Point contacts between normal and ferromagnetic metals are investigated using magnetoresistance 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 observations in terms of surface spin valve and spin vortex states, originating from a substantially modified spin structure at the ferromagnetic interface in the 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.
2009. Vol. 21, no 35