The tiniest version yet of a superconducting device that measures faint magnetic fields has been created by researchers in the Netherlands. It could be used to improve the resolution of magnetic microscopes important for the future of electronics and biology.
SQUIDs (superconducting quantum interference devices) can measure vanishingly small magnetic fields. The most sensitive type is made from a loop of superconducting metal with two junctions.
The junctions present an obstacle to superconducting current or 鈥渟upercurrent鈥� flowing through the loop and, thanks to quantum properties of superconducting materials, this effect is closely related to the magnetic field of the loop. So, monitoring the current provides a roundabout way of measuring a nearby magnetic field.
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So-called Scanning SQUID microscopes (SSMs) run a small SQUID over a sample to build up an image of its magnetic properties. These can be used to investigate the properties of micro-electronics and measure the tiny magnetic fields produced by living cells.
Better resolution
鈥淢aking SQUIDs smaller makes it possible to increase the resolution,鈥� says Aico Troeman, of the Mesa+ Institute for Nanotechnology, part of the University of Twente in the Netherlands. Troeman says the SQUID developed by his team is 鈥渂y far the smallest device currently out there.鈥� The device is around 180 nanometres in diameter.
Previously, so-called nanoSQUIDs have been used to 鈥渞ecord鈥� the magnetic flux over an area of around one micron square. But the device created by the Twente team has been used to look at areas more than thirty times smaller, just a few hundredths of a micron. 鈥淲e think this could be important in the future for SQUID microscopes,鈥� Troeman says.
The devices were made from strips of niobium metal, which superconducts when chilled to 9.3 kelvin (-263.85 degrees Celsius).
鈥淲e focus a stream of high energy gallium ions onto the niobium,鈥� Troeman told New 杏吧原创, 鈥渋t鈥檚 like sanding away material in a workshop.鈥�
Nanobridges
This cuts through the metal leaving only two thin 鈥渘anobridges鈥�, each 80 nanometres across, holding the strip together and forming a completed loop. The two bridges constitute the junctions that obstruct the supercurrent and the remainder of the strip.
Wolfgang Wernsdorfer, at the Louis N茅el National Centre for Scientific Research in Grenoble, France, was part of a team that demonstrated a nanotube-based nanoSQUIDs at the end of 2006. Reducing the loop size of a SQUID further, as the Dutch group has done, 鈥渋s interesting for imaging applications,鈥� he says.
But shrinking the loop could make the device less useful for measuring individual magnetic structures, he points out, because they become more complex to use.
For such applications, shrinking the size of the junctions in the loop is more important, Wernsdorfer explains, something that his group鈥檚 design does well.
Journal reference: Nanoletters (DOI: 10.1021/nl070870f)
