Anisotropic magnetoresistance (AMR) is the difference in the resistivity of ferromagnetic materials in external magnetic field when the field is applied along or perpendicular to the current. In macroscopic materials the conductance is diffusive (the mean free path of the electron is much smaller than the device dimensions) and AMR is due to spin dependent scattering of impurities. Until recently AMR used to be the primary way of detecting magnetic fields (as in hard-drive read heads). By using first-principles electronic structure techniques and symmetry arguments a new ballistic AMR effect was predicted in atomic size wires (BAMR). The physics of BAMR is completely different because the electron traverses the wire without scattering. Thus, BAMR is an essentially quantum effect. In addition, BAMR can be an order of magnitude larger than AMR, of either sign, and has a characteristic stepwise dependence of the direction of the magnetic field. In-situ measurements of the angular dependence of the conductance in magnetically saturated Ni point contacts show the signature of this new phenomenon. Understanding electron conductance on the quantum level is essential for future electronics as the individual elements approach the atomic size dimensions. [Phys. Rev. Lett. 94, 127203 (2005)]
See below a schematic view of the atomic contact. The profile of the contact conductance as a function of the magnetic field direction.