Effects of size and spin-polarized current on magnetic nanostructures.

摘要

Recent advances in nanofabrication have allowed scientists to produce structures smaller than 50 nm, creating interest in the effect of size reduction on physical systems. Many of these structures are for electronics applications, creating a need to know how the electrical properties are changed. In this dissertation the effects of size and current on magnetic materials are studied under multiple circumstances.;The linearity of current-induced magnetic oscillations is studied. A full set of the properties of such oscillations are measured and compared to a theory of the power and lineshape of nonlinear oscillators. In the single-mode regime the lineshape of a nonlinear current-driven magnetic oscillator is found to deviate from a simple Lorentzian and is determined by the temperature, driving force and damping parameter.;The effect of quantization on spin waves is studied. Confinement of spin waves due to size quantizes the spin wave excitations into distinct branches. The experimental data is compared to theory for dipole-exchange spin waves, which results in an experimentally-determined boundary parameter which is intermediate between pinned and free magnetization and different from what theory predicts. The quantized nature of the spin waves also leads to distinct scattering channels which open and close at distinct fields. A study of the damping parameter in narrow magnetic nanowires shows that these channels do open and close at certain fields corresponding to a confluence process involving three spin waves.;The effect of current on domain walls is studied. Spin-polarized current applied perpendicular to the thin-film plane containing a domain wall can move the domain wall more efficiently than current applied in the plane. Asymmetries in the functional dependence of spin-torque can also be exploited in the motion of domain walls. Asymmetric spin-torque can create a net force on the domain wall, which is not predicted for symmetric spin-torque. Asymmetric spin-torque can also excite internal degrees of freedom within the domain wall which correspond to a localized spin wave bound to the domain wall whose interaction with the domain wall creates very high domain wall velocities. This mode affects domain wall pumping, where high-frequency current can create long-range, high-speed motion.