Resonant switching and vortex dynamics in spin-flop bi-layers

Sammanfattning: This thesis is a study of the static and dynamic behavior of the magne-tization in spin-?op bi-layers, which consist of two soft ferromagnetic layerscoupled by dipolar forces through a thin nonmagnetic spacer. The focus ofthe work is three fold: collective spin dynamics in the anti-parallel groundstate; resonant switching in the presence of thermal agitation; and static anddynamic behavior of the system in the vortex-pair state, with a particularemphasis on the interlayer core-core interaction.Two collective spin-?op resonance modes are observed and interpreted asacoustical and optical spin precessions, in which the moments of the two lay-ers oscillate in phase and out of phase, respectively. An analytical macrospinmodel is developed to analyze the experimental results and is found to ac-curately predict the resonance frequencies and their ?eld dependence in thelow-?eld anti-parallel state and the high-?eld near saturated state. A micro-magnetic model is developed and successfully explains the static and dynamicbehavior of the system in the entire ?eld range, including the C- and S-typespin-perturbed scissor state of the bi-layer at intermediate ?elds.The optical spin-?op resonance at 3-4 GHz is used to demonstrate resonantswitching in the system, in the range of the applied ?eld where quasi-staticswitching is forbidden. An o?-axis ?eld of relatively small amplitude canexcite large-angle scissor-like oscillations at the optical resonance frequency,which can result in a full 180-degree reversal, with the two moments switchingpast each other into the mirror anti-parallel state. It is found that the switch-ing probability increases with increasing the duration of the microwave ?eldpulse, which shows that the resonant switching process is a?ected by thermalagitation. Micromagnetic modeling incorporating the e?ect of temperature isperformed and is in good agreement with the experimental results.Vortex pair states in spin-?op bi-layers are produced using high amplitude?eld pulses near the optical spin resonance in the system. The stable vortex-pair states, 16 in total, of which 4 sub-classes are non-degenerate in energy, areidenti?ed and investigated using static and dynamic applied ?elds. For AP-chirality vortex-pair states, the system can be studied while the two vortexcores are coupled and decoupled in a single ?eld sweep. It is found thatthe dynamics of the AP-chirality vortex pairs is critically determined by thepolarizations of the two vortex cores and the resulting attractive or repulsivecore-core interaction. The measured spin resonance modes in the system areinterpreted as gyrational, rotational, and vibrational resonances with the helpof the analytical and micromagnetic models developed herein.A signi?cant e?ort during this project was made to build two instrumentsfor surface and transport characterization of magnetic nanostructures: a high-current Scanning Tunneling Microscope for studying transport in magneticpoint contacts, and a Current In Plane Tunneling instrument for characteriz-ing unpatterned magnetic tunnel junctions. The design and implementationof the instruments as well as the test data are presented.