Breakthrough in Nanoscale Magnetic Sensors by UB Researchers May Make Ultra-High Density Storage Practical

BUFFALO, N.Y. -- A simpler and more reliable manufacturing method has allowed two materials researchers from the University at Buffalo to produce nanoscale magnetic sensors that could increase the storage capacity of hard disk drives by a factor of 1,000.

Building on results obtained last summer, the new sensors are up to 100 times more sensitive than any current alternative technology, according to researchers Harsh Deep Chopra, UB associate professor of mechanical and aerospace engineering, and Susan Hua, director of UB's Bio-Micro-Electro-Mechanical-Systems Facility and adjunct professor of mechanical and aerospace engineering.

The breakthrough could impact significantly the multi-billion-dollar storage industry. Their work is supported by the National Science Foundation (NSF).

As reported in the February issue of Physical Review B, Hua and Chopra's latest experiments with nanoscale sensors produce, at room temperature, unusually large electrical resistance changes in the presence of small magnetic fields.

"We first saw a large effect of over 3,000 percent resistance change in small magnetic fields last July," Chopra explains. "These latest results show that what we reported at the time was just the tip of the iceberg, pointing to beautiful science that remains to be discovered."

The largest signal they have seen is 33 times larger than the effect they reported last summer, which corresponds to a 100,000 percent change in resistance, the researchers say.

As stored "bits" of data get smaller, their magnetic fields get weaker, which makes individual bits harder to detect and "read." Packing more bits onto the surface of a disk, therefore, requires reliable sensors that are smaller, yet more sensitive to the bit's magnetic field.

Hua and Chopra's nanoscale sensors seem to be ideally suited to the task. The sensors produce much more distinct and reliable signals than current technologies do, which would enable the bit size to be shrunk dramatically.

Chopra and Hua's sensors have another advantage over other experimental techniques: Because of the sensors' high sensitivity at room temperature, they could be adapted more easily to work with existing hard-disk drive technologies used by the $25 billion data storage industry. Chopra predicts that their sensors would permit disk capacities on the order of terabits (trillions of bits) per square inch.

Hua and Chopra's novel work with magnetic sensors already has attracted industry interest. The Forbes/Wolfe Nanotech Report cited their research as one of the top five nanotechnology breakthroughs of 2002, and Industry Week selected Hua and Chopra as one of the 21 top research and development stars of 2002.

"There are many scientists pursing this type of research," Chopra says. "Our continued success is due largely to our (Chopra and Hua's) overlapping backgrounds in materials synthesis and magnetics, which creates a unique synergy within our research group."

Their success builds on an effect called "ballistic magnetoresistance" (BMR). "Magnetoresistance" measures the change in electrical resistance when a device is placed in a magnetic field, and many types of magnetoresistance are being explored for sensors that might find use in hard-disk drives. The magnetoresistance effect goes "ballistic" when an electron must cross a channel so narrow that the electron shoots straight through without scattering. In a normal wire, an electron zigzags its way through the material, called "diffusive" transport.

Chopra and Hua created their ballistic-effect sensors by forming nanoscale nickel "whiskers" between two larger nickel electrodes. Their current experiments include confirmation of the structure and composition of the whiskers with scanning electron microscopy.

The researchers suspect that the ballistic effect stems from pinch points, or constrictions, in the whiskers produced during manufacturing. The new manufacturing method, which also allowed them to reliably produce nanosensors with the desired effect, is a key to Chopra and Hua's latest success.

Chopra and Hua modified and adapted a method of producing controlled nanoscale wires originally developed by Arizona State University's Nongjian Tao. Tao's electrodeposition method allowed Chopra and Hua to specify in advance the resistance they wanted from their nanoscale whiskers. They now can reproduce their contacts reliably and simply, as opposed to the hit-or-miss method they had used previously. "We have been consistently able to produce contacts with BMR effects of several thousand percent," Chopra says.

These types of sensors also may have biomedical applications. For example, the sensor's electrical properties might be used to detect biomolecules in solution, even in low concentrations, according to Chopra. By attaching itself to the sensor, each type of biomolecule would impart its own "fingerprint" by changing the electrical signal of the nanocontact.