1. Artificial magnets and spintronic heterostructures

Two-dimensional (2D) van der Waals (vdW) materials have brought fresh prospects for spintronics, as evidenced by the rapid scientific progress made in this frontier over the past decade. Recently, literature reports that 2D vdW materials can serve as intrinsic magnets, bringing emerging opportunities for spintronic devices and applications. In our group, we are interested in (a) the epitaxial growth of 2D vdW materials as artificial magnets by ultra-high vacuum molecular beam epitaxy (MBE); (2) their exotic magnetism and control as well as their heterostructures; and (3) advanced spintronic devices.

Figure1 Artificial magnets and spintronic heterostructures[1-6]
Related publications:

[1] Lei Zhang#, Tong Yang#, Xiaoyue He, Wen Zhang, Giovanni Vinai, Chi Sin Tang, Xinmao Yin, Piero Torelli, Yuan Ping Feng, Ping Kwan Johnny Wong*, and Andrew T. S. Wee*, Molecular Beam Epitaxy of Two-Dimensional Vanadium-Molybdenum Diselenide Alloys, ACS Nano 14, 11140 (2020).

[2] Rui Zhu#, Wen Zhang#*, Wei Shen, Ping Kwan Johnny Wong, Qixing Wang, Qijie Liang, Zhen Tian, Ya Zhai, Cheng-wei Qiu, and Andrew T. S. Wee*, Exchange Bias in van der Waals CrCl3/Fe3GeTe2 Heterostructures, Nano Letters 20, 5030 (2020).

[3] Wen Zhang*, Lei Zhang, Ping Kwan Johnny Wong*, Jiaren Yuan, Giovanni Vinai, Piero Torelli, Gerrit van der Laan, Yuan Ping Feng, and Andrew T. S. Wee*, Magnetic Transition in Monolayer VSe2 via Interface Hybridization, ACS Nano 13, 8897 (2019).

[4] Ping Kwan Johnny Wong#*, Wen Zhang#, Jun Zhou, Fabio Bussolotti, Xinmao Yin, Lei Zhang, Alpha T. NDiaye, Simon A. Morton, Wei Chen, Johnson Goh*, Michel P. de Jong, Yuan Ping Feng, and Andrew T. S. Wee*, Metallic 1T Phase, 3d1 Electronic Configuration and Charge Density Wave Order in Molecular Beam Epitaxy Grown Monolayer Vanadium Ditelluride , ACS Nano 13, 12894 (2019).

[5] Wen Zhang#*, Ping Kwan Johnny Wong#, Xiaochao Zhu, Ashutosh Rath, Zhaocong Huang, Hongyu Wang, Simon A. Morton, Jiaren Yuan, Lei Zhang, Rebekah Chua, Shengwei Zeng, Er Liu, Feng Xu, Ariando, Daniel H. C. Chua, Yuan Ping Feng, Gerrit van der Laan, Stephan J. Pennycook, Ya Zhai, and Andrew T. S. Wee*, Ferromagnet/Two-Dimensional Semiconducting Transition-Metal Dichalcogenide Interface with Perpendicular Magnetic Anisotropy, ACS nano 13, 2253 (2019).

[6] Ping Kwan Johnny Wong#, Wen Zhang#, Fabio Bussolotti, Xinmao Yin, Tun Seng Herng, Lei Zhang, Yu Li Huang, Giovanni Vinai, Sridevi Krishnamurthi, Danil W. Bukhvalov, Yu Jie Zheng, Rebekah Chua, Alpha T. N’Diaye, Simon A. Morton, Chao-Yao Yang, Kui-Hon Ou Yang, Piero Torelli, Wei Chen, Kuan Eng Johnson Goh, Jun Ding, Minn-Tsong Lin, Geert Brocks, Michel P. de Jong*, Antonio H. Castro Neto*, and Andrew Thye Shen Wee*, Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet, Advanced Materials, 31, 1901185 (2019)

2. Intelligent spintronic devices and applications

Using brain-inspired spintronics devices as artificial neural networks is expected to offer a complementary approach to deal with complex computational problems, compared to conventional digital integrated circuits with von Neumann architectures. Much effort has been devoted to creating artificial neurons and synapses using various solid-state systems with ferroelectric materials, phase-change materials, oxide-based memristive materials, spin-torque nano-oscillators, and so on. In our group, we will extend our research interests to spintronic devices based on artificial vdW materials and magnetic materials for novel neuromorphic computing and nanoscale magnetic storage/sensors.

