I09-Surface and Interface Structural Analysis
|
Jieyi
Liu
,
Yiheng
Yang
,
Jianlei
Shen
,
Defa
Liu
,
Gohil Singh
Thakur
,
Charles
Guillemard
,
Alevtina
Smekhova
,
Houke
Chen
,
Deepnarayan
Biswas
,
Manuel
Valvidares
,
Enke
Liu
,
Claudia
Felser
,
Tien-Lin
Lee
,
Thorsten
Hesjedal
,
Yulin
Chen
,
Gerrit
Van Der Laan
Diamond Proposal Number(s):
[37930]
Open Access
Abstract: The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co3Sn2S2 is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co3Sn2S2 single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co0+ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7–9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.
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Feb 2025
|
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NONE-No attached Diamond beamline
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Emily
Heppell
,
Ryuji
Fujita
,
Gautam
Gurung
,
Jheng-Cyuan
Lin
,
Andrew
May
,
Michael
Foerster
,
M. Waqas
Khaliq
,
Miguel Angel
Niño
,
Manuel
Valvidares
,
Javier
Herrero-Martin
,
Pierluigi
Gargiani
,
Kenji
Watanabe
,
Takashi
Taniguchi
,
Dirk
Backes
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
Open Access
Abstract: The exploration of two-dimensional (2D) van der Waals ferromagnets has revealed intriguing magnetic properties with significant potential for spintronics applications. In this study, we examine the magnetic properties of Co-doped Fe5GeTe2 using X-ray photoemission electron microscopy (XPEEM) and X-ray magnetic circular dichroism (XMCD), complemented by density functional theory (DFT) calculations. Our XPEEM measurements reveal that the Curie temperature (TC) of a bilayer of (CoxFe1-x)5-δGeTe2 (with x = 0.28) reaches ∼300 K — a notable enhancement over most 2D ferromagnets in the ultrathin limit. Interestingly, the TC shows only a small dependence on film thickness (bulk TC ≈ 340 K), in line with the observed in-plane magnetic anisotropy and robust in-plane exchange coupling. XMCD measurements indicate that the spin moments for both Fe and Co are significantly reduced compared to the theoretical values. These insights highlight the potential of Co-doped Fe5GeTe2 for stable, high-temperature ferromagnetic applications in 2D materials.
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Dec 2024
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I10-Beamline for Advanced Dichroism - scattering
|
Xiaodong
Xie
,
Kejing
Ran
,
Yizhou
Liu
,
Raymond
Fan
,
Wancong
Tan
,
Haonan
Jin
,
Manuel
Valvidares
,
Nicolas
Jaouen
,
Haifeng
Du
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Shilei
Zhang
Diamond Proposal Number(s):
[20437, 26148, 22629]
Abstract: We identify a three-dimensional skyrmion side-face state in chiral magnets that consists of a thin layer of modulated surface spirals and an array of phase-locked skyrmion screws. Such chiral spin structures lead to a characteristic X-shaped magnetic diffraction pattern in resonant elastic x-ray scattering, reminiscent of Photo 51 of the DNA double-helix diffraction. By measuring both thin plates and bulk
Cu
2
OSeO
3
crystals in the field-in-plane geometry, we unambiguously identify the modulated skyrmion strings by retrieving their chirality and helix angle. The breaking of the translational symmetry along the side faces suppresses the bulk-favored conical state, providing a stabilization mechanism for the skyrmion lattice phase that has been overlooked so far.
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Feb 2023
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I06-Nanoscience (XPEEM)
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Allan S.
Johnson
,
Daniel
Perez-Salinas
,
Khalid M.
Siddiqui
,
Sungwon
Kim
,
Sungwook
Choi
,
Klara
Volckaert
,
Paulina E.
Majchrzak
,
Soeren
Ulstrup
,
Naman
Agarwal
,
Kent
Hallman
,
Richard F.
Haglund
,
Christian M.
Günther
,
Bastian
Pfau
,
Stefan
Eisebitt
,
Dirk
Backes
,
Francesco
Maccherozzi
,
Ann
Fitzpatrick
,
Sarnjeet S.
Dhesi
,
Pierluigi
Gargiani
,
Manuel
Valvidares
,
Nongnuch
Artrith
,
Frank
De Groot
,
Hyeongi
Choi
,
Dogeun
Jang
,
Abhishek
Katoch
,
Soonnam
Kwon
,
Sang Han
Park
,
Hyunjung
Kim
,
Simon E.
Wall
Diamond Proposal Number(s):
[22048]
Open Access
Abstract: Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.
