Conference Programme

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Overview
Session
S-01: Spin Orbitronics
Time:
Monday, 19/Jun/2017:
1:30pm - 3:30pm

Session Chair: Wen Siang Lew, Nanyang Technological University
Location: Rm 326

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Presentations
1:30pm - 2:00pm
Invited

Spin Orbitronics for Advanced Magnetic Memories

Stuart PARKIN

Max Planck Institute of Microstructure Physics, Germany

Over the past few years there have been remarkable discoveries in spin-based phenomena that rely on spin-orbit coupling that could spur the development of advanced magnetic memory devices. These include the formation of chiral spin textures in the form of Néel domain walls and topological spin textures, skyrmions, that are stabilized by a Dzyaloshinskii-Moriya exchange interaction. The Dzyaloshinskii-Moriya exchange interaction is derived from broken symmetries and spin-orbit interactions at interfaces or within the bulk of materials. Another important consequence of spin-orbit effects are the unexpectedly high conversion efficiencies of charge current to chiral spin current from topological spin textures and in conventional metals, via the spin Hall effect. Such spin currents lead to giant spin-orbit torques that can be used to switch the magnetization in three terminal magnetic tunnel junction memory elements or can be used to move domain walls in Racetrack Memory memory-storage devices[1]. Indeed record-breaking current-induced domain wall speeds exceeding 1,000 m/sec have recently been reported in atomically engineered synthetic antiferromagnetic racetracks in which the domain walls are “invisible” with no net magnetization[2]. I will discuss some of these exciting developments in the emerging field of spin orbitronics in my talk.

References:

[1] Parkin, S.S.P. & Yang, S.-H. Memory on the Racetrack. Nat. Nano. 10, 195-198 (2015).

[2] Yang, S.-H., Ryu, K.-S. & Parkin, S.S.P. Domain-wall velocities of up to 750 ms−1 driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nano. 10, 221-226 (2015).


2:00pm - 2:30pm
Invited

Spin and Charge Conversion in 2D Quantum Materials

Wei HAN

Peking University, China

Spintronics aims to use the spin degrees of freedom for future information technologies. A key challenge for spintronics is the efficient generation of spin currents from charge currents. Recently, emergent 2D quantum materials are very attractive for spintronics since they provide new mechanisms of spin currents generation [1].

In this talk, I will present the spin and charge conversion in three quantum materials. The first part of my talk will discuss the facet-dependent spin Hall effect and spin orbit torque in single crystalline antiferromagnetic IrMn3[2], This result suggests a novel mechanism of the spin current generation arising from the triangular spin configurations in the single crystalline IrMn3. The second part of my talk will focus on the gate tunable charge to spin conversion at room temperature using the Rashba-split 2DEG at the complex oxides (SrTiO3 and LaAlO3) interface [3]. At the end, I will discuss the spin to charge conversion in the spin-momentum locked surface states of the topological Kondo insulator (SmB6), of which pure surface states exist at low temperatures [4].

References:

[1] Wei Han, APL Materials 4, 032401 (2016).

[2] Weifeng Zhang*, Wei Han*, See-hun Yang, Yan Sun, Yang Zhang, Binghai Yan, and Stuart S. P. Parkin, Science Advances, 2: e1600759 (2016).

[3] Qi Song, Hongrui Zhang, Tang Su, Wei Yuan, Yangyang Chen, Wenyu Xing, Jing Shi, Ji Rong Sun, and Wei Han, arXiv:1609.06207.

[4] Qi Song, Jian Mi, Dan Zhao, Tang Su, Wei Yuan, Wenyu Xing, Yangyang Chen, Tianyu Wang, Tao Wu, Xian Hui Chen, X. C. Xie, Chi Zhang, Jing Shi, and Wei Han, Nature Communications 7, 13485 (2016).


2:30pm - 2:45pm
Oral

Enhanced Current-induced Torques in Ferrimagnets due to Negative Exchange

Rahul MISHRA1, Jiawei YU1, Xuepeng QIU2, Hyunsoo YANG1

1Department of Electrical and Computer Engineering, National University of Singapore, Singapore; 2Institute of Solid State Physics and School of Physics Science, Tongji University, China

Spin-orbit torques (SOTs) have been studied recently for its potential applications in memory and logic devices [1,2]. So far they have been widely studied in thin films with perpendicular magnetic anisotropy such as (Pt or Ta)/Co/(MgO or Al2O3). These thin films typically have a very low coercivity and are thermally unstable due to their sub nano-meter thicknesses, which is not ideal for device applications. We study SOT-induced magnetization switching and effective fields in rare earth-transition metal (RE-TM) ferrimagnet CoGdfor different compositions.

Devices from the structure of Pt (10 nm)/CoxGd1-x (6 nm)/TaOx (1 nm) show huge switching efficiency (ψ) and maximum spin-orbit effective fields near the compensation composition. The longitudinal effective field has a value of 6.1 KOe/10-8 Am-2 while ψ has a peak value of ~ 6x10-9 Oe/Am-2 near the compensation point. It is observed that ψ and SOT effective fields increase by around ~8 times as the ferrimagnet approaches compensation. This anomalous increase of ψ and SOT effective fields is attributed to the negative exchange interaction field due to the antiferromagnetic ordering between the Co and Gd sub-lattices. The negative exchange interaction makes the ferrimagnet thermally stable near compensation by increasing its anisotropy and also provides the exchange coupling torque that assists in switching, effectively increasing the overall switching efficiency of ferrimagnetic SOT devices.

