Occurrence of the vortex state for magnetic phase plates Abstract. Tunable magnetic phase plates may be realized as nanorings magnetized in the vortex state, where tunability would be granted by the temperature dependence of the saturation magnetization. Here, we study the statistical occurrence of the magnetic vortex state in circular and pentagonal rings within their magnetic phase diagram. We outline the useful operational range of parameters that may be utilized in practice. IntroductionIn addition to a large number of promising applications in the field of magnetic memories [1] and spintronics [2], nanorings may be utilized as magnetic phase plates in electron microscopy to achieve Zernike-type phase contrast [3,4]. In fact, thanks to the Aharonov-Bohm effect experienced by the electrons traveling through a nanoring in the vortex state, a phase shift is induced between the transmitted and scattered beams. The main benefit is visible in-focus contrast associated with weak (or strong) phase objects such as biological samples [5]. Furthermore, it was hinted in [6] that employing Zernike mode may also result in a substantial decrease of the electron dose necessary to image single atoms or molecules, so that imaging radiation sensitive materials becomes more practical. Aiming at the development of magnetic phase plates we focus here on the stability region of the vortex state in rings within a suitable parameter space. This is a necessary preliminary step to ascertain the range of dimensions and shapes that are compatible with an operational phase plate. Furthermore, we analyze with micromagnetic simulations the statistics of obtaining a vortex from a random initial configuration, and how the statistics varies with shape and size. This is useful to select the appropriate parameters that results in more frequent vortex states and, hence, reliable operability.
We report progress towards optimisation of artificial magnetic domain structures for efficient spin transfer torque domain wall (DW) motion. Co/Pt multilayer samples have been sputtered on (100) Si/SiO 2 substrates and perpendicular magnetic anisotropy confirmed using polar magneto-optical Kerr effect (MOKE) measurements. The influence of the thickness of Co and Pt layers on the coercivity and switching behaviour was systematically investigated and the conditions established for realising well-suited structures with medium coercivity (~100 Oe) and sharp switching fields. Optimised Co/Pt multilayer films have been lithographically patterned into nanowire devices for time-resolved extraordinary Hall effect (EHE) measurements. Our devices are based on 50 coplanar waveguides incorporating single and double Hall cross structures. The coercivity of the region surrounding the Co/Pt Hall crosses was reduced by local focussed ion beam (FIB) irradiation allowing the controlled nucleation of domain walls at the edges of these regions by application of an appropriate field sequence. We describe polar MOKE experiments that show how DC currents lead to asymmetric switching of these artificial domains due to current-assisted DW motion across them.
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