Field-Driven Domain-Wall Dynamics Related with Dzyaloshinskii-Moriya Interactions and Magnetic Bubblecade Memory

Abstract

For application as a data storage entitled to magnetic hard disk drives, the magnetic domains are widely used. Each domain represents unit of stored information, and the region between domains is known as domain walls (DWs). For the promising memory and logic devices, it has been proposed to shift the information by coherent moving these DWs through a magnetic nanowire, called racetrack memory. This concept has led to the establishment of a quickly evolving new research field which investigates the mechanisms moving the DWs. This mechanism is operated by the use of an externally applied magnetic field, or injecting an electric currents through the nanowire in which the DWs reside. Recently, perpendicularly magnetized anisotropy (PMA) materials are not only particularly interesting for these applicational devices, but also new physics is being found in these materials since finding DW motion driven by spin-orbit-torque and chiral DW by the Dzyaloshinskii-Moriya interaction (DMI). This thesis contains several contributions which are right at the heart of recent developments on DW physics in PMA materials such as Pt/Co/Pt. The first part of the thesis focuses on maximizing the motion of DWs by adjustment of thickness of metal (upper and bottom Pt layers) contacted by magnetic layer. In particular, it is shown that by changing the each Pt thickness, the PMA is altered in such a way that DW speed is affected. This phenomena is successfully explained by the creep law with scaling law between PMA and DW speed. So this results offer a method of maximizing the DW speed without changing the thickness of the magnetic Co layer. In the second part of the thesis, it deals with topic of fast DW motion called, the flow regime. This regime has been widely studied since 1950. All of the study have been performed the relation between DW speed and driving force (applied external field). In this thesis, main topic is the DW speed with respect to in-plane magnetic field (changing the DW energy related DW profile) with fixed out-of-plane magnetic field (driving force). Based on the 1-dimensional model, analytic solution is suggested and the result from analytic solution is well-satisfied with micromagnetic simulation. Furthermore, this analytic result is well-explaining experimental result, qualitatively: variation of the speed. Important point is the minimum speed at compensating the field induced by DMI, and so it can be quantitatively determined the DMI-induced field. The third part suggests new technique to measure the DMI. This is advanced technique to overcome such field-strength limit. The core idea is to utilize the dependence of minimum speed on the tilting an- gle of the DWs with respect to the direction of the in-plane magnetic field. The present idea can be also demonstrated to other lms (Pt/Co/AlOx and Pt/Co/MgO), where DMI induced field is even much larger than the field-strength limit of equipment, and thus enables the development of an advanced DMI meter based on asymmetric DW motion. The final part of the thesis is composed of application device by use of magnetic-field oscillation. For this application, unidirectional motion of magnetic DWs is the key concept underlying next-generation DW-mediated memory and logic devices. In this part, new scheme is suggested and it utilizes the recently discovered chiral DWs, which exhibit asymmetry in their speed with respect to magnetic fields. Because of this asymmetry, an alternating magnetic field results in the coherent motion of the DWs in one direction. Such coherent unidirectional motion is achieved even for an array of magnetic bubble domains, enabling the design of a new device prototype—magnetic bubblecade memory—with two-dimensional data-storage capability. And then the study about the optimization of magnetic bubblecade speed is performed. In the operation of the magnetic bubblecade memory, two major control factors were found to play the decisive role in determination of the bubblecade speed strength and tilting angle of the magnetic field. Here, the relation of bubble speed with respect to strength and tilting angle of the magnetic field and obtain the analytic solution from the creep scaling law is examined. The proposed analytic equation thus offers the way to predict the optimization condition for the maximum bubblecade speed in the operation of the magnetic bubblecade memory.