Among the spintronics devices, GMR-based readhead, STT/SOT-MRAM are commercialized spin memory devices. Spin-torque nano-oscillators (STNOs) are foreseen to be the next to appear in the market for microwave generators, magnetic field sensors, microwave diodes, and neuromorphic computing, etc. In our group, we are interested in (a) studying STNOs generated magnetodynamics for magnonic applications; (b) engineering the microwave performance of STNOs for signal generators; (c) developing electrical-, magnetic field-, voltage-controlled STNOs.

Figure2 Magnetodynamic in intelligent spintronic devices[7-10]
Related publications:

[7] Wen Zhang*, Dong Zhang, Ping Kwan Johnny Wong, Honglei Yuan, Sheng Jiang, Gerrit van der Laan, Ya Zhai*, and Zuhong Lu , Selective Tuning of Gilbert Damping in Spin-Valve Trilayer by Insertion of Rare-Earth Nanolayers , ACS Applied Materials & Interfaces 7, 17070 (2015).

[8] Sheng Jiang, Sunjae Chung*; Quang Tuan Le; Hamid Mazraati; Afshin Houshang; JohanÅkerman*, Using MagneticDroplet Nucleation to Determine the Spin Torque Efficiency and Asymmetry in Cox(Ni,Fe)1−x Thin Films, Physical Review Applied 10, 054014 (2018).

[9] Sheng Jiang; Roman Khymyn; Sunjae Chung; Tuan Quang Le; Liza Herrera Diez; AfshinHoushang; Mohammad Zahedinejad; Dafiné Ravelosona; Johan Åkerman*; Reduced Spin torque Nano-Oscillator Linewidth Using He+ Irradiation, Applied Physics Letters 116, 72403 (2020).

[10] Gaolong Cao#, Sheng Jiang#, Johan Åkerman*, and Jonas Weissenrieder*, Femtosecond Laser Driven Precessing Magnetic Gratings, Nanoscale 13, 3746 (2021).

3. Nanomagnetism explored by advanced X-ray techniques

Magnetism is one of the oldest scientific disciplines, but nanomagnetism, dealing with magnetic phenomena specific to structures having dimensions in the submicron range, is at the forefront of the emerging nanotechnology era. To explore nanomagnetism, sensitive techniques are required. To date, detecting nanomagnetism by X-ray is one of the most sensitive and vigorous methods, such as X-ray photoemission electron microscopy (XPEEM) and X-ray magnetic circular dichroism (XMCD). In our group, we investigate the magnetism in atomic thick magnetic thin films, nano-particles, and also organic molecular materials.

Figure3 Nanomagnetism explored by X-ray techniques[11-14]
Related publications:

[11] Wen Zhang*, Ping Kwan Johnny Wong, Dong Zhang, Jinjin Yue, Zhaoxia Kou, Gerrit van der Laan, Andreas Scholl, Jian-Guo Zheng, Zuhong Lu, and Ya Zhai*, XMCD and XMCD-PEEM Studies on Magnetic-Field-Assisted Self-Assembled 1D Nanochains of Spherical Ferrite Particles , Advanced Functional Materials 27, 1701265 (2017).

[12] Ping Kwan Johnny Wong*, Wen Zhang, Gerrit van der Laan, Michel P. de Jong *, Hybridization-Induced Charge Rebalancing at the Weakly Interactive C60/Fe3O4(001) Spinterface , Organic Electronics 29, 39 (2016).

[13] Ping Kwan Johnny Wong*, Wen Zhang, Kai Wang, Gerrit van der Laan, Yongbing Xu, Wilfred van der Wiel, and Michel P. de Jong*, Electronic and Magnetic Structure of C60/Fe3O4(001): A Hybrid Interface for Organic Spintronics, Journal of Materials Chemistry C 1, 1197 (2013).

[14] Wen Zhang, Ping Kwan Johnny Wong, Peng Yan, Jing Wu, Simon A. Morton, Xiangrong Wang, Xuefeng Hu, Yongbing Xu*, Andreas Scholl, Antony Young, Igor Barsukov, Michael Farle, and Gerrit van der Laan, Observation of Current-Driven Oscillatory Domain Wall Motion in Ni80Fe20/Co Bilayer Nanowire, Applied Physics Letters 103, 042403 (2013).