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Abstract: Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena
building on time-reversal symmetry-broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment Dy:Bi2Te3 with high transition temperature Cr:Sb2Te3 in a superlattice, and we found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the Dy:Bi2Te3 layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin-film stack or is surface-sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry, which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.
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Aug 2019
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Open Access
Abstract: Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic
topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure.
Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular
magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and
soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films.
X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the
local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.
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Jul 2019
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Theoretical Physics
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Open Access
Abstract: In 1878, the Dutch physicist Hendrik Antoon Lorentz first addressed the calculation of the local electric field at an atomic site in a ferroelectric material, generated by all the other electric dipoles within the sample. This calculation, which applies equally well to ferromagnets, is taught in Universities around the World. Here we demonstrate that the Lorentz concept can be used to speed up calculations of the local dipolar field in square, circular, and elliptical shaped monolayers and thin films, not only at the center of the film, but across the sample. Calculations show that long elliptical and rectangular films should exhibit the narrowest ferromagnetic resonance linewidth. In addition, discrete dipole calculations show that the Lorentz cavity field does not hold in tetragonal films. Depending on the ratio (b/a), the local field can be either less/greater than : an observation that has implications for ferromagnetic resonance. 3D simple cubic (SC) systems are also examined. For example, while most texts discuss the Lorentz cavity field in terms of a Lorentz sphere, the Lorentz cavity field still holds when a Lorentz sphere is replaced by a the Lorentz cube, but only in cubic SC, FCC and BCC systems. Finally, while the primary emphasis is on the discrete dipole-dipole interaction, contact is made with the continuum model. For example, in the continuous SC dipole model, just one monolayer is required to generate the Lorentz cavity field. This is in marked contrast to the discrete dipole model, where a minimum of five adjacent monolayers is required.
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Jul 2019
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Xiyang
Li
,
Shilei
Zhang
,
Hang
Li
,
Diego
Alba Venero
,
Jonathan S
White
,
Robert
Cubitt
,
Qingzhen
Huang
,
Jie
Chen
,
Lunhua
He
,
Gerrit
Van Der Laan
,
Wenhong
Wang
,
Thorsten
Hesjedal
,
Fangwei
Wang
Open Access
Abstract: A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee‐bean‐shaped objects are observed in real space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface‐confined objects, or, analogous to skyrmions in non-centrosymmetric helimagnets, 3D tube‐like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single‐ and polycrystalline‐MnNiGa samples using small‐angle neutron scattering. It is found that biskyrmions are not long‐range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in‐plane symmetry axis aligned along certain directions, governed by the magneto-crystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.
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Mar 2019
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I10-Beamline for Advanced Dichroism
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Wenjing
Li
,
Iuliia
Bykova
,
Shilei
Zhang
,
Guoqiang
Yu
,
Riccardo
Tomasello
,
Mario
Carpentieri
,
Yizhou
Liu
,
Yao
Guang
,
Joachim
Gräfe
,
Markus
Weigand
,
David M.
Burn
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Zhengren
Yan
,
Jiafeng
Feng
,
Caihua
Wan
,
Jinwu
Wei
,
Xiao
Wang
,
Xiaomin
Zhang
,
Hongjun
Xu
,
Chenyang
Guo
,
Hongxiang
Wei
,
Giovanni
Finocchio
,
Xiufeng
Han
,
Gisela
Schütz
Diamond Proposal Number(s):
[18898]
Abstract: Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real-space spin configuration. Here, two advanced X-ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO]n multilayers. First, by using ptychography, a high-resolution diffraction imaging technique, the 2D out-of-plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X-ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depthdependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole–dipole interactions together with the external magnetic field play an important role in stabilizing sub-100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. Our combined X-ray-based approach opens the door for in-depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.
