\label{pre}
\thispagestyle{empty}
\begin{center}
\vskip 30pt
{\large\bfseries PREFACE}
\vskip 5pt
\end{center}
\begin{spacing}{1.5}
\noindent Exciting developments in magnetic materials research started with the computer revolution in the late 1950s and the more recent quantum leap beginning around 1990 associated with the electron’s quantum property-spin based electronics or simply \textit{Spintronics}. The field of spintronics was largely triggered with both Albert Fert's and Peter Gr\"unberg’s independent and simultaneous pioneering discovery of Giant Magnetoresistance (GMR) in 1988. Another breakthrough in spintronics came from both Slonczewski's and Berger’s independent prediction of \textit{Spin transfer torque} in 1996. The spin transfer torque arises due to an exchange interaction and angular momentum transfer between the injected spin polarized ‘itinerant’ electrons and the local moments in the ferromagnet, when the current passes through the ultrathin ferromagnetic films separated by a non-magnetic metal. This spin transfer torque will switch the magnetization of the free layer in the multilayer nanopillar structure for a sufficiently large current. Spin transfer torque induced magnetization switching has recently attracted much interest due to its potential applications in magnetic random access memory (MRAM), fast programmable logic, high-density magnetic storage devices, magnetic sensors and in high frequency devices for telecommunications. Magnetic storage devices and magnetic sensors based on GMR and Tunnelling Magnetoresistance (TMR) effects require high quality multilayers constructed out of ultrathin ferromagnetic and non-magnetic films. The performance of these devices strongly depend on the morphological and structural properties of the films and their physical characteristics. Among them, the crucial factor is the interlayer coupling between the two ferromagnetic layers separated by a non-magnetic spacer. Various interlayer coupling mechanisms have been reported. Among them, pinhole coupling, orange peel coupling, biquadratic coupling and RKKY coupling are important ones. These interlayer couplings play an important role in the understanding of magnetization switching mechanism and GMR of spin valves and TMR of magnetic tunnel junction structures. 

The main objective of this work is to understand the physics of various couplings especially orange peel coupling and biquadratic coupling and their effect on the spin transfer torque induced magnetization switching. Further, the reduction of critical current density and switching time are two important issues in the development of potential applications. In order to reduce the switching time, the effect of biasing on spin transfer torque switching in the pentalayer nanopillar structure is additionally studied. The magnetization dynamics and the switching process of the free layer magnetization in nanopillar devices are basically nonlinear in nature and is governed by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation which is a highly nontrivial vector nonlinear evolution equation. By solving this LLGS equation, we can understand the underlying magnetization dynamics. But, it can not be solved analytically in the more general case due to its high nonlinear nature. In the time-independent case, the LLGS equation is analytically solved to calculate the critical current density required to switch the magnetization. Then  the full LLGS equation is solved numerically by using Runge-Kutta fourth order procedure in each case and from the numerical results we understand the effect of the interlayer coupling and biasing on spin transfer torque switching.

The contents of thesis is divided into seven chapters in four parts. The first two chapters forms the introductory part (Part I) which provide the basis for our investigations. In Chapter 1, a brief introduction and an overview of the magnetization switching process, details about various methods available to achieve magnetization switching and important  magnetoresistance effects are presented. The dynamics of magnetization switching can be understood from constructing appropriate model and the dynamical equation and the switching process can be understood from by solving the equation. In Chapter 2, dynamical equation for magnetization switching process is constructed. 

The results of the investigations on the effect of interlayer coupling on spin transfer torque switching in trilayer structure are presented in chapters 3 and 4 under Part II. In Chapter 3, the impact of orange peel coupling on spin torque magnetization switching in a trilayer nanopillar device is investigated by solving the switching dynamics of magnetization of the free layer governed by the LLGS equation. Chapter 4, concentrates on the study of the effect of biquadratic coupling on spin torque switching in a trilayer nanopillar structure.

Part III containing chapters 5 and 6 describes the results of our study on the magnetization switching dynamics in a pentalayer nanopillar structure. Chapter 5 deals in a greater detail with the numerical results about the role of interlayer coupling (both orange peel coupling and biquadratic coupling) on the magnetization switching in the pentalayer nanopillar. Chapter 6 is devoted to the investigations on the effect of biasing for reducing the switching time in the pentalayer structure. 

Part IV consists of Chapter 7, which provides the summary and conclusions of our investigations carried out in this thesis and the future ~outlook.\\[0.5cm]
\noindent{Tiruchirappalli}\\
\noindent{September 2016 \hfill \sc \bfseries D. Aravinthan}
\end{spacing}
