Analysis of magnetic tunnel junction-based spin-torque oscillator using NEGF-LLGS calculation

Abstract

Over the past few decades the remarkable development of spintronics has been witnessed by a number of researchers. Spintronics basically accompanies a phenomenon called \spin-orbit coupling," but attempts to figure out its physical description is still ongoing. An interfacial effect between ferromagnet and non-magnet has been investigated, which has brought about substantial studies of the spin-transfer. Spin-torque oscillator (STO) is an unusual type of spin device that converts the DC signals to RF signals. Due to its superior scalability and high frequency (GHz), the application of STOs has been sought recently, especially in telecommunication area. Several preliminary studies on STO and its integration on the circuits have been conducted actively since 2000s. As a material for the STO, magnetic tunnel junction (MTJ) or giant magneto-resistance (GMR) structure has been utilized. Numerous existing investigations on the MTJ-based STOs have been employed Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation, which describes dynamics of magnetization on the MTJ, however, the transport mechanism inside the MTJ should be clarified by considering quantum transport. Today one generally explains charge transport by introducing non-equilibrium Green's function (NEGF) approach to convey its quantum mechanical characteristics. In this thesis, simulation works to model MTJ-based STO were presented introducing (NEGF) transport calculation. The formulating steps for the LLGS equation and the NEGF transport calculation were presented with providing several principles and our assumptions for tri-layer MTJ simulation. By solving NEGF formalism and LLGS equation, the consecutive movement of magnetization could be described in every pico-second. The frequencies that we obtained from FFT analyses on the oscillating MTJs were compared with the previous paper. It is expected that this work would be significantly required in modeling advanced spin devices, in particular, the future nanoscaled telecommunication devices.