I06-Nanoscience
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S.
Duttagupta
,
A.
Kurenkov
,
O. A.
Tretiakov
,
G.
Krishnaswamy
,
G.
Sala
,
V.
Krizakova
,
F.
Maccherozzi
,
S. S.
Dhesi
,
P.
Gambardella
,
S.
Fukami
,
H.
Ohno
Diamond Proposal Number(s):
[20413]
Open Access
Abstract: The ability to represent information using an antiferromagnetic material is attractive for future antiferromagnetic spintronic devices. Previous studies have focussed on the utilization of antiferromagnetic materials with biaxial magnetic anisotropy for electrical manipulation. A practical realization of these antiferromagnetic devices is limited by the requirement of material-specific constraints. Here, we demonstrate current-induced switching in a polycrystalline PtMn/Pt metallic heterostructure. A comparison of electrical transport measurements in PtMn with and without the Pt layer, corroborated by x-ray imaging, reveals reversible switching of the thermally-stable antiferromagnetic Néel vector by spin-orbit torques. The presented results demonstrate the potential of polycrystalline metals for antiferromagnetic spintronics.
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Nov 2020
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I06-Nanoscience
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Abstract: The `water window', covering 2.4–4.4 nm, is an important wavelength range particularly essential to biology research. Cr/Ti multilayers are one of the promising reflecting elements in this region because the near-normal-incidence reflectivity is theoretically as high as 64% at 2.73 nm. However, due to multilayer imperfections, the reported reflectivity is lower than 3% for near-normal incidence. Here, B and C were intentionally incorporated into ultra-thin Cr/Ti soft X-ray multilayers by co-deposition of B4C at the interfaces. The effect on the multilayer structure and composition has been investigated using X-ray reflectometry, X-ray photoelectron spectroscopy, and cross-section electron microscopy. It is shown that B and C are mainly bonded to Ti sites, forming a nonstoichiometric TiBxCy composition, which hinders the interface diffusion, supresses the crystallization of the Cr/Ti multilayer and dramatically improves the interface quality of Cr/TiBxCy multilayers. As a result, the near-normal-incidence reflectivity of soft X-rays increases from 4.48% to 15.75% at a wavelength of 2.73 nm.
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Nov 2020
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I06-Nanoscience
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M.
Ghidini
,
R.
Pellicelli
,
R.
Mansell
,
D.
Pesquera
,
B.
Nair
,
X.
Moya
,
S.
Farokhipoor
,
F.
Maccherozzi
,
C. H. W.
Barnes
,
R. P.
Cowburn
,
S. S.
Dhesi
,
N. D.
Mathur
Diamond Proposal Number(s):
[11843]
Open Access
Abstract: Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.
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Oct 2020
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I06-Nanoscience
|
T.
Janda
,
J.
Godinho
,
T.
Ostatnicky
,
E.
Pfitzner
,
G.
Ulrich
,
A.
Hoehl
,
S.
Reimers
,
Z.
Šobáň
,
T.
Metzger
,
H.
Reichlová
,
V.
Novák
,
R. P.
Campion
,
J.
Heberle
,
P.
Wadley
,
K. W.
Edmonds
,
O. J.
Amin
,
J. S.
Chauhan
,
S. S.
Dhesi
,
F.
Maccherozzi
,
R. M.
Otxoa
,
P. E.
Roy
,
K.
Olejník
,
P.
Němec
,
T.
Jungwirth
,
B.
Kaestner
,
J.
Wunderlich
Diamond Proposal Number(s):
[22437, 16376, 20793]
Abstract: Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible
180
∘
switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent
90
∘
switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.
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Sep 2020
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I05-ARPES
I06-Nanoscience
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Sung Won
Jung
,
Sangyeon
Pak
,
Sanghyo
Lee
,
Sonka
Reimers
,
Saumya
Mukherjee
,
Pavel
Dudin
,
Timur K.
Kim
,
Mattia
Cattelan
,
Neil
Fox
,
Sarnjeet S.
Dhesi
,
Cephise
Cacho
,
Seungnam
Cha
Diamond Proposal Number(s):
[24367, 22901]
Abstract: The recent rise of van der Waals (vdW) crystals has opened new prospects for studying versatile and exotic fundamental physics with future device applications such as twistronics. Even though the recent development on Angle-resolved photoemission spectroscopy (ARPES) with Nano-focusing optics, making clean surfaces and interfaces of chemically transferred crystals have been challenging to obtain high-resolution ARPES spectra. Here, we show that by employing nano-ARPES with submicron sized beam and polystyrene-assisted transfer followed by annealing process in ultra-high vacuum environment, remarkably clear ARPES spectral features such as spin-orbit splitting and band renormalization of CVD-grown, monolayered MoS2 can be measured. Our finding paves a way to exploit chemically transferred crystals for measuring high-resolution ARPES spectra to observe exotic quasi-particles in vdW heterostructures.
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Jul 2020
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I06-Nanoscience
|
D.
Pesquera
,
E.
Khestanova
,
M.
Ghidini
,
S.
Zhang
,
A. P.
Rooney
,
F.
Maccherozzi
,
P.
Riego
,
S.
Farokhipoor
,
J.
Kim
,
X.
Moya
,
M. E.
Vickers
,
N. A.
Stelmashenko
,
S. J.
Haigh
,
S. S.
Dhesi
,
N. D.
Mathur
Diamond Proposal Number(s):
[14745]
Open Access
Abstract: Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces.
