Gradiometric micro-SQUID susceptometer for scanning measurements of mesoscopic samples.

We have fabricated and characterized micro-SQUID susceptometers for use in low-temperature scanning probe microscopy systems. The design features the following: a 4.6 mum diameter pickup loop; an integrated field coil to apply a local field to the sample; an additional counterwound pickup-loop/field-coil pair to cancel the background signal from the applied field in the absence of the sample; modulation coils to allow setting the SQUID at its optimum bias point (independent of the applied field), and shielding and symmetry that minimizes coupling of magnetic fields into the leads and body of the SQUID. We use a SQUID series array preamplifier to obtain a system bandwidth of 1 MHz. The flux noise at 125 mK is approximately 0.25 mu Phi 0/ sqrt Hz above 10 kHz, with a value of 2.5 mu Phi 0/ sqrt Hz at 10 Hz. The nominal sensitivity to electron spins located at the center of the pickup loop is approximately 200 muB/ sqrt Hz above 10 kHz, in the white-noise frequency region.

[1]  Alex I. Braginski,et al.  The SQUID handbook , 2006 .

[2]  Dick Veldhuis,et al.  NanoSQUIDs Based on Niobium Constrictions , 2007 .

[3]  John Clarke,et al.  dc SQUID: Noise and optimization , 1977 .

[4]  C. Burroughs,et al.  Superconducting integrated circuit fabrication with low temperature ECR-based PECVD SiO/sub 2/ dielectric films , 1995, IEEE Transactions on Applied Superconductivity.

[5]  Clarke,et al.  Hot-electron effects in metals. , 1994, Physical review. B, Condensed matter.

[6]  J. C. Price,et al.  Susceptibility of a mesoscopic superconducting ring , 1997 .

[7]  William J. Gallagher,et al.  High‐resolution scanning SQUID microscope , 1995 .

[8]  Claudia D. Tesche,et al.  dc SQUID: Current noise , 1979 .

[9]  Design and performance aspects of pickup loop structures for miniature SQUID magnetometry , 1995, IEEE Transactions on Applied Superconductivity.

[10]  Mattan Kamon,et al.  FASTHENRY: a multipole-accelerated 3-D inductance extraction program , 1994 .

[11]  Dominique Mailly,et al.  DC-SQUID Magnetization Measurements of Single Magnetic Particles , 1995 .

[12]  John M. Martinis,et al.  DC SQUID series array amplifiers with 120 MHz bandwidth (corrected) , 2001 .

[13]  John Clarke,et al.  QUANTUM NOISE THEORY FOR THE dc SQUID , 1981 .

[14]  J. Clarke,et al.  Low‐frequency noise in dc superconducting quantum interference devices below 1 K , 1987 .

[15]  D. Mailly,et al.  Experimental observation of persistent currents in GaAs-AlGaAs single loop. , 1993, Physical review letters.

[16]  M. Ketchen,et al.  Scanning superconducting quantum interference device susceptometry , 2001 .

[17]  R. L. Peterson,et al.  Analysis of threshold curves for superconducting interferometers , 1979 .

[18]  Naia Venturi,et al.  Compact large‐range cryogenic scanner , 1995 .

[19]  J. Waldram,et al.  Superconducting Quantum Interference Devices and their Applications , 1982 .

[20]  M. Huber,et al.  Fluctuation Superconductivity in Mesoscopic Aluminum Rings , 2007, Science.

[21]  D. L. Tilbrook,et al.  Development of a niobium nanosuperconducting quantum interference device for the detection of small spin populations , 2003 .

[22]  M. Monthioux,et al.  Carbon nanotube superconducting quantum interference device , 2006, Nature nanotechnology.

[23]  D. Awschalom,et al.  Design, fabrication, and performance of integrated miniature SQUID susceptometers , 1989 .