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Investigation and Control of Ultrafast Magnetic Phenomena


Spintronic devices have shown a lot of promise in low power and non-volatile memory applications. However, conventional spintronic devices are limited by the speed of equilibrium magnetization reversal. For more than two decades, the field of ultrafast magnetism, wherein magnetic processes in (sub)picosecond timescales are triggered by the ultrafast non-equilibrium heating of magnetic thin films with femtosecond laser pulses, has provided us with the tantalizing prospect of controlling magnetism in unprecedentedly fast timescales. This dissertation will detail the research conducted over the last 6 years in understanding ultrafast magnetic phenomena, and in controlling and integrating them with conventional spintronic processes to realize fast, non-volatile spintronic devices.

The first part of the dissertation will focus on work done to understand the fundamental limitations of some spintronic and ultrafast magnetic phenomena. This will include experiments on detecting the current induced spin accumulation due to the spin-orbit effects in heavy metals directly on the heavy metal surface using an optical technique called the magnetization-induced second harmonic generation (MSHG). Insight into the dynamics and timescales of current induced spin accumulation in conventional spin-orbit torque (SOT) devices gained from these experiments will help understand the speed limitations of such devices. The dissertation then focuses on the ultrafast helicity-independent all-optical switching (HI-AOS) in ferrimagnetic GdFeCo and GdTbCo alloys. These experiments shed a light on the underlying mechanism of such a process, and unravel the complex interplay of exchange coupling, elemental damping and other parameters in ultrafast magnetization switching events. The upper limit for the pulse duration of optical excitation that triggers HI-AOS, which has important implications when it comes to integrating these processes on-chip, is also studied.

The second part of the dissertation will introduce ways to build up on the experimental results of the first part, thereby moving towards the integration of ultrafast magnetic phenomena into conventional spintronic devices. Experiments performed to extend the ultrafast HI-AOS capabilities of GdFeCo to Co/Pt multilayers by controlling the exchange interaction between these two films are presented. This is of technological significance because HI-AOS had thus far only been reported in Gd-based ferrimagnetic films, which are not very attractive for device integration due to their ferrimagnetic nature. Co/Pt multilayers, on the other, are ferromagnetic and are well suited for application in spintronic devices. Then, the ultrafast control of magnetism by picosecond heat current and electrical current pulses will be introduced. Finally, the dissertation will present recent results on demonstrating the deterministic spin-orbit torque switching of a Co/Pt ferromagnet by short, 6 ps electrical pulses.

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