I05-ARPES
|
A. Garrison
Linn
,
Peipei
Hao
,
Kyle N.
Gordon
,
Dushyant
Narayan
,
Bryan S.
Berggren
,
Nathaniel
Speiser
,
Sonka
Reimers
,
Richard P.
Campion
,
Vít
Novák
,
Sarnjeet S.
Dhesi
,
Timur K.
Kim
,
Cephise
Cacho
,
Libor
Šmejkal
,
Tomáš
Jungwirth
,
Jonathan D.
Denlinger
,
Peter
Wadley
,
Daniel S.
Dessau
Diamond Proposal Number(s):
[24224]
Open Access
Abstract: Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of ≈− 390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. In addition, 2×1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.
|
May 2023
|
|
I06-Nanoscience
|
O. J.
Amin
,
S. F.
Poole
,
S.
Reimers
,
L. X.
Barton
,
A.
Dal Din
,
F.
Maccherozzi
,
S. S.
Dhesi
,
V.
Novák
,
F.
Krizek
,
J. S.
Chauhan
,
R. P.
Campion
,
A. W.
Rushforth
,
T.
Jungwirth
,
O. A.
Tretiakov
,
K. W.
Edmonds
,
P.
Wadley
Diamond Proposal Number(s):
[26255, 27845]
Open Access
Abstract: Topologically protected magnetic textures are promising candidates for information carriers in future memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. These textures—nanoscale whirls in the magnetic order—include skyrmions, half-skyrmions (merons) and their antiparticles. Antiferromagnets have been shown to host versions of these textures that have high potential for terahertz dynamics, deflection-free motion and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin-film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The merons and antimerons are localized on 180° domain walls, and move in the direction of the current pulses. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high-density, high-speed magnetic memory devices.
|
May 2023
|
|
I06-Nanoscience
|
Diamond Proposal Number(s):
[14135]
Abstract: The size of the orbital moment in
Fe
3
O
4
has been the subject of a long-standing and contentious debate. In this paper, we make use of ferromagnetic resonance (FMR) spectroscopy and x-ray magnetic circular dichroism (XMCD) to provide complementary determinations of the size of the orbital moment in “bulklike” epitaxial
Fe
3
O
4
films grown on yttria-stabilized zirconia (111) substrates. Annealing the 100 nm as-grown films to
1100
∘
C
in a reducing atmosphere improves the stoichiometry and microstructure of the films, allowing for bulklike properties to be recovered as evidenced by x-ray diffraction and vibrating sample magnetometry. In addition, in-plane angular FMR spectra exhibit a crossover from a fourfold symmetry to the expected sixfold symmetry of the (111) surface, together with an anomalous peak in the FMR linewidth at
∼
10
GHz; this is indicative of low Gilbert damping in combination with two-magnon scattering. For the bulklike annealed sample, a spectroscopic splitting factor
g
≈
2.18
is obtained using both FMR and XMCD techniques, providing evidence for the presence of a finite orbital moment in
Fe
3
O
4
.
|
Feb 2023
|
|
I06-Nanoscience
|
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.
|
Dec 2022
|
|
|
|
Ian
Vidamour
,
Matthew O. A.
Ellis
,
David
Griffin
,
Guru
Venkat
,
Charles
Swindells
,
Richard W. S.
Dawidek
,
Thomas J
Broomhall
,
Nina-Juliane
Steinke
,
Joshaniel
Cooper
,
Francesco
Maccherozzi
,
Sarnjeet
Dhesi
,
Susan
Stepney
,
Eleni
Vasilaki
,
Dan A.
Allwood
,
Tom James
Hayward
Open Access
Abstract: Devices based on arrays of interconnected magnetic nano-rings with emergent magnetization dynamics have recently been proposed for use in reservoir computing applications, but for them to be computationally useful it must be possible to optimise their dynamical responses. Here, we use a phenomenological model to demonstrate that such reservoirs can be optimised for classification tasks by tuning hyperparameters that control the scaling and input-rate of data into the system using rotating magnetic fields. We use task-independent metrics to assess the rings' computational capabilities at each set of these hyperparameters and show how these metrics correlate directly to performance in spoken and written digit recognition tasks. We then show that these metrics, and performance in tasks, can be further improved by expanding the reservoir's output to include multiple, concurrent measures of the ring arrays' magnetic states.
|
Aug 2022
|
|
I06-Nanoscience
|
Andrea
Ronchi
,
Paolo
Franceschini
,
Andrea
De Poli
,
Pia
Homm
,
Ann
Fitzpatrick
,
Francesco
Maccherozzi
,
Gabriele
Ferrini
,
Francesco
Banfi
,
Sarnjeet S.
