Studies on dynamic thermal reversal and transition jitter in magnetic materials

SNR, magnetization switching speed at high field and thermal decay at finite temperature are important issues determining achievable magnetic recording density and data rate. In this dissertation, dynamic thermal processes of the magnetization and transition jitter variance are studied with a view towards a physical understanding of limitations on the system issues.; Chapter 3 discusses an analytical model to calculate transition jitter variance. The model includes a transition parameter formula and a cross-track correlation length formula that include finite grain size and inter-granular interactions.; Chapter 4 presents models to study dynamic thermal processes of magnetization. A new dynamic thermal magnetization model is introduced, where damping and thermal fluctuations are added to normal modes of the magnetic system. The damping in this model can also be directly related to microscopic relaxation mechanisms. Thermal magnetization noise as measured, for example in GMR head, is studied in Chapter 5 using conventional stochastic Landau-Lifshitz-Gilbert model and the normal mode approach. The two models give different results. The normal mode approach is justified using a system-reservoir interaction model. Chapter 6 introduces the study of large angle dynamic thermal magnetization reversal by considering the simplest case of independent magnetic grains with crystal anisotropy. Dynamic coercivity versus pulse time is obtained for the whole time range, from purely dynamic reversal to long time thermal excitations. The nonlinear oscillator model generally predicts a faster reversal behavior compared to that of LLG for high field dynamic reversal.; The dynamic thermal reversals for ferromagnetic and anti-ferromagnetic coupled magnetic grains are studied in Chapter 7. The results can be used to characterize the performance of perpendicular media and anti-ferromagnetic coupled media.; Chapter 8 focuses on long time thermal decay of many interacting magnetic grains. The concept of optimal path for weakly perturbed stochastic dynamic system is combined with micro-magnetic simulation to obtain the reversal barrier for interacting magnetic systems. Nonuniform thermal reversal behavior for one dimensional nanostructures and three dimensional elongated ferromagnetic particles are studied. The effect of interactions on thermal decay of perpendicular media is quantified. The thesis ends with a discussion of future work in Chapter 9.