Interfacial engineering to reduce switching current in perpendicularly magnetized Pt/Co/Pt system for memory applications

Abstract

Tuning the interfacial properties of heavy metal (HM)/ Ferromagnet (FM)/ heavy metal (HM) thin film systems with perpendicular magnetic anisotropy (PMA) has attracted massive attention in the recent past because of their ability to construct next-generation high-density magnetic data storage devices. The surface anisotropy dominates over the bulk anisotropy at the HM/FM interface to realize a PMA system in which the easy axis of magnetization lies perpendicular to the film plane. The PMA systems are known to host chiral domain walls of Nèel type supported by the interfacial Dzyaloshinskii-Moriya interaction (iDMI) that also emerges from the HM/FM interface. The HM layer converts longitudinal charge current (+ ~Jx) to a transverse spin current (+ ~Jz ) of a fixed direction of spin polarization (−ˆy) due to the spin Hall effect (SHE). This charge-to-spin conversion ratio is characterized by the spin Hall angle (θsh). Injection of this spin-polarized current into an adjacent FM layer applies spin-orbit torque on the magnetization of the FM layer to manipulate its magnetization. Minimization of current density (Jc) required for magnetization switching is a major challenge to construct memory devices with lower power consumption. Another challenge is to eliminate the requirement of an in-plane bias magnetic field to achieve magnetization switching and realize field-free switching. In this thesis, the interfacial properties of Pt/Co/Pt based PMA systems have been modified to accomplish field-free switching of magnetization at reduced Jc. The thesis is organized into 7 chapters.

In chapter 1, a brief history of magnetic data storage devices has been discussed. How the discovery of the giant magnetoresistance-based read heads revolutionised the hard disk drive sector has also been briefed. The relevance of the works presented in this thesis can be visualized by comparing them with the recent trends of spintronics research presented in this chapter.

In chapter 2, the theory of ferromagnetic material (FM) arising from the discrete moments in a crystalline solid has been discussed. The assumptions of micromagnetism have been presented to simulate the magnetization of the ferromagnetic thin film under external perturbations (magnetic field and current) in the micromagnetic framework by energy minimization technique and solving the Landau-Lifshitz-Gilbert equation. An introduction to the field-induced domain wall motion has been given. Finally, an overview of the micromagnetic simulation technique used in this thesis is introduced.

In chapter 3, the experimental techniques optimized to deposit the multilayered thin films and characterize them have been discussed, and their working principle has been presented in detail. The experimental protocol of some commonly used techniques is introduced at the end.

In chapter 4, The chirality of the Néel type domain walls formed in FM thin films with perpendicular magnetic anisotropy (PMA) has been determined by studying asymmetric domain wall expansion under the application of a biasing magnetic field in-plane of the sample. The connection between the chirality and interfacial Dzyaloshinskii-Moriya interaction (iDMI) has been established. It has been observed that the Ta/Pt/Co/Pt multilayer stabilizes Néel walls of right-handed chirality. However, with the introduction of a thin Au layer between the top Co/Pt interface, the iDMI strength (Def f ) increases with the increase of Au layer thickness as well as the chirality of DW reverses. The domain wall with left-handed chirality has been found in the Ta/Pt/Co/Au(t)/Pt multilayers for t = 0.3, 0.5 and 0.7 nm. It has been observed that the Def f also changes its sign from negative in the Ta/Pt/Co/Pt multilayer to positive in the Ta/Pt/Co/Au(t)/Pt multilayers. This reversal has been explained considering the individual Def f of the Pt/Co, Au/Co interfaces.

In chapter 5, The current-induced magnetization reversal (CIMR) of the Ta/Pt/Co/Pt based PMA systems has been studied to estimate the current density (Jc) required for magnetization reversal. At first, An 1 nm thick Au layer has been introduced at the top Co/Pt interface (Ta/Pt/Co/Au/Pt) and bottom Pt/Co interface (Ta/Pt/Au/Co/Pt) and Jc has been evaluated. The reduction of Jc upto ∼ 34% has been recorded in the Ta/Pt/Au/Co/Pt system with respect to the Ta/Pt/Co/Pt system. Whereas the Ta/Pt/Co/Au/Pt system does not show any significant reduction in Jc. To verify further, Ta/Pt/Co/Au and Ta/Pt/Au/Co/Au multilayers have been deposited. The reduction of Jc upto ∼ 30% has been observed in the Ta/Pt/Au/Co/Au system compared to the Ta/Pt/Co/Pt system. To further reduce the Jc a Ta capping layer has been used. The Jc reduced upto ∼ 58% in the Ta/Pt/Au/Co/Pt/Ta system with respect to the Ta/Pt/Co/Pt system. The field-free switching has been realized by introducing an in-plane magnetized Co layer on the top Co layer in the Ta/Pt/Au/Co/Pt/Ta/Au/Pt multilayer at Jc = 1.55 × 1011 A/m2. Micromagnetic simulations have been carried out to understand the role of Def f and effective spin Hall angle of the multilayer thin film system (θsh) in Jc. From the simulations, the qualitative values of θsh of the multilayers have been evaluated for comparison.

In chapter 6, the deterministic switching has been achieved without an in-plane bias magnetic field by introducing a buffer layer of Cu in between the Ta and Pt layer in Ta/Pt/Co/Pt based PMA system. Ta/Cu(t)/Pt/Co/Pt multilayers have been prepared for t = 0, 1, 2, and 4 nm to study CIMR and estimate the Jc. The Jc reduces with an increase in Cu layer thickness. The Jc has been reduced upto 16%, 35% and 55% compared to the Ta/Pt/Co/Pt system in multilayers with t = 1, 2, and 4 nm, respectively. The orbital spin Hall effect of Cu contributes to the θsh by enhancing the spin current generation. In addition, the field-free switching has been observed in the samples with x = 2 and 4 nm.

In chapter 7, A comprehensive overview has been presented, summarizing the key findings derived from the investigations. An overview that will assist readers in recognizing the most important research problems in this topic has been incorporated in this thesis.

In the appendices, a list of works that have been used extensively to support the thesis has been included. In appendix A, the Python code developed for micromagnetic simulation has been incorporated. Appendix B contains the MATLAB codes used to perform data analysis.