Ballistic Magnetoresistance

Ballistic magnetoresistance (BMR) is yet another way in which spin orientation, encoding information on a storage medium such as a hard drive, can modify electrical resistance in a nearby circuit, thereby accomplishing the sensing of that orientation. The sensitive part of the circuit might consist of sandwiches of alternating magnetic and nonmagnetic layers (giant magnetoresistance, GMR; and tunnel junctions, TMR) or might have no magnetic materials at all (extraordinary magnetoresistance, or EMR; see Update 589).

In ballistic magnetoresistance, the sensor size is reduced to just a cluster of ferromagnetic atoms, joined together by, say, two lead wires. "Ballistic" means that the sensor is smaller than the typical scattering path length for the electron, which therefore moves in a straight trajectory. This means that the scattering the electron suffers will be owing to magnetic effects and not to general scattering from atoms in the sensor itself, making the readout process very sensitive.

If the electrons flowing in the circuit have been spin-polarized then when they flow through the sensor they will scatter more or less (meaning larger or lesser resistance) depending on the magnetization state within the sliver of atoms constituting the contact, and on the faint force exerted by the tiny magnetic storage domain being read out by the sensor (see figure).

In a new BMR experiment conducted at SUNY-Buffalo (Harsh Deep Chopra, hchopra@eng.buffalo.edu, 716-645-2593, x2310, and Susan Hua), the size of the sensor is so small (only nm in width and length) that the electron, on its way through the contact, will have less of a chance to accommodate itself to the spin regime of the second electrode (if it is different from that of the first electrode) and will consequently scatter more prominently, translating into a large magnetoresistance effect.

In the Buffalo experiment a remarkably large magnetoresistance effect (change in resistance) of 3150% is observed at room temperature (compared to 100% for GMR, and 1300% for EMR, or 1300% for room temperature "colossal magnetoresistance," or CMR). This represents the highest room temperature spin dependent MR effect ever observed for a spintronic device.

And this was accomplished in a very weak magnetic fields (less than 160 gauss), which means that as the size of the domain being read out shrinks (as more and more data is crammed onto smaller spaces on the recording medium) the signal will continue to be strongly felt in the sensor. Since the size of the sensor is only a sliver of atoms, bits can be reduced to comparable size, which could lead to storage capacities approaching terabits/sq in. (Chopra and Hua, Physical Review B, rapid communications, 1 July).