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Instruments / Technical Area	Publication Details	Published
I10-Beamline for Advanced Dichroism	Experimental observation of dual magnetic states in topological insulators Wenqing Liu , Yongbing Xu , Liang He , Gerrit Van Der Laan , Rong Zhang , Kang Wang Science Advances 5 DOI: 10.1126/sciadv.aav2088 Diamond Proposal Number(s): [20748] Open Access Show Abstract ▼ Abstract: The recently discovered topological phase offers new possibilities for spintronics and condensed matter. Even insulating material exhibits conductivity at the edges of certain systems, giving rise to an anomalous quantum Hall effect and other coherent spin transport phenomena, in which heat dissipation is minimized, with potential uses for next-generation energy-efficient electronics. While the metallic surface states of topological insulators (TIs) have been extensively studied, direct comparison of the surface and bulk magnetic properties of TIs has been little explored. We report unambiguous evidence for distinctly enhanced surface magnetism in a prototype magnetic TI, Cr-doped Bi2Se3. Using synchrotron-based x-ray techniques, we demonstrate a “three-step transition” model, with a temperature window of ~15 K, where the TI surface is magnetically ordered while the bulk is not. Understanding the dual magnetization process has strong implications for defining a physical modelof magnetic TIs and lays the foundation for applications to information technology.	Feb 2019
	Spin-ARPES EUV beamline for ultrafast materials research and development Zhonghui Nie , Ion Cristian Edmond Turcu , Yao Li , Xiaoqian Zhang , Liang He , Jian Tu , Zhiqiang Ni , Huangfeng Xu , Yequan Chen , Xuezhong Ruan , Fabio Frassetto , Paolo Miotti , Nicola Fabris , Luca Poletto , Jing Wu , Qiangsheng Lu , Chang Liu , Thorsten Kampen , Ya Zhai , Wenqing Liu , Cephise Cacho , Xuefeng Wang , Fengqiu Wang , Yi Shi , Rong Zhang , Yongbing Xu Applied Sciences 9 DOI: 10.3390/app9030370 Open Access Show Abstract ▼ Abstract: A new femtosecond, Extreme Ultraviolet (EUV), Time Resolved Spin-Angle Resolved Photo-Emission Spectroscopy (TR-Spin-ARPES) beamline was developed for ultrafast materials research and development. This 50-fs laser-driven, table-top beamline is an integral part of the “Ultrafast Spintronic Materials Facility”, dedicated to engineering ultrafast materials. This facility provides a fast and in-situ analysis and development of new materials. The EUV source based on high harmonic generation process emits 2.3 × 1011 photons/second (2.3 × 108 photons/pulse) at H23 (35.7 eV) and its photon energy ranges from 10 eV to 75 eV, which enables surface sensitive studies of the electronic structure dynamics. The EUV monochromator provides the narrow bandwidth of the EUV beamline while preserving its pulse duration in an energy range of 10–100 eV. Ultrafast surface photovoltaic effect with ~650 fs rise-time was observed in p-GaAs (100) from time-resolved ARPES spectra. The data acquisition time could be reduced by over two orders of magnitude by scaling the laser driver from 1 KHz, 4W to MHz, KW average power.	Jan 2019
I06-Nanoscience	The atomic-scale magnetism of Co 2 FeAl Heusler alloy epitaxial thin films Xiaoqian Zhang , Wenqing Liu , Yu Yan , Wei Niu , Bolin Lai , Yafei Zhao , Wei Wang , Liang He , Hao Meng , Yongbing Xu Applied Physics Letters 113 DOI: 10.1063/1.5056193 Diamond Proposal Number(s): [17930] Show Abstract ▼ Abstract: The atomic-scale magnetism of Co2FeAl Heusler alloys has long been an outstanding question, and with the thickness down to the nanometer scale, this becomes even more sophisticated. Here, we report a direct measurement of the Co2FeAl epitaxial thin films on the GaAs(001) substrate with the in-situ magneto-optic Kerr effect and the synchrotron-based X-ray magnetic circular dichroism techniques. Strong uniaxial magnetic anisotropy has been observed from all thicknesses of the Co2FeAl thin films between 3 unit cells (uc) and 20 uc. A critical thickness of 3 uc has been identified, below which an anti-parallel spin component of the Co atoms occurs. This anti-parallel spin component can be responsible for the significantly reduced magnetic moment and the low spin-polarization near the Fermi level of the Co2FeAl.	Nov 2018
