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The Scanning SQUID Microscope

Introduction
Examples of past research
Current research
Reference and bibliography

Links:

Scanning SQUID Microscope Data Archive
New Scanning SQUID Microscope Data
Vortex Distributions near surface steps
 

In laboratories around the world there has been significant interest in developing magnetic imaging techniques that can measure microscopic distributions of magnetic fields, with the smallest possible spatial resolution (of the order of microns) and highest field sensitivity (small fractions of the flux quantum f0 = 2x10-15 Webers).  The primary motivations have been the desire to study mesoscopic devices, superconducting and magnetic materials, and to understand the dynamics and interactions of vortices, for example, in high temperature superconductors.  A very successful microscopic imaging technique meeting these requirements is the scanning SQUID microscope (SSM), typically based on low critical temperature superconducting SQUIDs. In such a system a superconducting pickup loop is coupled magnetically to a dc-SQUID, which is scanned over the surface of a sample to map out the magnetic field.

Our research group has built a SSM, which operates down to 2.5K with a Nb tri-layer dc SQUID and pickup loop. The pickup loop is scanned approximately 2mm above the surface and has a diameter of 10mm. The SSM is capable of detecting magnetic fields as small as 10-10T, which corresponds to a flux sensitivity of 10-6F0 in the loop.  The detector loop, fabricated near the end of the edge of a quartz substrate, is inductively coupled to the SQUID in a flip-chip configuration and is held by gravity against the surface as it is scanned.  We scan using computer-driven stepper motors achieving spatial positioning accuracy of better than 1mm at a scan range of 1cm x 1cm.

Sensitivity of different types of magnetic probes

 To improve the performance of the SSM, in our recently modified setup the sample is scanned under the pickup coil and a Ketchen-type SQUID gradiometer is used.  Further improvements include replacing the pickup coil and the SQUID by a fully integrated detector system with a pickup loop diameter of 4mm.  A similar system will be built for use in an ultra-low temperature system (dilution refrigerator).  This will enable us to study the variation of currents around vortices in unconventional superconductors and spontaneous currents in mesoscopic superconductor samples.

Examples of past research

• Superconductor networks and clusters

Lan Vu, Studies of magnetic vortices in superconductor networks and clusters by scanning SQUID microscopy, PhD thesis, University of Illinois at Urbana-Champaign, 1993

Scanning SQUID Microscope images of arrays:
 square array at 1/2-filling, triangle array at 1/4-filling,  square array at 2/5-filling, ensemble of 3x3 cluster arrays at 2/3 filling.

We have imaged vortex configurations in large square superconductor arrays and cluster arrays having a small number of cells,[i] as illustrated in the figure above.  One of the most fundamental properties of arrays is the ground state configuration of vortices when cooled in a magnetic field.  The resulting distribution reflects the competition between the vortex repulsion, which tries to space out the vortices into a triangular pattern, and the underlying array geometry, which forces the vortices into the pinning sites provided by the cells.  In square and triangle arrays, the expected ground state configurations have been calculated for low rational values of the frustration parameter f, the applied magnetic flux quanta per cell.  However, the observed configuration is complicated by disorder in the junction critical currents, thermal fluctuations that excite metastable states, inhomogeneous magnetic fields, and vortex entry barriers at the edges of the array.

• Imaging the flux structure in NbSe2, and measurement of the flux dynamics

Britton L. Plourde, Vortex distributions and dynamics in superconductor near surface and sample edges studied by SSM and critical current measurements, PhD thesis, University of Illinois at Urbana-Champaign, 2000

SSM image of the  flux distribution around a surface step in NeSe2 after  removing a 13.8Oe cooling field. The low side of the step is at the left.

The SSM was used to image both static and dynamic flux structures in NbSe2 crystals cooled in magnetic fields up to 14Oe. Static images of the vortex distribution in small magnetic fields show a substantial distortion by steps on the crystal surface. Vortex motion is induced by removing the cooling field abruptly, causing vortices to leave the sample. Using the excellent flux sensitivity of the SSM, we can image the changing flux patterns around the steps and we are also able to monitor the local temporal dynamics of the flux exit.

