Magnetic thin film Fe ring structures and devices

Thesis (Ph.D.)--University of Washington, 2014Magnetic thin film ring structures show distinct magnetic states and highly reproducible switching behavior, which makes them candidates for multiple magnetoelectronic and sensing devices. The magnetic states and switching behaviors of thin film ring structures are closely related to their physical properties such as the lateral size, thickness, width and the material anisotropy. In order to systematically study the magnetic properties of thin film ring structures, we use different techniques such as photolithography and electron-beam lithography to fabricate magnetic thin film Fe ring structures with different outer diameter (Dout), thickness (tFe) and inner-to-outer diameter ratio (Din/Dout). The magnetization reversal processes of these Fe ring structures are studied using magneto-optical Kerr effect and magnetic force microscopy. To explain the magnetic behavior of the Fe ring structures observed in experiments, we use LLG micromagnetic simulation to model the domain configuration of the Fe ring during switching. The evolution of magnetic reversal behaviors in Fe ring structures is also explained by calculating the energy densities of vortex state, single-domain state and axial state with micromagnetic simulator. To control the magnetic switching behavior of the Fe ring structures, exchange bias interfacial coupling is introduced into the ring structure by growing a bilayer of IrMn/Fe in the ring structure. By studying the angular dependence of the hysteresis loop shift, we find that exchange bias induces a magnetic unidirectional anisotropy and a collinear magnetic uniaxial anisotropy in the ring structure. Exchange bias induced magnetic anisotropies cause anisotropic magnetic reversal modes in IrMn/Fe (10 nm) ring and a higher one-step to two-step transitional thickness in IrMn/Fe (x nm) ring structures. After study the magnetic properties of the Fe rings and exchange biased IrMn/Fe rings, we incorporate them into one ring shape magnetic tunneling junction device with current perpendicular to plane configuration. In the ring-MTJ device, a Fe ring free layer and an exchange biased IrMn/Fe ring pinned layer are sandwiched by an MgO barrier layer between them. Depending on the relative orientation of the magnetization in the Fe ring and IrMn/Fe ring, the resistance of the device is higher when they are antiparallel than parallel, which may be used as memory element in a magnetoresistive random-access memory