Static and dynamic properties of uniform- and vortex-states in synthetic nanomagnets

Synthetic antiferromagnets (SAFs) consist of two thin ferromagnetic particles separated by a thin nonmagnetic spacer. The magnetic moments of the two particles couple antiparallel via dipolar interactions, with the interlayer exchange interaction suppressed by a suitable choice of the spacer material. The SAF system studied in this thesis contains thin elliptical-in-the-plane permalloy particles magnetized uniformly and mutually antiparallel in the ground state. A SAF can also exhibit long-lived metastable nonuniform magnetization states, such as spin-vortex pairs. The thesis explores hysteresis and spin dynamics in: (i) uniformly magnetized SAFs and (ii) SAFs in the vortex-pair state. The uniformly magnetized antiparallel ground state of a symmetrical SAF, having identical ferromagnetic particles, is double degenerate. The resonance modes are in-phase (acoustical) and out-of-phase (optical) oscillations of the magnetic moments. Two forms of asymmetry are studied: a thickness imbalance between the two ferromagnetic layers and an asymmetric bias-field acting on only one of the layers. The asymmetries are shown to lift the degeneracy of the antiparallel ground state, which in the static regime results in unequal stability of the two states. In the dynamic regime, the asymmetries are shown to result in a splitting of the resonance frequency of the new non-degenerate ground states. The resulting resonant-mode splitting can be used to selectively switch between the antiparallel ground states by resonant microwave or thermal activation of the system. A microwave-driven switching cycle is demonstrated, where the switched-to state becomes off-resonant, thus requiring a pulse of a different frequency for switching back into the initial state. Such resonant switching is found to be tunable by a weak external bias field, which is practical for memory applications. Spin vortex pairs were obtained by applying specific large-amplitude high-frequency field excitations to a SAF. The static and dynamic properties of the vortex pairs were found to be strongly dependent on the relative orientation of the vortex chiralities and vortex-core polarizations in the two ferromagnetic particles of the SAF. For parallel vortex-core polarizations, a strong monopole-like core-core interaction is found to dominate the magnetic properties of the system, increasing the characteristic resonance frequency by an order of magnitude. The measured field and frequency dependent behavior of the magnetization in the vortex-pair state is explained using analytical theory and numerical micromagnetic simulations. A number of new results obtained on strongly coupled pairs of vertically stacked spin vortices should benefit the growing field of physics focused on topological spin states in nanostructures.