Study on chiral domain wall in ultra-thin films: Tailoring of spin orbit torque and Dzyaloshinskii-Moriya interaction

Abstract

As the circuit wire width of DRAM used today got near to the scale of 10nm which is the physical lower limit, there has aroused various problems regarding cost efficiency, structure-design and so on. To overcome these new challenges, the interest in new-generation memory device has increased drastically. The magnetic memory device using spintronics have proven itself to be one of the promising candidates, as it excels in solving various problems regarding writing/reading time, endurance, non-volatility, power consumption and so on. This is the reason why spintronics is being studied hard both industrially and scholarly. This paper focuses on solving physical problems hindering the mass production of Spin torque device. Those problems are as listed: 1)High speed, 2)Read-Write reliability, 3)High density & low energy consumption. Some new interesting physical phenomena such as spin orbit torque(SOT) and Dzyaloshinskii-Moriya interaction(DMI) have been discovered and studied in the course of time. In chapter 1, the start and expected future of new memory device using spintronics will be introduced. Recently various kinds of new-generation memory have suggested. We will specify the unique properties of spintronics memory device. Device using spin-torque has been studied in 2 ways which will be introduced in details. First the STT-MRAM with current flowing through 2 magnetic layers separated by nonmagnetic layer, and second the SOT-MRAM with current flowing in the plane of domain walls will be discussed. In chapter 2, we discuss the ultrathin film with perpendicular magnetic anisotropy that minimizes the influence of sputtering defects that hinder the domain wall movement. We developed various manufacturing methods with sputtering system and they will be discussed in detail here. To measure the width of the film layers, we used XRT/AFM to analyze the structure of ultrathin film. Set of films with structure modification by annealing effect have been produced. Measuring these films led to the empirical correlation between structure property and magnetic property. In chapter 3, the Gilbert damping constant is the main subject. This is related with the high performance of spin torque device. The definition and significance of Gilbert damping is explained. To optimize this constant, we studied various related magnetic properties and finally achieved the method to control it via structure tailoring. In chapter 4, we investigate the SOT and DMI in Co films sandwiched by various 3d, 4d, and 5d transition metals. Recently, it has been found that the efficiency of domain-wall motion driven by current can be largely enhanced by the SOT combined with the DMI. It is therefore important to analyze the sign and magnitude of the DMI and SOT to understand their physical origin as well as to achieve memory and logic devices. In this study, we report the DMI and SOT of various metallic ferromagnetic films, of which the structures are Ta(5 nm)/Pt(2.5 nm)/Co(0.6 nm)/X(5nm) films with various choice of X by Ti(3d), Cu(3d), Ru(4d), Pd(4d), Ta(5d), W(5d), Au(5d), and Mg. The sign and magnitude of the DMI and SOT are then measured from the asymmetric domain-wall expansion and ω-2ω measurement method, respectively. The overall trend depending on the material combination will be discussed. In chapter 5, we control the SOT and DMI by use of Mg insertion to Pt/Co/Pt structure. The Mg insertion layer thickness dependence of DMI sign and magnitude was measured by asymmetric DW expansion method. From the measured dependence, it has been shown that the DMI field can be controlled via film structure tailoring. In chapter 6, the whole dissertation is concluded. The physical origin of SOT and DMI has been studied scientifically. The DMI and SOT of various films have been measured and classified in hierarchical manner. The material design rule from it has suggested the method to tailor spin torque devices. This hierarchical approach to tailor spin torque memory devices is becoming a stepping stone to the future of nanomagnetism.