Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L38: Spin Transport and SpintronicsFocus Live
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Sponsoring Units: GMAG DMP Chair: Manfred Albrecht, Univ Augsburg |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L38.00001: Multi-domain memristive magnetization switching in [Co/Ni]X/PtMn heterostructures Gunasheel Kauwtilyaa Krishnaswamy, Alexander Kurenkov, Giacomo Sala, Manuel Baumgartner, Viola Krizakova, Corneliu Nistor, Francesco Maccherozzi, Sarnjeet Dhesi, Shunsuke Fukami, Hideo Ohno, Pietro Gambardella Current-induced spin-orbit torques (SOT) enable the switching of ferromagnet (FM)/ antiferromagnet (AFM) multilayers for memory applications and potential use in neuromorphic computing [1]. We use x-ray photoemission electron microscopy to image the FM and AFM domains of [Co/Ni]X/PtMn layers at intermediate switching states as a function of injected currents. The AFM PtMn layer has a granular texture, with majority of the domains smaller than 100 nm, whereas the FM domains in Co/Ni are typically larger than 200 nm. We find no strict correlation between the current-induced switching of FM domains and the underlying AFM domains. Imaging of Co/Ni domains shows that switching occurs by the incremental domain expansion driven by SOTs starting from the edges of blocked domains and proceeding by domain wall displacements rather than grain-by-grain [2]. Highly reproducible domain patterns are found between consecutive switching cycles. Together, the step-wise expansion of the domains and the domain pattern reproducibility determine the memristive properties of the bilayer. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L38.00002: Magnetic domain-wall inverter and diode Zhaochu Luo, Schären Stefan, Aleš Hrabec, Phuong Dao, Giacomo Sala, Simone Finizio, Junxiao Feng, Sina Mayr, Jörg Raabe, Pietro Gambardella, Laura J Heyderman Magnetic domain-walls (DWs) can be used for data storage as well as for performing logic operations. Whereas the research into current-driven DW motion has made tremendous progress in recent years, the realization of DW logic circuits has faced more challenges. In order to realize efficient and scalable DW logic circuits, it is desirable to perform logic operations using an electric current, which would allow for the individual addressing of the gates and minimal energy consumption. In recent work, we have demonstrated a current-driven DW logic scheme based on the chiral coupling between adjacent magnets induced by interfacial Dzyaloshinskii–Moriya interaction [1,2]. In this talk, we will first show how to construct a magnetic field- and current-driven DW inverter in a system with perpendicular anisotropy and then tailor the symmetry of the inverter to introduce non-reciprocity in the DW inversion process. We will show that such a device can work as a DW diode, thus extending the operation of DW logic to the AC signal regime. Our work illustrates how chirally coupled structures offer a versatile platform to design efficient DW logic devices [3]. |
Wednesday, March 17, 2021 8:24AM - 9:00AM Live |
L38.00003: The impact of morphology and hybridization on the Dzyaloshinskii-Moryia Interaction Invited Speaker: Hans Nembach The Dzyaloshinskii-Moriya interaction (DMI) gives rise to chiral magnetic structures, which include chiral spin chains and skyrmions. DMI requires broken inversion symmetry and can exist in the bulk of a material as well as at interfaces. We used Brillouin light scattering spectroscopy to determine the DMI strength from the non-reciprocal frequency-shift of spin waves [1]. In order to gain deeper insight into the underlying physics of DMI and explore ways on how to tune the DMI through interface modifications, we prepared multiple samples to study different aspects of the DMI [2], [3], [4]. Specifically, Cu/Co90Fe10, Pt/Co90Fe10 and Co90Fe10/Ta(t) series were in-situ oxidized for different times and subsequently capped to prevent further oxidation. Density functional theory (DFT) calculations have demonstrated that 2p-3d and 3d-5d hybridization and the associated charge transfer is important for DMI. We determined that the spectroscopic splitting factor g is correlated to the DMI in the Cu/Co90Fe10 and Pt/Co90Fe10 oxide sample series. This is an indirect confirmation of the DFT predictions. We introduced a Cu dusting layer at a CoFeB/Pt interface to modify the Heisenberg exchange locally. SQUID magnetometry shows that the Cu layer reduces the proximity magnetization in the Pt as well, which can be seen as a measure for the strength of the exchange coupling. This demonstrates the correlation between the exchange directly at the interface and the DMI. Finally, we irradiated CoFeB/Pt samples with He+ to modify the interface morphology and measured an increase of 20 % for the DMI before it decreases for the highest fluences. