I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[20748]
Open Access
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.
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Feb 2019
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I06-Nanoscience
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Diamond Proposal Number(s):
[17930]
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.
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Nov 2018
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I06-Nanoscience
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Wei
Niu
,
Wenqing
Liu
,
Min
Gu
,
Yongda
Chen
,
Xiaoqian
Zhang
,
Minhao
Zhang
,
Yequan
Chen
,
Ji
Wang
,
Jun
Du
,
Fengqi
Song
,
Xiaoqing
Pan
,
Nini
Pryds
,
Xuefeng
Wang
,
Peng
Wang
,
Yongbing
Xu
,
Yunzhong
Chen
,
Rong
Zhang
Diamond Proposal Number(s):
[15239]
Abstract: Atomically engineered oxide heterostructures provide a fertile ground for creating novel states, for example, a 2D electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 µB/Mn) in LaMnO3‐based heterostructures, the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films are found. Combining spatially resolved electron energy‐loss spectroscopy, X‐ray absorption spectroscopy, and X‐ray magnetic circular dichroism techniques, it is determined unambiguously that the ferromagnetism results from a conventional Mn3+‐O‐Mn4+ double‐exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto‐unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all‐oxide spintronic devices.
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Apr 2018
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I06-Nanoscience
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Andy
Quindeau
,
Can O.
Avci
,
Wenqing
Liu
,
Congli
Sun
,
Maxwell
Mann
,
Astera S.
Tang
,
Mehmet C.
Onbasli
,
David
Bono
,
Paul M.
Voyles
,
Yongbing
Xu
,
Jason
Robinson
,
Geoffrey S. D.
Beach
,
Caroline A.
Ross
Diamond Proposal Number(s):
[12664]
Abstract: With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film.
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Jan 2017
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I06-Nanoscience
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Diamond Proposal Number(s):
[12664]
Abstract: Artificial heterostructures based on LaNiO3 (LNO) have been widely investigated with the aim to realize the insulating antiferromagnetic state of LNO. In this work, we grew [(La0.7Sr0.3MnO3)5-(LaNiO3)n]12 superlattices on (001)-oriented SrTiO3 substrates by pulsed laser deposition and observed an unexpected exchange bias effect in field-cooled hysteresis loops. Through X-ray absorption spectroscopy and magnetic circular dichroism experiments, we found that the charge transfer at the interfacial Mn and Ni ions can induce a localized magnetic moment. A remarkable increase of exchange bias field and a transition from metal to insulator were simultaneously observed upon decreasing the thickness of the LNO layer, indicating the antiferromagnetic insulator state in 2 unit cells LNO ultrathin layers. The robust exchange bias of 745 Oe in the superlattice is caused by an interfacial localized magnetic moment and an antiferromagnetic state in the ultrathin LNO layer, pinning the ferromagnetic La0.7Sr0.3MnO3 layers together. Our results demonstrate that artificial interface engineering is a useful method to realize novel magnetic and transport properties.
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Jan 2017
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I10-Beamline for Advanced Dichroism
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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
Diamond Proposal Number(s):
[12660]
Open Access
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.
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May 2016
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[9979]
Abstract: Understanding magnetism in ferromagnetic metal/semiconductor (FM/SC) hetero-structures is important to the development of the new generation spin field-effect transistor (SpinFET). Here, we report an element-specific x-ray magnetic circularly dichroism study of the interfacial magnetic moments for two FM/SC model systems, namely Co/GaAs and Ni/GaAs, which was enabled using a specially designed FM1/FM2/SC superstructure. We observed a robust room temperature magnetization of the interfacial Co, whilst that of the interfacial Ni was strongly diminished down to 5 K due to hybridization of the Ni d(eg) and GaAs sp3 states. The validity of the selected method was confirmed by first-principles calculations, showing only small deviations (<0.02 and <0.07 μB/atom for Co/GaAs and Ni/GaAs, respectively) compared to the real FM/SC interfaces. Our work proved that the electronic structure and magnetic ground state of the interfacial FM2 is not altered when the topmost FM2 is replaced by FM1 and that this model is applicable generally for probing the buried magnetic interfaces in the advanced spintronic materials.
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Feb 2016
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Magnets
Theoretical Physics
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Abstract: The rise of spintronics has been closely linked with the development of instrumentation in nano-characterization over the past 20 years. The experimental side of spintronic research today has moved to a point where the paramount urgency is to use materials of the highest perfection and homogeneity as well as analysis tools with atomic sensitivity. Such criteria require usually exclusive techniques, dedicated equipment, and extreme physical conditions, such as ultrahigh vacuum, low temperatures, high fields, etc. This chapter presents some of the most advanced experimental tools, i.e., synchrotron-based magnetic dichroism techniques, which have facilitated the studies of many cutting-edge subjects of spintronics, such as the heterojunction interfacial magnetism, magnetic proximity effect, magnetism in diluted magnetic semiconductors (DMSs), doped topological insulators, half-metallic alloys, magnetic domain structures, and spin transfer torque (STT) effec
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Jan 2016
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I10-Beamline for Advanced Dichroism
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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
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.
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Oct 2015
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I10-Beamline for Advanced Dichroism
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Wenqing
Liu
,
W. Y.
Wang
,
J. J.
Wang
,
F. Q.
Wang
,
C.
Lu
,
F.
Jin
,
A.
Zhang
,
Q. M.
Zhang
,
Gerrit
Van Der Laan
,
Y. B.
Xu
,
Q. X.
Li
,
R.
Zhang
Diamond Proposal Number(s):
[9979]
Open Access
Abstract: Successful spin injection into graphene makes it a competitive contender in the race to become a
key material for quantum computation, or the spin-operation-based data processing and sensing. Engineering ferromagnetic metal (FM)/graphene heterojunctions is one of the most promising avenues to realise it, however, their interface magnetism remains an open question up to this day. In any proposed FM/graphene spintronic devices, the best opportunity for spin transport could only be achieved where no magnetic dead layer exists at the FM/graphene interface. Here we present a comprehensive study of the epitaxial Fe/graphene interface by means of X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) calculations. The experiment has been performed using a specially designed FM1/FM2/graphene structure that to a large extent restores the realistic case of the proposed graphene-based transistors. We have quantitatively observed a reduced but still sizable magnetic moments of the epitaxial Fe ML on graphene, which is well resembled by simulations and can be attributed to the strong hybridization between the Fe 3dz2 and the C 2pz orbitals and the sp-orbital-like behavior of the Fe 3d electrons due to the presence of graphene.
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Jul 2015
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