Spin Transfer Torques by Point-Contact Spin injection

摘要

Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is 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 substantial current density (e.g., 10~8 A/cm~2) is required to observe the effect of the spin transfer torques. Consequently, switching by spin-polarized currents is often realized in small 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 with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of STT effect which is the inverse effect of domain wall magnetoresistance effect rather than giant magnetoresistance effect. We further show that in granular solids, the STT effect that can be exploited to 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.