Conference Programme

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Overview
Session
S-02: Memory and Storage
Time:
Monday, 19/Jun/2017:
4:00pm - 6:15pm

Session Chair: S. N. Piramanayagam, Nanyang Technological University
Location: Rm 326

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Presentations
4:00pm - 4:30pm
Invited

Nano Spintronics Device for Artificial Neural Network

Hideo OHNO1,2

1Research Institute of Electrical Communication (RIEC), Tohoku University, Japan; 2Center for Spintronics Integrated Systems (CSIS), Center for Innovative Integrated Electronic Systems (CIES), WPI Advanced Institute for Materials Research (WPI-AIMR), Center for Spintronics Research Network (CSRN), Tohoku University, Japan

Three-terminal spintronics device separates current paths for magnetization switching and for reading the state of magnetization, allowing fast operation in a relaxed design configuration, which are desirable for digital applications [1]. We have recently reported that by employing an antiferromagnet Pt0.4Mn0.6 layer beneath perpendicular easy-axis [Co(0.3)/Ni(0.6)] layer, one can exert spin-orbit torque from the antiferromagnet to the ferromagnet in a three-terminal device and observe current induced magnetization switching in the absence of external magnetic fields as long as there is an exchange bias [2]. At the same time, we have observed analog switching, i.e. the degree of reversal at the zero-magnetic field measured by the anomalous Hall effect depends on how far the device was driven by the switching current. This property has been utilized for an artificial synapse in associative memory operations [3]. Here, I summarize the proof-of-concept experiment we conducted on artificial-neural-network-based associative memory operation using spin-orbit torque devices, reports the properties of the current material system, and discuss the scalability of the present approach and its prospect.

Work supported in part by the R & D for Next-Generation Information Technology of MEXT and by ImPACT from JST. The spin-orbit device was developed in collaboration with S. Fukami and the CSIS team. The artificial neural network demonstration was done in collaboration with W. A. Borders, H. Akima, S. Fukami, S. Moriya, S. Kurihara, Y. Horio, and S. Sato.

References:

[1] S. Fukami et al. Nature Nanotechnology doi:10.1028/nnano2016.29 (2016); 2016 Symp. on VLSI Tech., T06-5 (2016).

[2] S. Fukami et al. Nature Materials 15, 535 (2016); doi:10.1038/nmat4566.

[3] W. A. Borders et al., Appl. Phys. Express 10, 013007 (2017)


4:30pm - 5:00pm
Invited

Experimental Demonstration of Non-Volatile and Programmable Multi-Functional Spin Logic

Xiufeng HAN

Institute of Physics, Chinese Academy of Sciences, China

Confronting with the gigantic volume of data produced every day, raising integration density by reducing the size of devices becomes harder and harder to meet the ever-increasing demand for high-performance computers. One feasible path is to actualize more logic functions in one cell. Many efforts have been dedicated to explore the prospective candidates of spin logic gate in different systems, such as semiconductors, Oersted-field controlled magnetic tunnel junctions, magnetic domain engineered nanowires, phase-change material, graphene and magnetoelectric oxides, etc. However, only a few of them are compatible with complementary metal oxide semiconductor (CMOS) architecture, which limits their practical applications.

Among of them, Spin Logic is of great interest as its desired property of nonvolatility and subsequentially the potential for realizing the idea of processing in memory architecture which is regarded to play increasingly important role in today’s fast-growing volumes of data. In this respect, we experimentally demonstrate a prototype Spin-Orbit Torque (SOT) based Spin Logic cell integrated with five frequently used logic functions (AND, OR, NOT, NAND and NOR). The cell can be easily programmed and reprogrammed to perform desired function. Furthermore, the information stored in cells is symmetry-protected, making it possible to expand into logic gate array where the cell can be manipulated one by one without changing the information of other undesired cells [1]. This work provides a prospective example of multi-functional Spin Logic cell with reprogrammability and nonvolatility, which will advance the application of Spin Logic devices.

