I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[17612]
Open Access
Abstract: Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic X-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field, we find that a state of perpendicularly ordered skyrmions forms, in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined to a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface guiding makes the robust state particular interesting for racetracklike devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.
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Jan 2020
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Katharina
Zeissler
,
Simone
Finizio
,
Craig
Barton
,
Alexandra J.
Huxtable
,
Jamie
Massey
,
Jörg
Raabe
,
Alexandr V.
Sadovnikov
,
Sergey A.
Nikitov
,
Richard
Brearton
,
Thorsten
Hesjedal
,
Gerrit
Van Der Laan
,
Mark C.
Rosamond
,
Edmund H.
Linfield
,
Gavin
Burnell
,
Christopher H.
Marrows
Open Access
Abstract: Magnetic skyrmions are topologically non-trivial nanoscale objects. Their topology, which originates in their chiral domain wall winding, governs their unique response to a motion-inducing force. When subjected to an electrical current, the chiral winding of the spin texture leads to a deflection of the skyrmion trajectory, characterised by an angle with respect to the applied force direction. This skyrmion Hall angle is predicted to be skyrmion diameter-dependent. In contrast, our experimental study finds that the skyrmion Hall angle is diameter-independent for skyrmions with diameters ranging from 35 to 825 nm. At an average velocity of 6 ± 1 ms−1, the average skyrmion Hall angle was measured to be 9° ± 2°. In fact, the skyrmion dynamics is dominated by the local energy landscape such as materials defects and the local magnetic configuration.
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Jan 2020
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I05-ARPES
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V. A.
Rogalev
,
F.
Reis
,
F.
Adler
,
M.
Bauernfeind
,
J.
Erhardt
,
A.
Kowalewski
,
M. R.
Scholz
,
L.
Dudy
,
L. B.
Duffy
,
T.
Hesjedal
,
M.
Hoesch
,
G.
Bihlmayer
,
J.
Schäfer
,
R.
Claessen
Diamond Proposal Number(s):
[10244, 10289, 12892, 15285]
Abstract: We report on the electronic structure of
α
-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of
α
-Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.
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Dec 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[18898]
Abstract: Collective spin excitations of ordered magnetic structures offer great potential for the development of novel spintronic devices. The present approach relies on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic X-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR technique builds on X-ray detected FMR that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.
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Dec 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[20437]
Abstract: A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector,
q
, is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase,
q
only varies with normalized field but not with temperature—in sharp contrast to the skyrmion phase, for which
q
is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.
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Nov 2019
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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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