Characteristics Of Magnetization Dynamics In Perpendicular Magnetic Tunnel Junctions

With advancement in information technology and the arrival of the big dataera, data resource has become one of the indispensable necessities of life.Meanwhile, the development of data storage devices that are of smaller size,larger capacity, and higher reliability has been unabated. Therefore, constantlyimproving the storage density and enhancing the performance of storage devicesare of great significance. The perpendicular spin transfer torque magneticrandom access memory (STT-MRAM) is one of the promising candidates for thenext generation memory devices due to its fast speed, non-volatile features andits nearly infinite number of write-read cycles, and indeed, its potential forachieving extremely high level of storage density as compared to the otheralternatives for solid-state data storage devices.In nanoscale magnetic structures, magnetization switching dynamics is adirect measure of the storage device performance. Thus, research into themagnetization dynamics is of great significance to magnetic storage technologyand spintronics. In the previous STT-MRAM devices, an in in-plane structurehas been used. In order to reduce the current density, the magnetic memory has to have a larger cross section. Therefore, in analysis of the magnetizationdynamics in such structure, a simplified macrospin model can be used, in thatonly the effect induced by demagnetization factor in the vertical direction needsto be taken into consideration, or the cross section is assumed to be infinitelylarge. However, due to relatively smaller switching current for the perpendicularstructure of MTJ, the cross-sectional area can be further reduced while retainingthe thermal stability of the device. In this case, the simplified macrospin modelappears to ineffective with the effect of cross-sectional area being neglected.Additionally, the recent research has shown that changing the free layer orpolarization layer thickness can result in a small tilt angle of the magneticmoment with respect to the vertical direction, as observed in the magneticstructures using CoFeB thin film or Co/Pd multilayer film, for example.However, the influence of such tilt angle on the magnetization under the finitesize of the cross-sectional area, remains to be studied. To this end, this papermainly studies the following aspects around dynamic features of magnetizationswitching:1. Chapter1introduces the significance and development history of themagnetic storage technology, along with the present status and futuredevelopment direction of such technology.2. Chapter2introduces the demagnetizing field and demagnetizing energy.With the point of the magnetic dipole interaction, the formula of demagnetizingfield is derived in detail. Finally, generalizations about the components of demagnetization factor of the ellipsoid and the rectangular structure are given.3. In Chapter3, it concretely analyzes the methods of dealing with themacro spin model, and summarizes its advantages and disadvantages, byutilizing numerical simulation method, especially focusing on the effect the spintransfer torque (STT) effect.4. Base on the Landau-Lifshitz-Gilbert (LLG) equation which hasintroduced the component of demagnetizing factor of in-plane structure, thechapter4mainly investigate the effects of magnetization tilt angle of pinnedlayer on the dynamic characteristics of the magnetization reversal in a magnetictunnel junction (MTJ) having a free layer with the perpendicular magneticanisotropy (PMA). The results demonstrates that the switching time and therequired threshold current density can be significantly reduced for aperpendicular MTJ structure when using a relatively small magnetization tiltangle of the pinned layer.5. Usingmacrospin model, the influences of the free layer’s shape and sizeson the magnetization reversal characteristics have been discussed. Thedependence of the threshold current density and the switching time as a functionof the ratio of the free layer thickness over its characteristic length has beenstudied. The influences of the magnetization tilt angle of free layer on thethreshold current density and the switching time have also been investigated,and compared with the result in which dynamic characteristics of themagnetization reversal is influenced by the effect of magnetization tilt angle of the pinned layer. The results demonstrates that the switching time and therequired threshold current density can be significantly reduced for aperpendicular MTJ structure when using a relatively small magnetization tiltangle of free layer and appropriate aspect ratio of the free layer. Moreover,comparing with conventional arithmetic, larger and more accurate switchingtime and threshold current density can be obtained under the same condition.