Determining the vibrations between sensor and sample in SQUID microscopy

Abstract

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 ...

Cryogen-free variable temperature scanning SQUID microscope

“…We characterize the frequency spectrum of relative sensorsample vibrations by measuring the SQUID noise spectrum as a function of position in a region of sharp flux gradient. 24 An isolated vortex in a Nb film has a spatial extent given by the London penetration depth λ ≈ 80 nm, well below the spatial resolution of the SQUID sensors used in this work (which have a pickup loop with an inner diameter of 600 nm 22 ). As such, we can treat the vortex as a magnetic point source with Simulations were performed using Autodesk Fusion 360 with room temperature material parameters to gain a qualitative understanding of the microscope's main vibration modes.…”

Versatile Millikelvin Hybrid Cooling Platform for Superconductivity Research

“…Thus far, vibrations between SQUID sensor and the imaged device or material are at best at 10's of nanometers level. While this level of vibrations is tolerable by micro-SQUIDs [12,[24][25][26], for nano SQUIDs [27][28][29][30] significant improvement needs to be realized in vibration mitigation approaches.…”

The on-chip scanning probe with dual niobium nitride nanoscale superconducting quantum interference devices for magnetic imaging at the high temperature

“…The SQUID loop of a few millimeters long was usually designed in a gradiometric structure to reduce the influence of the background magnetic field. However, most of these SQUID probes are made of superconducting materials like aluminum [22,23], indium [24], lead [25,26], and niobium (Nb) [27]. These probes have relatively low critical temperatures (T c ), which limits the exploration of the superconductors with high-transition temperatures by the SSM.…”