The role of electron and phonon temperatures in the helicity-independent all-optical switching of GdFeCo

Ultrafast optical heating of the electrons in ferrimagnetic metals can result in all-optical switching (AOS) of the magnetization. Here we report quantitative measurements of the temperature rise of GdFeCo thin films during helicity-independent AOS. Critical switching fluences are obtained as a function of the initial temperature of the sample and for laser pulse durations from 55 fs to 15 ps. We conclude that non-equilibrium phenomena are necessary for helicity-independent AOS, although the peak electron temperature does not play a critical role. Pump-probe time-resolved experiments show that the switching time increases as the pulse duration increases, with 10 ps pulses resulting in switching times of ~sim 13 ps. These results raise new questions about the fundamental mechanism of helicity-independent AOS.Comment: 18 pages, 6 figures and supplementary material

Optical Switching in Tb/Co-Multilayer Based Nanoscale Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal by an ultrashort laser pulse is extremely important. We demonstrate optical switching of Tb/Comultilayer-based nanoscale MTJs by combining optical writing and electrical read-out methods. A 90 fs-long laser pulse switches the magnetization of the storage layer (SL). The change in magnetoresistance between the SL and a reference layer (RL) is probed electrically across the tunnel barrier. Single-shot switching is demonstrated by varying the cell diameter from 300 nm to 20 nm. The anisotropy, magnetostatic coupling, and switching probability exhibit cell-size dependence. By suitable association of laser fluence and magnetic field, successive commutation between high-resistance and low-resistance states is achieved. The switching dynamics in a continuous film is probed with the magneto-optical Kerr effect technique. Our experimental findings provide strong support for the growing interest in ultrafast spintronic devices.Comment: total pages 22, Total figure

Picosecond Spin Orbit Torque Switching

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications.Comment: Includes article + supplementary information. Latest version uses full name of the first author. Nature Electronics (2020

Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal (\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic \textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, and elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing \textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that acounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS

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

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

Progress toward picosecond on-chip magnetic memory

We offer a perspective on the prospects of ultrafast spintronics and opto-magnetism as a pathway to high-performance, energy-efficient, and non-volatile embedded memory in digital integrated circuit applications. Conventional spintronic devices, such as spin-transfer-torque magnetic-resistive random-access memory (STT-MRAM) and spin-orbit torque MRAM, are promising due to their non-volatility, energy-efficiency, and high endurance. STT-MRAMs are now entering into the commercial market; however, they are limited in write speed to the nanosecond timescale. Improvement in the write speed of spintronic devices can significantly increase their usefulness as viable alternatives to the existing CMOS-based devices. In this article, we discuss recent studies that advance the field of ultrafast spintronics and opto-magnetism. An optimized ferromagnet-ferrimagnet exchange-coupled magnetic stack, which can serve as the free layer of a magnetic tunnel junction (MTJ), can be optically switched in as fast as ∼3 ps. Integration of ultrafast magnetic switching of a similar stack into an MTJ device has enabled electrical readout of the switched state using a relatively larger tunneling magnetoresistance ratio. Purely electronic ultrafast spin-orbit torque induced switching of a ferromagnet has been demonstrated using ∼6 ps long charge current pulses. We conclude our Perspective by discussing some of the challenges that remain to be addressed to accelerate ultrafast spintronics technologies toward practical implementation in high-performance digital information processing systems

Picosecond spin-orbit torque–induced coherent magnetization switching in a ferromagnet

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices

Single-shot switching in Tb/Co-multilayer based nanoscale magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal of the magnetization by an ultrashort laser pulse is extremely important. We demonstrate single-shot switching of Tb/Co-multilayer based nanoscale MTJs by combining the optical writing and the electrical read-out methods. A 90-fs-long laser pulse switches the magnetization of the storage layer (SL). The change in the tunneling magnetoresistance (TMR) between the SL and a reference layer (RL) is probed electrically across the oxide barrier. Single-shot switching is demonstrated by varying the cell diameter from 300 nm to 20 nm. The anisotropy, magnetostatic coupling, and switching probability exhibit cell-size dependence. By suitable association of laser fluence and magnetic field, successive commutation between high-resistance and low-resistance states is achieved. The nature of the magnetization reversal of both SL and RL in a continuous film is probed with a depth-resolved magneto-optical Kerr effect (MOKE) magnetometry. The ultrafast dynamics in the continuous full-MTJ stack is investigated with the time-resolved pump–probe technique. Our experimental findings provide strong support for the growing interest in ultrafast spintronic devices