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L38.00004: A Low Energy-Barrier Magnet based Analog Stochastic Neuron Samiran Ganguly, Avik Ghosh Neuromorphic hardware built using emerging nano-materials technology can provide enormous scalability and energy-efficiency over conventional digital circuit designs for both edge and cloud AI devices. In this work, we propose a compact energy-efficient Analog Stochastic Neuron device built from a low energy-barrier magnetic tunnel junction and a few silicon transistors that can function as a compact drop-in replacement for stochastic sigmoidal neurons in a “software” model of a neural network (NN), allowing building of large scale NN circuits where neurons are first class objects. We discuss in details the physics that allows us to build such a device and using a comprehensive coupled stochastic magneto-dynamics and charge and spin transport, demonstrate multiple applications of this device using illustrative examples. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L38.00005: Controlling magnon interaction by a nanoscale dipole switch Arezoo Etesamirad, Rodolfo Rodriguez, Boris Ivanov, Jordan A Katine, Ilya N Krivorotov, Roman V. Verba, Vasyl Tyberkevych, Igor Barsukov The ability to control and tune magnetic dissipation is a key concept of emergent spintronic technologies. Magnon scattering processes [1,2] constitute a major dissipation channel in nanomagnets, can redefine their response to spin-torques, and hold the promise for manipulating magnetic states on the quantum level. Controlling these processes, while being imperative for spintronic applications, has remained difficult to achieve. Here, we propose an approach for controlling magnon scattering by a nanoscale dipole switch. We demonstrate an experimental proof-of-concept in magnetic tunnel junction nanodevices. By triggering the spin-flop transition in the synthetic antiferromagnet layers and utilizing their dipole field, a three-magnon process in the free layer is toggled. The switching of the synthetic antiferromagnet tunes the strength of the magnon interaction by at least one order of magnitude, leading to two distinct dissipative states. The results allow for controlling dissipation by external stimuli at nanoscale and show promise for spin-torque applications and hybrid quantum information technologies. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L38.00006: Energy dissipation in macroscopic tunnel junctions Leonardo Rios E, Edgar Patino, Neelima Govind Kelkar Tunnel junctions constitute a remarkable research area due to their potential applications in information storage systems and quantum computing. Tunneling currents research projects can cover the study of superconducting, magnetic or spintronic devices. Most of these devices have a tunnel barrier that can be theoretically studied with Simmons model for low or intermediate voltages. However, fitting experimental data for high voltages can be challenging for some devices. One of the reasons for this theory to not agree with experimental data is that energy dissipation in the tunnel barrier is often neglected. In this work, we show an I-V characterization of a macroscopical high quality tunnel junction of the form Al/Al2O3/Al and its deviation with the theoretical Simmons model for certain voltages. We include a dissipation factor with a friction coefficient, η, as it is included in previous works for the study of tunneling times. Results show that the friction coefficient depends on the voltage, which implies that less energy is lost in the process of tunneling when the incident energy of the particle is reaching the height of the barrier. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L38.00007: Highly efficient spin-orbit torque switching of perpendicular magnetization using topological insulators with a high thermal stability Quanjun Pan, Yuting Liu, Hao Wu, Peng Zhang, Hanshen Huang, Xiaoyu Che, Yingying Wu, Bingqian Dai, Qiming Shao, Kang-Lung Wang Recent advances in using topological insulators (TIs) with ferromagnets (FMs) at room temperature have opened up an innovative avenue in spin-orbit torque non-volatile magnetic memory and low dissipation electronics applications. However, a major challenge of direct integration of TIs with a perpendicularly magnetized FM remains in retaining an extraordinary charge-to-spin conversion efficiency in TIs. In addition, the indispensable thermal compatibility with modern CMOS technologies has not yet been achieved in TI-based structures. In this talk, we will demonstrate the high-quality integration of perpendicularly magnetized FM with TI through a light metal insertion layer and achieved efficient magnetization switching at ambient temperature. The energy efficiency of TIs is at least an order magnitude larger than those typical values of heavy metals (e.g., Ta, Pt, etc.). Moreover, we demonstrate that the current-induced magnetization switching and perpendicular magnetic anisotropy of the integrated FM can be well preserved for an annealing temperature up to 400C. The compatibility of our TI-based thin films with the modern CMOS back-end-of-line process paves the way towards TI-based low-dissipation spintronic applications. