Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S39: Magnetic Memories and ComputingFocus
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Sponsoring Units: GMAG DMP Chair: Jonathan Sun, IBM Thomas J. Watson Research Center Room: BCEC 207 |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S39.00001: Ultrafast p-bits using synthetic antiferromagnets Jan Kaiser, Kerem Y Camsari, Supriyo Datta, Pramey Upadhyaya Various applications ranging from cryptography to stochastic computing need random number generators (RNGs) which can be implemented in hardware using magnetic tunnel junctions (MTJs) with low barrier unstable ferromagnets (LBMs). It has been shown that with proper design such ferromagnets can show random fluctuations at GHz speeds. In this talk using stochastic LLG simulations, we will show that synthetic antiferromagnets (SAFs) built out of LBMs can fluctuate with even higher speeds. Using full SPICE simulations, we show that MTJs using SAFs can be combined with standard transistors to provide a fast tunable RNG or p-bit which can speed up numerous applications. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S39.00002: Physical meaning of the full width half maximum of switching time distribution in ferromagnetic nano-structure Jung-Hwan Moon, Tae Young Lee, Chun-Yeol You Generally, line width or full width half maximum of physically measurable quantity. For example, the line with of RLC is related with quality factor of the circuit, and one of ferromagnetic resonance is related with damping constant of the ferromagnetic materials. The switching time of the ferromagnetic nano-structure has been actively studied because of its importance in the physics and device applications. And the switching behavior itself is well described by Fokker Planck Equation (FPE) [1]. Recently, we found that the full width of half maximum (FWHM) of switching time distribution is closely related with the damping constant [2]. We solved FPE with high barrier approximation, we obtained the FWHM is inversely proportional to the external applied field, and the damping constant can be obtained from the slope. Surprisingly enough, the obtained relation is quite similar to the equation for the damping constant and line width of the ferromagnetic resonance, while its underlying physics are totally different. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S39.00003: Energy Efficient Voltage Controlled Domain Wall Neuron and Synapse Md Ali Azam, Dhritiman Bhattacharya, Caroline Anne Ross, Damien Querlioz, Jayasimha Atulasimha We present micromagnetic simulation of voltage-controlled spintronic neuron and synapse by utilizing domain wall (DW) motion in the free layer of a Magnetic Tunnel Junction (MTJ). The free layer is a soft ferromagnet (CoFeB) exchange coupled to a magnetostrictive Rare Earth Iron Garnet (REIG) racetrack with perpendicular magnetic anisotropy (PMA) deposited on a heavy metal layer. The whole stack is fabricated on a piezoelectric substrate. Chiral domain walls can be introduced in the free layer due to PMA and Dzyaloshinskii-Moriya Interaction (DMI) [1, 2], translated with spin orbit torque by applying a current pulse to the heavy metal layer and arrested at specific location along the racetrack domain by tuning the bulk PMA of the magnetic layer using a highly localized voltage-generated strain from the piezoelectric layer. Micromagnetic simulations with realistic defects and thermal noise will be presented for an energy efficient neuron and non-volatile programmable synapse using this device. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S39.00004: Unit cell thick perpendicularly-magnetized ferrimagnetic Mn3Z (Z=Ge, Sn, Sb) Heusler films prepared by chemical templating Panagiotis Ch. Filippou, Jaewoo Jeong, Yari Ferrante, See-Hun Yang, Teya Topuria, Mahesh G. Samant, Stuart S P Parkin The task of improving the performances of next-generation spin-transfer torque magnetoresistive random access memory (STT-MRAM) requires the use of new magnetic materials with high perpendicular magnetic anisotropy (PMA) and low magnetic moment within the magnetic tunnel junction, i.e. MRAM storage element. Heusler alloys are a large family of compounds with tunable magnetic properties. Some of these have a tetragonal structure with its elongated axis perpendicular to the film plane and thus they may display PMA due to their structure broken symmetry. However, so far, PMA has been observed only for 50Å-thick films, i.e. too thick for technological applications. In this talk we demonstrate that X3Z (X=Mn; Z=Ge,Sn,Sb) tetragonal Heusler films, as thin as only 1 unit cell, can be developed by using a novel chemical templating technique. We show this can be achieved by growing the Heusler films, even at room temperature, onto atomically-ordered X’Z’ (X’=Co; Z’=Al,Ga,Ge,Sn) underlayers that promote chemical ordering within the Heusler films. Excellent PMA with square hysteresis is found in Mn3Z layers that are only 1-2 unit cells thick. The possibility of preparing ultrathin Heusler films with PMA makes possible their technological application for a wide range of spintronic devices. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S39.00005: Neuromorphic Signal Encoding and Decoding using Antiferromagnetic Artificial Neurons James Voorheis, Vasyl S Tyberkevych Neuromorphic signal processing is one of the most promising post-Von Neumann computational paradigms [1]. Recently, it was proposed that antiferromagnetic (AFM) spin Hall oscillators, operating in a sub-critical regime, can function as ultra-fast artificial neurons [2, 3]. One of the characteristic features of AFM neurons is the inertial nature of their dynamics: a sufficiently large input spike may induce not one, but several output spikes. Here, we show that this feature can be used for effective encoding and decoding of information from conventional binary to neuromorphic format. The neuromorphic encoder has several inputs, which represent information in parallel binary format, and one output neuron, which produces a train of several spikes. The number of spikes produced by the output neuron equals the input binary code. The decoder reverses this operation, splitting one multi-train input into several binary outputs. The proposed neuromorphic encoder/decoder can be used as an interface between neuromorphic and conventional circuits. