Abstract
A novel memory device based on the magnetoreisitive effect is known as magnetic random access memory (MRAM) and is also called “dream memory”. In this dissertation, we investigate the key issue of the MRAM － writing efficiency. Firstly, in the concept of spin torque transfer MRAM, we can manipulate the damping constant and saturation magnetization of CoFeB by simply adjusting the capping layers. We found that the Ta capping layer caused the intermixing, and increased the damping constant of CoFeB. By inserting various metals, the intermixing can be improved and the lower damping constant with Cu/Ta capping layers was obtained. Moreover, the results of micromagnetic simulation indicate that the optimal switching current density can be achieved by reducing the damping constant with a Cu capping layer. In our simulation results, we demonstrate that by changing the capping layer from Ta to Cu/Ta, we can effectively reduce the critical current density by 27 %. Secondly, in the concept of magnetic domain wall RAM (so called “race-track memory”), we demonstrate that the depinning probability of the transverse-type domain wall strongly depends on the domain wall configuration when the domain wall moves to the notch. We found that when the current density is larger than threshold current density, the domain wall structure periodically changes between transverse-type domain wall and antivortex-type domain wall. Due to this transition behavior, part of the spin torque energy contributes to the transformation of the domain wall structure. Since the antivortex-type domain wall stores more energy, the stored energy can help the domain wall to be depinned from the notch and increases the depinning probability. Finally, we take advantage of the magnetic domain wall and we propose a method which combines either conventional current- or field- driven MRAM with the magnetic domain wall. We demonstrate that the domain wall can be artificially created in patterned antiferromagnetic/ferromagnetic exchange bias system using ion irradiation. The magnetic domain wall was formed by the different switching fields in the irradiated and non-irradiated areas and could assist the magnetization switching. Furthermore, this study proposes a new type of MRAM design. A comparison of the proposed magnetoresistive device with the conventional ellipse one shows that the new design reduces the switching field and the critical current density by about 90.0 % and 68.8 %, respectively, in field- and current- driven cases. In both of these cases, we demonstrated the domain wall assisted magnetization reversal. This study provides an effect way to increase the writing efficiency of MRAM.