Spin Polarized Current Induced Magnetization Dynamics Of A Magnetic Vortex In A Permalloy Nanodisk

The spin transfer torque (STT) induced by spin polarized current provides a method for manipulating the magnetization direction by the current rather than the external magnetic field. For the magnetic vortex, the polarity and chirality could be the signal carrier, so the magnetic vortex induced by STT holds great promise in the applications of non-volatile magnetic random access memory, ultra-high density recording, magnetic sensors and the high-frequency microwave oscillators. In this thesis, we perform the micromagnetic simulations and analytical calculations to explore the STT-induced magnetic vortex dynamics in a confined nanocontact strucrure.The main contents of this thesis are as follows:The spin polarized currents are injected into the Permalloy nanodisk through dozens of nanometer-size contacts to excite the vortex dynamics. We first investigate the vortex dynamics induced by one nanocontact which locates at the center of the nanodisk, including polarity reversal, chirality reversal and polarity-plus-chirality reversal. However, we observe that the polarity reversal behavior is different when the nanocontact is designed not at the center of the nanodisk and find out a method for reducing the reversal current density and switching time. Furthermore, we study in detail the vortex dynamics induced by three nanocontacts located at a diameter of the nanodisk, mainly focusing on the polarity reversal, transient magnetic configurations, the trajectory and instantaneous angular frequency of vortex core gyrotropic motion.The micromagnetic simulations reveal that the vortex could be excited when the nanocontact locates at the center of the confined nanodisk, and the gyrotropic motion, the polarity and chirality reversal are all fulfilled through altering the magnitude of current for different current directions and current polarized directions. The polarity can be switched via the creation and annihilation of vortex-antivortex pairs when the polarity of vortex core is antiparallel to the polarized direction of the current; the chirality can be switched when the orientation of the Oersted field is antiparallel to the direction of the chirality, and there are two kinds of chirality-reversal processes according to the current density range: for lower current density, the creation and annihilation of several vortices and antivortices result in the chirality reversal. For higher current density, radial spin wave and azimuthal spin wave lead to the chirality reversal, however, the vortex core is still in the center of the nanodisk during the chirality reversal processes.Comparing with the case that the nanocontact locates at the center of the nanodisk, we observe that the polarity reversal behavior is different when the nanocontact is designed not at the center of the nanodisk. The micromagnetic simulations demonstrate that the vortex core is easier to be excited at the asymmetric total effective potential energy and the vortex core velocities yield a periodic fluctuation with increasing amplitudes, leading to some very high instantaneous velocities. Thus, such velocity fluctuation makes the vortex core easy to attain a threshold velocity required for the vortex core switching, reducing greatly the critical reversal current density and switching time. Meanwhile, it is found that properly choosing the nanocontact position can reduce 50% of the reversal current density. From an application point of view, the concentrated-switching positions of vortex core might be applicable to encrypted storage devices.It is observed that much more dynamics behaviors are induced when the vortex core is excited with three nanocontacts. In addition to the peculiar vortex core gyrotropic motion and the polarity reversal, the transient magnetic configurations are formed when the current densities are high and the inter-distances are large between the nanocontacts. The vortex core displays an irregular increasing orbital radius and velocity, and finally attaining the steady motion with periodic changes in velocity and radius. What’s more, the trajectory of vortex core is not circle anymore and the trajectory is affected by the distributions of Oersted field energy. The periodic changes in instantaneous angular frequency are ascribed to the non-uniform field forces in the vortex core motion, which results in periodic changes in the instantaneous velocity of the vortex core.The simulations demonstrate that the polarity can be switched with the increased current density. However, not only the switching time but also the after-switching positions are various for three different current combinations. The analytical calculations for total effective potential energy reveal that the asymmetric distributions of total effective potential energy cause the different switching time and final positions of the vortex core. For higher current density and larger inter-distances between the nanocontacts, the richer transient magnetic configurations are formed in the nanodisk, implying the Oersted field exhibits great influence on the magnetic configurations, meanwhile, the larger inter-distances between the nanocontacts provide enough space for forming magnetic configurations.