Localized detection of very small regularly placed magnetic systems, such as
an array of tiny magnetic islands, has been of great interest to scientists for years
because of their applications to data storage media. One such detection device,
the micro-SQUID (microscopic superconducting quantum interference device), can
be used to detect very small changes in magnetic flux. Most low Tc micro-SQUIDs
(LTS) are made from aluminum or niobium. While an aluminum SQUID is relatively
easy to fabricate, one often needs a low temperature system than can be cooled to
1K to see the critical current phenomenon. As a contrast, niobium which has a
higher critical temperature, and a more complicated fabrication procedure due to its
need to be fabricated in a extremely clean environment to achieve a reproducible
value for its critical temperature, which is about 9.25 K. Such a SQUID will only
need to be immersed in a helium bath for the superconducting transition to occur.
Alternatives such as tin, indium and lead, which are soft superconductors, do not
wet a silicon/silicon dioxide surface as easily as niobium and aluminum. However,
the benefits of a successful implementation of these soft superconductors as SQUIDs
could greatly outweigh their drawbacks in terms of reducing the amount of time
necessary for fabrication and measurement as well as the low temperature system
requirements.
In this dissertation, the successful development of functional square and rectangular tin sub-microscopic SQUIDs for use as magnetometers is reported. The
application of a germanium pre-nucleation layer, as a means of creating a electrically
continuous path, offers an alternative to micro-SQUIDs fabricated under more in-
volved methods as used in niobium SQUIDs. An image of the device surface showed
that the roughness consisted of defects such as holes which gives rise to critical current
fluctuations and vortex pinning due to magnetic hysteresis. However, the oscillations
observed from several of these devices, were found to be smooth with sharp edges
but with a diminished period of oscillation. Several devices were tested and their
fabrication, measurement and characterization methods are described.
Another important study incorporated in our analysis of these tin germanium
SQUID included its reduction from the micrometer regime to its lowest functional ge-
ometry. Moreover, to avoid the operational breakdown of a SQUID due to magnetic
hysteresis and a diminution in sensitivity, the condition 2IcL / 0 had to be satisfied,
where Ic is the critical current and L is the inductance of the device and 0 is one flux
quantum 0 = h
2e = 20:86 gauss mm2. Experimental measurements showed that all
of these devices had magnetic hysteresis and operated outside of this constraint. In
addition, several devices exhibited extremely high critical currents when the temper-
ature was lowered a few milli-Kelvin past the transition temperature. Furthermore,
unstable regions were present in the minima of the modulations indicating that ad-
ditional quantum effects were incorporated into the device as a result of screening
currents and magnetic hysteresis behavior.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-05-8048 |
| Date | 2010 May 1900 |
| Creators | Ford, Arlene C. |
| Contributors | Teizer, Winfried |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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