I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[28727]
Open Access
Abstract: The low-temperature electronic structure of the van der Waals ferromagnet CrSiTe3 has been investigated. This ferromagnetic semiconductor has a magnetic bulk transition temperature of 33 K, which can reach up to 80 K in single- and few-layer flakes. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L2,3 and Te Mb edges on in vacuo cleaved single crystals, give strong evidence for hybridization-mediated super-exchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of the XMCD with the calculated Cr L2,3 multiplet spectra, confirm a strongly covalent bond between the Cr 3d(eg) and Te 5p states. Application of the XMCD sum rules gives a non-vanishing orbital moment, supporting a partial occupation of the eg states, apart from the t2g. Also, the presence of a non-zero XMCD signal at the Te Mb edge confirms a Te 5p spin polarization due to mixing with the Cr eg bonding states. The results strongly suggest that superexchange, instead of the previously suggested single ion anisotropy, is responsible for the low-temperature ferromagnetic ordering of 2D materials such as CrSiTe3 and CrGeTe3. This demonstrates the interplay between electron correlation and ferromagnetism in insulating two-dimensional materials.
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Dec 2021
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I10-Beamline for Advanced Dichroism
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Open Access
Abstract: Single-crystal exchange biased multilayer thin-films consisting of an Fe layer on single-crystal chemically ordered L12-IrMn3 and disordered γ-IrMn3 were investigated using circularly and linearly polarised soft X-ray reflectivity to de- termine the interfacial magnetic structure between Fe and IrMn3. In ordered L12-IrMn3, we found that the Mn uncompensated moments at the interface are strongly pinned and only Fe rotational moments are observed. In disor- dered γ-IrMn3 the uncompensated moments are partially pinned, where the Mn moments are rotatable but the rotations are restricted. These findings are a crucial development in exchange bias theory to understand the force that drives the magnetic reversal process in ordered and disordered IrMn3.
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Nov 2021
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[14915]
Abstract: Electrostatically defined perovskite oxide heterostructures, in which carriers are confined by the modulation of the A-site ion charge, offer new possibilities of tuning the magnetic properties of manganite oxides. We investigate the preferential orientation of ferromagnetic and antiferromagnetic moments in ultrathin
La
0.7
Sr
0.3
MnO
3
layers embedded in antiferromagnetic
Sr
MnO
3
as they undergo a metal-to-insulator transition with decreasing thickness. Our results evince the role of orbital occupation, metallicity, and competition of different magnetic phases, in absence of spurious effects occurring in thin films as a result of symmetry breaking at
La
0.7
Sr
0.3
MnO
3
interfaces and of incorporation of oxygen vacancies.
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Aug 2021
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[22157]
Open Access
Abstract: Tunnel junctions based on Fe storing globular proteins are an interesting class of biomolecular tunnel junctions due to their tunable Fe ion loading, symmetrical structure and thermal stability, and are therefore attractive to study the mechanisms of charge transport (CT) at the molecular level. This paper describes a temperature-induced change in the CT mechanism across junctions with large globular (∼25 nm in diameter) E2-proteins bioengineered with Fe-binding peptides from ferritin (E2-LFtn) to mineralise Fe ions in the form of iron oxide nanoparticles (NPs) inside the protein's cavity. The iron oxide NPs provide accessible energy states that support high CT rates and shallow activation barriers. Interestingly, the CT mechanism changes abruptly, but reversibly, from incoherent tunnelling (which is thermally activated) to coherent tunnelling (which is activationless) across the E2-LFtn-based tunnel junctions with the highest Fe ion loading at a temperature of 220–240 K. During this transition the current density across the junctions increases by a factor of 13 at an applied voltage of V = −0.8 V. X-ray absorption spectroscopy indicates that the iron oxide NPs inside the E2-LFtn cages undergo a reversible phase transition; this phase transition opens up new a tunnelling pathway changing the mechanism of CT from thermally activated to activationless tunnelling despite the large size of the E2-LFtn and associated distance for tunnelling.
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Mar 2021
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I10-Beamline for Advanced Dichroism
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Abstract: Nanoparticles that combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core–shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which has been scarcely reported yet. These nanoparticles consist of a Fe3−δO4 core covered by a first shell of CoFe2O4 and a second shell of Fe3−δO4, e.g., a Fe3−δO4@CoFe2O4@Fe3−δO4 onion-like structure. They were synthesized through a multistep seed-mediated growth approach which consists consists in performing three successive thermal decomposition of metal complexes in a high-boiling-point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increasing. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by 57Fe Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, whereas the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3−δO4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core–shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles to accurately tune the magnetic properties of nanoparticles according to the desired applications.
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Mar 2021
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I10-Beamline for Advanced Dichroism
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A.
Tcakaev
,
V. B.
Zabolotnyy
,
C. I.
Fornari
,
P.
Rüßmann
,
T. R. F.
Peixoto
,
F.
Stier
,
M.
Dettbarn
,
P.
Kagerer
,
E.
Weschke
,
E.
Schierle
,
P.
Bencok
,
P. H. O.
Rappl
,
E.
Abramof
,
H.
Bentmann
,
E.
Goering
,
F.
