Spin-transfer torque induced domain wall ferromagnetic resonance in nanostrips

Abstract

The frequency response of a Néel domain wall in a nanostrip excited by alternating spin-polarized current is studied by micromagnetic simulations. Several internal normal modes of the domain wall are excited and the corresponding spatial distributions of oscillation power are imaged. In the case of current perpendicular to the wall, the excited normal modes are mainly concentrated at the wall center and/or wall boundaries, forming edge modes, center modes, standing-wave modes and their mixed modes. The localization and spatial distribution of the modes have a close relationship with the total internal field, especially its inhomogeneity. With the decrease of saturation magnetizations Ms, the spatial inhomogeneities of the total field are gradually weakened and some domain wall normal modes disappear. In the case where the current is parallel to the wall, the wall thickness oscillation mode (or breathing mode) is excited. Furthermore, due to the geometrical confinement, high-order thickness modes such as edge- and standing wave-thickness modes are observed. The magnetization dependence of the eigen-frequency exhibits different forms for different normal modes, which can be qualitatively explained based on an approximate dispersion equation for spin-wave modes of a quasi-uniformly magnetized rectangular element.