Superconducting Quantum Interference Device

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Superconducting Quantum Interference Device
SQUID
C. P. Sun
National Sun Yat Sen University
Department of Physics
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Introduction
? Essential Mechanism
Josephson junction
?
A Josephson junction is an superconductor-insulato
r- superconductor (SIS) layer structure placed
between two electrodes
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DC Josephson A dc current flows across the
junction in the absence of
any electric or magnetic field.
AC Josephson A dc voltage applied across the
junction causes rf current
oscillations across the junction. This effect
has been utilized in a
precision determination of the value of
Further, an rf voltage applied
with the dc voltage can
then cause a dc current across the junction.
Macroscopic long-range quantum interference
A dc magnetic field applied
through a superconducting
circuit containing two junctions causes the
maximum supercurrent to
show interference effect as a function
of magnetic field intensity.
?Magnetometer
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DC Measurement
When the external dc field is applied to the
sample, the magnetization changed with
temperature will be obtained due to the
spin-spin, spin- orbital and orbital-orbital
effect interacting with magnetic field.
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AC Measurement An oscillating AC magnetic
field is applied to the sample. The change in
flux seen by the detection circuitry is caused
only by the magnetic moment of the sample as it
responses the applied AC field.
Xac dM/dH obtained from these measurement
is described as having both real and imaginary
components X and X , where the imaginary
component is proportional to the energy losses in
the sample.
What can be derived from AC Measurement? ?Structur
e details of materials, resonance phenomena,
electrical conductivity by induced currents,
relaxation processes such as flux creep in
SC and energy exchange between magnetic spins
and the lattice in the paramagnetic materials.
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Advantage of MPMS AC System

• Conventional AC suceptometers measure the
  voltage induced
• in an inductive detection coil by an oscillating
  AC magnetic moment.
• The most common systems use mutual inductance
  bridge to measure the voltage induced.
• These systems measure only signals with
  frequencies at or very near the applied
  excitation.

• ? High pressure effect

• ? The natural constraint

Voltage induced is proportional to the
frequencies of the oscillating drive field
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How does MPMS solve it?
? MPMS AC option combining an AC drive field with
a SQUID-based detection system.
?The SQUID is an extremely sensitive
flux-to-voltage converter that directly
measures the change in flux as the sample moves
through a superconducting detection coil
coupled to the SQUID circuit.
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RSO Measurement (Reciprocating Sample Option)

• RSO measure a sample by moving it rapidly and
  sinusoidally through the SQUID pickup coil.

The options use of a high-quality servo motor
and a digital signal processor (DSP) allow rapid
measurement. The servo motor, unlike the stepper
motor performing DC measurements, doesnt stop
sample movement for each data reading. Lock-in
techniques that use the DSP reduce the
contribution of low-frequency noise to the
measurement.
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Measure consideration of RSO
? Center position To ensure absolutely accurate
measurements even when temperature drifts. To
oscillate sample through most or all of pickup
coils while taking a high number of readings.
?Maximum slope To take measurements quickly. To
move sample through small section of pickup
coils. To perform hysteresis measurement.
sensitive to position
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Thank you for attention
Sun 2005.05.18
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Conclusion
Tc Curie Temperature? Ths phase transition
between paramagnetic
and ferromagnetic behavior.
TN Neel Temperature? Ths phase transition
between paramagnetic
and Antiferromagnetic behavior.
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Collect More Data After More Experience
More Experience can help us think more details
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The minimum value of magnetic flux
Fluxon
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Spin Glass? Material exhibiting a high magnetic
frustration and its
magnetic structure is disordered even at low
temperature.
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Tc Curie Temperature? Ths phase transition
between paramagnetic
and ferromagnetic behavior.
TN Neel Temperature? Ths phase transition
between paramagnetic
and Antiferromagnetic behavior.