Superconducting QUantum Interference Device (SQUID) microscopy has excellent magnetic field sensitivity, but suffers from modest spatial resolution when compared with other scanning probes. This spatial resolution is determined by both the size of the field sensitive area and the spacing between this area and the sample surface.In this paper we describe scanning SQUID susceptometers that achieve sub-micron spatial resolution while retaining a white noise floor flux sensitivity of ≈ 2µΦ 0 /Hz 1/2 .This high spatial resolution is accomplished by deep sub-micron feature sizes, well shielded pickup loops fabricated using a planarized process, and a deep etch step that minimizes the spacing between the sample surface and the SQUID pickup loop. We describe the design, modeling, fabrication, and testing of these sensors. Although submicron spatial resolution has been achieved previously in scanning SQUID sensors, our sensors not only achieve high spatial resolution, but also have integrated modulation coils for flux feedback, integrated field coils for susceptibility measurements, and batch processing. They are therefore a generally applicable tool for imaging sample magnetization, currents, and susceptibilities with higher spatial resolution than previous susceptometers.PACS numbers: 85.25.Dq,07.55.-w
In the past, magnetic images acquired using scanning Superconducting Quantum Interference Device (SQUID) microscopy have been interpreted using simple models for the sensor point spread function. However, more complicated modeling is needed when the characteristic dimensions of the field sensitive areas in these sensors become comparable to the London penetration depth. In this paper we calculate the response of SQUIDs with deep sub-micron pickup loops to different sources of magnetic fields by solving coupled London's and Maxwell's equations using the full sensor geometry. Tests of these calculations using various field sources are in reasonable agreement with experiments. These calculations allow us to more accurately interpret sub-micron spatial resolution data obtained using scanning SQUID microscopy. arXiv:1607.03950v1 [cond-mat.supr-con]
Vibrations can cause noise in scanning probe microscopies. Relative vibrations between the scanning sensor and the sample are important but can be more difficult to determine than absolute vibrations or vibrations relative to the laboratory. We measure the noise spectral density in a scanning SQUID microscope as a function of position near a localized source of magnetic field, and show that we can determine the spectra of all three components of the relative sensor-sample vibrations. This method is a powerful tool for diagnosing vibrational noise in scanning microscopies.There is a large literature on detecting vibrational motion in scanning probe microscopy. Vibrations of the microscope as a whole have been determined using an acceleromater; 1 of the cantilever using piezoelectric sensing 2 or interferometry 3 ; of the sample using stroboscopic optical microscopy, 4 non-linear effects in atomic force microscopy, 5 or the cantilever deflection in scanning force microscopy. 6 In addition, a standard technique for analyzing resolution and stability in electron beam lithography is to move an anisotropically etched silicon edge relative to the beam. 7 However there has been relatively less work on using sensor-sample vibrations as a diagnostic tool of vibrations within the microscope itself.In this paper we show how one can determine all three components of the vibrations between sensor and sample by measuring the time dependence of the flux through a scanning Superconducting QUantum Interference Device (SQUID) pickup loop due to a superconducting vortex.Our measurements were made in a scanning microscope developed in a collaboration between Attocube and Stanford. Briefly, in this system the vibration isolation is provided by suspending the entire system from springs. The microscope is housed in a vacuum can that is inserted into a liquid helium dewar; cooling is provided by He 4 exchange gas. The coarse positioning and scanning of the sample are performed by an Attocube piezoelectric stack. The SQUID is mounted on a cantilever consisting of a 3 mm wide, 10 mm long, and 25 µm thick copper shim, with wire bonds making electrical contacts. The SQUID mount and Attocube stack are mounted in a massive titanium housing. The titanium housing is suspended from a copper support which is firmly clamped to the sides of the vacuum can for thermalization of the microscope wiring. All measurements reported here were made at 4.2K.The SQUID susceptometer 8 used for these measurements has an integrated pickup loop and one turn field coil in the geometry indicated by the solid lines in Fig. 1a. We scan the sample relative to the SQUID's pickup loop by applying a variable DC-Voltage to our piezobased scanners. In our coordinate system the long axis of the cantilever is in theŷ direction, as are the leads to the pickup loop in the SQUID, and the sample plane is the xy plane.The studied sample is a 0.4 µm thick superconducting niobium film (T c = 9.2K). An isolated superconducting vortex was located by repeatedly cooling the sample in ...
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