• Characterization of vortex arrays in patterned MoGe thin films.

Britton L. Plourde, Vortex distributions and dynamics in superconductor near surface and sample edges studied by SSM and critical current measurements, PhD thesis, University of Illinois at Urbana-Champaign, 2000

Vortex patterns in MoGe films imaged by SSM:
 uniform film, and film with surface trenches showing excess vortices pinned along inner step edges.

We have also used the SSM to study vortex distributions and dynamics in homogeneous superconducting thin films. [ii],[iii]   Most interesting are MoGe films, which have weak enough vortex pinning to form vortex lattices even at very low magnetic fields and for which critical currents in microfabricated strips are dominated by edge barriers and surface steps rather than by bulk pinning.  The figure shows an example of a vortex lattice in a uniform MoGe film and in a film with surface trenches at which vortices are pinned.  We have studied these effects extensively, answering some questions about the vortex arrangement and motion.

Current research:

• Characterization of vortex arrays in patterned MoGe thin films.

Bunching of vortices has been observed near the edges of patterned trenches in these films, such that the average magnetic field near these trench edges is larger than the applied fields. This may represent a "paramagnetic" Meissner effect, the physical basis of which is not yet theoretically understood.

• Unconventional superconductors.

We will use the SSM system to study the microscopic effects that occur at interfaces of high temperature superconductors that arise because of their unconventional d-wave symmetry. These intrinsic effects significantly modify the order parameter and quasiparticle density of states at free surfaces, interfaces, and grain boundaries. It is predicted that the suppression of the d-wave order parameter allows for the formation of zero-energy bound states on surfaces of (110) orientation, i.e., associated with the nodes in the gap.  It has been further predicted that under some circumstances, a secondary, subdominant order parameter with a p/2 phase shift with respect to the underlying d-wave order parameter can develop at much lower temperature than the bulk, d-wave transition. This results in a complex superconductor order parameter of the form d+is or d+id' at the surface that breaks time-reversal symmetry, and that has observable consequences, including the generation of spontaneous supercurrents and fractional vortices.

Therefore, one of the strongest confirmations of the existence of a subdominant order parameter that breaks time-reversal symmetry would be the direct observation of these surface currents.  These currents are estimated to generate fields that are a significant fraction of a single flux quantum over dimensions of order a micron, readily measurable by SSM.  To date, c-axis-oriented films of YBCO that have been ion-beam patterned with edges oriented along the (110) direction have not shown evidence of such currents. It is possible that damage to the edges during processing is responsible for the absence of such fields.

References and bibliography:

[i] L.N. Vu, M. S. Wistrom, and D. J. Van Harlingen.  Imaging of Vortex Configurations in Superconducting Networks and Clusters by Scanning SQUID Microscopy.  Appl. Phys. Lett. 63, 1693-1695 (1993).

 [ii] B. L. T. Plourde and D. J. Van Harlingen.  Scanning SQUID Microscopy of Flux Distributions and Motion near Surface Features in NbSe2.  Physics of Vortices, Flux Pinning, and Dynamics, NATO ASI 356, 281-290 (1999).

 [iii] B.L.T. Plourde, D.J. Van Harlingen, R. Besseling, M.B.S. Hesselberth and P.H. Kes.  Vortex dynamics in superconducting strips observed by Scanning SQUID Microscopy. Physica C: Superconductivity, 341-348 , 1023-1026 (2000).

Lan Vu, Studies of magnetic vortices in superconductor networks and clusters by scanning SQUID microscopy, PhD thesis, University of Illinois at Urbana-Champaign, 1993

Britton L. Plourde, Vortex distributions and dynamics in superconductor near surface and sample edges by SSM and critical current measurements, PhD thesis, University of Illinois at Urbana-Champaign, 2000