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Feb 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[17402, 13539, 11503, 11119, 10256, 10207]
Abstract: Magnetic topological insulators (MTIs) are a novel materials class in which a topologically nontrivial electronic band structure coexists with long‐range ferromagnetic order. The ferromagnetic ground state can break time‐reversal symmetry, opening a gap in the topological surface states whose size is dependent on the magnitude of the magnetic moment. Doping with rare earth ions is one way to introduce higher magnetic moments into a material, however, in Bi2Te3 bulk crystals, the solubility limit is only a few percent. Using molecular beam epitaxy for the growth of doped (Sb,Bi)2(Se,Te)3 TI thin films, high doping concentrations can be achieved while preserving their high crystalline quality. The growth, structural, electronic, and magnetic properties of Dy, Ho, and Gd doped TI thin films will be reviewed. Indeed, high magnetic moments can be introduced into the TIs, which are, however, not ferromagnetically ordered. By making use of interfacial effects, magnetic long‐range order in Dy doped Bi2Te3, proximity‐coupled to the MTI Cr:Sb2Te3, has been achieved. Clearly, engineered MTI heterostructures offer new possibilities that combine the advantageous properties of different layers, and thus provide an ideal materials platform enabling the observation new quantum effects at higher temperatures.
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Jan 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[11784, 12958]
Abstract: Magnetic skyrmions are particle-like, topologically protected magnetization entities that are promising candidates for information carriers in racetrack-memory schemes. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Recently, we demonstrated experimentally that chiral skyrmions in Cu2OSeO3 can be effectively manipulated by a magnetic field gradient, leading to a collective rotation of the skyrmion lattice with well-defined dynamics in a radial field gradient. Here, we employ a skyrmion particle model to numerically study the effects of resultant shear forces on the structure of the skyrmion lattice. We demonstrate that anisotropic peak broadening in experimentally observed diffraction patterns can be attributed to extended linear regions in the magnetic field profile. We show that topological (5-7) defects emerge to protect the six-fold symmetry of the lattice under the application of local shear forces, further enhancing the stability of proposed magnetic field driven devices.
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Jan 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[9210, 11500]
Abstract: We studied the magnetodynamics of ferromagnetic films coupling across a topological insulator (TI)
Bi2Se3 layer using ferromagnetic resonance (FMR). TIs have attracted much attention across the physics community as they hold the potential for dissipationless carrier transport, extremely high spin-orbit torques, and are host to novel quantum effects. To investigate the coupling between the ferromagnetic (FM) layers, vector network analyzer (VNA)-FMR measurements of the resonance linewidth were performed as a function of bias field angle. By bringing the resonances of the two FM layers into close proximity, it was possible to observe antidamping torques that lead to a narrowing of linewidth, a characteristic of spin pumping. The element- and hence layer-specific technique of x-ray detected ferromagnetic resonance (XFMR) was used to circumvent the difficulty of obtaining accurate fits to the two overlapping resonances in close proximity. Our results confirm that the interaction across the TI is a dynamic exchange mediated by spin pumping, as opposed to a self-coupling of the surface state or similar, more unconventional mechanisms.
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Oct 2018
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[16162]
Abstract: The recently confirmed monolayer ferromagnet CrI3 is a frisky example of a two-dimensional ferromagnetic material with great application potential in van der Waals heterostructures. Here we present a soft x-ray absorption spectroscopy study of the magnetic bulk properties of CrI3, giving insight into the magnetic coupling scenario which is relevant for understanding its thickness-dependent magnetic properties. The experimental Cr
L2,3
x-ray magnetic circular dichroism spectra show a good agreement with calculated spectra for a hybridized ground state. In this high-spin Cr ground state the Cr–I bonds show a strongly covalent character. This is responsible for the strong superexchange interaction and increased spin-orbit coupling, resulting in the large magnetic anisotropy of the two-dimensionally layered CrI3 crystal.
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Aug 2018
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I05-ARPES
I10-Beamline for Advanced Dichroism
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J. M.
Riley
,
F.
Caruso
,
C.
Verdi
,
L. B.
Duffy
,
M. D.
Watson
,
L.
Bawden
,
K.
Volckaert
,
G.
Van Der Laan
,
T.
Hesjedal
,
M.
Hoesch
,
F.
Giustino
,
P. D. C.
King
Diamond Proposal Number(s):
[15481, 13539, 16162]
Open Access
Abstract: Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio
many-body calculations, we demonstrate how a strong coupling of conduction electrons with
collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.
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Jun 2018
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