Reinert
,
V.
Hinkov
Diamond Proposal Number(s):
[19994]
Abstract: Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet in a recent photoemission study of Eu-doped
Bi
2
Te
3
films, the spectra remained gapless down to
T
=
20
K. Here we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the
M
4
,
5
x-ray absorption and magnetic circular dichroism spectra reveals a
Eu
2
+
valence and confirms a large magnetic moment, consistent with a
4
f
7
8
S
7
/
2
ground state. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth-doped
Bi
2
Te
3
and call for spectroscopic studies in the Kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.
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Nov 2020
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I10-Beamline for Advanced Dichroism
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Myron S.
Huzan
,
Manuel
Fix
,
Matteo
Aramini
,
Peter
Bencok
,
J. Frederick W.
Mosselmans
,
Shusaku
Hayama
,
Franziska A.
Breitner
,
Leland B.
Gee
,
Charles J.
Titus
,
Marie-Anne
Arrio
,
Anton
Jesche
,
Michael L.
Baker
Diamond Proposal Number(s):
[21117, 23982]
Open Access
Abstract: Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine FeI dopant ions to be linearly coordinated, occupying a D6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L2,3-edge XAS from which the energy reduction of 3dz2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L3-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm−1), that corresponds with Orbach relaxation via the first excited, MJ = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.
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Oct 2020
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I10-Beamline for Advanced Dichroism
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Sheng-Qun
Su
,
Shu-Qi
Wu
,
Michael L.
Baker
,
Peter
Bencok
,
Nobuaki
Azuma
,
Yuji
Miyazaki
,
Motohiro
Nakano
,
Soonchul
Kang
,
Yoshihito
Shiota
,
Kazunari
Yoshizawa
,
Shinji
Kanegawa
,
Osamu
Sato
Diamond Proposal Number(s):
[17723]
Abstract: Orbital angular momentum plays a vital role in various applications, especially magnetic and spintronic properties. There-fore, controlling orbital angular momentum is of paramount importance to both fundamental science and new technologi-cal applications. Many attempts have been made to modulate the ligand-field induced quenching effects of orbital angular momentum to manipulate magnetic properties. However, to date, reported changes in the magnitude of orbital angular momentum are small in both molecular and solid-state magnetic materials. Moreover, no effective methods currently exist to modulate orbital angular momentum. Here, we report a dynamic bond approach to realize a large change in orbital angu-lar momentum. We have developed a Co(II) complex that exhibits coordination number switching between six and seven. This cooperative dynamic bond switching induces considerable modulation of the ligand field, thereby leading to substantial quenching and restoration of the orbital angular momentum. This switching mechanism is entirely different from those of spin-crossover and valence tautomeric compounds, which exhibit switching in spin multiplicity.
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Jun 2020
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I10-Beamline for Advanced Dichroism
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Shu-Qi
Wu
,
Meijiao
Liu
,
Kaige
Gao
,
Shinji
Kanegawa
,
Yusuke
Horie
,
Genki
Aoyama
,
Hajime
Okajima
,
Akira
Sakamoto
,
Michael L.
Baker
,
Myron S.
Huzan
,
Peter
Bencok
,
Tsukasa
Abe
,
Yoshihito
Shiota
,
Kazunari
Yoshizawa
,
Wenhuang
Xu
,
Hui-Zhong
Kou
,
Osamu
Sato
Diamond Proposal Number(s):
[21117]
Open Access
Abstract: Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds.
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Apr 2020
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I09-Surface and Interface Structural Analysis
I10-Beamline for Advanced Dichroism
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Georgios
Araizi-Kanoutas
,
Jaap
Geessinck
,
Nicolas
Gauquelin
,
Steef
Smit
,
Xanthe H.
Verbeek
,
Shrawan K.
Mishra
,
Peter
Bencok
,
Christoph
Schlueter
,
Tien-Lin
Lee
,
Dileep
Krishnan
,
Jarmo
Fatermans
,
Jo
Verbeeck
,
Guus
Rijnders
,
Gertjan
Koster
,
Mark S.
Golden
Abstract: We report charge transfer up to a single electron per interfacial unit cell across nonpolar heterointerfaces from the Mott insulator
LaTi
O
3
to the charge transfer insulator
LaCo
O
3
. In high-quality bi- and trilayer systems grown using pulsed laser deposition, soft x-ray absorption, dichroism, and scanning transmission electron microscopy-electron energy loss spectroscopy are used to probe the cobalt-
3
d
electron count and provide an element-specific investigation of the magnetic properties. The experiments show the cobalt valence conversion is active within 3 unit cells of the heterointerface, and able to generate full conversion to
3
d
7
divalent Co, which displays a paramagnetic ground state. The number of
LaTi
O
3
/
LaCo
O
3
interfaces, the thickness of an additional, electronically insulating “break” layer between the
LaTi
O
3
and
LaCo
O
3
, and the
LaCo
O
3
film thickness itself in trilayers provide a trio of control knobs for average charge of the cobalt ions in
LaCo
O
3
, illustrating the efficacy of
O
−
2
p
band alignment as a guiding principle for property design in complex oxide heterointerfaces.
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Feb 2020
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