Spin transfer in magnetic nanopillar junctions.

摘要

This thesis describes experimental investigations of charge current induced spin angular momentum transfer in magnetic multilayer pillar junctions, which are approximately 100 nm in diameter. Large currents traversing a magnetic multilayer strongly influence the magnetic state of the latter. Above a threshold current, angular momentum transfer from the conduction band electrons to the background magnetization leads to fundamentally new types of magnetic excitations.; We have fabricated junctions by means of a new wedge growth nanostencil mask process. This approach allows the fabrication of large arrays of junctions where the layer thicknesses can be varied continuously across a single wafer. Such samples are essential for the systematic study of spin transfer effects. Detailed magnetotransport measurements with such samples at room temperature and 4.2 K have answered fundamental question on spin transfer concerning its efficiency, nature and the minimum requirement for it to occur.; In bilayer junctions consisting of a thin and a thick ferromagnetic layers separated by a normal metal layer we have observed current induced hysteretic changes in the device resistance in large applied fields. Our results demonstrate that spin transfer torques induce a complete reversal of the thin ferromagnetic layer to alignment antiparallel to the applied field---that is, to a state of maximum magnetic energy even in large applied fields.; Detailed measurements with similar samples in low applied fields show that spin transfer changes the magnetization dynamics not only by means of a torque term but also by means of an effective field acting on the background magnetization.; Transport measurements in large applied fields in junctions containing only a single ferromagnetic layer show that spin transfer effects take place even when the charge current impinging on the ferromagnetic layer is unpolarized. Our results show that a strong asymmetry in longitudinal spin accumulation is sufficient and reveal a new magnetoresistance effect, by which spin transfer induced excitations reduce the junction resistance. Similar excitations are observed in bilayer junctions for both current polarities. Such bipolar excitations are not expected in a single domain model of spin-transfer and demonstrate the importance of asymmetries in longitudinal spin accumulation even in bilayer junctions.