I21-Resonant Inelastic X-ray Scattering (RIXS)
|
S.
Agrestini
,
F.
Borgatti
,
P.
Florio
,
J.
Frassineti
,
D.
Fiore Mosca
,
Q.
Faure
,
B.
Detlefs
,
C. j.
Sahle
,
S.
Francoual
,
J.
Choi
,
Mirian
Garcia-Fernandez
,
K.-J.
Zhou
,
V. f.
Mitrović
,
P. m.
Woodward
,
G.
Ghiringhelli
,
C.
Franchini
,
F.
Boscherini
,
S.
Sanna
,
M.
Moretti Sala
Open Access
Abstract: A supposedly nonmagnetic 5𝑑1 double perovskite oxide is investigated by a combination of spectroscopic and theoretical methods, namely, resonant inelastic x-ray scattering, x-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand-field calculations. We found that the large spin-orbit coupling admixes the 5𝑑 𝑡2𝑔 and 𝑒𝑔 orbitals, covalency raises the 5𝑑 population well above the nominal value, and the local symmetry is lower than 𝑂ℎ. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5𝑑1 ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.
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Aug 2024
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I10-Beamline for Advanced Dichroism - scattering
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M. T.
Littlehales
,
S. H.
Moody
,
P. J.
Bereciartua
,
D. A.
Mayoh
,
Z. B.
Parkin
,
T. J.
Blundell
,
E.
Unsworth
,
S.
Francoual
,
G.
Balakrishnan
,
D.
Alba Venero
,
P. D.
Hatton
Diamond Proposal Number(s):
[34192]
Open Access
Abstract: The Eu(Ga1−𝑥Al𝑥)4 series is composed of centrosymmetric structures which exhibit a wide range of rich topological phenomena, including some members hosting magnetic skyrmions. In this letter, we investigate the previously unreported intermediate compound EuGa2.4Al1.6, which hosts two distinct phase transitions under zero applied magnetic field. We have used resonant elastic x-ray scattering with full linear polarization analysis to unambiguously determine the zero-field magnetic structures, which consist of a transition between a basal plane transverse spin density wave at higher temperatures into a noncollinear helical ground state. Furthermore, we demonstrate a phase coexistence regime below the transition and reveal an elliptically modulated helical magnetic structure emerging from wavevector splitting.
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Jul 2024
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I16-Materials and Magnetism
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M. C.
Rahn
,
M. N.
Wilson
,
T. J.
Hicken
,
F. L.
Pratt
,
C.
Wang
,
F.
Orlandi
,
D. D.
Khalyavin
,
P.
Manuel
,
L. S. I.
Veiga
,
A.
Bombardi
,
S.
Francoual
,
P.
Bereciartua
,
A. S.
Sukhanov
,
J. D.
Thompson
,
S. M.
Thomas
,
P. F. S.
Rosa
,
T.
Lancaster
,
F.
Ronning
,
M.
Janoschek
Open Access
Abstract: Eu
5
In
2
Sb
6
is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of
Eu
5
In
2
Sb
6
single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic x-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of
Eu
5
In
2
Sb
6
results in coplanar noncollinear magnetic structures with an Ising-like net magnetization along the
a
axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-
q
structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.
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May 2024
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I06-Nanoscience (XPEEM)
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Diamond Proposal Number(s):
[29620]
Abstract: Here, we report on the structural, electronic, and magnetic properties of a polycrystalline sample of the LaCaCoIrO double-perovskite investigated by means of synchrotron x-ray powder diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism at the Co and Ir edges, magnetometry, and electrical transport. Our results indicate a configuration of nearly Co/Ir configuration for the transition-metal ions, with spin canting within the Co antiferromagnetic superstructure responsible for the ferromagnetic-like behavior observed below 100 K. The highly insulating character of LaCaCoIrO and its positive magnetoresistance further suggest that this antiferromagnetic superexchange interaction occurs through an indirect hybridization between the Co orbitals.
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Dec 2023
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I16-Materials and Magnetism
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Diamond Proposal Number(s):
[27353]
Open Access
Abstract: Magnetic topological insulators and semimetals are a class of crystalline solids whose properties are strongly influenced by the coupling between non-trivial electronic topology and magnetic spin configurations. Such materials can host exotic electromagnetic responses. Among these are topological insulators with certain types of antiferromagnetic order which are predicted to realize axion electrodynamics. Here we investigate the highly unusual helimagnetic phases recently reported in EuIn2As2, which has been identified as a candidate for an axion insulator. Using resonant elastic x-ray scattering we show that the two types of magnetic order observed in EuIn2As2 are spatially uniform phases with commensurate chiral magnetic structures, ruling out a possible phase-separation scenario, and we propose that entropy associated with low energy spin fluctuations plays a significant role in driving the phase transition between them. Our results establish that the magnetic order in EuIn2As2 satisfies the symmetry requirements for an axion insulator.
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Jun 2023
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I06-Nanoscience (XPEEM)
|
Diamond Proposal Number(s):
[29620]
Open Access
Abstract: Here we present a detailed investigation of the Co and Ir local electronic structures in
La
1.5
A
0.5
CoIrO
6
(
A
=
Ba
, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir
L
2
,
3
-edge and Co
K
-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for
A
=
Ca
below
∼
96
K
, the competing magnetic phases, and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the
A
=
Ba
, on the other hand, there is no evidence of long-range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds are discussed in terms of the structural distortion that alters the strength of the Co-Ir couplings, with a relevant role played by the Co
3
d
e
g
–
Ir
5
d
j
eff
=
1
/
2
hybridization.
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Jun 2023
|
|
|
|
Jung-Hwa
Kim
,
Darren C.
