Nonlinear Characterization and Modeling of Magnetic Tunnel Junction (MTJ)-Based Magnetic Sensors

A magnetic tunnel junction (MTJ) consists of the free layer (FL), pinned layer (PL), and the antiferromagnetic (AFM) layer. The AFM layer fixes the PL’s magnetization orientation via exchange coupling. The FL and PL are separated by a thin, crystalline MgO barrier. Electrons tunnel through the MgO insulating layer resulting in electrical conductivity, which depends on the relative orientation between the magnetizations of the FL and PL. The PL is assumed to be perfectly fixed while the FL is free to move in response to the external magnetic field. Even though it is assumed that the PL is fixed, it is in fact pinned by a finite energy. Room-temperature thermal magnetization fluctuations (i.e. linear magnetization dynamics) of both the FL and PL are the main source of magnetic noise and can be quantified by their ferromagnetic resonance (FMR) modes. Another source of magnetization dynamics in MTJs is the current-induced spin-torque effect, which describes a direct transfer of angular momentum from the spin-polarized electrons to the local magnetization. This effect influences the thermal magnetization fluctuations, and its contribution can be observed directly in the measured FMR spectrum. MTJs also exhibit nonlinear characteristics—nonlinear bias voltage dependence of the tunneling magnetoresistance (TMR) and spin-torque effects. The interaction between the MTJs’ linear magnetization dynamics and nonlinear characteristics results in nonlinear magnetization dynamics. The latter can be quantified by the sub-harmonics of the FL and PL FMR modes as well as the “splitting mode” located above the frequency of the FL. The resultant nonlinear magnetization dynamics are interpreted via micromagnetic modeling. The author provides physical explanations of the observed nonlinear effects and elaborates on their applications. This work will be continued in the course of postdoctoral research.