Magnetization Processes of Permalloy Thin Films and Its Array Systems

In this thesis, we investigate the metastable state and magnetization processes of Permalloy thin film by means of numerical micromagnetic simulation and experiment. Besides, we study the effect of interparticle dipole interaction on Permalloy thin film arrays. A metastable state for the elongated permalloy elements is found to exist within a short field range before magnetization reversal. From the micromagnetic simulation it exhibits a wave-like structure, and the corresponding magnetic pole density distribution shows a pattern with periodic spots along the boundary. We also use magnetic force microscopy imaging and magnetoresistance measurement, which measures the nucleation field and coercive field, to study the non-coherent magnetization reversal properties of Permalloy ellipse. The metastable state before switching is found to be repeatable. The magnetic structures and hysteresis loops of permalloy thin film arrays are investigated here using magnetic force microscopy and vibrating sample magnetometer. The strength of interparticle dipole interaction can be revealed by the number of single-domain pairs with antiparallel magnetizations when the array is relaxed from a strong hard-axis field. Besides, hysteresis loops obtained by vibrating sample magnetometer measurements show that arrays with narrower interparticle spacings have lower coercivities and remanences. The results obtained from vibrating sample magnetometer are in very good agreement with magnetic force microscopy imaging. In the research on magnetic thin films with special geometries, we investigate the possible equilibrium states existing in the square network composed of four-arm junctions. Our result shows that different magnetization states in the junctions can be classified by counting the net magnetic pole densities accumulated over there. The pattern which has higher energy density, is for the first time observed experimentally in the square network. This provides a promising chance to use Permalloy network junctions as bit ultrahigh-density storage media.Abstract i Contents iii List of Figures vi 1 Introduction 1 References 7 2 Review of theoretical fundamental to magnetic materials 9 2.1 Hamiltonian of the system 9 2.1.1 Exchange energy 10 2.1.2 Magnetostatic energy 11 2.1.3 Anisotropy energy 12 2.1.4 Zeeman energy 13 2.2 Brown's static equilibrium condition 13 2.3 Theoretical studies of the magnetization reversal of a single domain particle 15 2.3.1 Stoner-Wohlfarth model: coherent rotation 15 2.3.2 Nucleation problem 20 2.4 Micromagnetic simulation 24 References 28 3 Sample fabrication and experimental techniques 30 3.1 Electron beam lithography 31 3.2 Vibrating sample magnetometer 37 3.3 Magnetic force microscopy 39 3.4 Magnetoresistance measurement 45 References 47 4 Planar buckling metastable state before magnetization reversal in elongated permalloy thin films 49 4.1 Metastable state before magnetization reversal in single-domain elongated thin films 49 4.1.1 Introduction 49 4.1.2 Background 50 4.1.3 Micromagnetic simulation 51 4.1.4 Sample preparation and experiments 55 4.1.5 Results and discussion 57 4.2 Ab Initio studies of magnetization reversal via planar buckling mode 62 References 79 5 Magnetic properties of periodic permalloy elliptical arrays 81 5.1 Interparticle spacing dependence of magnetization reversal properties on permalloy thin film arrays 81 5.1.1 Introduction 81 5.1.2 Experimental method 81 5.1.3 Results and discussion 84 5.1.4 Summary 88 5.2 Angular dependence of magnetization reversal properties on permalloy thin film arrays 89 References 96 6 Micromagnetic states of four-armed square network junctions 97 6.1 Introduction 97 6.2 Simulation method 98 6.3 Experimental method 98 6.4 Results and discussion 99 References 103 7 Conclusion 10