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Jun 2020
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I06-Nanoscience
|
Diamond Proposal Number(s):
[17563]
Open Access
Abstract: Exchange-bias has been reported in bulk nanocrystalline Fe2MnAl, but individual thin films of this Heusler alloy have never been studied so far. Here we study the structural and magnetic properties of nanocrystalline thin films of Fe2–xMn1+xAl (x = –0.25, 0, 0.25) obtained by sputtering and ex-situ post-deposition annealing. We find that Fe2MnAl films display exchange-bias, and that varying Mn concentration determines the magnitude of the effect, which can be either enhanced (in Fe1.75Mn1.25Al) or suppressed (in Fe2.25Mn0.75Al). X-ray diffraction (XRD) shows that our films present a mixed L21-B2 Heusler structure where increasing Mn concentration favors the partial transformation of the L21 phase into the B2 phase. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) reveal that this composition-driven L21→B2 transformation is accompanied by phase segregation at the nanoscale. As a result, the Fe2–xMn1+xAl films that show exchange-bias (x = 0, 0.25) are heterogeneous, with nanograins of an Fe-rich phase embedded in a Mn-rich matrix (a non-negative matrix factorisation algorithm was used to give an indication of the phase composition from EDX data). Our comparative analysis of XRD, magnetometry and X-ray magnetic circular dichroism (XMCD), shows that Fe-rich nanograins and Mn-rich matrix are composed of a ferromagnetic L21 phase and an antiferromagnetic B2 phase, respectively, thus revealing that exchange-coupling between these two phases is the cause of the exchange-bias effect. Our work should inspire the development of single-layer, environmentally-friendly spin valve devices based on nanocomposite Heusler films.
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May 2020
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I06-Nanoscience
|
Diamond Proposal Number(s):
[8876, 6230]
Abstract: We review our work on continuous Ni films coupled via strain to ferroelectric substrates of BaTiO3 (BTO) and 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 (PMN-PT). We show that magnetic force microscopy (MFM) and photoemission electron microscopy (PEEM) of the Ni films (during or after electrical treatment) permit to reveal nanoscale converse magnetoelectric effects (CMEs) that are novel and elude macroscopic measurements. As examples, we discuss magnetization reversal without applied field in multilayer capacitors (MLCs), shear-strain-mediated CMEs in thin Ni films on PMN-PT and reversible switching of perpendicular magnetization from out-of-plane to in-plane in Ni films on BTO. In this latter case, we show that PEEM can be used to measure both magnetic and ferroelectric domains, thus providing key mechanistic insight in the magnetoelectric coupling mechanism.
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May 2020
|
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I06-Nanoscience
|
M.
Wang
,
C.
Andrews
,
S.
Reimers
,
O. J.
Amin
,
P.
Wadley
,
R. P.
Campion
,
S. F.
Poole
,
J.
Felton
,
K. W.
Edmonds
,
B. L.
Gallagher
,
A. W.
Rushforth
,
O.
Makarovsky
,
K.
Gas
,
M.
Sawicki
,
D.
Kriegner
,
J.
Zubáč
,
K.
Olejník
,
V.
Novák
,
T.
Jungwirth
,
M.
Shahrokhvand
,
U.
Zeitler
,
S. S.
Dhesi
,
F.
Maccherozzi
Diamond Proposal Number(s):
[11846, 20793]
Abstract: We report magnetic-field-induced rotation of the antiferromagnetic Néel vector in epitaxial CuMnAs thin films. First, using soft x-ray magnetic linear dichroism spectroscopy as well as magnetometry, we demonstrate spin-flop switching and continuous spin reorientation in films with uniaxial and biaxial magnetic anisotropies, respectively, for applied magnetic fields of the order of 2 T. The remnant antiferromagnetic domain configurations are determined using x-ray photoemission electron microscopy. Next, we show that the Néel vector reorientations are manifested in the longitudinal and transverse anisotropic magnetoresistance. Dependencies of the electrical resistance on the orientation of the Néel vector with respect to both the electrical current direction and the crystal symmetry are identified, including a weak fourth-order term evident at high magnetic fields. The results provide characterization of key parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, and spin-flop field in epitaxial films of tetragonal CuMnAs, a candidate material for antiferromagnetic spintronics.
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Mar 2020
|
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I06-Nanoscience
|
Massimo
Ghidini
,
Rhodri
Mansell
,
Raffaele
Pellicelli
,
David
Pesquera
,
Bhaskaran
Nair
,
Xavier
Moya
,
Saeedeh
Farokhipoor
,
Francesco
Maccherozzi
,
Crispin
Barnes
,
Russell
Cowburn
,
Sarnjeet
Dhesi
,
Neil
Mathur
Diamond Proposal Number(s):
[11843]
Abstract: Using photoemission electron microscopy (PEEM) to image ferromagnetism in polycrystalline Ni disks, and ferroelectricity in their single-crystal BaTiO3 substrates, we find that voltage-driven 90 ferroelectric domain switching serves to reversibly annihilate each magnetic vortex via uniaxial compressive strain, and that the orientation of the resulting bi domain reveals the chirality of the annihilated vortex. (Micromagnetic simulations reveal that only 60% of this strain was required for annihilation.) Voltage control of magnetic vortices is novel, and should be energetically favourable with respect to the use of a magnetic field or an electrical current. In future, stray field from bi domains could be exploited to read vortex chirality. Given that core polarity can already be read via stray field, our work represents a step towards four-state low power memory applications.
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Feb 2020
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