ARLINGTON, Va. -- A simpler and more reliable manufacturing method has allowed two materials researchers to produce nanoscale magnetic sensors that could increase the storage capacity of hard disk drives by a factor of a thousand. Building on results reported last summer, the new sensors are up to 100 times more sensitive than any current alternative technology.
Susan Hua and Harsh Deep Chopra, both professors at the State University of New York at Buffalo, report in the February issue of Physical Review B on their latest experiments with nanoscale sensors that produce, at room temperature, unusually large electrical resistance changes in the presence of small magnetic fields. The work is supported by the National Science Foundation (NSF), an independent federal agency that supports fundamental research and education across all fields of science and engineering.
"We first saw a large effect of over 3,000 percent resistance change in small magnetic fields last July," Chopra said. "That was just the tip of the iceberg. These results point to the 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.
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 computer disk, therefore, requires reliable sensors that are smaller, yet more sensitive to the bitŐs magnetic field. Hua and ChopraŐs nanoscale sensor seems to be ideally suited to the task.
For comparison, the technology in today's hard disk drives relies on signals as weak as a 20 percent change in resistance. In other words, if sensor has a baseline signal of 1, an "off" bit causes Chopra and Hua's sensors to spike at signal strength of -1,000, and an "on" bit registers +1,000. Current sensors, which only work on much larger bit sizes, would swing between an "off" signal of 0.8 and "on" of 1.2. The larger changes mean that the new 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 that are currently being studied: Because of the sensors' high sensitivity at room temperature, they would be straightforward to adapt to work with existing technologies used by the $25 billion hard disk drive industry. Chopra predicts that their sensors would permit disk capacities on the order of terabits (trillions of bits) per square inch.
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. 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 in a process 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 therefore a key to Chopra and Hua's latest success.
Chopra and Hua modified and adapted a method of producing controlled nanoscale wires originally developed b y Arizona State University's Nongjian Tao, whose work is also supported by NSF. Tao's electrodeposition method allowed Chopra and Hua to specify in advance the resistance they wanted from their nanoscale whiskers. They can now 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 said.
Besides disk drives, these types of sensors may also 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.