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LMX: Laboratory for Multiscale materials eXperiments

The Laboratory for Multiscale materials eXperiments (LMX) focusses on designing novel functional materials in poly- and single crystalline form, as thin films and as multilayers. Read more about LMX


30 August 2018

Claire Donnelly dissertation research awards

In August 2018, Claire Donnelly was awarded the SPS Award in Computational Physics, sponsored by COMSOL, and the Werner Meyer-Ilse Memorial Award. We congratulate her on these awards as well as for the two awards received earlier this year: the ETH Medal for an outstanding doctoral thesis and the American Physical Society Richard L. Greene Dissertation Award, recognizing doctoral thesis research of exceptional quality and importance. These prizes are for her dissertation on “Hard X-ray Tomography of Three Dimensional Magnetic Structures”. Claire carried out her dissertation in the Laboratory for Mesoscopic Systems (ETH Zurich – Paul Scherrer Institute) in collaboration with the CXS group and the OMNY project, with experiments conducted at the cSAXS beamline, SLS, and Sebastian Gliga, a Marie Curie Fellow at the University of Glasgow. She will continue this research at the University of Cambridge with a Leverhulme Fellowship supported by the Newton Trust. We wish her every success! - Picture courtesy of the APS.

Scientific Highlights

11 February 2019

Electronic localization in CaVO3 films via bandwidth control

Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO3, a correlated metal in its bulk form that has only a single electron in its V4+ 3d manifold, as a representative example. We find that thick films and ultrathin films (≤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10 u.c. CaVO3 film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V3+ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10 u.c. CaVO3 film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3d–O 2p hybridization.
Facility: SLS

Reference: D.E. McNally et al, npj Quantum Materials 4, 6 (2019)

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8 February 2019

Emergent magnetic monopole dynamics in macroscopically degenerate artificial spin ice

Magnetic monopoles, proposed as elementary particles that act as isolated magnetic south and north poles, have long attracted research interest as magnetic analogs to electric charge. In solid-state physics, a classical analog to these elusive particles has emerged as topological excitations within pyrochlore spin ice systems. We present the first real-time imaging of emergent magnetic monopole motion in a macroscopically degenerate artificial spin ice system consisting of thermally activated Ising-type nanomagnets lithographically arranged onto a pre-etched silicon substrate. A real-space characterization of emergent magnetic monopoles within the framework of Debye-Hückel theory is performed, providing visual evidence that these topological defects act like a plasma of Coulomb-type magnetic charges. In contrast to vertex defects in a purely two-dimensional artificial square ice, magnetic monopoles are free to evolve within a divergence-free vacuum, a magnetic Coulomb phase, for which features in the form of pinch-point singularities in magnetic structure factors are observed.
Keywords: artificial spin ice; monopoles; XMCD; thin films;

Facility: Thin Films and Interfaces, LMX, ETHZ, Advanced Light Source, Lawrence Berkeley National Laboratory (LBNL) USA

Reference: A. Farhan et al., Science Advances 5 (2), eaav6380 (2019)

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21 December 2018

Time-Reversal Symmetry Breaking in Re-Based Superconductors

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation (μSR) studies of superconducting noncentro-symmetric Re0.82Nb0.18 (Tc=8.8 K) and centrosymmetric Re (Tc=2.7 K). In Re0.82Nb0.18, the low–temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron- phonon coupling. In both Re0.82Nb0.18 and pure Re, the spontaneous magnetic fields revealed by zero-field μSR below Tc indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T=transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg10Ir19B16 and Nb0.5Os0.5, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.
Facility: SμS

Reference: T. Shang et al, Physical Review Letters 121, 257002 (2018)

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10 December 2018

Poling of an artificial magneto-toroidal crystal

Although ferromagnetism is known to be of enormous importance, the exploitation of materials with a compensated (for example, antiferromagnetic) arrangement of long-range ordered magnetic moments is still in its infancy. Antiferromagnetism is more robust against external perturbations, exhibits ultrafast responses of the spin system and is key to phenomena such as exchange bias, magnetically induced ferroelectricity or certain magnetoresistance phenomena. However, there is no conjugate field for the manipulation of antiferromagnetic order, hindering both its observation and direct manipulation. Only recently, direct poling of a particular antiferromagnet was achieved with spintronic approaches. An interesting alternative to antiferromagnetism is ferrotoroidicity—a recently established fourth form of ferroic order. This is defined as a vortex-like magnetic state with zero net magnetization, yet with a spontaneously occurring toroidal moment. As a hallmark of ferroic order, there must be a conjugate field that can manipulate the order parameter. For ferrotoroidic materials, this is a toroidal field—a magnetic vortex field violating both space-inversion and time-reversal symmetry analogous to the toroidal moment. However, the nature and generation of the toroidal field remain elusive for conventional crystalline systems. Here, we demonstrate the creation of an artificial crystal consisting of mesoscopic planar nanomagnets with a magneto-toroidal-ordered ground state. Effective toroidal fields of either sign are applied by scanning a magnetic tip over the crystal. Thus, we achieve local control over the orientation of the toroidal moment despite its zero net magnetization.
Reference: J. Lehmann et al, Nature Nanotechnology, adv. online publication (Dec 2018)

