THESIS
2018

xv, 106 pages : illustrations ; 30 cm

**Abstract**
In this thesis, the magnetization manipulation has been investigated in two aspects, namely
magnetization reversal of nanoparticle by microwave pulse & current and 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 circularly/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 pul...[

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In this thesis, the magnetization manipulation has been investigated in two aspects, namely
magnetization reversal of nanoparticle by microwave pulse & current and 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 circularly/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 DW always propagates towards
hotter regions accompanying the rotation of DW plane (i.e., turbulent mode of DW propagation)
around easy axis. DW rotation in uniaxial nanowire indicates the existence of breakdown limit.
DW linear 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, DW linear and angular velocities decrease with damping constant,
α. 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 DW dynamics under a thermal gradient
to that under electric STT, the coefficient of magnonic non-adiabatic STT, β is calculated. The
magnitude of β does not depend on the thermal gradient. In addition, we observe that the β
increases (decreases) with the uniaxial anisotropy K

_{x} (damping α).

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