Peets
,
Manfred
Reehuis
,
Peter
Adler
,
Andrey
Maljuk
,
Tobias
Ritschel
,
Morgan C.
Allison
,
Jochen
Geck
,
Jose R. L.
Mardegan
,
Pablo J.
Bereciartua Perez
,
Sonia
Francoual
,
Andrew
Walters
,
Thomas
Keller
,
Paula M.
Abdala
,
Philip
Pattison
,
Pinder
Dosanjh
,
Bernhard
Keimer
Open Access
Abstract: Since the discovery of charge disproportionation in the
FeO
2
square-lattice compound
Sr
3
Fe
2
O
7
by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained “hidden” to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe
K
-edge resonant x-ray scattering to demonstrate checkerboard charge order in the
FeO
2
planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.
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Aug 2021
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I16-Materials and Magnetism
|
Max
Burian
,
Michael
Porer
,
Jose R. L.
Mardegan
,
Vincent
Esposito
,
Sergii
Parchenko
,
Bulat
Burganov
,
Namrata
Gurung
,
Mahesh
Ramakrishnan
,
Valerio
Scagnoli
,
Hiroki
Ueda
,
Sonia
Francoual
,
Federica
Fabrizi
,
Yoshikazu
Tanaka
,
Tadashi
Togashi
,
Yuya
Kubota
,
Makina
Yabashi
,
Kai
Rossnagel
,
Steven L.
Johnson
,
Urs
Staub
Diamond Proposal Number(s):
[15742]
Open Access
Abstract: In this work, we use ultrafast pump-probe nonresonant and resonant x-ray diffraction to track the periodic lattice distortion and the electronic charge density wave in
1
T
−
TiSe
2
upon optical excitation. We observe a fluence regime in which the periodic lattice deformation is strongly suppressed but the charge density wave related Se
4
p
orbital order remains mostly intact. Complete melting of both structural and electronic order occurs four to five times faster than expected from a purely electronic charge-screening process, strongly suggesting a structurally assisted weakening of excitonic correlations. Our experimental data provide insight on the intricate coupling between structural and electronic order in stabilizing the periodic-lattice-distortion/charge-density-wave state in
1
T
−
TiSe
2
. The results further show that electron-phonon coupling can lead to different, energy dependent phase-transition pathways in condensed matter systems, opening different possibilities in the conception of nonequilibrium phenomena at the ultrafast scale.
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Feb 2021
|
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Zhehong
Liu
,
Xiao
Wang
,
Xubin
Ye
,
Xudong
Shen
,
Yuecheng
Bian
,
Wei
Ding
,
Stefano
Agrestini
,
Sheng-Chieh
Liao
,
Hong-Ji
Lin
,
Chien-Te
Chen
,
Shih-Chang
Weng
,
Kai
Chen
,
Philippe
Ohresser
,
Lucie
Nataf
,
Francois
Baudelet
,
Zhigao
Sheng
,
Sonia
Francoual
,
Jose R. L.
Mardegan
,
Olaf
Leupold
,
Zefang
Li
,
Xuekui
Xi
,
Wenhong
Wang
,
Liu Hao
Tjeng
,
Zhiwei
Hu
,
Youwen
Long
Abstract: A quadruple perovskite oxide
Ca
Cu
3
Co
2
Re
2
O
12
was synthesized by high-pressure annealing. This compound crystallizes in an
A
- and
B
-site ordered quadruple perovskite structure with space group
P
n
−
3
. The charge combination is determined to be
Ca
Cu
2
+
3
Co
2
+
2
Re
6
+
2
O
12
by bond valence sum analysis and x-ray absorption spectroscopy. In contrast to other isostructural
A
Cu
3
B
2
B
′
2
O
12
compounds with a single magnetic transition, a long-range antiferromagnetic phase transition originating from the
A
′
-site
Cu
2
+
sublattice is found to occur at
T
N
≈
28
K
. Subsequently, the spin coupling between the
B
-site
Co
2
+
and
B
′
-site
Re
6
+
ions contributes to a ferrimagnetic transition around
T
C
≈
20
K
. Strong electrical insulating behavior is identified by optical measurement with an energy gap of approximately 3.75 eV. The mechanisms of the spin interactions are discussed in detail.
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Jan 2021
|
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I16-Materials and Magnetism
|
Stephan
Geprags
,
Christoph
Klewe
,
Sibylle
Meyer
,
Dominik
Graulich
,
Felix
Schade
,
Marc
Schneider
,
Sonia
Francoual
,
Stephen P.
Collins
,
Katharina
Ollefs
,
Fabrice
Wilhelm
,
Andrei
Rogalev
,
Yves
Joly
,
Sebastian T. B.
Goennenwein
,
Matthias
Opel
,
Timo
Kuschel
,
Rudolf
Gross
Diamond Proposal Number(s):
[12772]
Abstract: The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator
Y
3
Fe
5
O
12
has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we compare thin film heterostructures with different orders of the layer stacking and different interface properties. For standard Pt layers on
Y
3
Fe
5
O
12
thin films, we do not detect any static magnetic polarization in Pt. These samples show an angle-dependent magnetoresistance behavior, which is consistent with the established spin Hall magnetoresistance. In contrast, for the inverted layer sequence,
Y
3
Fe
5
O
12
thin films grown on Pt layers, Pt displays a finite induced magnetic moment comparable to that of all-metallic Pt/Fe bilayers. This magnetic moment is found to originate from finite intermixing at the
Y
3
Fe
5
O
12
/
Pt
interface. As a consequence, we found a complex angle-dependent magnetoresistance indicating a superposition of the spin Hall and the anisotropic magnetoresistance in these types of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution.
|
Dec 2020
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