, 2863 (1999).128 T.S. Lee, Y.R. Chemla, E. Dantsker, J. Clarke, IEEE Trans. Appl. Supercond. AS-7, 3147 (1997).129 R.C. Black, F. C. Wellstood, E. Dantsker, A. H. Miklich, J. J. Kingston, D. T. Nemeth, and J. Clarke, Appl. Phys. Lett. 64,1 (1994).130 R.C. Black, F. C. Wellstood, E. Dantsker, A. H. Miklich, D. T. Nemeth, D. Koelle, F. Ludwig, and J. Clarke, Appl. Phys.Lett. 66, 1267 (1995).131 D. J. Bishop, P. L. Gammel, D. A. Huse, and C. A. Murray, Science 255, 165 (1992).132 S. Gotoh and N. Koshizuka, Phys. C 176, 300 (1991).133 D. Rugar, H. J. Mamin, P. Guethner, S. E. Lambert, J. E. Stern, I. McFadyen, and T. Yogi, J. Appl. Phys. 68, 1169 (1990).134 A. M. Chang, H. D. Hallen, L. Harriott, H. F. Hess, H. L. Kao, J. Kwo, R. E. Miller, R. Wolfe, and J. van der Ziel, Appl.2005