Physicists Build Ultrasensitive Magnetic Sensors

The researchers have used the materials, called silver chalcogenides, to make extremely precise measurements of magnetic fields as high as 600,000 Gauss, or more than a million times the Earth's magnetic field, at the National High Magnetic Field Laboratory at Los Alamos National Laboratory. Conventional techniques have difficulty measuring magnetic fields more than approximately 250,000 Gauss, and they do so with less accuracy.

In the medical arena, the megagauss sensors would serve as a more sensitive detector for high-field Magnetic Resonance Imaging applications.

"If you need to know the magnetic field very accurately, and if the field changes a lot over a small region of space, then these sensors would be very good," said Thomas Rosenbaum, the James Franck Professor in Physics at the University of Chicago.

The sensors also could be useful for studying the physics that underlies high-temperature superconductors, which have been touted for potential applications ranging from superfast computers to levitating trains.

"One way to do that is to apply a magnetic field that kills the superconductivity and ask, what's left? What does that so-called normal state look like? The magnetic fields you need for that are huge," Rosenbaum said. "These large magnetic fields let you get at physics that you otherwise just can't get at."

The study was led by Anke Husmann, formerly a postdoctoral researcher at the University of Chicago, now with Toshiba Research Laboratory in Cambridge, England. Her co-authors also included Jonathan Betts, Greg Boebinger and Al Migliori of the National High Magnetic Field Laboratory at Los Alamos and Marie-Louise Saboungi at Argonne National Laboratory.

"The drive to obtain the highest possible magnetic fields cuts across disciplinary boundaries, but it has particularly potent implications for materials science," wrote Husmann and her colleagues in their Nature article. That's because magnetic fields change the way electrons move through solids. "The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications," they wrote.

The research team knew from previous experiments that silver chalcogenides-which consist of two parts silver, one part selenium and one part tellurium-showed no response to magnetic field. "But if you went with a tiny bit extra silver, say one part in 10,000, or a little more tellurium or selenium, all of a sudden the material was incredibly sensitive to magnetic field," Rosenbaum said.

The team had originally planned to study silver chalcogenides to see if they could probe the structure of glassy materials by looking at the movement of electrical charge. But when they applied a magnetic field to the material, they observed an unexpectedly large response.

"This is a classic case of serendipity. We were looking for something different," Rosenbaum said.

Rosenbaum noted that the sensors are small, cheap and sensitive. The sensors measure approximately one cubic millimeter, about the size of a pencil tip. The materials cost only pennies in those quantities, and they are capable of measuring extremely large magnetic fields with an accuracy of a fraction of a percent.

Last year Husmann and Rosenbaum (University of Chicago), patented their sensors along with David Price and Marie-Louise Saboungi (Argonne), and the late Rong Xu (Chicago and Argonne). The research was funded by the U.S. Department of Energy.