SQUID on tip sensors
SQUIDs
A
Superconducting QUantum Interference Device (SQUID) is the most
sensitive device for
measuring magnetic field. The main part of the device consists of a
superconducting loop with two Josephson junctions or weak links coupled
to two electrical leads. Since the complex superconducting order
parameter, ψ(r)=|ψ(r)|eiφ(r), has
to be single valued, the phase of the order parameter should change by
a multiple of 2π around the loop, resulting in quantization of the flux
in the loop in units of flux quantum Φ0 = hc/2e = 20.7
[Gauss μ m2]. As a result, the properties of the
loop, including its critical current and the resistance, become
periodic with magnetic field or more precisely with the flux in the
loop. The typical flux sensitivity of a SQUID is 10-6 Φ0,
allowing measurement of magnetic field with extremely high sensitivity.
Because the field sensitivity improves with increasing the area of the
loop, most of the SQUIDs are large. Decreasing the size of the loop
decreases field sensitivity but improves spatial resolution and
increases the sensitivity for detecting microscopic magnetic moments.
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| A SQUID is schematically
described by a superconducting loop with two weak links. |
The
maximal current that can
flow in the SQUID without dissipation is periodic in flux in the loop Ic(Φ) = I0|cos(π Φ/Φ0)|. |
nanoSQUID on a tip
In
recent years
there is a growing interest in development of microSQUIDs for imaging
and study of quantum magnetism. Imaging magnetic fields on a nanoscale
is a major challenge in nanotechnology, physics, chemistry, and
biology. One of the milestones in this endeavor is to achieve
sensitivity sufficient for detection of the magnetic moment of a single
electron. There are three main technological challenges: fabrication of
a sensor with a high magnetic flux sensitivity, reducing the size of
the sensor, and the ability to scan the sensor nanometers above the
sample. Most of the current microSQUIDs are based on planar technology
using lithographic or focused ion beam patterning methods. The large
in-plane size of the planar devices, however, prevents bringing the
SQUID loop into sufficiently close proximity to the sample (due to
alignment issues) to scan it with optimal sensitivity.
We have developed a very simple self-aligned fabrication method which
results in the smallest SQUIDs to date. The layout of the nanoSQUID is
simply set by the geometry of a pulled quartz tip with no lithographic
steps and no processing involved. In addition, the major advantage is
that this nanoSQUID resides on a very sharp tip that is ideally suited
for scanning probe microscopy. Another significant advantage of the
SQUID on tip is that it can operate at fields as high as 0.5 T.
Finally, the fact that the I-V characteristics of the SQUID on tip can
be made asymmetric allows high sensitivity operation of the SQUID
essentially at any field by proper selection of the bias voltage and
polarity. The following table presents figures of merit of an aluminum
SQUID on
tip with a diameter of 200 nm.
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SQUID on tip fabrication
The
sensor
preparation is conceptually very simple. A quartz tube of 1 mm outside
diameter is pulled to form a pair of sharp pipettes with tip diameter
that can be controllably varied between 100 and 400 nm using a
commercial pipette puller (a Sutter Instrument P-2000 in our case).
Then, Au leads are deposited or Indium leads are painted on two sides
of the cylindrical base of the pipette. Finally, the pipette is mounted
on a rotator and put into a vacuum chamber for three "self-aligned"
steps of thermal evaporation of Aluminum. In the first step, 25 nm of
Al are deposited on the tip tilted at an angle of -100° with respect to
the line to the source. Then the tip is rotated to an angle of 100°,
followed by a second deposition of 25 nm. As a result, two leads on
opposite sides of the quartz tube are formed. In the last step 17 nm of
Al are evaporated at an angle of 0°, coating the apex ring of the tip.
The resulting structure has two leads connected to a ring. "Strong"
superconducting regions are formed in the areas where the leads make
contact with the ring, while the two parts of the ring in the region of
the gap between the leads constitute two weak links, thus forming the
SQUID. The resulting nanoSQUID requires no lithographic processing, its
size is controlled by a conventional pulling procedure of a quartz
tube, and it is located at the apex of a sharp tip fit for scanning
probe microscopy.
  nanoSQUID on tip fabrication recipe |
Measurement circuitry
SQUIDs
are usually
current biased and their voltage is measured. We apply voltage bias
instead, using a low resistance bias resistor and measure the current
of the SQUID on tip using low temperature series SQUID array current to
voltage amplifier (SSAA) fabricated at NIST. This is a chip that
consists
of an array of a hundred SQUIDs in series that are inductively coupled
to the current from the SQUID and operate in a closed feedback loop
using an inductively coupled feedback coil. The noise level of the
series SQUID array amplifier is few pA/Hz1/2.
 The SQUID on tip measurement circuit. The
current through the SQUID on tip is inductively coupled to an SSAA - a
cryogenic current
amplifier.
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SQUID on tip characteristics
The I-V
characteristics of SQUID on tip display several interesting features.
First, the I-V characteristics show no hysteresis which is a
significant advantage of our nanoSQUIDs and of the voltage bias setup,
which avoids the need for complicated pulsed measurements. Second, a
large negative differential resistance is present over a wide range of
biases, which is consistent with the resistively shunted model of a
Josephson junction taking into account our voltage bias circuit. Third,
the SQUIDs on tip show large modulation of the critical current Ic
with
field that is required for a high field sensitivity. Finally, the I-V
characteristics display fine structure at high biases which apparently
results from some resonances.
 
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SQUID-on-tip as a sample topography sensor
The main advantage of the
SQUID-on-tip sensors is the ability to bring them very close to the
sample and to scan within nanometers above the surface of a sample. In
order to prevent crashing the tip and to maintain a constant height
over the sample one needs a method for sensitive sensing of tip-sample
distance and a feedback mechanism. We achieve this by gluing the tip to
a quartz crystal oscillator in the shape of a tuning fork that is
commonly used in quartz watches. When the tip is brought to within a
few nanometers from sample it dampens the tuning fork resonance. The
frequency shift or the reduction in amplitude of the resonance peak of
the tuning fork is used as a feedback mechanism that drives a piezo
device for maintaining a predefined tip-sample distance. Using this
approach we integrated the SQUID on tip into a scanning probe
microscope operating at 300 mK which provides a simultaneous imaging of
sample topography and of the local magnetic field on the nanoscale.
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For additional information, see:
- Self-aligned nanoscale SQUID on a tip
A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, L. Ne'eman, D. Vasyukov, E. Zeldov, M. E. Huber, J. Martin and A. Yacoby
Nano Letters 10, 1046 (2010) - dc SQUID: Noise and optimization
C. D. Tesche and J. Clarke
J. Low Temp. Phys. 29, 301 (1977) - A series array of DC SQUIDs
R. P. Welty and J. M. Martinis
IEEE Trans. Magn. 27, 2924 (1991) - DC SQUID series array amplifiers with 120 MHz bandwidth
M. E. Huber, P. A. Neil, R. G. Benson, D. A. Burns, A. F. Corey, C. S. Flynn, Y. Kitaygorodskaya, O. Massihzadeh, J. M. Martinis and G. C. Hilton
IEEE Trans. Appl. Supercond. 11, 4048 (2001)