As stored "bits" of data get smaller their magnetic field gets weaker, making the bits harder to detect and "read." Reliable reading of the data depends on producing a large enough magnetically-induced change in the electrical resistance of the sensor. Producing a detectable change at room temperature is another challenge.
In an experiment at the State University of New York at Buffalo, Harsh Deep Chopra and Susan Hua demonstrated that their tiny sensor produces an unusually large change in resistance in an ultra-small magnetic field, at room temperature. The magnitude of the magnetic effect they created surpasses all previous records. The results will be published in the July 1 issue of Physical Review B.
The effect is based on spintronics, a rapidly growing field that employs not only the charge but also the spin of electrons in making electrical devices.
The current technology used in the heads, or sensors, that read bits from a storage disk is based on an effect called "giant" magnetoresistance (GMR). GMR refers to the change in the sensor resistance when placed in a magnetic field; the effect is typically less than 100 percent. Inside a hard drive, a GMR device senses the local magnetic field of a stored bit of data. Such sensors have enabled commercial hard drives that can store the amount of data contained in a DVD full-length movie in a space the size of a credit card.
The effect created with the new nickel device is called "ballistic" magnetoresistance (BMR) and employs an electrical conductor that is only a few atoms wide and long. The BMR experiment exhibited a record change in sensor resistance of more than 3,000 percent. Chopra predicts the ultimate capacity will be about a terabit per square inch. This could enable the storage of 50 or more DVDs on a hard drive the size of a credit card.
Besides being useful for the multi-billion-dollar data storage industry, the BMR techniques could improve magnetic measurements and the study of magnetic effects in individual atoms, molecules and nanoscale clusters. It could also greatly enhance resolution and sensitivity of scanning probe imaging techniques that are widely used to characterize magnetic materials.
K. L. Murty