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Dec 2022
|
|
I10-Beamline for Advanced Dichroism - scattering
|
Diamond Proposal Number(s):
[19996]
Abstract: We report a comprehensive study on the magnetic ground state of
La
1.5
Ca
0.5
Co
O
4
combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the
Co
L
2
,
3
edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at
T
N
≈
52
K
. Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic
A
-centered lattice but with
a
and
b
axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group
A
2
m
m
where two nonequivalent compressed and expanded
Co
O
6
octahedra are ordered showing a checkerboard pattern in the
a
b
plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as
(
1
/
4
,
1
/
4
,
0
)
t
,
(
1
/
4
,
1
/
4
,
1
)
t
, and
(
1
/
4
,
1
/
4
,
1
/
2
)
t
in the parent tetragonal cell were observed at low temperature at the
Co
L
2
,
3
-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the
Co
L
2
edge, indicates that only
Co
2
+
-like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the
a
b
plane of the parent tetragonal cell with a slight tilt in the
c
axis. Despite the onset temperatures for the three resonant magnetic reflections being the same,
≈
55
K
, different thermal behavior is observed between
(
1
/
4
,
1
/
4
,
1
/
2
)
t
and
(
1
/
4
,
1
/
4
,
L
)
t
(
L
=
integer
) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the
(
1
/
4
,
1
/
4
,
1
/
2
)
t
magnetic reflection is at least ten times larger than that of the
(
1
/
4
,
1
/
4
,
L
)
t
(
L
=
integer
)
ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector
k
=
(
1
/
4
,
1
/
4
,
1
/
2
)
t
that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in
La
1.5
Ca
0.5
Co
O
4
corresponds then to this propagation vector
k
=
(
1
/
4
,
1
/
4
,
1
/
2
)
t
while
(
1
/
4
,
1
/
4
,
0
)
t
and
(
1
/
4
,
1
/
4
,
1
)
t
magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.
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May 2021
|
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Abstract: We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)2(Se,Te)3 family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10−3 μB/at.
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Nov 2020
|
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Yangkun
He
,
Gerhard H.
Fecher
,
Chenguang
Fu
,
Yu
Pan
,
Kaustuv
Manna
,
Johannes
Kroder
,
Ajay
Jha
,
Xiao
Wang
,
Zhiwei
Hu
,
Stefano
Agrestini
,
Javier
Herrero-Martin
,
Manuel
Valvidares
,
Yurii
Skourski
,
Walter
Schnelle
,
Plamen
Stamenov
,
Horst
Borrmann
,
Liu Hao
Tjeng
,
Rudolf
Schaefer
,
Stuart S. P.
Parkin
,
John Michael D.
Coey
,
Claudia
Felser
Open Access
Abstract: The development of high‐density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat‐assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2CoSb is introduced as a new hard magnet with potential for thin‐film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m−3 is combined with a saturation magnetization of μ0Ms = 0.52 T at 2 K (2.2 MJ m−3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare‐earth‐free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m−1 K−1, make Rh2CoSb a candidate for the development of heat‐assisted writing with a recording density in excess of 10 Tb in.−2.
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Oct 2020
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I05-ARPES
|
Raphael C.
Vidal
,
Alexander
Zeugner
,
Jorge I.
Facio
,
Rajyavardhan
Ray
,
M. Hossein
Haghighi
,
Anja U. b.
Wolter
,
Laura T.
Corredor Bohorquez
,
Federico
Caglieris
,
Simon
Moser
,
Tim
Figgemeier
,
Thiago R. F.
Peixoto
,
Hari Babu
Vasili
,
Manuel
Valvidares
,
Sungwon
Jung
,
Cephise
Cacho
,
Alexey
Alfonsov
,
Kavita
Mehlawat
,
Vladislav
Kataev
,
Christian
Hess
,
Manuel
Richter
,
Bernd
Büchner
,
Jeroen
Van Den Brink
,
Michael
Ruck
,
Friedrich
Reinert
,
Hendrik
Bentmann
,
Anna
Isaeva
Diamond Proposal Number(s):
[22468]
Open Access
Abstract: Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized.
MnBi
2
Te
4
has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular
(
Bi
2
Te
3
)
n
(
MnBi
2
Te
4
)
series. Here, for
n
=
1
, we confirm a nonstoichiometric composition proximate to
MnBi
4
Te
7
. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the
MnBi
4
Te
7
(
0001
)
surface, analogous to the nonmagnetic parent compound
Bi
2
Te
3
. Our results establish
MnBi
4
Te
7
as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.
|
Dec 2019
|
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I05-ARPES
|
R. C.
Vidal
,
H.
Bentmann
,
T. R. F.
Peixoto
,
A.
Zeugner
,
S.
Moser
,
C.-H.
Min
,
S.
Schatz
,
K.
Kissner
,
M.
Unzelmann
,
C. I.
Fornari
,
H. B.
Vasili
,
M.
Valvidares
,
K.
Sakamoto
,
D.
Mondal
,
J.
Fujii
,
I.
Vobornik
,
S.
Jung
,
C.
Cacho
,
T. K.
Kim
,
R. J.
Koch
,
C.
Jozwiak
,
A.
Bostwick
,
J. D.
Denlinger
,
E.
Rotenberg
,
J.
Buck
,
M.
Hoesch
,
F.
Diekmann
,
S.
Rohlf
,
M.
Kalläne
,
K.
Rossnagel
,
M. M.
Otrokov
,
E. V.
Chulkov
,
M.
Ruck
,
A.
Isaeva
,
F.
Reinert
Diamond Proposal Number(s):
[19278, 22468]
Abstract: The layered van der Waals antiferromagnet
MnBi
2
Te
4
has been predicted to combine the band ordering of archetypical topological insulators such as
Bi
2
Te
3
with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of
MnBi
2
Te
4
(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
|
Sep 2019
|
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