SQUID Gradiometer Arrays for Ultra-Low Temperature Magnetic Micro-Calorimeter

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SQUID Gradiometer Arrays for Ultra-Low
Temperature Magnetic Micro-Calorimeter
V. Zakosarenko, R. Stolz, S. Anders, L. Fritzsch,
and H.-G. Meyer Institute for Physical High
Technology, Albert-Einstein-Str. 10, D-07745
Jena, Germany A. Fleischmann and C.
Enns Kirchhoff Institute of Physics, Ruprecht
Karls University of Heidelberg, Im Neuenheimer
Feld 227, D-69120 Heidelberg, Germany. The work
is supported by the German BMBF under the
contract No. 13N8225.
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Contents

• Principles of magnetic calorimeter
• Experiments in KIP Uni Heidelberg
• SQUID technology at IPHT Jena
• Layout of SQUID gradiometers as sensors
• Integrated field coil
• 8-pixel SQUID array with integrated field coil
• Conclusions

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Principles of Magnetic Calorimeter
Eg gt DT gt DM
Sensor AuEr AuYb
Bi2Te3Er PbTeEr
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Integrated SQUID Gradiometer
Optimal sensor parameters and demands to SQUID?
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Magnetization and Heat Capacity
Optimal magnetic field 5 mT Challenge for SQUID !
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Magnetic Susceptibility of AuEr
AuEr 600ppm
Optimal working temperature 50 mK
spin glass
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Double-SQUID Concept
Noise can be very low
Detector SQUID read out by a current-sensor SQUID
Low noise Large Slewrate Small power dissipation
on detector SQUID chip (Voltage bias)
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Experiments at KIP (University of Heidelber)
Two-pixel detector Detector SQUID Ketchen, IBM
Two AuEr 300 ppm sensors ? 50 ?m, h 25
?m Magnetic field 3 mT
Two gold absorbers 160 x 160 x 5 ?m3
Double SQUID scheme SQUID amplifier standard
current sensor Model CCblue from Supracon
(Jena) current noise 2pA/ Hz1/2
Directly coupled SQUID electronics from Supracon
voltage noise 0.3nV/ Hz1/2 very low
temperature dependence
ADR VeriCold Technologies, Munich base
temperature 21 mK, holdtime below 30 mK
2 days
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Results of the Experiment
raw data
Total heat capacity of 2.5 pJ/K . Performance of
both sensors are almost identical.
Energy resolution 3.4 eV _at_ 6 keV
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Baseline Noise
Resolution is constant over the whole energy
range, Small temperature drifts, Small position
dependence.
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Thermalization
Sensor 1000x larger, spot-welded to copper block
Heat capacity 10-9 J/K
Sensor bonded with vacuum grease to Si Heat
capacity 1.2 x 10-12 J/K
Magnetic calorimeters can be made very fast.
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SQUID-Technologie am IPHT Jena
SQUID current sensor SC8B, fabricated in the
standard-Nb/Al2O3/Nb technology at IPHT Jena.
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Gradiometer MC1 - Layout

• Integrated field coil
• in the washer 50mT/A
• in the center 38mT/A
• The magnetic field at both washers is equal ?
  drift compensation.

SQUID inductance 60 pH Critical current
12µA Voltage swing 50µV Feedback coil
coupling
27.5mA/F0 White noise level (4.2K)
1.3mF0/Hz1/2
Superconducting short with thermal switch for
operation with Persistant Current.
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Gradiometer MC3 Coil
Problem Critical current of the coil in all
layouts does not exceed 20mA Field1...1,5 mT
Steps reason for low Jc (?)
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Conclusions for the layout
? Josephson junctions operates in the desirable
magnetic fields. ? SQUIDs show good
parameters. ? Superconducting shorts with
thermal switches operate well.
? Critical current of the field coils is to low.
gt needs of further dewelopment of the technology
and/or layout.
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New Detector-SQUID layout
Field coil in the first wiring. Coil with more
turns.
Current leads in the upper wiring is much wider.
SQUID as a serial gradiometer.
Washer size optimized to dimensions of Au/Er
pill.
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Adapted Technology

• Special etching technique to get flat edges in
  the first wiring.
• Larger film thickness of the second wiring.

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Thermalization
Gold strips for better thermalization of the
sensor
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Eight-Pixels Array
4 SQUIDS (8 pixels in 200 200 µm grid),
Bond pads for thermal contact,
Field coil common for two SQUIDs
92mT/A, Magnetic field up to 6mT,
Optimized persistent current switch 4mW in
liquid He, SQUID remains superconducting!
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Conclusions

• Gradiometer SQUIDs for magnetic
  micro-calorimeters are developed and fabricated.
• Josephson junctions are able to operate in the
  desirable magnetic fields.
• SQUID parameters correspond to the design
  values.
• The integrated field coils were developed and
  tested.
• Superconducting shorts with thermal switch
  operate well, the desired persistent current
  could be frozen in the coil.
• Eight-pixel SQUID arrays are designed and
  fabricated.
• SQUIDs and SQUID arrays can be used for real
  measurements.