Spin Transfer Torque Induced Switching In Magnetic Tunnel Junction For Stt-Ram Application

Abstract

This thesis describes experimental studies of the spin transfer torque induced switching in magnetic tunnel junctions (MTJ) for the application of spin transfer torque random access memory (STT-RAM). In the material development; the in-plane MTJ was optimized in order to meet the requirement of STT-RAM application. Perpendicular magnetocrystalline anisotropy was obtained in the L10 phase FePd designed for the top MTJ electrode and bottom MTJ electrode. Moreover, full CoFeB MTJ with interface perpendicular anisotropy was developed. An average of 48% reduction in the intrinsic critical current density was found by increasing the interface perpendicular anisotropy. Sub 200 ps ultrafast STT induced switching was also demonstrated in those CoFeB MTJs where the out-of-plane demagnetizing field was partially canceled by the interface perpendicular anisotropy. High J/Jc0 ratio and magnetization nucleation at the edge of free layer are possibly the two major factors that contribute to the ultrafast spin transfer torque switching. In the spin transfer torque (STT) induced switching study; systematic characterization of the probabilistic STT induced switching process was done. It includes the three STT induced switching modes, the switching energy, the switching speed and the high precision switching probability density function (PDF). The temperature dependent MTJ properties and STT switching distribution was also studied. Those results provided key parameters for the STT-RAM design. In the end, direct and compelling experimental evidence was provided to show the large dynamic energy barrier reduction induced by high frequency spin current excitations. The concept of magnetization logarithmic susceptibility was proposed to describe this dynamic effect. By comparing with the simulation results, the measured logarithmic susceptibility frequency response was used to reveal the magnetic properties of MTJs and understand the spin-transfer torque induced magnetization switching dynamics.