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Open Access Publications from the University of California

Scholarly Works (14 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2020)

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.

    Cover page: Investigation and Control of Ultrafast Magnetic Phenomena
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    The discovery of ultrafast helicity-independent all-optical switching (HI-AOS), as well as picosecond all-electrical switching of a ferrimagnet, has inspired the ultrafast spintronics community to explore ultrafast switching of a ferromagnet to achieve practical ultrafast storage and memory devices. Two explored mechanisms of HI-AOS of a ferromagnet in ferromagnet-ferrimagnet heterostructure are: a) exploiting the indirect exchange coupling with and b) injection of non-local spin current originated from a switching ferrimagnet. In this manuscript, exchange mediated HI-AOS of a Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange coupled “[Co/Pt]-multilayers/Pt spacer/CoGd” heterostructure is demonstrated. The authors have measured layer-resolved static magnetic properties, single-shot HI-AOS, and magnetization dynamics of the ferromagnetic Co/Pt multilayers (MLs), that are ferromagnetically or antiferromagnetically coupled with ferrimagnetic CoGd layers. Time-resolved magnetization dynamics reveal a 3.5 ps switching time of the Co/Pt MLs, which is the fastest switching of a ferromagnet reported to date. Employing an extended microscopic three-temperature model, the temporal dynamics of the exchange coupled ferromagnet–ferrimagnet heterostructure are simulated, qualitatively and quantitatively explaining the experimental switching phenomena. This work experimentally as well as theoretically establishes the mechanism of exchange mediated all-optical switching of ferromagnet-ferrimagnet heterostructures, which can be integrated with a magnetic tunnel junction for efficient reading after ultrafast energy-efficient switching.
      Cover page: RKKY Exchange Bias Mediated Ultrafast All‐Optical Switching of a FerromagnetCreative Commons 'BY-NC' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2022)
      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.
        Cover page: Progress toward picosecond on-chip magnetic memoryCreative Commons 'BY-NC' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        The ability to control magnetism in sub-picosecond timescales using laser pulses has potential applications in fast magnetic devices. It has been demonstrated that a single femto-second laser pulse can reverse -The magnetization of a film within a few picoseconds [1], a phenomenon called as all-optical switching (AOS). However, this phenomenon thus far has been observed only in ferrimagnetic GdFeCo films. On -The o-Ther hand, ferromagnetic films, which are of greater interest than ferrimagnets in many technological applications, need multiple laser pulses for magnetization reversal [2], making -The overall effect slow and energy inefficient. Fast, singleshot switching of a ferromagnet can open up new avenues for spintronic devices that operate at unprecedented picosecond timescales, -Thereby making -Them viable non-volatile replacements for silicon based RAMs and logic devices.
          Cover page: Single Shot Ultrafast all Optical Magnetization Switching of Ferromagnetic Co/Pt Multilayers
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          A single femto-second optical pulse can fully reverse the magnetization of a film within picoseconds. Such fast operation hugely increases the range of application of magnetic devices. However, so far, this type of ultrafast switching has been restricted to ferri-magnetic GdFeCo films. In contrast, all optical switching of ferro-magnetic films require multiple pulses, thereby being slower and less energy efficient. Here, we demonstrate magnetization switching induced by a single laser pulse in various ferromagnetic Co/Pt multilayers grown on GdFeCo, by exploiting the exchange coupling between the two magnetic films. Table-top depth-sensitive time-resolved magneto-optical experiments show that the Co/Pt magnetization switches within 7 ps. This coupling approach will allow ultrafast control of a variety of magnetic films, which is critical for applications.
            Cover page: Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            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 nonequilibrium 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 ∼13 ps. These results raise new questions about the fundamental mechanism of helicity-independent AOS.
              Cover page: Role of electron and phonon temperatures in the helicity-independent all-optical switching of GdFeCo
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              Up until now, magnetic nanodots used for magnetic random access memory have required spin-polarized currents to transfer the angular momentum needed to switch the magnetization and thereby switch the magnetic memory bit. This particular switching process, however, is limited to nanosecond or greater timescales-too slow for use as low-level cache in energy efficient electronics systems. On the other hand, this work aims to achieve ultrafast femtosecond switching of nanomagnetic dots without the use of spin-polarized currents. Using just linearly polarized light, several research groups have demonstrated all-optical magnetization switching in large GdFeCo magnetic dots, ranging from several microns [1-4] down to 400 nm [5]; this work characterizes the switching behavior as these dots are scaled down further in size, with the aim of minimizing the energy required for switching the magnetic memory bit. The fabrication process, magnetization behavior and optical switching behavior are additionally characterized to better understand how size affects the functionality of these optically-switchable ferrimagnets. Knowledge of this behavior will allow future developments of simultaneously ultrasmall and ultrafast magnetic memory systems, thereby enabling increased data storage in future electronics.
                Cover page: Ultrafast Magnetic Memory Bits Using All-Optical Magnetic Switching
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2023)
                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.
                  Cover page: Picosecond spin-orbit torque–induced coherent magnetization switching in a ferromagnet
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2021)
                  We investigate the influence of dislocations and twin walls in BaTiO3 on its ferroelectric response and the resulting effect on the perpendicular magnetic anisotropy (PMA) of a strain-coupled [Co Ni]n film. A dense twinned structure in conjunction with a high dislocation density significantly reduces the converse piezoelectric effect of BaTiO3 by hindering the propagation of newly nucleated domains with an applied electric field. This, in turn, results in a modest reduction of the PMA of the ferromagnetic layer. On the other hand, the ferroelectric polarization reorients from [100] to [001] direction in a dislocation-free BaTiO3, inducing the maximum achievable in-plane compressive strain of 1.1%. A large fraction of this uniaxial strain is transferred to the magnetoelastically coupled ferromagnetic layers whose magnetization switches to in plane via the inverse magnetostriction effect. This work reveals the critical role of the interplay between twin walls and dislocations within a ferroelectric substrate in the performance of multiferroic heterostructures and provides insight into the development of highly energy-efficient magnetoelectric devices.
                    Cover page: Influence of dislocations and twin walls in BaTiO3 on the voltage-controlled switching of perpendicular magnetization
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2021)
                    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 (a-) rare earth-transition metal (a-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic a-Gd22-xTbxCo78 thin films, from x=0 to 18, and elucidate the role of Gd in HI-AOS in 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 a-Gd10Tb12Co78 leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting 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.
                      Cover page: Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films
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