SQUID On Tip sensors


A Superconducting QUantum Interference Device (SQUID) is the most sensitive device for measuring magnetic field. The main part of the device consists of a superconducting loop with two Josephson junctions or weak links coupled to two electrical leads. Since the complex superconducting order parameter, ψ(r)=|ψ(r)|eiφ(r), has to be single valued, the phase of the order parameter should change by a multiple of 2π around the loop, resulting in quantization of the flux in the loop in units of flux quantum Φ0 = hc/2e = 20.7 [Gauss μm2]. As a result, the properties of the loop, including its critical current and the resistance, become periodic with magnetic field or more precisely with the flux in the loop. The typical flux sensitivity of a SQUID is 10-6 Φ0, allowing measurement of magnetic field with extremely high sensitivity. Because the field sensitivity improves with increasing the area of the loop, most of the SQUIDs are large. Decreasing the size of the loop decreases field sensitivity but improves spatial resolution and increases the sensitivity for detecting microscopic magnetic moments.
SQUID schematic

A SQUID is schematically described by a superconducting loop with two weak links. The maximal current that can flow in the SQUID without dissipation is periodic in flux in the loop
 Ic(Φ) = I0|cos(π Φ/Φ0)|.


nanoSQUID on a tip

In recent years there is a growing interest in development of microSQUIDs and nanoSQUIDs for imaging and study of quantum magnetism. Imaging magnetic fields on a nanoscale is a major challenge in nanotechnology, physics, chemistry, and biology. One of the milestones in this endeavor is to achieve sensitivity sufficient for detection of the magnetic moment of a single electron. There are three main technological challenges: fabrication of a sensor with a high magnetic flux sensitivity, reducing the size of the sensor, and the ability to scan the sensor nanometers above the sample. Most of the current microSQUIDs are based on planar technology using lithographic or focused ion beam patterning methods. The planar geometry, however, prevents bringing the SQUID loop into sufficiently close proximity to the sample (due to alignment issues). We have developed a self-aligned fabrication method of smallest SQUIDs in the world which does not require any lithographic or processing steps. In addition, the major advantage is that this nanoSQUID resides on the apex of a very sharp tip that is ideally suited for scanning probe microscopy. The resulting SQUID-on-tip (SOT) made of Pb has an effective diameter of below 50 nm and flux noise of down to Φn = 50 nΦ0/Hz1/2 at 4.2 K that is operational up to unprecedented high fields of 1 T. The corresponding spin sensitivity of the device is Sn = 0.38 μB/Hz1/2, which is about two orders of magnitude more sensitive than any other scanning SQUID to date.


SQUID on tip fabrication

The fabrication technique of the SOT is very simple conceptually. A quartz tube of 1 mm outside diameter is pulled to form a pair of sharp pipettes with tip diameter that can be controllably varied between 25 and 400 nm using a commercial pipette puller (a Sutter Instrument P-2000 in our case). Then, Au leads are deposited or Indium leads are painted on two sides of the cylindrical base of the pipette. Finally, the pipette is mounted on a rotator and put into a vacuum chamber for three "self-aligned" steps of thermal evaporation of a superconducting film such as Al or Pb. In the first step, ~25 nm of Pb are deposited on the tip tilted at an angle of -110° with respect to the line to the source. Then the tip is rotated to an angle of 110°, followed by a second deposition of ~25 nm. As a result, two leads on opposite sides of the quartz tube are formed. In the last step ~17 nm of Pb are evaporated at an angle of 0°, coating the apex ring of the tip. The resulting structure has two leads connected to a ring. "Strong" superconducting regions are formed in the areas where the leads make contact with the ring, while the two parts of the ring in the region of the gap between the leads constitute two weak links, thus forming the SQUID. The resulting nanoSQUID-on-tip requires no lithographic processing, its size is controlled by a conventional pulling procedure of a quartz tube, and it is located at the apex of a sharp tip fit for scanning probe microscopy.

nanoSQUID on tip fabrication recipe.


Additional information

    1. A scanning superconducting quantum interference device with single electron spin sensitivity
      D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber and E. Zeldov
      Nat. Nano 8, 639 (2013)
    2. Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena
      A. Finkler, D. Vasyukov, Y. Segev, L. Ne'eman, E. O. Lachman, M. L. Rappaport, Y. Myasoedov, E. Zeldov, and M. E. Huber
      Rev. Sci. Instrum. 83, 073702 (2012).
    3. Self-aligned nanoscale SQUID on a tip
      A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, L. Ne'eman, D. Vasyukov, E. Zeldov, M. E. Huber, J. Martin and A. Yacoby
      Nano Letters 10, 1046 (2010)
    4. Electrically tunable multi-terminal SQUID-on-tip
      Aviram Uri, Alexander Y. Meltzer, Yonathan Anahory, Lior Embon, Ella O. Lachman, Dorri Halbertal, Naren HR, Yuri Myasoedov, Martin E. Huber, Andrea Young, Eli Zeldov
    5. Three-junction SQUID-on-tip with tunable in-plane and out-of-plane magnetic field sensitivity
      Y. Anahory , J. Reiner , L. Embon, D. Halbertal, A. Yakovenko, Y. Myasoedov, M. L. Rappaport , M. E. Huber, and E. Zeldov
      Nano Lett. 14, 6481-6487 (2014)