I10-Beamline for Advanced Dichroism	Evidence for ferromagnetic coupling at the doped topological insulator/ferrimagnetic insulator interface Wenqing Liu , Liang He , Yan Zhou , Koichi Murata , Mehmet C. Onbasli , Caroline A. Ross , Ying Jiang , Yong Wang , Yongbing Xu , Rong Zhang , Kang. L. Wang Aip Advances 6 DOI: 10.1063/1.4943157 Diamond Proposal Number(s): [12660] Show Abstract ▼ Abstract: One of the major obstacles of the magnetic topological insulators (TIs) impeding their practical use is the low Curie temperature (Tc). Very recently, we have demonstrated the enhancement of the magnetic ordering in Cr-doped Bi2Se3 by means of proximity to the high-Tcferrimagneticinsulator (FMI) Y3Fe5O12 and found a large and rapidly decreasing penetration depth of the proximity effect, suggestive of a different carrier propagation process near the TI surface. Here we further present a study of the interfacial magnetic interaction of this TI/FMI heterostrucutre. The synchrotron-based X-ray magnetic circular dichroism (XMCD) technique was used to probe the nature of the exchange coupling of the Bi2−xCrxSe3/Y3Fe5O12 interface. We found that the Bi2−xCrxSe3grown on Y3Fe5O12(111) predominately contains Cr3+ cations, and the spin direction of the Cr3+ is aligned parallel to that of tetrahedral Fe3+ of the YIG, revealing a ferromagnetic exchange coupling between the Bi2−xCrxSe3 and the Y3Fe5O12.	May 2016
I10-Beamline for Advanced Dichroism	Atomic-Scale Magnetism of Cr-Doped Bi2Se3 Thin Film Topological Insulators Wenqing Liu , Damien West , Liang He , Yongbing Xu , Jun Liu , Kejie Wang , Yong Wang , Gerrit Van Der Laan , Rong Zhang , Shengbai Zhang , Kang. L. Wang Acs Nano 9 (10), 10237 - 10243 DOI: 10.1021/acsnano.5b03980 PMID: 26348798 Show Abstract ▼ Abstract: Magnetic doping is the most common method for breaking time-reversal-symmetry surface states of topological insulators (TIs) to realize novel physical phenomena and to create beneficial technological applications. Here we present a study of the magnetic coupling of a prototype magnetic TI, that is, Cr-doped Bi2Se3, in its ultrathin limit which is expected to give rise to quantum anomalous Hall (QAH) effect. The high quality Bi2–xCrxSe3 epitaxial thin film was prepared using molecular beam epitaxy (MBE), characterized with scanning transimission electron microscopy (STEM), electrical magnetotransport, and X-ray magnetic circularly dichroism (XMCD) techniques, and the results were simulated using density functional theory (DFT) with spin–orbit coupling (SOC). We observed a sizable spin moment mspin = (2.05 ± 0.20) μB/Cr and a small and negative orbital moment morb = (−0.05 ± 0.02) μB/Cr of the Bi1.94Cr0.06Se3 thin film at 2.5 K. A remarkable fraction of the (CrBi–CrI)3+ antiferromagnetic dimer in the Bi2–xCrxSe3 for 0.02 < x < 0.40 was obtained using first-principles simulations, which was neglected in previous studies. The spontaneous coexistence of ferro- and antiferromagnetic Cr defects in Bi2–xCrxSe3 explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 μB/Cr predicted by Hund’s rule.	Oct 2015
I10-Beamline for Advanced Dichroism	Enhancing Magnetic Ordering in Cr-Doped Bi2Se3 Using High-TC Ferrimagnetic Insulator Wenqing Liu , Liang He , Yongbing Xu , Koichi Murata , Mehmet C. Onbasli , Murong Lang , Nick Maltby , Shunpu Li , Xuefeng Wang , Caroline A. Ross , Peter Bencok , Gerrit Van Der Laan , Rong Zhang , Kang. L. Wang Nano Letters 15 (1), 764 - 769 DOI: 10.1021/nl504480g Diamond Proposal Number(s): [9953] Show Abstract ▼ Abstract: We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface. KEYWORDS: Magnetic topological insulator, XMCD, proximity effect, ferrimagnetic insulator YIG, spintronics	Jan 2015

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