Dhesi
,
Mariela
Menghini
,
Michele
Fabrizio
,
Jean-Pierre
Locquet
,
Claudio
Giannetti
Diamond Proposal Number(s):
[18897, 21700]
Open Access
Abstract: Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V2O3, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.
|
Jun 2022
|
|
I06-Nanoscience
|
G.
Awana
,
R.
Fujita
,
A.
Frisk
,
P.
Chen
,
Q.
Yao
,
A. J.
Caruana
,
C. J.
Kinane
,
N.-J.
Steinke
,
S.
Langridge
,
P.
Olalde-Velasco
,
S. S.
Dhesi
,
G.
Van Der Laan
,
X. F.
Kou
,
S. L.
Zhang
,
T.
Hesjedal
,
D.
Backes
Diamond Proposal Number(s):
[23748]
Open Access
Abstract: An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in
MnTe
/
Bi
2
Te
3
heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the
Bi
2
Te
3
layer. Furthermore, the interface between MnTe and
Bi
2
Te
3
is not abrupt, but extending over
∼
2.2
nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.
|
May 2022
|
|
I06-Nanoscience
|
Diamond Proposal Number(s):
[19060]
Open Access
Abstract: The authors describe and compare two complementary techniques that are habitually used to image ferromagnetic and ferroelectric materials with sub-micron spatial resolutions (typically 50 nm, at best 10 nm). The first technique is variable-temperature photoemission electron microscopy with magnetic/antiferromagnetic/polar contrast from circularly/linearly polarized incident X-rays (XPEEM). The second technique is magnetic force microscopy (MFM). Focusing mainly on the authors’ own work, but not exclusively, published/unpublished XPEEM and MFM images of ferroic domains and complex magnetic textures (involving vortices and phase separation) are presented. Highlights include the use of two XPEEM images to create 2D vector maps of in-plane (IP) magnetization, and the use of imaging to detect electrically driven local reversals of magnetization. The brief and simple descriptions of XPEEM and MFM should be useful for beginners seeking to employ these techniques in order to understand and harness ferroic materials.
|
May 2022
|
|
I06-Nanoscience
|
Filip
Krizek
,
Sonka
Reimers
,
Zdeněk
Kašpar
,
Alberto
Marmodoro
,
Jan
Michalička
,
Ondřej
Man
,
Alexander
Edström
,
Oliver J.
Amin
,
Kevin W.
Edmonds
,
Richard P.
Campion
,
Francesco
Maccherozzi
,
Sarnjeet S.
Dhesi
,
Jan
Zubáč
,
Domink
Kriegner
,
Dina
Carbone
,
Jakub
Železný
,
Karel
Výborný
,
Kamil
Olejník
,
Vít
Novák
,
Jan
Rusz
,
Juan-Carlos
Idrobo
,
Peter
Wadley
,
Tomas
Jungwirth
Diamond Proposal Number(s):
[22437]
Open Access
Abstract: The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field–insensitive neuromorphic functionalities.
|
Apr 2022
|
|
I06-Nanoscience
|
Dong
Li
,
Bonan
Zhu
,
Dirk
Backes
,
Larissa S. I.
Veiga
,
Tien-Lin
Lee
,
Hongguang
Wang
,
Qian
He
,
Pinku
Roy
,
Jiaye
Zhang
,
Jueli
Shi
,
Aiping
Chen
,
Peter A.
Van Aken
,
Quanxi
Jia
,
Sarnjeet S.
Dhesi
,
David O.
Scanlon
,
Kelvin H. L.
Zhang
,
Weiwei
Li
Diamond Proposal Number(s):
[25425, 26901, 29616]
Abstract: Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of
La
0.9
Ba
0.1
MnO
3
thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn
e
g
orbitals can be switched from the
d
3
z
2
−
r
2
orbital to the
d
x
2
−
y
2
orbital by varying the strain from compressive to tensile. The change of orbital occupancy associated with Mn
3
d
-O
2
p
hybridization leads to dramatic modulation of the magnetic and electronic properties of strained
La
0.9
Ba
0.1
MnO
3
thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the
La
0.9
Ba
0.1
MnO
3
system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.
|
Apr 2022
|
|
