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UH Researcher Developing New Type of Magnetic Field Sensor
Toby Weber

Stanko Brankovic, assistant professor of electrical & computer engineering with the University of Houston Cullen College of Engineering, has won a grant worth up to $1.6 million from the United States Department of Defense.

Brankovic, along with co-principle investigator Paul Ruchhoeft, also an assistant professor of electrical & computer engineering with the college, will use the grant to create a new type of magnetic field sensor that, if successful, will be hundreds, perhaps thousands of times more sensitive than anything currently available.

Such sensors would have multiple uses. On the military front, which is where the DOD’s interest lies, hundreds of thousands or more of these sensors could be the key components in a low-cost system that maps mine fields quickly and accurately. In the medical arena, the sensors could be applied to magnetic resonance imaging, yielding highly detailed images of, for example, a tumor or an injured knee.

The funding for the project, “Single Ferromagnetic Nanocontact-Based Devices as Magnetic Field Sensors,” will be delivered in two stages. The first stage, valued at $100,000 for one year, requires a proof of concept, in which Brankovic must construct a working sensor. To do this, he will utilize new ideas in the nanoengineering of novel materials and the development of nanofabrication processes for devices smaller than 10 nanometers.

Should he succeed, the DOD will then consider awarding him an extra $1.5 million for the completion of an entire system that incorporates multiple sensors, data-transmission equipment, and equipment and software that translates data into an easily understandable format.

Brankovic’s sensors will be based upon the phenomenon known as “ballistic magnetoresistance,” which is the effect of a magnetic field on the ability of electrons to flow between magnetic electrodes through a nanocontact—a tiny wire measuring in the nanometers that forms naturally between magnetic electrodes.

If the two elctrodes’ magnetic orientations (the direction in which a material’s magnetism pushes or pulls) are different, some of the electrons flowing between them will be repelled by the spot in the nanocontact where the two different magnetizations meet. This physical space is known as the magnetic domain wall.

When exposed to a magnetic field, however, electrons’ ability to travel through the nanocontact changes due to the resulting change in magnetic orientation of the electrodes. Depending on the size and material of the nanocontact and magnetization of the electrodes, the electrons will flow through either more or less easily. This change can be measured by simple tools such as a voltmeter.

On the bulk scale, magnetoresistance is only one factor in determining how easily electrons travel between electrodes. On the nanoscale, in which these magnetic field sensors will be constructed, it is the only cause of fluctuation in the flow of electrons.

“When you reduce the size of the system to the level below a certain point, the electrons go through nanocontact without any thermal scattering [the scattering that occurs when electrons bounce off atoms in the conductive material],” said Brankovic. “For magnetic materials, the electron scattering at that nanoscale is caused only by magnetic domain walls introduced by a magnetic field. Otherwise, electrons travel more or less ballistically.”

The heart of Brankovic’s system, then, will consist of two magnetic electrodes, connected by a very small magnetic nanocontact. When exposed to a magnetic field, the flow of electrons through the nanocontact will change, yielding a measurable result.

Exactly how magnetoresistance works on this scale is unknown, and will be one of the subjects of Brankovic’s research. Two of the main theories to explain the phenomenon—both of which are supported by limited physical evidence—are incompatible.

Brankovic has developed his own theory that, if it proves correct, would account for the both sets of evidence. In this theory, the nanocontact that connects the two electrodes is composed of non-conductive metal oxide that has metal channels that act as conductive pathways for electrons. When exposed to a magnetic field, some, but not all, of the channels of conductive material are altered either by the magnetic domain wall or by magnetostriction, the phenomena of material’s shape changing slightly when exposed to a magnetic field. Either of these explanations would result in a small but measurable change in the flow of electrons.

Whether this theory proves correct or if magnetic resistance on the nanoscale works in some other manner, Brankovic’s goal will remain the same: to build a first-of-its kind magnetic field sensor that is far more powerful than any other sensor to date. Should he succeed, his invention will be a sea of change in the arena of magnetic field detection.

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