Ga(^+) focused Ion beam irradiated Ni(_81)Fe(_19) thin films and Planar nanostructures investigated by the Magneto-Optical Kerr Effect

Abstract

Patterned magnetic films are of interest for storing and sensing information, and possible logic applications, and find commercial applications in consumer goods such as personal computers. This thesis addresses the fast magnetic patterning of capped ultrathin Ni(_81)Fe(_19) films in Chapter 5, and the patterning and controlled magnetic switching of planar nanowires in Chapter 4. Controlled domain wall switching of complex wire geometries with comer structures, artificial trapping sites, 3-terminal junctions and more complex wire circuits is described in Chapters 6-7.The magnetic switching of planar Ni(_81)Fe(_19) nanowires fabricated by 30 keV, focused ion beam Ga(^+) ions was investigated, in the width range 60-500 nm. Experimentally measured wire easy axis coercivity is inversely proportional to width, similar to Stoner-Wohlfarth switching behaviour. Angular switching data for wires is presented. Significantly, wire coercivity and anisotropy field are shown to be strongly dependent on the ion beam raster direction during wire fabrication. The controlled propagation of head-to-head domain walls in a 27 Hz anticlockwise rotating magnetic field, through smoothly rounded comers is experimentally demonstrated. Domain wall propagation fields, 7 ± 3 Oe, just above the intrinsic domain wall coercivity were found. Using an L-shaped rounded comer geometry, the magnetic fields at which domain walls are introduced into wires and the domain wall propagation field were separated. Reproducible pinning and depinning of single domain walls on artificial domain wall traps with depths from 35-125 nm is demonshated.3-Terminal continuous Ni(_81)Fe(_19) wire junctions, suitable for AND/OR domain wall logic operations are described, in which the magnetic switching field of the device output is strongly dependent on the number of domain walls (0, 1, or 2), at the junction. An operating field phase diagram is presented in the context of junction integration with existing domain wall logic elements. Capped NigiFei9 films were ferromagnetically quenched by radiation induced transport of bilayer interfacial atoms. For Si/ Ni(_81)Fe(_19)/Al or Si/ Ni(_81)Fe(_19)/Au bilayers, the critical Ga(^+) ion dose to quench ferromagnetic ordering (Φ), measured by the magneto-optical Kerr (MOKE) effect, is demonstrated to be linearly proportional to the square of NigiFei9 thickness, (tNiFe)(^2) Therefore ultrathin-capped Ni(_81)Fe(_19) films can be magnetically quenched at ion doses ~ an order of magnitude lower then Ni(_81)Fe(_19)/Si samples, which are typically patterned by radiation sputtering from the vacuum- surface interface. Bilayer coercivity, uniaxial anisotropy field, remanent magnetization, and saturation magnetization as measured by MOKE, were tailored by controlled localized ion doses. Ga(^+) ion doses as low as 8 x 10(^13) ions.cm(^-2) reproducibly quenched measured room temperature ferromagnetism in 2 nm thick buried (_81)Fe(_19) films. Patterning of 200 nm wide in-plane magnetized wires embedded between a non magnetic cap and substrate is demonstrated.