Research:
nanoSQUIDs
nanoSQUID 
publications
We also use our fabrication technology for Josephson voltage standards for fabricating other circuits. Especially the fabrication of very small structures or junctions allows additional and new applications. Recently, we have enhanced our technology and fabricated different kinds of nanoSQUIDs within the framework of co-operations and projects with the Physical Institute, section 
solid state physics of the University Tübingen as well as with the 
department 7.2 of PTB Berlin (SQUID: Superconducting QUantum Interference Device). These nanoSQUIDs are based on SNS Josephson junctions with the normal metal HfTi as the barrier. The goal of these research activities are nanoSQUIDs for detection of small spin systems (e.g. magnetic nanoparticles) with high spatial resolution and sensitivity and, simultaneously, with high resistivity against external magnetic fields.
For fabricating the nanoSQUIDs we use a technology, which is based on electron beam lithography and chemical-mechanical polishing (CMP). We need very small Josephson junction with dimensions in the nanometer scale for the nanoSQUIDs; we aim for typical dimensions below 100 nm x 100 nm. Simultaneously, a sufficient critical current is required. We meet these demands by using SNS Josephson junctions with a HfTi barrier; these junctions enable very high critical current densities in a wide range from about 50 kA/cm2 to 600 kA/cm2.
We have fabricated nanoSQUIDs of different designs, which are very complex in part. An important advantage of our planar technology is the flexibility to integrate the SQUID loop in-plane and / or out-of-plane. We have also reduced these SQUID loops down to the nanometer size, which results in a high spatial resolution and in a high resistivity against external magnetic fields. Two scanning electron microscope (SEM) pictures show structures of our nanoSQUIDs.
SEM picture of some details of a nanoSQUID circuit (design: PTB Berlin) after finishing two steps of the fabrication process. The Josephson junctions are clearly visible as small squares on top of the Niobium wires in the middle of the picture.
The fabricated nanoSQUIDs show an extremely low flux noise in the order SΦ1/2 < 200 nΦ0/Hz1/2. The spin sensitivity of about 20 μB/Hz1/2 is very high and shall be optimised in direction of single spin sensitivity. The nanoSQUIDs have been operated in magnetic fields up to 0.5 T. The nanoSQUIDs have been already used in first application for detecting magnetic nanoparticles, as, for example, in a scanning SQUID microscope for visualisation of single Abrikosov vortices as well as for direct illustration of the magnetisation of a nanotube. Additional details are given in the publications listed below.
The discussed research activities have been performed in part within the framework of national and international projects, e.g. in the EMRP project "MetNEMS" or the DFG project "Hochempfindliche nanoSQUIDs zur Detektion kleiner Spin-Systeme" (ultrasensitive nanoSQUIDs for the detection of small spin systems; additional information (in German) here).
SEM picture of details of a planar coil (design: PTB Berlin). The Niobium wiring has a width of 200 nm and a spacing of 200 nm. The picture clearly shows the high quality of the fabrication process.
nanoSQUID publications
J. Nagel, O.F. Kieler, T. Weimann, R. Wölbing, J. Kohlmann, A.B. Zorin, R. Kleiner, D. Koelle, and M. Kemmler, "Superconducting quantum interference devices with submicron Nb/HfTi/Nb junctions for investigation of small magnetic particles," Appl. Phys. Lett. 99 (2011) 032506 (3 pp). (doi:10.1063/1.3614437)
R. Wölbing, J. Nagel, T. Schwarz, O. Kieler, T. Weimann, J. Kohlmann, A. B. Zorin, M. Kemmler, R. Kleiner, and D. Koelle, "Nb nano superconducting quantum interference devices with high spin sensitivity for operation in magnetic fields up to 0.5 T," Appl. Phys. Lett. 102 (2013) 192601 (4 pp). (doi:10.1063/1.4804673)
A. Buchter, J. Nagel, D. Rüffer, F. Xue, D.P. Weber, O.F. Kieler, T. Weimann, J. Kohlmann, A.B. Zorin, E. Russo-Averchi, R. Huber, P. Berberich, A. Fontcuberta i Morral, M. Kemmler, R. Kleiner, D. Koelle, D. Grundler, and M. Poggio, "Reversal mechanism of an individual Ni nanotube simultaneously studied by torque and SQUID magnetometry," Phys. Rev. Lett. 111 (2013) 067202 (5 pp). (doi:10.1103/PhysRevLett.111.067202)
J. Nagel, A. Buchter, F. Xue, O. F. Kieler, T. Weimann, J. Kohlmann, A. B. Zorin, D. Rüffer, E. Russo-Averchi, R. Huber, P. Berberich, A. Fontcuberta i Morral, D. Grundler, R. Kleiner, D. Koelle, M. Poggio, and M. Kemmler, "Nanoscale multifunctional sensor formed by a Ni nanotube and a scanning Nb nanoSQUID," Phys. Rev. B 88 (2013) 064425 (7 pp). (doi:10.1103/PhysRevB.88.064425)
S. Bechstein, F. Ruede, D. Drung, J.-H. Storm, C. Kohn, O.F. Kieler, J. Kohlmann, T. Weimann, T. Patel, Bo Li, D. Cox, J.C. Gallop, L. Hao, and T. Schurig, "Design and fabrication of coupled nanoSQUIDs and NEMS," IEEE Trans. Appl. Supercond. 25 (2015) 1602604 (4 pp). (doi:10.1063/1.4909523)
S. Bechstein, F. Ruede, D. Drung, J.-H. Storm, O. F. Kieler, J. Kohlmann, T. Weimann, and T. Schurig, "HfTi-nanoSQUID gradiometers with high linearity," Appl. Phys. Lett. 106 (2015) 072601 (4 pp). (doi:10.1063/1.4909523)
J. Beyer, M. Klemm, J.-H. Storm, O. Kieler, T. Weimann, and V. Morosh, "Noise of dc-SQUIDs with planar sub-micrometer Nb/HfTi/Nb junctions," Supercond. Sci. Technol. 28 (2015), 085011 (8 pp). (doi:10.1088/0953-2048/28/8/085011)
A. Buchter, R. Wölbing, M. Wyss, O.F. Kieler, T. Weimann, J. Kohlmann, A.B. Zorin, D. Rüffer, F. Matteini, G. Tütüncüoglu, F. Heimbach, A. Kleibert, A. Fontcuberta i Morral, D. Grundler, R. Kleiner, D. Koelle, and M. Poggio, "Magnetization reversal of an individual exchange-biased permalloy nanotube," Phys. Rev. B 92 (2015) 214432 (7 pp). (doi:10.1103/PhysRevB.92.214432)
M.J. Martínez-Pérez, D. Gella, B. Müller, V. Morosh, R. Wölbing, J. Sesé, O. Kieler, R. Kleiner, and D. Kölle, "Three-axis vector nano superconducting quantum interference device", ACS Nano 10 (2016) 8308-8315. (doi:10.1021/acsnano.6b02218)
S. Bechstein, C. Köhn, D. Drung, J.-H. Storm, O. Kieler, V. Morosh, and T. Schurig, "Investigation of nanoSQUID designs for practical applications", Supercond. Sci. Technol. 30 (2017) 034007 (9pp). (doi:10.1088/1361-6668/aa557f)