I10-Beamline for Advanced Dichroism
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N.-J.
Steinke
,
S. L.
Zhang
,
P. J.
Baker
,
L. B.
Duffy
,
F.
Kronast
,
J.
Krieger
,
Z.
Salman
,
T.
Prokscha
,
A.
Suter
,
S.
Langridge
,
Gerrit
Van Der Laan
,
T.
Hesjedal
Diamond Proposal Number(s):
[11503]
Abstract: Chromium-doped
Sb
2
Te
3
is a magnetic topological insulator (MTI), which belongs to the
(
Sb
,
Bi
)
2
(
Se
,
Te
)
3
family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below
≈
2
K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation
(
μ
SR
)
, and magnetic soft x-ray spectroscopy and imaging.
μ
SR
provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static
(
≈
μ
s
)
and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.
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Dec 2022
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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.
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Aug 2022
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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.
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May 2022
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Abstract: We identify room-temperature converse magnetoelectric effects (CMEs) that are non-volatile by using a single-crystal substrate of PMN–PT (001)pc (pc denotes pseudocubic) to impart voltage-driven strain to a polycrystalline film of Ni. An appropriate magnetic-field history enhances the magnetoelectric coefficient to a near-record peak of ∼10−6 s m−1 and permits electrically driven magnetization reversal of substantial net magnetization. In zero magnetic field, electrically driven ferroelectric domain switching produces large changes of in-plane magnetization that are non-volatile. Microscopically, these changes are accompanied by the creation and destruction of magnetic stripe domains, implying the electrical control of perpendicular magnetic anisotropy. Moreover, the stripe direction can be rotated by a magnetic field or an electric field, the latter yielding the first example of electrically driven rotatable magnetic anisotropy. The observed CMEs are associated with repeatable ferroelectric domain switching that yields a memory effect. This memory effect is well known for PMN–PT (110)pc but not PMN–PT (001)pc. Given that close control of the applied field is not required as for PMN–PT (110)pc, this memory effect could lead the way to magnetoelectric memories based on PMN–PT (001)pc membranes that switch at low voltage.
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Apr 2021
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I06-Nanoscience
|
Richard W.
Dawidek
,
Thomas J.
Hayward
,
Ian T.
Vidamour
,
Thomas J.
Broomhall
,
Guru
Venkat
,
Mohanad Al
Mamoori
,
Aidan
Mullen
,
Stephan J.
Kyle
,
Paul W.
Fry
,
Nina-Juliane
Steinke
,
Joshaniel F. K.
Cooper
,
Francesco
Maccherozzi
,
Sarnjeet S.
Dhesi
,
Lucia
Aballe
,
Michael
Foerster
,
Jordi
Prat
,
Eleni
Vasilaki
,
Matthew O. A.
Ellis
,
Dan A.
Allwood
Diamond Proposal Number(s):
[24205]
Open Access
Abstract: Emergent behaviors occur when simple interactions between a system's constituent elements produce properties that the individual elements do not exhibit in isolation. This article reports tunable emergent behaviors observed in domain wall (DW) populations of arrays of interconnected magnetic ring‐shaped nanowires under an applied rotating magnetic field. DWs interact stochastically at ring junctions to create mechanisms of DW population loss and gain. These combine to give a dynamic, field‐dependent equilibrium DW population that is a robust and emergent property of the array, despite highly varied local magnetic configurations. The magnetic ring arrays’ properties (e.g., non‐linear behavior, “fading memory” to changes in field, fabrication repeatability, and scalability) suggest they are an interesting candidate system for realizing reservoir computing (RC), a form of neuromorphic computing, in hardware. By way of example, simulations of ring arrays performing RC approaches 100% success in classifying spoken digits for single speakers.
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Feb 2021
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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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I10-Beamline for Advanced Dichroism
|
Diamond Proposal Number(s):
[17480]
Abstract: The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped three-dimensional topological insulator (TI). The pioneering transition-metal doped (Sb,Bi)2(Se,Te)3 thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth-doped (Sb,Bi)2(Se,Te)3 thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allowfor the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:Bi2Te3/Cr:Sb2Te3 thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:Sb2Te3 can introduce long-range magnetic order in high-moment Dy:Bi2Te3—upto a temperature of 17 K—in excellent agreement with first-principles calculations,which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.
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May 2018
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L. B.
Duffy
,
N.-J.
Steinke
,
J. A.
Krieger
,
A. I.
Figueroa
,
K.
Kummer
,
T.
Lancaster
,
S. R.
Giblin
,
F. L.
Pratt
,
S. J.
Blundell
,
T.
Prokscha
,
A.
Suter
,
Sean
Langridge
,
V. N.
Strocov
,
Z.
Salman
,
G.
Van Der Laan
,
T.
Hesjedal
Abstract: Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI’s exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped Bi2Te3 remained elusive.Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.
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May 2018
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B18-Core EXAFS
|
Diamond Proposal Number(s):
[15702]
Open Access
Abstract: Using soft x-ray absorption spectroscopy we determined the chemical and magnetic properties of the magnetic topological insulator (MTI) Cr:Sb2Te3. X-ray magnetic circular dichroism (XMCD) at the Cr L2,3, Te M4,5, and Sb M4,5 edges shows that the Te 5p moment is aligned antiparallel to both the Cr 3d and Sb 5p moments, which is characteristic for carrier-mediated ferromagnetic coupling. Comparison of the Cr L2,3 spectra with multiplet calculations indicates a hybridized Cr state, consistent with the carrier-mediated coupling scenario. We studied the enhancement of the Curie temperature, TC, of the MTI thin film through the magnetic proximity effect. Arrott plots, measured using the Cr L3 XMCD, show a TC ≈ 87 K for the as-cleaved film. After deposition of a thin layer of ferromagnetic Co onto the surface, the TC increases to ∼93 K, while the Co and Cr moments are parallel. This increase in TC is unexpectedly small compared to similar systems reported earlier. The XMCD spectra demonstrate that the Co/MTI interface remains intact, i.e., no reaction between Co and the MTI takes place. Our results are a useful starting point for refining the physical models of Cr-doped Sb2Te3, which is required for making use of them in device applications.
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Jun 2017
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Open Access
Abstract: Styrene-alt-maleic acid lipid particles (SMALPs) are self-assembled discoidal structures composed of a polymer belt and a segment of lipid bilayer, which are capable of encapsulating membrane proteins directly from the cell membrane. Here we present evidence of the exchange of lipids between such “nanodiscs” and lipid monolayers adsorbed at either solid–liquid or air–liquid interfaces. This behavior has important implications for the potential uses of nanodiscs.
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Nov 2016
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