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S39.00006: Voltage controlled memory device based on fixed magnetic skyrmions Dhritiman Bhattacharya, Seyed Armin Razavi, Hao Wu, Kang L. Wang, Jayasimha Atulasimha Manipulating static magnetic skyrmions with voltage control of magnetic anisotropy (VCMA) can be utilized to design energy efficient memory devices with reduced device footprint. Using micromagnetic simulation, we demonstrate such devices. With application of a sequential positive and negative voltage pulse, two skyrmionic (core-up and core-down) and two ferromagnetic (up and down) states can be achieved [1]. Further, starting from a ferromagnetic state, a voltage pulse that reduces PMA can induce reversal via skyrmion breathing [2]. As a proof of concept experiment, we demonstrate such VCMA induced manipulation of magnetic skyrmions in an antiferromagnet/ferromagnet/oxide heterostructure film. We observed annihilation of skyrmions while increasing PMA and formation of more skyrmions by reducing PMA. This reversible creation and annihilation of skyrmions could potentially lead to novel skyrmion based memory devices. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S39.00007: YIG-based microwave magnonic circuits at millikelvin temperatures Alexy Karenowska, Sandoko Kosen, Arjan F. Van Loo The experimental exploration of microwave magnonic systems at millikelvin temperatures has attracted significant recent interest. Work in this area is motivated both by the desire to better understand the behaviour and physics of these systems at very low temperatures, and their potential application in solid-state quantum information processing. In this talk, we explore recent experimental results from a range of investigations into the low-temperature properties of yttrium iron garnet (YIG) -based magnonic systems [1-4]. We discuss related technical and materials considerations relevant to successful quantum magnonic experiments, and offer a perspective on future work in this area |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S39.00008: Broadband communication and information processing with edge spin waves Farkhad Aliev, Antonio Lara, Diego Caso, Cesar Gonzalez-Ruano, Konstantin Guslienko, Jose Luis Prieto Spin waves, being usually reflected by domain walls, could also be channeled along them. Here we discuss quasi one-dimensional spin waves in Permalloy dots of different geometries and in different magnetic states. Recent studies allowed observation of spin waves along domain walls in rectangular, circular [1] and triangular dos in the ground or metastable states. Triangular dots could also present edge pinned inhomogeneous magnetic states, depending on the direction of the external magnetic field. These edge domain walls yield the interesting, and potentially applicable to real devices property of broadband spin waves confinement to the edges of the structure [2,3] with capabilities to be redirected at angles exceeding 100 degrees. We also show how these waves could be generalized for arbitrary shapes and propose few devices (such as edge spin wave interferometers, controllers or splitters) where edge spin waves could be implemented. [1] F. G. Aliev, et al., Phys. Rev. B84, 144406 (2011); [2] A. Lara, V. Metlushko, F. G. Aliev, J. Appl. Phys. 114, 213905 (2013); [3] A. Lara, J. Robledo, K.Y. Guslienko, F. G. Aliev, Scientific Reports, 7: 5597 (2017). |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S39.00009: Mutual control of coherent spin waves and magnetic domain walls in a magnonic device Jiahao Han, Pengxiang Zhang, Justin Hou, Luqiao Liu Spin waves, the collective excitation of electronic spins inside magnetic materials, offer new opportunities for wave-based computing. The successful implementation of spin wave devices requires efficient modulation of spin wave propagation. Here we experimentally demonstrate that nanometer wide magnetic domain walls can be used to manipulate the phase and magnitude of coherent spin waves in a non-volatile manner. Besides using magnetic domain walls to control the transmission of spin waves, we further show that a spin wave can in turn be used to move the position of magnetic domain walls, via the spin transfer torque effect generated from magnon spin current. This mutual interaction between spin waves and magnetic domain walls opens up the possibility of realizing all-magnon spintronic devices, where one spin wave signal can be used to control others via reconfiguring magnetic domain structures. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S39.00010: Voltage Controlled Logic-in-memory Architecture in Manganite Nanowire Qian Shi, Fengxian Jiang, Yang Yu, Hanxuan Lin, Yunfang Kou, Tian Miao, Hao Liu, Wenting Yang, Wenbin Wang, Hangwen Guo, Lifeng Yin, Jian Shen Logic-in-memory Architecture in one single device unit is highly attractive in developing next-generation nonvolatile devices and shows potential to overcome the von Neumann bottleneck in conventional computers. However, most non von Neumann prototype devices require high current density, resulting in substantial heat and power consumption when miniaturized. Here, we utilize the electric field induced metal-insulator transitions in manganites and demonstrate that the electric field effect can be exploited to implement a logic-in-memory architecture in manganite nanowire. Highly repeating eight-level resistive states along with Boolean logic operations can be simultaneously performed in one single device unit. This architecture only requires current density of 4×101 A/cm2, which is four orders of magnitude smaller than state-of-the-art designs. Such device is promising for future computing systems beyond von Neumann architecture with low heat dissipation and power consumption. