A spin-polarized dc current can induce steady-state, microwave frequency magnetization dynamics in a nanoscale ferromagnet. The torque that drives these dynamics originates from the exchange of spin angular momentum between conduction electrons and the magnetization. We present measurements of current perpendicular to the plane (CPP) giant magnetoresistance (GMR) nanopillar devices in which this phenomenon occurs. We focus on devices that contain one reference ferromagnetic layer that has a fixed magnetization and one free ferromagnetic layer with a magnetization that responds to spin torque. The resulting spin transfer induced magnetization dynamics combined with GMR lead to resistance noise, which we measure in both the frequency- and time-domain. The appearance of these dynamical states is consistent with spin transfer in that dynamics are observed only for those combinations of current direction and magnetic configuration in which spin torque opposes the FL configuration set by the magnetic field. Furthermore, the amplitude of the resultant resistance noise increases rapidly with increasing current until saturating at a value that is a large fraction of the magnetoresistance between parallel and antiparallel states. This behaviour is contrasted with similar measurements of a current-in-plane (CIP) GMR device in which the magnetic resistance noise is thermally activated.
Planar tunneling spectroscopy is used to investigate the quasi- particle density of states of YBCO. The tunneling conductance, taken as a function of temperature, magnetic field, crystallographic orientation, Pr, Zn and Ni doping and ion-induced damage confirms that the observed zero-bias conductance peak (ZBCP) is an Andreev Bound State (ABS). This ABS occurs at the interface of an unconventional superconductor, that breaks the order parameter reflectional symmetry: in this case, the (110) surface of YBCO. An applied magnetic filed causes a splitting of the ZBCP, which is due to the Doppler shift arising from the scalar product of the quasiparticle velocity with the superfluid momentum, VF(DOT)PS. A dramatic dependence of the ZBCP splitting with the direction of the applied field demonstrates that the transport properties of the ABS are highly anisotropic with respect to the crystal axes. In zero field, the ZBCP splits below Ts approximately 8K, which is a manifestation of a phase transition into a state that breaks time-reversal symmetry.
We have measured the conductance of Y1-xPrxBa2Cu3O7/Pb planar tunnel junctions fabricated on thin films where x equals 0, 0.2 and 0.4. When tunneling into the ab-plane, there is a weak decrease in conductance at eV approximately 2kBTc, where a superconducting gap is expected. With increasing Pr concentration, the energy where this gap-like feature appears is reduced by an amount commensurate with the reduction in Tc. The gap-like structure in the c-axis tunneling conductance becomes weaker, but displays no change in energy when YBCO is doped with Pr.
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