Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropy
Sammanfattning: Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements.This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, >0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism.
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