I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[25166]
Open Access
Abstract: We present the synthesis of a novel binary metal oxide material: Ba7Mn4O15. The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100–300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr7Mn4O15 that forms its own structural class. However, we find that Ba7Mn4O15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr7Mn4O15.
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Jun 2022
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I11-High Resolution Powder Diffraction
I15-Extreme Conditions
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Diamond Proposal Number(s):
[14061, 17673, 16390]
Open Access
Abstract: The similar electronic structures of Bi3+ and Pb2+ have motivated researchers to explore bismuth-based perovskite compounds, which in the past decade has been further fuelled by the demand for developing lead-free piezoceramics. The difficulty in stabilizing the perovskite phase in bismuth based compounds has directed most research activities towards exploring two main compounds - multiferroic BiFeO3 and relaxor ferroelectric Na1/2Bi1/2TiO3 and their derivatives. In recent years, quenching these materials from the sintering temperature or from the paraelectric phase (above the Curie temperature, Tc) has resulted in a plethora of fundamentally interesting and technologically relevant advances, including enhanced thermal depolarization temperature, high Tc, giant strain and control over the atomic structure and electrical conductivity at the domain wall. In this contribution, a brief overview of quenching piezoceramics is presented, with majority of the discussion encompassing salient features of quenching lead-free perovskite structured Na1/2Bi1/2TiO3- and BiFeO3- based materials. For each material system, the influence of quenching on phase transitions, domain switching behavior and electromechanical properties are presented, apart from outlining the current understanding of the underlying mechanisms. The review provides guidelines for further exploration of the quenching strategy for improving the functionality of Bi-based piezoceramics.
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May 2022
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I06-Nanoscience
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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.
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May 2022
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I16-Materials and Magnetism
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Dorin
Rusu
,
Jonathan J. P.
Peters
,
Thomas P. A.
Hase
,
James A.
Gott
,
Gareth A. A.
Nisbet
,
Jörg
Strempfer
,
Daniel
Haskel
,
Samuel D.
Seddon
,
Richard
Beanland
,
Ana M.
Sanchez
,
Marin
Alexe
Diamond Proposal Number(s):
[25569]
Open Access
Abstract: Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices1 and skyrmions when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric–insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii–Moriya interaction (DMi) has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.
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Feb 2022
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I21-Resonant Inelastic X-ray Scattering (RIXS)
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Suhan
Son
,
Youjin
Lee
,
Jae Ha
Kim
,
Beom Hyun
Kim
,
Chaebin
Kim
,
Woongki
Na
,
Hwiin
Ju
,
Sudong
Park
,
Abhishek
Nag
,
Ke-Jin
Zhou
,
Young-Woo
Son
,
Hyeongdo
Kim
,
Woo-Suk
Noh
,
Jae-Hoon
Park
,
Jong Seok
Lee
,
Hyeonsik
Cheong
,
Jae Hoon
Kim
,
Je-Geun
Park
Diamond Proposal Number(s):
[29554]
Abstract: Matter-light interaction is at the center of diverse research fields from quantum optics to condensed matter physics, opening new fields like laser physics. A magnetic exciton is one such rare example found in magnetic insulators. However, it is relatively rare to observe that external variables control matter-light interaction. Here, we report that the broken inversion symmetry of multiferroicity can act as an external knob enabling the magnetic exciton in van der Waals antiferromagnet NiI2. We further discover that this magnetic exciton arises from a transition between Zhang-Rice-triplet and Zhang-Rice-singlet's fundamentally quantum entangled states. This quantum entanglement produces an ultra-sharp optical exciton peak at 1.384 eV with a 5 meV linewidth. Our work demonstrates that NiI2 is two-dimensional magnetically ordered with an intrinsically quantum entangled ground state.
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Dec 2021
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Open Access
Abstract: Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations.
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Aug 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[7758, 8615]
Abstract: In this work we present the study of rare earth molybdates RE=(Eu, Tb and
Ho) with formula RE2(MoO4)3. Under ambient conditions, Eu- and Tb-containing
compounds can be found in the phases α and β0
, while holmium-containing can be
found in the phase γ and β0 under these conditions. In the study of the β0
-Tb2(MoO4)3
compression by the CCDD group, the hypothesis of a transition to a new phase, called
the phase δ, was considered. Subsequently, in the study of Y2(MoO4)3 synthesised
unconventional conditions, non-stoichiometric oxide and molybdate phases with dif fractograms very similar to those of the phase δ were obtained.
This opened the door to another possible hypothesis about the β0 → δ transition,
that it was a decomposition induced by high pressure. For its verification, the synt hesis of the named compounds was carried out, modifying the solid state synthesis,
applying a pressure of 0.66 GPa for compacting powder samples and increasing or
decreasing the synthesis temperature, which was different for each case.
We carry out a study of the crystalline structures of the most relevant phases
involved: chelite-α, β-β0 and γ. Additionally, symmetry relationships provide clarity
in understanding the phase transitions.
A routine diffractogram was performed for each compound synthesised by using
the X-ray diffractometer available at SIDIX (X-ray Facilitiy of the La Laguna Univer sity). Diffraction data collected under pressure were provided by the CCDD (group
with which the supervisors of this work are researching). Note that, in addition to
the β0
-Tb2(MoO4)3 data, β0
-phase of Ho and Eu molybdates data were also available.
