(A) micromagnetic study on the critical current density of spin transfer torque

Abstract

A great deal of research on spin transfer torque has been carried out for a decade, since it was initiated by the predictions of Slonczewski and Berger in 1996. It provides a novel means to manipulate the magnetization by electric current itself, not by the magnetic field it induces. As well as its rich physics we have to explore, spin transfer torque has high potential for important applications such as write scheme in Magnetic Random Access Memory (MRAM), spin torque nano-oscillator, and memory device using domain wall motion. One important issue in the research field of spin transfer torque is to lower the critical current density for the excitation of magnetization. The critical current density is too high for practical use, so that the reduction of the current is needed. In this dissertation, we study variations of critical current density for magnetization switching in high density MRAM cell arrays with longitudinal magnetization. Also, the critical current density for domain wall motion in nanowire with perpendicular anisotropy as a function of nanowire geometry is studied, and perpendicular magnetic tunnel junction (MTJ) with soft polarization-enhancement layers as function of saturation magnetization and interlayer exchange coupling strength. In the first part, we investigate the change of the switching current density in the conventional rectangular MRAM cell array. When the cell spacing is within substantial magnetostatic interaction, the magnetostatic interaction has two different kinds of effect on the switching current density. First, the overall stray field from the neighboring cells increases or decreases the switching current density, according to the relative direction of the switching to the stray field. It follows the theoretical description using single domain model. The other one is the increase of the switching current density due to the resonant precession of the magnetic cells, regardless of the switching direction. It becomes problem...