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L38.00008: Resonance-enhanced exchange coupling for voltage-controlled magnetic switching in the scaled magnetic memory device Shehrin Sayed, Cheng-Hsiang Hsu, Niklas Roschewsky, See-Hun Yang, Sayeef Salahuddin We will discuss a voltage-controlled magnetic tunnel junction that uses a resonant tunneling barrier instead of a single oxide barrier. A voltage across the structure leads to a resonant enhancement of the interlayer exchange coupling. The peak equivalent exchange field is strong enough to switch typical ferromagnets used in scaled magnetic memory devices. The resonance-enhanced coupling exhibits a voltage-dependent oscillation that can enable a bidirectional switching with the same voltage polarity, unlike conventional magnetic devices, where a bidirectional current or a magnetic field is necessary. The switching threshold is decoupled from the speed due to the conservative nature of the exerted torque, unlike the conventional spin-torque devices that exhibit a trade-off due to the nonconservative nature of the switching torque. We further show that the magnetoresistance is higher for smaller voltages, while the exchange field is higher for larger voltages—this is promising for efficient read and write operations in potential memory applications. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L38.00009: Manipulating Berry curvature of SrRuO3 thin films via epitaxial strain Zhiwei Liu, Hanghui Chen, Di Tian, Shengchun Shen, Yu Zhou, Pu Yu Berry curvature of an electronic band structure underlies the calculation of intrinsic anomalous Hall conductivity (AHC) and ferroelectric polarization of quantum materials. However, Berry curvature is highly sensitive to subtle changes in electronic band structure and therefore can be manipulated through external stimulus. Transition metal oxide SrRuO3 provides an ideal material platform to study fine-tuning of Berry curvature. In this work, we carry out systemic transport studies and first-principles calculations to investigate the change in AHC of SrRuO3 thin films both under compressive and tensile strains. We find that as the applied strain changes from compressive to tensile, the magnetic easy axis of SrRuO3 rotates from out-of-plane to in-plane. In the meantime, the sign of AHC changes from negative to positive at low temperatures. In addition, we find that under tensile strain, the AHC of SrRuO3 thin films exhibits a characteristic hump feature, which we attribute to the rotation of Ru magnetic moment. Using Berry curvature calculations, our first-principles simulation reproduces the strain-driven sign change in AHC and the non-monotonic dependence of AHC on Ru magnetic moment when SrRuO3 thin films are under tensile strain. |
Wednesday, March 17, 2021 10:12AM - 10:48AM Live |
L38.00010: Ultrafast spintronics with terahertz radiation Invited Speaker: Tobias Kampfrath To take advantage of the electron spin in future electronics, spin angular momentum needs to be transferred and detected. Heat gradients and electric fields have been shown to efficiently drive spin transport at megahertz and gigahertz frequencies. However, to probe the initial elementary steps that lead to the formation of spin currents, we need to launch and measure transport on femtosecond time scales. To achieve this goal, we transfer spintronic concepts to the terahertz frequency range by employing both ultrashort optical and terahertz electromagnetic pulses. This experimental strategy provides new insights into important transport phenomena, for instance the spin Seebeck effect as well as anisotropic and spin Hall magnetoresistance. Interesting applications such as the efficient generation of ultrashort terahertz electromagnetic pulses and the high-throughput characterization of spin-to-charge conversion emerge. |
Wednesday, March 17, 2021 10:48AM - 11:00AM On Demand |
L38.00011: Spin Orbit Torque Domain Wall-Magnetic Tunnel Junction Devices and Circuits for In-Memory and Neuromorphic Computing Mahshid Alamdar, Thomas Leonard, Can Cui, Bishweshwor P. Rimal, Lin Xue, Otitoaleke G. Akinola, Tianyao Patrick Xiao, Joseph S. Friedman, Christopher H. Bennet, Matthew J. Marinella, Jean Anne C. Incorvia Domain wall-magnetic tunnel junction (DW-MTJ) in-memory computing devices can address major data processing bottlenecks with traditional computing, especially for accomplishing data-intensive and real-time tasks. We propose three-terminal DW-MTJ in-memory computing devices that resolve challenges with traditional DW-MTJs by using perpendicular magnetic anisotropy (PMA) with more robustness to thermal fluctuations than in-plane magnetic anisotropy, spin-orbit torque switching that requires less switching current than spin transfer torque, and an optimized lithography process to produce average device tunnel magnetoresistance TMR = 164%, close to the expected highest TMR seen in PMA MTJs, and resistance-area product RA = 31 Ω.μm2 , close to the RA of the unpatterned film. A two-device circuit shows bit propagation between devices. Switching voltage cycle-to-cycle variation is measured as a tunable probabilistic function and is shown to be curtailed to 7% by controlling the DW initial position, which we show corresponds to 96% accuracy in a DW-MTJ full adder simulation. These results make major strides in using DW-MTJs for in-memory and neuromorphic computing applications. |
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