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|Posted: Fri Mar 28, 2008 4:03 pm Post subject: 'NanoSQUIDs' to improve magnetic microscopes
|19 June 2007 NewScientistTech
'NanoSQUIDs' to improve magnetic microscopes
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 "supercurrent" 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.
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.
"Making SQUIDs smaller makes it possible to increase the resolution," says Aico Troeman, of Mesa+ , part of the University of Twente in the Netherlands. Troeman says the SQUID developed by his team is "by far the smallest device currently out there." The device is around 180 nanometres in diameter.
Previously, so-called nanoSQUIDs have been used to "record" the magnetic flux over an area of around one micron square. But the device created by the Delft team has been used to look at areas more than thirty times smaller, just a few hundredths of a micron. "We think this could be important in the future for SQUID microscopes," Troeman says.
The devices were made from strips of niobium metal, which superconducts when chilled to 9.3 kelvin (-263.85 degrees Celsius).
"We focus a stream of high energy gallium ions onto the niobium," Troeman said, "It's like sanding away material in a workshop."
This cuts through the metal leaving only two thin "nanobridges", 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 Néel 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, "is interesting for imaging applications," 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's design does well.