Berlin 2018 – scientific programme

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MA: Fachverband Magnetismus

MA 21: Poster I

MA 21.62: Poster

Tuesday, March 13, 2018, 09:30–13:00, Poster A

The influence of the Hall-bar geometry on the apparent spin Hall angle in harmonic Hall voltage measurements — •Lukas Neumann and Markus Meinert — Center for Spinelectronic Materials and Devices, Faculty of Physics, Bielefeld University, D-33501 Bielefeld, Germany

We investigate the influence of the Hall-bar geometry on the apparent spin Hall angle (SHA) in harmonic Hall voltage measurements which is a well established method to determine the SHA of a nonmagnetic metal/ferromagnet bilayer structure like Ta/CoFeB. Tantalum is a heavy metal with large spin-orbit coupling such that an in-plane current generates a substantial spin-orbit torque acting on the magnetization orientation of the ferromagnetic layer. The samples are patterned into Hall bars using electron beam lithography. Being located in an in-plane magnetic field an AC current through the Hall-bar generates a Hall-voltage whose second harmonic gives the apparent SHA. In the simplest model, the influence of the voltage pickup arms is neglected. Obviously, the current density distribution in the vicinity of the voltage pickup arms is not homogeneous and depends on the width of these arms. To systematically investigate this effect we varied the pickup arm width and observe a strong change of the apparent SHA as the pickup arm width is increased. In a symmetric Hall cross with four-fold rotational symmetry, the apparent SHA is reduced by about 30% with respect to the maximum value at narrow pickup arm width. We compare the measured values with simulations of the current density distribution.