References:

[1] I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandiera, B. Rodmacq, A. Schuhl, and P. Gambardella, Nature 476, 189 (2011).

[2] L. Liu, C. F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman, Science 336, 555 (2012).


2:45pm - 3:00pm
Oral

Spin Orbit Torque in a Single Ferromagnetic Layer

Jihang YU1,2, Liang LIU1, Weinan LIN1, Jinyu DENG1, Herng Yau YOONG1, Han WANG1, Hongxi LIU2, Francis POH2, Danny SHUM2, Jingsheng CHEN1

1National University of Singapore, Singapore; 2GLOBALFOUNDRIES Singapore Pte Ltd, Singapore

Spin orbit torque (SOT) has attracted much attention for its possible application in high density, low power consumption current assisted switching magnetic random access memories (MRAM). Recently, SOT has been explored in different systems like heavy metal(HM)/ferromagnetic(FM)/oxide trilayers, bulk multilayer material with perpendicular magnetic anisotropy (PMA), Heusler alloys and topological insulators. Yet, there is no study on the SOT of single layer ordered ferromagnetic material with PMA such as L10 FePt.

In current work, L10 FePt films were epitaxially grown on (001) SrTiO3 (STO) single crystal substrate using magnetron sputtering. A thin layer (4 nm) of amorphous silicon dioxide (SiO2) was capped on top of the FePt layer as a protection layer. The film qualities were confirmed by Vibrating Sample Magnetometer (VSM) and High Resolution X-Ray Diffractometer (HRXRD). The films were patterned to Hall bars with 10um widths to study the anomalous hall voltages of single layer FePt films.

AC and DC input current was injected by 6221 from I1 to I2 electrodes with AC current frequency 661.7 Hz. 1st and 2nd harmonic hall voltage signals were measured across V1 and V2 electrodes using two lock-in amplifier. SOT were calculated using 2nd harmonic signal model with both AC and DC input current.

The longitudinal and transverse components of torque efficiencies were found to be 3 and 26.2 kOe at 108 A/cm2 respectively. The torque efficiencies are significant larger than the traditional HM/FM/oxide trilayers and bulk multilayer material with PMA. The fact that large SOT can be generated in L10 FePt and act on its own magnetization provides a possibility of future MRAM with simpler structure and faster switching speed. The simple structure could eliminate many disturbing factors in the study of origins of SOT. It may also lead to a new way to generate and engineer the strength of SOT.


3:00pm - 3:15pm
Oral

Doping Effects on Heisenberg and Dzyaloshinskii-Moriya Exchange Interactions in Fe(1-x)Mn(x)Ge

Jarvis LOH, Chee Kwan GAN

Institute of High Performance Computing, Agency for Science Technology and Research (A*STAR), Singapore

Exotic spin textures, in their plethora of forms and sizes, emanate largely from the intricate and often intense competition between spin interaction mechanisms. In this study, we examine the Heisenberg and Dzyaloshinskii-Moriya (DM) exchange in Fe1-xMnxGe chiral magnets, and whether and how the exchange interaction in the binary form of Fe1-xMnxGe is affected by dopants of a second 3d transition metal species. Using first-principles calculations based on full-potential linearized augmented plane-wave (FLAPW)-based density functional theory (DFT), the ground-state spin order of bulk FeGe, MnGe, and two intermediate ternary compounds (Fe0.75Mn0.25Ge and Fe0.25Mn0.75Ge) is first ascertained, and subsequently their spiral periodicity is derived. The presence of the dopants generally increases the size of the ground-state spin spiral. The Heisenberg exchange is ferromagnetic between nearest metallic atom pairs, and anti-ferromagnetic between 2nd nearest metallic pairs in all four compounds. The exchange interaction between Mn pairs is slightly weaker than between Fe pairs. However, the dopants reduce the interaction strength considerably, especially for Mn dopants. The DM exchange is generally less sensitive to doping effects in MnGe than FeGe in the three highly symmetric directions. The Wannier-interpolated bands of the ternary compounds exhibit significant lifting of state degeneracy, especially around the R-, M-, and X- points. In contrast to Fe dopants, Mn dopants increase the exchange splitting of the 3d states. The spin-down Fermi surface of the ternary compounds contrasts starkly with that of the binary compounds; in the latter, the Fermi surface remains spherical, while in the former, it is highly distorted, likely due to electron screening effects. Nevertheless, the Fermi surface of Fe0.25Mn0.75Ge resembles that of its undoped form (MnGe) more as compared to the case of Fe0.75Mn0.25Ge and FeGe. Essentially, our calculations have amply demonstrated that the exchange interactions in FeGe are more susceptible to doping effects than in MnGe.


3:15pm - 3:30pm
Oral

Spin-orbitronics with Topological Insulators

Lei SHEN

Department of Mechanical Engineering, National University of Singapore, Singapore

Topological insulators (TIs) are "strange" materials behaving as insulators in their interior and conductors on the surface. They also have strong spin-orbit interactions (SOI) that make them excellent candidates for the development of spin-orbitronics, a new field of spintronics. In this talk I will present recent first-principles works on spin-orbitronics with TIs.



 
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