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4 December 2018

Highly selective surface acoustic wave e-nose implemented by laser direct writing

In this paper, we present an e-nose for the detection of volatile compounds based on an array of six surface acoustic wave (SAW) resonators coated with five different polymers (i.e. polyepichlorohydrin, polyisobutylene, polyethylenimine, (hydroxypropyl)methyl cellulose, and poly(styrene-co-maleic acid) partial isobutyl/methyl mixed ester, plus an uncoated SAW device used as reference. In particular, matrix assisted pulsed laser evaporation was used to deposit thin polymer layers which were subsequently used as donor films in the laser induced forward transfer process to selectively cover each SAW resonators of the array. The SAW e-nose was tested upon exposure to vapors of ethyl acetate, dimethyl methylphosphonate, dichloromethane, dichloropentane, and water. The frequency responses showed, for each of the sensors, a different sensitivity to the selected chemical agents and a good agreement with the theoretical sensitivities derived from the linear-solvation-energy-relationship between vapors and polymers. Specifically, the implemented SAW e-nose allowed the discrimination between all the considered vapors in the tested concentrations ranges as highlighted by the results of principal component analysis. Finally, the obtained results indicated that laser deposition of polymers onto SAW resonators is possible without significant modifications of their functionality and with a good reliability and reproducibility.
Keywords: LIFT; MAPLE; SAW; Polymer coating; Sensors; e-nose;

Facility: Thin Films and Interfaces, LMX, National Institute for Lasers, Plasma, and Radiation Physics, Romania, Institute for Photonics and Nanotechnologies & Institute for Microelectronics and Microsystems, Italian National Research Council, Italy

Reference: M. Benetti et al., Sensors & Actuators: B. Chemical 283, 154-162 (2019)

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29 November 2018

Spin triplet ground-state in the copper hexamer compounds A2Cu3O(SO4)3 (A = Na, K)

The compounds A2Cu3O(SO4)3(A=Na,K) are characterized by copper hexamers which are weakly coupled along the b axis to realize one-dimensional antiferromagnetic chains below TN≈3K, whereas the interchain interactions along the a and c axes are negligible. We investigated the energy-level splittings of the copper hexamers by inelastic neutron scattering below and above TN. The eight lowest-lying hexamer states could be unambiguously assigned and parametrized in terms of a Heisenberg exchange Hamiltonian, providing direct experimental evidence for an S=1 triplet ground-state associated with the copper hexamers. Therefore, the compounds A2Cu3O(SO4)3 serve as cluster-based spin-1 antiferromagnets to support Haldane's conjecture that a gap appears in the excitation spectrum below TN, which was verified by inelastic neutron scattering.
Keywords: Magnetism; one-dimensional antiferromagnetic chains; inelastic neutron scattering;

Facility: SSC, LMX, LNS, SINQ, Oak Ridge National Laboratory

Reference: A. Furrer et al., Phys. Rev. B 98, 180410(R) (2018)

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26 October 2018

Design of magnetic spirals in layered perovskites: Extending the stability range far beyond room temperature

In insulating materials with ordered magnetic spiral phases, ferroelectricity can emerge owing to the breaking of in- version symmetry. This property is of both fundamental and practical interest, particularly with a view to exploiting it in low-power electronic devices. Advances toward technological applications have been hindered, however, by the rel- atively low ordering temperatures Tspiral of most magnetic spiral phases, which rarely exceed 100 K. We have recently established that the ordering temperature of a magnetic spiral can be increased up to 310 K by the introduction of chemical disorder. Here, we explore the design space opened up by this novel mechanism by combining it with a targeted lattice control of some magnetic interactions. In Cu-Fe layered perovskites, we obtain Tspiral values close to 400 K, comfortably far from room temperature and almost 100 K higher than using chemical disorder alone. Moreover, we reveal a linear relationship between the spiral’s wave vector and the onset temperature of the spiral phase. This linear law ends at a paramagnetic-collinear-spiral triple point, which defines the highest spiral ordering temperature that can be achieved in this class of materials. On the basis of these findings, we propose a general set of rules for designing magnetic spirals in layered perovskites using external pressure, chemical substitutions, and/or epitaxial strain, which should guide future efforts to engineer magnetic spiral phases with ordering temperatures suitable for technological applications.
Facility: SINQ

