UC San Diego

Magnetic nanostructures are vital components of numerous existing and prospective magnetic devices, including hard disk drives, magnetic sensors, and microwave generators. The ability to examine and predict the behavior of magnetic nanostructures is essential for improving existing devices and exploring new technologies and areas of application. This thesis consists of three parts. In part I, key concepts of magnetism are covered (chapter 1), followed by an introduction to micromagnetics (chapter 2). Key interactions are discussed. The Landau-Lifshitz- Gilbert equation is introduced, and the variational approach of W. F. Brown is presented. Part II is devoted to computational micromagnetics. Interaction energies, fields and torques, introduced in part I, are transcribed from the continuum to their finite element form. The validity of developed models is discussed with reference to physical assumptions and discretization criteria. Chapter 3 introduces finite element modeling, and provides derivations of micromagnetic fields in the linear basis representation. Spin transfer torques are modeled in chapter 4. Thermal effects are included in the computational framework in chapter 5. Chapter 6 discusses an implementation of the nudged elastic band method for the computation of energy barriers. A model accounting for polycrystallinity is developed in chapter 7. The model takes into account the wide variety of distributions and imperfections which characterize true systems. The modeling presented in chapters 3-7 forms a general framework for the computational study of diverse magnetic phenomena in contemporary structures and devices. Chapter 8 concludes part II with an outline of powerful acceleration schemes, which were essential for the large- scale micromagnetic simulations presented in part III. Part III begins with the analysis of the perpendicular magnetic recording system (chapter 9). A simulation study of the recording process with readback analysis is presented. Heat-assisted magnetic recording is considered in chapter 10. The effects of optical spot size and switching rate on signal quality are investigated. Chapter 11 is devoted to bit patterned media (BPM). Reversal modes and thermal stability of Co/Pd multilayer islands are studied. A novel BPM design, called capped BPM, is shown to provide enhanced tunability of thermal stability, writability, switching field distributions, and readback. Chapter 12 discusses spin valve applications. An all- perpendicular composite spin valve is shown to significantly reduce the tradeoff between switching currents and thermal stability. The last chapter 13 covers domain wall (DW) devices. DW motion in magnetically frustrated nanorings is analyzed. Antiferromagnetically coupled nanowires are shown to extend DW velocities beyond the Walker breakdown limit. A crosswire architecture is proposed for Boolean operations and the study of disorder dynamics