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S39.00011: Electrical annealing and stochastic resonance in superparamagnets for oscillatory networks with dynamic connectivity Punyashloka Debashis, Pramey Upadhyaya, Zhihong Chen In this work, we report electrical annealing and stochastic resonance in thermally unstable nanomagnets with weak perpendicular anisotropy made from Ta/CoFeB/MgO stack. The magnetization of such nanomagnets shows random telegraphic fluctuation with a mean that is tunable by the giant spin Hall effect (GSHE) current generated in the Ta underlayer. These nanomagnets form stochastic oscillators with a natural frequency f0, given by f0 = (tup+tdn)-1, where tup and tdn are the average dwell time of the magnetization in the “up” and “down” state. We demonstrate tuning of f0 of individual nanomagnets by electrical feedback of the magnetization state to the GSHE underlayer. Depending on the polarity and strength of the feedback, the magnetization fluctuation becomes slower or faster, analogous to temperature annealing. We further demonstrate the phenomenon of stochastic resonance in our nanomagnets, where the magnetization follows a weak external periodic drive through the GSHE underlayer only when the drive frequency matches with f0. These novel phenomena can be used to realize coupled oscillatory nanomagnet networks with dynamic connectivity, which alleviates the full connectivity requirement in a network of N oscillators from N2 weighted connections to N homogenous connections. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S39.00012: Fabrication and programming of nanomagnet arrays for nanoscale magnetic field synthesis Tzu-Ming Lu, Ezra Bussmann Nanoscale magnetic field synthesis finds applications in band structure engineering, nano-particle manipulation, and local spin resonance. In the presence of a 1D spatially rotating magnetic field, the band structure of a 1D electron system without spin-orbit coupling can be engineered such that an effective spin-orbit (SO) gap appears in the energy dispersion. Inside the effective SO gap the spin degeneracy is removed. We present our results on fabricating nanomagnet arrays using two magnetic materials with distinct coercivities, programming the nanomagnet arrays using a global external magnetic field, and characterizing the magnetization configuration using magnetic force microscopy. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S39.00013: Magnetic tunnel junction synapses for neuromorphic computing Benjamin MADON, M A Mueed, Noel Arellano, Brian Hughes, Matthieu Grelier, Angelo Couto, Eric Billaud, Spencer Matonis, Aakash Pushp Deep learning algorithms are now widely used. However, the amount of computation power they require to run on conventional CMOS electronics remains high. Consequently, there is an important need for specialized fast and energy-efficient processors tailored for deep learning. We believe that magnetic tunnel junctions in a crossbar array potentially have all the required characteristics of an ideal synapse: high resistance (kOhms), gigahertz speed and symmetric and bi-directional partial switching behavior. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S39.00014: Magnetoelectric device read-out schemes based on electric resistivity measurements in heavy metal Hall bar structures Ather Mahmood, Will Echtenkamp, Junlei Wang, Christian Binek Controlling magnetism by electrical means is a key challenge in the field of spintronics and essential for energy efficient devices in computing. Previously suggested magnetoelectric MRAMs suffer from detrimental effects associated with the magnetization reversal of a ferromagnetic layer. Eliminating a ferromagnet reduces the energy-delay product by eliminating the energy needed to reverse a ferromagnet and us the intrinsically high switching speed of antiferromagnets (AFM). We study the electrically-controlled boundary magnetization of the AFM magnetoelectric (ME) chromia in thin films using Pt as a sensing layer. We provide evidence that the main contribution to the anomalous Hall effect (AHE)-signal in chromia/heavy metal (HM) devices originates from spin Hall magnetoresistance and not from the proximity induced magnetization in the HM. To accomplish this, we investigate the temperature, magnetic field, and Pt-thickness dependence of the transverse and longitudinal resistivity. Our results promise a pathway to optimize AHE readout, overcoming energy-delay constraints accompanying magnetization reversal in ferromagnets. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S39.00015: Anderson localization in transition metal oxides and its application in neuromorphic circuitry Christopher Singh, Keith T Butler, Wei-Cheng Lee It has become increasingly clear that the performance of modern computing architectures is reaching a quantum mechanical road-block, and that neuromorphic computing, a brain inspired computing model, is a very promising paradigm in going beyond the von Neumann computing architecture. At the very core of this technology, is the ability to induce fast, reversible metal-insulator transitions. We propose that Anderson localization in select transition metal oxides can be exploited to design a new generation of memristors. Utilizing toy models as well as first principles descriptions of niobium and vanadium oxide structures, we show that disordering the system can be leveraged to engineer the metal insulator switching, a key physical mechanism in the construction of modern day neuromorphic circuits. Furthermore, we demonstrate that a simple metric of eigenstate localization, the Gini vector, can be an efficient way to capture the localization properties in a first-principles tight binding model. |
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