Current Driven tri-stable Resistance States in Magnetic Point Contacts

Point contacts between normal and ferromagnetic metals are investigated using magneto-resistance and transport spectroscopy measurements combined with micromagnetic simulations. Pronounced hysteresis in the point-contact resistance versus both bias current and external magnetic field are observed. It is found that such hysteretic resistance can exhibit, in addition to bi-stable resistance states found in ordinary spin valves, tri-stable resistance states with a middle resistance level. We interpret these observation in terms of surface spin-valve and spin-vortex states, originating from a substantially modified spin structure at the ferromagnetic interface in contact core. We argue that these surface spin states, subject to a weakened exchange interaction, dominate the effects of spin transfer torques on the nanometer scale.