Program Highlights

Tunneling Anisotropic Magnetoresistance in Magnetic Break Junctions

J. D. Burton, R. F. Sabirianov, J. P. Velev, and E. Y. Tsymbal
Nebraska MRSEC
and
O. N. Mryasov
Seagate Technology, Pittsburgh

Anisotropic magnetoresistance (AMR) is the dependence of the electrical resistance of a ferromagnetic material on the orientation of its magnetization. AMR was discovered by Lord Kelvin in 1857 when he measured the resistance of a sample of nickel for current flowing along and perpendicular to the magnetization. Even now, more than 150 years after its discovery, researchers are continuing to uncover new regimes of AMR in nanoscale systems that would amaze Lord Kelvin.
In completely broken metal break-junctions, current is carried by electrons being transmitted by quantum mechanical tunneling through the vacuum. The magnitude of this tunneling current depends on the magnetization orientation, an effect called Tunneling Anisotropic Magnetoresistance (TAMR). Recently, researchers at the Nebraska MRSEC and Seagate Technology studied this effect theoretically. [Burton et al., Phys. Rev. B 76, 144430 (2007)] First-principles calculations of Ni and Co atomic-scale break junctions reveal a strong dependence of the conductance on the magnetization direction due to resonant electronic states localized at the tips of the electrodes. These resonances are highly sensitive to the orientation of the magnetization, causing TAMR to depend on the applied voltage on a scale of a few millivolts. These results bear a resemblance to recent experimental data reported by the group at Cornell and suggest that TAMR driven by resonant states is a general phenomenon typical for magnetic broken contacts.
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 0213808.

Geometry of magnetic Ni break-junction (left) and conductance as a function of magnetization angle and bias voltage (right).