Spin Transfer Torques by Point-Contact Spin injection
Tingyong Chen, Yi Ji, S X. Huang, C L. Chien, Mark D. Stiles
Spin-transfer torques (STT) provide a new mechanism to alter the magnetic configuration in magnetic heterostructures, a feat previously only achieved by an external magnetic field. A current flowing perpendicular through a noncollinear magnetic spin structure can induce torques on the magnetization because an electron carries angular momentum, or spin, part of which can be transferred to the magnetic layer as a torque. A spin-polarized current of a very high current density (e.g., 106 A/cm2 to 108 A/cm2) is required to observe the effect of the spin transfer torques. Consequently, switching by spin-polarized currents is typically observed in structures with sub-micron cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin injection involving no lithography. In a continuous Co/Cu/Co trilayer, we have observed hysteretic reversal of sub-100 nm magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom Co layer. In an exchange-biased single ferromagnetic layer, we have observed a new form of STT effect which we interpret as the inverse effect of domain wall magnetoresistance. We further show that in granular solids, the STT effect that can induce a large spin disorder when combined with a large magnetic field. As a result, we have obtained a spectacular MR effect in excess of 400 %, the largest ever reported in any metallic systems.