Magnetoresistance of atomic-size contacts realized with mechanically controllable break junctions

Abstract

We present a comprehensive study of the conductance behavior of atomic-size contacts made of ferromagnetic metals (Co) or noble metals (Au) with ferromagnetic electrodes (Co). In order to separate the influence of the large electrodes from the influence of the contacts themselves, we used different sample geometries. These include combinations of nonmagnetic electrodes connected to magnetic bridges and vice versa as well as different orientations of the magnetic field. The magnetoresistance (MR) curves show very rich behavior with strong MR ratios (MRR). In all geometries the MRR values are of comparable size, reaching up to a few thousand percent in the tunneling regime. We study the possible influence of the micromagnetic order of the domains in the vicinity of the contact as well as ballistic MR, giant MR, tunnel MR, atomically enhanced anisotropic MR (AAMR), and magnetostriction. We conclude that AAMR is the most important origin for the MR at high magnetic fields (|B|>2 T), while magnetostriction, tunnel MR, and giant MR govern the low-field regime (|B|