Reference: T. Shang et al, Science Advances 4, eaau6386 (2018)

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25 October 2018

Rolling dopant and strain in Y-doped BiFeO3 epitaxial thin films for photoelectrochemical water splitting

We report significant photoelectrochemical activity of Y-doped BiFeO3 (Y-BFO) epitaxial thin films deposited on Nb:SrTiO3 substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2Θ-ω scans of the symmetric (00l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L||) is preserved on larger nanoregions/nanodomains. For higher thickness values L|| is decreasing while domains tilt angles (αtilt) is increasing. The photocurrent value obtained for the thinnest sample was as high as Jph = 0.72 mA/cm2, at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film’s thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.
Reference: F. Haydous et al, Scientific Reports 8, 15826 (2018)

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15 October 2018

Observation of the out-of-plane magnetization in a mesoscopic ferromagnetic structure superjacent to a superconductor

The geometry of magnetic flux penetration in a high temperature superconductor at a buried interface was imaged using element-specific x-ray excited luminescence. We performed low tem- perature observation of the flux penetration in YBa2Cu3O7–δ (YBCO) at a buried interface by imaging of the perpendicular magnetization component in square Permalloy (Py) mesostructures patterned superjacent to a YBCO film. Element specific imaging below the critical temperature of YBCO reveals a cross-like geometry of the perpendicular magnetization component which is decorated by regions of alternating out-of-plane magnetization at the edges of the patterned Py structures. The cross structure can be attributed to the geometry of flux penetration originating from the superconductor and is reproduced using micromagnetic simulations. Our experimental method opens up possibilities for the investigation of flux penetration in superconductors at the nanoscale.
Facility: SLS

Reference: A.K. Suszka et al, Applied Physics Letters 113, 162601 (2018)

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25 September 2018

Evidence of a Coulomb-Interaction-Induced Lifshitz Transition and Robust Hybrid Weyl Semimetal in Td-MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td-MoTe2. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe2, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.
Facility: SLS

Reference: N. Xu et al, Physical Review Letters 121, 136401 (2018)

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18 September 2018

Influence of Plume Properties on Thin Film Composition in Pulsed Laser Deposition

Despite the apparent simplicity of pulsed laser deposition, consistent deposition of thin films with the desired thickness, composition, crystallinity, and quality still remains challenging. This article explores the influence of process parameters with respect to film thickness and composition, two key aspects for thin films which have a very strong effect on film properties, possible applications, and characterization. Using five perovskite materials, a systematic analysis of different process parameters, e.g., target material, deposition pressure, fluence, substrate temperature or target to substrate distance, is performed. The results are classified under target ablation, plasma expansion, and substrates effects, which provide vital guidance to reduce the degree of trial and error when producing thin films. Moreover, they enable the understanding of what should be considered, and avoided for the deposition of thin films.
Keywords: Pulsed Laser Ablation; Laserinduced plasma; Plasma spectroscopy; Thin films; Thin film properties;

Facility: Thin Films and Interfaces, LMX, ETHZ

Reference: A. Ojeda, M. Döbeli, and T. Lippert, Adv. Mat. Interfaces 5, 1701062 (2018)

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28 August 2018

Magnetoelectric inversion of domain patterns

The inversion of inhomogeneous physical states has great technological importance; for example, active noise reduction relies on the emission of an inverted sound wave that interferes destructively with the noise of the emitter1, and inverting the evolution of a spin system by using a magnetic-field pulse enables magnetic resonance tomography2. In contrast to these examples, inversion of a distribution of ferromagnetic or ferroelectric domains within a material is surprisingly difficult: field poling creates a single-domain state, and piece-by-piece inversion using a scanning tip is impractical. Here we report inversion of entire ferromagnetic and ferroelectric domain patterns in the magnetoelectric material Co3TeO6 and the multiferroic material Mn2GeO4, respectively. In these materials, an applied magnetic field reverses the magnetization or polarization, respectively, of each domain, but leaves the domain pattern intact. Landau theory indicates that this type of magnetoelectric inversion is universal across materials that exhibit complex ordering, with one order parameter holding the memory of the domain structure and another setting its overall sign. Domain-pattern inversion is only one example of a previously unnoticed effect in systems such as multiferroics, in which several order parameters are available for combination. Exploring these effects could therefore advance multiferroics towards new levels of functionality.
Reference: N. Leo et al, Nature 560, 466 (2018)

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