Using the ICSD database we simulated the different phases that would be expec ted to be found, and with which a visual identification of the phases was performed.
Applying the Le Bail refinement method, the intensities of the full profile were refined
as a verification of the existence of the expected phases.
The synthesis of europium molybdate was carried out at 500oC, 550oC and
600oC. After the analysis and refinements, different mixtures of phases with struc tural types Sm2O3, MoO3, Eu4Mo7O27, Eu2Mo4O15 and the phase α-Eu2(MoO4)3
were detected. Furthermore, by analysing the pure β0
-Eu2(MoO4)3 phase under pres sure, it was observed that around 2.23 GPa a phase transition occurs, interpreted
as a decomposition into the β0
, Eu2O3 and Eu2Mo4O15 phases, while at 5 GPa an
amorphisation undergoes.
Holmium oxide, β0
-Ho2(MoO4)3, nonstoichiometric Y2Mo4O15 phases were iden tified for the holmium molybdated synthetised at 600oC. Under pressure, as in the
case of the europium molybdate, a phase transition occurs around 2.3 GPa in which
the β0
-phase, the europium oxide and the Y2Mo4O15 phase are involved and the
non-reversible amorphous phase starts at around 5 GPa.
In these cases as well as the one studied by the CCDD group on Tb molybdate,
when the decompression is carried out, the initial β0
-RE2(MoO4)3 phase is not com pletely recovered, so the phase transition is not reversible, which leads us to think
that it is a decomposition induced by pressure.
In addition to the experimental work and the analysis and discussion of the
results, we would like to highlight the literature review carried out, which is a very
important part of this dissertation. On the one hand, the research was contextualised,
its interest was explained and an exhaustive description of the crystal structures of the
materials studied was made. It was also necessary to: 1) review and introduce some
crystallographic terms that were later used in the description of these structures. 2)
Describe the experimental techniques used, reviewing their physical foundations. 3)
Explain the tools used in the analysis of the data.
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Jun 2021
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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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Moein
Adnani
,
Melissa
Gooch
,
Liangzi
Deng
,
Stefano
Agrestini
,
Javier
Herrero-Martin
,
Hung-Cheng
Wu
,
Chung-Kai
Chang
,
Taha
Salavati-Fard
,
Narayan
Poudel
,
Jose Luis
Garcia-Munoz
,
Samira
Daneshmandi
,
Zheng
Wu
,
Lars C.
Grabow
,
Yen-Chung
Lai
,
Hung-Duen
Yang
,
Eric
Pellegrin
,
Ching-Wu
Chu
Abstract: We have investigated the multiferroicity and magnetoelectric (ME) coupling in
Ho
Fe
W
O
6
. With a noncentrosymmetric polar structure (space group Pna
2
1
) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature (
T
N
) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements. The metamagnetic behavior at low temperatures was found to be directly related to the dielectric properties of the compound. In particular, field-dependent measurements of capacitance show a magnetocapacitance (MC) effect with double-hysteresis loop behavior in direct correspondence with the magnetization. Our x-ray diffraction results show the Pna
2
1
structure down to 8 K and suggest the absence of a structural phase transition across
T
N
. Soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism (XMCD) measurements at the Fe
L
2
,
3
and Ho
M
4
,
5
edges revealed the oxidation state of Fe and Ho cations to be
3
+
. Fe
L
2
,
3
XMCD further shows that
Fe
3
+
cations are antiferromagnetically ordered in a noncollinear fashion with spins arranged
90
∘
with respect to each other. Our findings show that
Ho
Fe
W
O
6
is a type-II multiferroic exhibiting a MC effect. The observed MC effect and the change in polarization by the magnetic field, as well as their direct correspondence with magnetization, further support the strong ME coupling in this compound.
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Mar 2021
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I11-High Resolution Powder Diffraction
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Henrik
Jacobsen
,
Hai L.
Feng
,
Andrew J.
Princep
,
Marein C.
Rahn
,
Yanfeng
Guo
,
Jie
Chen
,
Yoshitaka
Matsushita
,
Yoshihiro
Tsujimoto
,
Masahiro
Nagao
,
Dmitry
Khalyavin
,
Pascal
Manuel
,
Claire A.
Murray
,
Christian
Donnerer
,
James G.
Vale
,
Marco
Moretti Sala
,
Kazunari
Yamaura
,
Andrew T.
Boothroyd
Diamond Proposal Number(s):
[9839]
Abstract: We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure,
Pb
2
CaOsO
6
and
Pb
2
ZnOsO
6
. Upon cooling below 80 K,
Pb
2
CaOsO
6
simultaneously undergoes a metal-to-insulator transition and develops antiferromagnetic order.
Pb
2
ZnOsO
6
, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structures. The Os atoms in
Pb
2
ZnOsO
6
are arranged on an approximately face-centered cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of
Pb
2
CaOsO
6
is heavily distorted up to at least 500 K including antiferroelectriclike displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which, in turn, drives the metal-insulator transition. Our results suggest that the phase transition in
Pb
2
CaOsO
6
is spin driven and could be a rare example of a Slater transition.
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Dec 2020
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