Nanosquid magnetometers and high resolution scanning squid microscopy


Troeman, Aico Giardano Paul (2007) Nanosquid magnetometers and high resolution scanning squid microscopy. thesis.

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Abstract:Recent interest in the development of small sized superconducting quantum interference devices (SQUIDs) has been motivated by the applicability of these sensors for the investigation of small, local, magnetic signals, such as the magnetization reversal of small magnetic clusters and the observation of local magnetic structures using a scanning SQUID microscope (SSM). Further miniaturization of the sensors offers a possibility to enhance the sensitivity and spatial resolution in these experiments. In the first part of this thesis the development of miniature SQUIDs based on Niobium nanobridges was
described. From a fabrication point of view the realization of such devices offers less practical limitations compared to the development of miniature sensors based on “classical” Josephson tunnel junctions.

The nanobridge based SQUIDs were patterned by means of focused ion beam milling. Since the ion beam profile is inhomogeneous, structures with widths that are smaller than the beam diameter can be created by letting two beam profiles overlap. Simulations and experiments aimed towards the determination of the effects of the ion induced damage on patterned devices have proven that, in Niobium structures patterned with a 25 keV Ga focused ion beam, superconductivity is suppressed as far as 35 nm inwards from the surface at T = 4.2 K. This result was taken into account when modeling the dimensions
of realized nanobridges.
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