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230 items found (0 items not approved), displaying 1 to 10.[First/Prev] 1, 2, 3, 4, 5, 6, 7, 8 [Next/Last]

Instruments / Technical Area	Publication Details	Published
Theoretical Physics	Circular dichroism of second harmonic generation response S. W. Lovesey , G. Van Der Laan Physical Review B 100, 245112 DOI: 10.1103/PhysRevB.100.245112 Open Access Show Abstract ▼ Abstract: Polarization-dependent photon spectroscopy (dichroism) of the second-harmonic generation response is shown to reveal chiral and magnetic properties of a sample. Two dichroic signals are allowed with electric-dipole and electric-quadrupole scattering events, and both require circular polarization in the primary beam. Explicit expressions for the signals are derived using theoretical techniques from atomic physics.	Dec 2019
I10-Beamline for Advanced Dichroism	Helical magnetic ordering in thin FeGe membranes D. M. Burn , S. L. Zhang , S. Wang , H. F. Du , Gerrit Van Der Laan , T. Hesjedal Physical Review B 100 DOI: 10.1103/PhysRevB.100.184403 Diamond Proposal Number(s): [20437] Show Abstract ▼ Abstract: A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector, q , is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase, q only varies with normalized field but not with temperature—in sharp contrast to the skyrmion phase, for which q is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.	Nov 2019
I10-Beamline for Advanced Dichroism	Effect of Fe-metabolizing bacteria and humic substances on magnetite nanoparticle reactivity towards arsenic and chromium Anneli Sundman , Anna-lena Vitzhum , Konstantin Adaktylos-surber , Adriana I. Figueroa , Gerrit Van Der Laan , Birgit Daus , Andreas Kappler , James M. Byrne Journal Of Hazardous Materials DOI: 10.1016/j.jhazmat.2019.121450 Diamond Proposal Number(s): [9565] Show Abstract ▼ Abstract: Magnetite is a magnetic, Fe(II)-Fe(III)-mineral formed through abiogenic and biogenic pathways. It constitutes an attractive material for remediation due to its reactivity, large surface-area-to-volume ratio when present as nanoparticles, and magnetic recoverability. Magnetite can be repeatedly microbially oxidized or reduced, but it is unclear how this influences the reactivity of magnetite towards toxic metal or metalloid contaminants. In this study, magnetite (both abiogenic and biogenic) was exposed to microbial Fe(II) oxidation and Fe(III) reduction, before reacted with hexavalent chromium (Cr(VI)) or pentavalent arsenic (As(V)). Results showed microbial reduction of both magnetite types improved the removal rate of Cr(VI) from solution, though surprisingly microbial Fe(II)-oxidation also showed enhanced reactivity towards Cr(VI) compared to un-treated magnetite. Synchrotron based analysis confirmed the formation of Cr(III) at the surface of the magnetite. Reactivity with As was less dramatic and showed un-treated material was able to remove As(V) from solution faster than microbially Fe(III)-reduced and Fe(II)-oxidized magnetite. The presence of humic substances was also shown to lead to a decreased reactivity of biogenic and abiogenic magnetite towards As(V) and Cr(VI). Our results imply that Fe-metabolizing bacteria influence the immobilization of contaminants and should be considered when evaluating remediation schemes, especially where Fe-metabolizing bacteria are active.	Oct 2019
	Magnetic profile of proximity-coupled (Dy,Bi)2Te3 / (Cr,Sb)2Te3 topological insulator heterostructures L. B. Duffy , N.-j. Steinke , D. M. Burn , A. Frisk , L. Lari , B. Kuerbanjiang , V. K. Lazarov , G. Van Der Laan , S. Langridge , T. Hesjedal Physical Review B 100, 054402 DOI: 10.1103/PhysRevB.100.054402 Show Abstract ▼ 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.	Aug 2019
Theoretical Physics	Expanding the Lorentz concept in magnetism Graham J. Bowden , Gerrit Van Der Laan , Thorsten Hesjedal , Robert J. Hicken New Journal Of Physics DOI: 10.1088/1367-2630/ab2f63 Open Access Show Abstract ▼ Abstract: In 1878, the Dutch physicist Hendrik Antoon Lorentz first addressed the calculation of the local electric field at an atomic site in a ferroelectric material, generated by all the other electric dipoles within the sample. This calculation, which applies equally well to ferromagnets, is taught in Universities around the World. Here we demonstrate that the Lorentz concept can be used to speed up calculations of the local dipolar field in square, circular, and elliptical shaped monolayers and thin films, not only at the center of the film, but across the sample. Calculations show that long elliptical and rectangular films should exhibit the narrowest ferromagnetic resonance linewidth. In addition, discrete dipole calculations show that the Lorentz cavity field does not hold in tetragonal films. Depending on the ratio (b/a), the local field can be either less/greater than : an observation that has implications for ferromagnetic resonance. 3D simple cubic (SC) systems are also examined. For example, while most texts discuss the Lorentz cavity field in terms of a Lorentz sphere, the Lorentz cavity field still holds when a Lorentz sphere is replaced by a the Lorentz cube, but only in cubic SC, FCC and BCC systems. Finally, while the primary emphasis is on the discrete dipole-dipole interaction, contact is made with the continuum model. For example, in the continuous SC dipole model, just one monolayer is required to generate the Lorentz cavity field. This is in marked contrast to the discrete dipole model, where a minimum of five adjacent monolayers is required.	Jul 2019