References:

[1] X. Zhang, C.H. Wan, Z.H. Yuan, C. Fang, W.J. Kong, H. Wu, Q.T. Zhang, B.S. Tao,X. F. Han, Experimental demonstration of programmable multi-functional Spin Logic cell based on spin Hall effect. J. Magn. Magn. Mater. 428 (2017) 401-405 (Letter to Editor).


5:00pm - 5:15pm
Oral

CoFeB Composition Dependence on Thermal Stability of Magnetic Tunnel Junctions

Shalabh SRIVASTAVA, Rajagopalan RAMASWAMY, Jaesung SON, Tanmay DUTTA, Kie Leong TEO, Hyunsoo YANG

National University of Singapore, Singapore

Among the major challenges in the development of perpendicular magnetic tunnel junctions (MTJs) with CoFeB magnetic layers are sustaining high annealing robustness up to the temperature of 400 ºC and maintaining operation at the temperature of 260 ºC for program retention. To solve this problem, there is a need to find the optimum composition of CoFeB with a high thermal stability. The property of annealing temperature dependence and operation temperature dependence of magnetic properties of CoFeB can be harnessed in applications to achieve high magnetocrystalline anisotropy and its low-temperature coefficient. In this work, we investigated the dependence of magnetic properties on the composition of CoFeB ultrathin films (double MgO MTJs with the W insert layer) at different annealing conditions. Temperature dependent magnetic properties were studied to determine the operation stability for program retention.

The dependence of magnetic properties on the composition of CoFeB was studied by simultaneous co-sputtering of two targets of with composition of Co20Fe60B20 and Co60Fe20B20. The samples with different compositions were thus achieved by changing the deposition power of CoFeBtargets. The anisotropy of CoFeB was found to be maximum for the samples deposited to achieve for the composition x = 32% to 42%. The damping coefficient shows a minimum at x = 35% for 400 ºC annealed samples. The operation stability measurements show that the change in the magnetization (MS) values for x = 32% and 35% samples is comparatively small and anisotropy energy (Keff) achieves maxima between x = 32% to 35%. Thus, Co composition between x = 32% to 35% should be considered for thermally stable MTJs.


5:15pm - 5:30pm
Oral

Crystal Structure Investigation in Bi1-xHoxFeO3 (x = 0.05, 0.1, 0.15, & 0.2) Multiferroics by Rietveld Analysis

Jogender SINGH1, Ashish AGARWAL1, Sujata SANGHI1, Ompal SINGH1, Shyam SUNDER1,2

1Guru Jambheshwar University of Science and Technology, India; 2University of Delhi, India

Bi1-xHoxFeO3 multiferroics (with x = 0.05, 0.1, 0.15, & 0.2) were synthesized via conventional solid state reaction method. X-ray patterns recorded by powder diffraction method depicts change in crystal structure at concentration x = 0.2. Rietveld refinement of the XRD patterns deduce the stabilisation of rhombohedral R3c phase upto concentration x = 0.15, while for concentration x = 0.2, the mixed phase model including rhombohedral R3c and orthorhombic Pnma space group was established to give the best fitting. The electron density contours obtained by G-Fourier program of Fullprof suite deduced the smooth spread of diffracting elements (electrons) throughout the refinement simulated crystal structure. The change in crystal structure is attributed to the distortion of FeO6 octahedra via doping of Ho at phase boundaries.


