Electrical determination of vortex state in submicron magnetic elements

Abstract

Vortex structures in confined geometries are currently under close scrutiny due to their unique properties associated with their spatial confinement and the non-uniform distribution of the magnetization. The magnetic vortex is characterized by two boolean topological quantities: circulation (clockwise or counterclockwise c = ± 1) of the in-plane magnetization and polarity (up or down, p = ± 1) of the vortex core. These four degenerate states are quite stable and can accelerate the development of more compact and high performance magnetic memory devices. Thus an understanding of their dynamical behavior and a way to electrically detect these states is a major requirement for their development. Progress can be achieved by combining theoretical calculations, micromagnetic simulations and experimental approaches. The phenomenon of spin transfer torque is exploited to excite the lowest frequency (gyro) mode of the vortex core confined in a submicron magnetic (Permalloy - Ni81Fe19) element. The gyrotropic motion of the vortex core leads to a periodic change in the magnetization and hence its resistance: due to the anisotropic magnetoresistance (AMR) effect. This periodic change in resistance combines with the excitation current and generates a periodic homodyne voltage signal. An external static magnetic field is applied to break the symmetry and to rectify the homodyne voltage signal which we measure in a nanovoltmeter. It is found that the sign of the rectified AMR signal depends upon the handedness (cp) of the vortex structure. Micromagnetic simulations provide better understanding and are in good agreement with our experimental results. Additionally, vortex dynamics in these samples is investigated in a Scanning Transmission X-ray Microscope (STXM) with a temporal (< 100 ps) and spatial (~ 30 nm) resolution which allows us to verify the resonance frequency of the magnetic element as well as the power range to excite the vortex core. The AMR based technique thus can be used to detect the circulation and the polarity of the vortex state electrically and could open a route to implement magnetic vortex elements in memory and storage hierarchies. The phenomenon of Spin Motive Force (SMF) has also been studied by micromagnetic simulations. It is found that, in a particular configuration, the SMF signal shows a phase difference of 180 degrees for two polarities of the vortex core, when the voltage probe contacts are located parallel to the excitation rf field direction. No phase shift is observed in the perpendicular case. In addition, a 180 degree phase difference is observed for different circulations of the vortex structure. Therefore, this could also be a possible way to determine polarity and circulation of the magnetic vortex by carefully examining the phase relation of the SMF generated voltage signals. An attempt has also been made to measure the SMF experimentally. However, due to the small expected signal unambiguous detection of SMF was not successful so far.