	Magnetic transition in monolayer VSe 2 via interface hybridization Wen Zhang , Lei Zhang , Ping Kwan Johnny Wong , Jiaren Yuan , Giovanni Vinai , Piero Torelli , Gerrit Van Der Laan , Yuan Ping Feng , Andrew T. S. Wee Acs Nano DOI: 10.1021/acsnano.9b02996 Show Abstract ▼ Abstract: Magnetism in monolayer (ML) VSe2 has attracted broad interest in spintronics, while existing reports have not reached consensus. Using element-specific X-ray magnetic circular dichroism, a magnetic transition in ML VSe2 has been demonstrated at the contamination-free interface between Co and VSe2. Through interfacial hybridization with a Co atomic overlayer, a magnetic moment of about 0.4 μB per V atom in ML VSe2 is revealed, approaching values predicted by previous theoretical calculations. Promotion of the ferromagnetism in ML VSe2 is accompanied by its antiferromagnetic coupling to Co and a reduction in the spin moment of Co. In comparison to the absence of this interface-induced ferromagnetism at the Fe/ML MoSe2 interface, these findings at the Co/ML VSe2 interface provide clear proof that the ML VSe2, initially with magnetic disorder, is on the verge of magnetic transition.	Jul 2019
	Cr2Te3 thin films for integration in magnetic topological insulator heterostructures D. M. Burn , L. B. Duffy , R. Fujita , S. L. Zhang , A. I. Figueroa , J. Herrero-martin , G. Van Der Laan , T. Hesjedal Scientific Reports 9, 10793 DOI: 10.1038/s41598-019-47265-7 Open Access Show Abstract ▼ Abstract: Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.	Jul 2019
	Curie temperature enhancement and cation ordering in titanomagnetites: Evidence from magnetic properties, XMCD, and Mössbauer spectroscopy J. A. Bowles , S.‐c. L. L. Lappe , M. J. Jackson , Elke Arenholz , G. Van Der Laan Geochemistry, Geophysics, Geosystems DOI: 10.1029/2019GC008217 Show Abstract ▼ Abstract: Previous work has documented time‐ and temperature‐dependent variations in the Curie temperature (Tc) of natural titanomagnetites, independent of any changes in sample composition. To better understand the atomic‐scale processes responsible for these variations, we have generated a set of synthetic titanomagnetites with a range of Ti, Mg, and Al substitution; a subset of samples was additionally oxidized at low temperature (150°C). Samples were annealed at temperatures between 325‐400 °C for up to 1000 h and characterized in terms of magnetic properties; Fe valence and site occupancy were constrained by X‐ray magnetic circular dichroism (XMCD) and Mössbauer spectroscopy. Annealing results in large (up to ~100 °C) changes in Tc, but Mössbauer, XMCD, and saturation magnetization data all demonstrate that intersite reordering of Fe2+/Fe3+ does not play a role in the observed Tc changes. Rather, the data are consistent with vacancy‐enhanced nanoscale chemical clustering within the octahedral sublattice. This clustering may be a precursor to chemical unmixing at temperatures below the titanomagnetite binary solvus. Additionally, the data strongly support a model where cation vacancies are predominantly situated on octahedral sites; Mg‐substitution is largely accommodated on octahedral sites; and Al‐substitution is split between the two sites.	Apr 2019
Theoretical Physics	Magnetic multipoles in a ruthenate Ca3Ru2O7 S. W. Lovesey , D. D. Khalyavin , G. Van Der Laan Physical Review B 99, 134444 DOI: 10.1103/PhysRevB.99.134444 Show Abstract ▼ Abstract: Compulsory Dirac multipoles in the bilayer perovskite Ca3Ru2O7 are absent in published analyses of experimental data. In a first step at correcting knowledge of the magnetic structure, we have analyzed existing Bragg diffraction patterns gathered on samples held well below the Néel temperature at which A-type antiferromagnetic order of axial dipoles spontaneously develops. Patterns were gathered with neutrons, and linearly polarized x rays tuned in energy to a ruthenium atomic resonance. Neutron diffraction data contain solid evidence of Dirac dipoles (anapoles or toroidal moments). No such conclusion is reached with existing x-ray diffraction data, which instead is ambiguous on the question. To address this shortcoming by future experiments, we calculated additional diffraction patterns. Chiral order of Dirac multipoles is allowed by magnetic space-group PCna21, and it can be exposed in Bragg diffraction using circularly polarized x rays. Likewise, a similar experiment can expose a chiral order of axial dipoles. A magnetic field applied parallel to the b axis creates a ferrimagnetic structure in which bulk magnetization arises from field-induced nonequivalent Ru sites (magnetic space-group Pmc21).	Apr 2019
I10-Beamline for Advanced Dichroism	Experimental observation of dual magnetic states in topological insulators Wenqing Liu , Yongbing Xu , Liang He , Gerrit Van Der Laan , Rong Zhang , Kang Wang Science Advances 5 DOI: 10.1126/sciadv.aav2088 Diamond Proposal Number(s): [20748] Open Access Show Abstract ▼ Abstract: The recently discovered topological phase offers new possibilities for spintronics and condensed matter. Even insulating material exhibits conductivity at the edges of certain systems, giving rise to an anomalous quantum Hall effect and other coherent spin transport phenomena, in which heat dissipation is minimized, with potential uses for next-generation energy-efficient electronics. While the metallic surface states of topological insulators (TIs) have been extensively studied, direct comparison of the surface and bulk magnetic properties of TIs has been little explored. We report unambiguous evidence for distinctly enhanced surface magnetism in a prototype magnetic TI, Cr-doped Bi2Se3. Using synchrotron-based x-ray techniques, we demonstrate a “three-step transition” model, with a temperature window of ~15 K, where the TI surface is magnetically ordered while the bulk is not. Understanding the dual magnetization process has strong implications for defining a physical modelof magnetic TIs and lays the foundation for applications to information technology.	Feb 2019

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