5:30pm - 5:45pm
Oral

Evolution of Crystal Structure with Sintering Time in Bi1-xPrxFeO3 (x = 0.05, 0.10, 0.15, & 0.20) Multiferroics

Ompal SINGH1, Ashish AGARWAL1, Sujata SANGHI1, Jogender SINGH1, Shyam SUNDER1,2

1Guru Jambheshwar University of Science and Technology, India; 2University of Delhi, India

Pr doped bismuth ferrite multiferroics; Bi1-xPrxFeO3 (x = 0.05, 0.1, 0.15, & 0.2) were synthesized by solid state reaction method with calcination at 400˚C and sintering at 800˚C. To investigate the stability of the crystal structure formulated in the samples, sintering time was varied from 1 to 12 hours. The XRD patterns for the sample at x = 0.05, indicates the suppression of impurity phases by 3 hour sintering time. It predicts pure rhombohedral R3c phase (which is characteristic phase of BiFeO3) upto 8 hours and thereafter traces of Bi2Fe4O9 were found in XRD patterns which increases in fraction by sintering time. At x = 0.1, the sample gets well synthesized in R3c phase by 3 hours and persists the same upto 10 hours. The XRD pattern at 12 hours again shows Bi2Fe4O9 compound but with lower intensity than the previous composition. For x = 0.15 sample, the XRD shows mixed structure with rhombohedral R3c and orthorhombic Pbam by 12 hours sintering. Below 12 hours the crystal structure shows mixed symmetry of rhombohedral R3c, orthorhombic Pnma and orthorhombic Pbam. In sample for x = 0.2, similar trend was observed for lesser sintering times. After 12 hours the pure orthorhombic Pbam phase was accomplished in the sample. Overall we conclude that the lower Pr concentrated BiFeO3 samples require lesser heat treatment to stabilise in single rhombohedral R3c phase, while for the higher concentration, the crystal structure changes towards the orthorhombic Pbam phase with longer durations of sintering times.


5:45pm - 6:00pm
Oral

Structural and Magnetic Properties of Exchange Coupled (CoPt)m/(FeCo)n Superlattices

Sandeep Kumar JAIN1,2, Vijay KUMAR2,3

1Indian Institute of Technology, Mandi, India; 2Dr Vijay Kumar Foundation, India; 3Center for Informatics, School of Natural Sciences (SoNS), Shiv Nadar University, India

We report results of ab initio electronic structure calculations on (CoPt)m/(FeCo)n superlattices [m = 4, 6, and 8 and n = 2-2m] within the framework of density functional theory. The superlattices are designed by combining different numbers of the layers of hard phase CoPt and soft phase FeCo. The effects of the variation of the thickness of each phase on the structural and magnetic properties have been studied. We find an increasing trend for maximum energy product with the increase in the thickness of the FeCo layers. The magnetization of the superlattices lies in the range of 1050-1430 emu/cm3. We find the maximum energy product to be 80 MGOe, This is significantly larger than the value for bulk CoPt. We have also included the effect of spin-orbit coupling to calculate the magnetic anisotropy energy and thereby anisotropy field. We predict in-plane anisotropy for all the systems. except (CoPt)6/(FeCo)2 and (CoPt)8/(FeCo)2superlattices.


6:00pm - 6:15pm
Oral

Evidence of Structural and Magnetic Correlation in (lll)-oriented La0.7Sr0.3MnO3/SrRuO3 Superlattices

Roshna S H1, P PADHAN1, W PRELLIER2

1Indian Institute of Technology Madras, India; 2Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, France

Artificial superstructure of (lll) oriented La0.7Sr0.3MnO3(LSMO) - SrRuO3(SRO) were grown on (111) oriented SrTiO3 substrates using pulsed laser deposition. The growth epitaxy, crystalline quality, crystal structure and artificial bilayer periodicity of these superlattices were studied using x-ray diffraction. The cumulative strain in these superlattices was studied from the reciprocal space mapping (RSM) using high-resolution x-ray diffractometer. The RSM image of these superlattices indicates the partial relaxation of the strain. The magnetic exchange coupling at the interfaces of the LSMO-SRO has been investigated with the in-plane and the out-of-plane oriented magnetic field. The magnetic anomaly of these superlattices were observed in the temperature dependent frequencies of the Raman lines. The observed peak at around 160 K and steplike anomaly at around 350 K of the Raman peak frequencies provide strong evidence of magnetic phase transition of SRO and LSMO respectively.



 
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