Ballistic magnetoresistance

(BMR) is yet another way in which spin orientation can modify electrical resistance in a circuit. The sensitive part of the circuit might consist of sandwiches of alternating magnetic and nonmagnetic layers (giant magnetoresistance and tunnel magnetoresistance) or might have no magnetic materials at all (extraordinary magnetoresistance; see Physics Today, July 2002, page 9 ). In BMR, the sensor is a quantum point contact of ferromagnetic atoms between two wires. The contact is narrower than the typical scattering path length for electrons, which therefore move ballistically in straight trajectories. Any scattering an electron suffers will be due only to magnetic effects. If the electrons in the circuit are spin polarized then they will scatter more or less (with greater or lesser resistance) at the contact, depending on the contact’s magnetization state and on the faint force exerted by any nearby tiny magnetic storage domain. In a new BMR experiment conducted at SUNY Buffalo, Harsh Chopra and Susan Hua found a remarkably large resistance change in nickel nanocontacts at room temperature. For example, they saw a change in resistance of more than a factor of 30 (from 8 to 260 ohms) in an applied magnetic field of less than 0.016 T (160 gauss). The researchers say that they can reliably reproduce the BMR effect in many samples. (H. D. Chopra, S. Z. Hua, Phys. Rev. B 66, 020403(R), 2002 http://dx.doi.org/10.1103/PhysRevB.66.020403 .)