Investigations of low dimensional magnetic structures for memory and storage device applications

• Main Author: Kumar, Durgesh
• Other Authors: S.N. Piramanayagam
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/146139
• Institution: Nanyang Technological University

The data storing devices are of great importance in the age of big data. Amongst several data storage devices, the hard disk drives (HDDs) share the maximum (~80% of the total data). Therefore, continuous advancements in the areal density of HDDs are required. However, the areal density growth of current HDD technology is not proportional to the generated big-data, and hence, alternative technologies need to be researched. The media industry is eyeing a potential alternative, namely, heated-dot magnetic recording (HDMR). However, one of the major challenges of HDMR is the optimization of a patterning technique suitable for the mass production of media. Between physical and magnetic patterning processes, magnetic patterning suits more for mass production of the recording disks, which requires ion implantation of non-magnetic species through the hard mask. Moreover, the usage of self-assembly based lithography technique for fabrication of the hard mask enhances the suitability of magnetic patterning for the mass production of recording disks. Therefore, we have studied the magnetic pattering of Co80Pt20 and FePt-C films. The self-assembly of polystyrene- polydimethylsiloxane di-block copolymer, which resulted in the patterns as small as ~20 nm, was utilized to fabricate the hard mask on the magnetic samples. The non-magnetic 14N+ and 40Ar+ ions with high and low energies (HE and LE) were utilized. Regarding Co80Pt20 samples, the structural and magnetic properties are more sensitive to LE 14N+ and HE 40Ar+ implantations. The change in magnetic properties might be arising from an increase in A or a decrease in Ku or both. However, for FePt-C samples, all the implantation cases except LE 40Ar+ ion implantation are significantly affecting the structural and magnetic properties. Domain wall (DW) memory is another potential replacement of the present storage scheme. Although there has been a significant advancement in the writing, DW motion, and reading operations of the DW memory, there are still many unsolved issues. One of them is lack of controlled and reliable motion of DWs. The concept of artificial pinning sites has been proposed for controlled and reliable DW motion. The artificial pinning sites are formed by locally changing the physical shape or the magnetic properties of the memory unit. There have been a few proposed concepts of fabricating artificial pinning sites in the past. However, the most common problem with these concepts is the non-uniform pinning strength of pinning sites. We have studied the concepts of (a) square wave shaped nanowire, and (b) local introduction of interfacial Dzyaloshinskii-Moriya interaction (iDMI) to fabricate pinning sites using micromagnetic simulations. The optimum pinning strength of the former was found to be limited in a small range of current density values. However, the latter shows the DW pinning in a large range of current density, geometrical, and magnetic parameters. Moreover, the pinning strength was found to be tunable as a function of the above-mentioned parameters. In addition, we have also observed the DW oscillations in the nanoscale iDMI region for a certain range of parameters. We have developed a theoretical model that excellently explains the nature of studied DW oscillations. The operational power consumption is another problem for DW memories. The experimental demonstration of spin-orbit torque (SOT) driven DW motion has accelerated the research towards reducing the power of DW devices. However, there has not been much success in the past. Noticeably, the β-W has been the most common choice for the HM layer due to its high SOT efficiency. However, β-W invites the problem of larger defects in the ferromagnetic (FM) layer, and hence, larger operational power. Thus, we have studied a novel design, where an α or (α+β)-W layer has been sandwiched between β-W and FM layer. The proposed insertion reduced the intrinsic pinning of the FM layer and helped to reduce the current density by a factor of 104 for driving the DWs. This observation lays the foundation for ultra-low power DW devices. In addition to memory applications, these devices were further studied for emerging neuromorphic computing.