Non Collinear Spin Torque Driven Excitation In Nanomagnets

This dissertation summarizes my investigations in non-collinear spin-transfertorque (ST) effects on nanomagnets. The Slonczewksi-like ST mostly competes with the damping torque in a device structure where the spin polarization (SP) is collinear to the magnetic moment of a free layer (FL) at equilibrium. Contrarily the ST has both (anti- )damping and an equivalent field torque effects in non-collinear configurations in which the SP is close to perpendicular to the FL. The additional field effect of the ST shifts the equilibrium or dynamic offset angle of the FL and thereby changes the characteristics of the excitations. In my first study, I demonstrated the ST-driven ultrafast nanomagnet switching in nanopillar spin-valve devices incorporating both an out-of-plane spinpolarizer and an in-plane polarizer/analyzer, with pulse widths down to 50 ps. I also explained the physical origin of the observed asymmetric threshold currents as functions of the switching direction and the pulse polarity, which is beneficial for a non-toggle write operation. In addition I proposed methods to suppress the magnetic ringing that occurs after the ballistic switching. In my second study, I discovered quasi-linear behavior of a spin-torque nano-oscillator under an external hard axis magnetic field that controls the precession axis of the FL and its offset angle from the SP. The observed linewidth (~ 5 MHz) was very close to the fundamental limit determined by thermal driven noise. The quasi-linearity, or the cancellation of the nonlinear coupling between the amplitude fluctuation and phase noise, was achieved by obtaining a small frequency agility together with a negative feedback effect from the ST causing the enhancement of the dynamical damping. In my third study, I investigated perpendicularly magnetized Co nanodot switching via spin-Hall-induced spin currents. In the thermally activated reversal regime I estimated the current-induced effective field (H s) that has a magnitude within the predicted range due to the large spin-Hall angle in Pt when the Joule heating effect is taken into account for the magnetic system. In addition an excitation of a subvolume drove the entire magnetization switching, thus even a small but heat-assisted H s was able to reverse such a Co dot with a strong perpendicular magnetic anisotropy.