ABSTRACT

In this thesis, the magnetization manipulation has been investigated in two aspects, namely magnetization reversal of nanoparticle by microwave pulse & current, and microscopic mechanism of domain wall (DW) dynamics along uniaxial nanowire under a thermal gradient. The magnetization reversal of high anisotropy magnetic nanoparticle (also known as Stoner particle) is studied by down-chirp pulse of circular/linearly (CP/LP) polarized microwaves and spin polarized current simultaneously. Numerical results reveal that solely short down-chirp pulse of CP/LP microwave with certain range of initial frequency induces fast magnetization reversal. For this fast magnetization reversal, the underlying principle is the down-chirp microwave pulse maintains the phase difference with magnetization in such that the pulse acts as energy source and sink for magnetic particle before and after crossing the barrier respectively. Afterwards, the switching amplitude of chirped pulse is being reduced regarding practically realizable limit by applying a spin-polarized current additionally. This model and its findings might be helpful to realize fast magnetization switching of high anisotropy material with low current to achieve high thermal stability. Domain wall (DW) dynamics under the thermal gradient along uniaxial nanowire is investigated numerically. We use micromagnetic model in which the magnetization is governed by the stochastic Landau-Lifshitz- Gilbert equation. It is observed that DWalways propagates towards hotter regions accompanying the rotation of DWplane (i.e., turbulent mode of DWpropagation) around easy axis. DW rotation in uniaxial nanowire indicates the existence of breakdown limit. DWlinear as well as angular velocity proportionately increases with the thermal gradient along the uniaxial nanowire. The spin/magnons current has been calculated and hence we estimate DW linear velocity. It is found the simulated DW velocity almost coincides with DW velocity which is calculated from magnon current density for small damping ( 0:002). Moreover, DW velocity weakly depends on DW width (or uniaxial anisotropy), whereas DW angular velocity linearly increases with the decrease of the DW width (increase of uniaxial anisotropy). In the case of being damping dependent, DWlinear and angular velocities decrease with damping constant, It is mentioned that all the features of DW dynamics under thermal gradient observed are analogous to that under electric STT, which justify the magnonic STT as underlying physics behind thermal gradient. Reasonably, by comparing the DWdynamics under a thermal gradient to that under electric STT, the coefficient of magnonic non-adiabatic STT, is calculated. The magnitude of does not depend on the thermal gradient. At the same time, we observe that the increases (decreases) with the uniaxial anisotropy Kx (damping ).