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86 items found (0 items not approved), displaying 41 to 50.[First/Prev] 1, 2, 3, 4, 5, 6, 7, 8 [Next/Last]

Instruments / Technical Area	Publication Details	Published
I10-Beamline for Advanced Dichroism	Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft x-ray scattering S. L. Zhang , G. Van Der Laan , T. Hesjedal Physical Review B 96, 094401 DOI: 10.1103/PhysRevB.96.094401 Diamond Proposal Number(s): [12958, 11784] Open Access Show Abstract ▼ Abstract: Long-wavelength spin spiral structures are ubiquitous in a large variety of magnetic materials. The detailed magnetic structure can take many variations owing to their different physical origins. Therefore, the unambiguous structural determination is crucial for understanding these spin systems, though such a task is experimentally challenging. Here, we show that ordered spin spiral structures can be fully determined in a single measurement by dichroic resonant elastic x-ray scattering using circularly polarized light. It is found that at certain geometrical conditions, the circular dichroism of the diffraction vanishes completely, revealing a one-to-one correspondence with the spin structure. We demonstrate both theoretically and experimentally this experimental principle, which allows for unambiguous structure determination immediately from the measured signal, whereby no modeling-based data refinement is needed. This largely expands the capabilities of conventional magnetic characterization techniques.	Sep 2017
	Synthesis of Superconductor-Topological Insulator Hybrid Nanoribbon Structures Piet Schönherr , Fengyu Zhang , Vesna Srot , Peter Van Aken , Thorsten Hesjedal Nano 12 DOI: 10.1142/S1793292017500953 Open Access Show Abstract ▼ Abstract: Superconductors in proximity to topological insulators (TIs) have the potential to unlock exotic quantum phenomena, such as Majorana fermions. Quasi-one-dimensional structures are particularly suited to host these quantum states. Despite the growth of TI nanostructures being relatively straightforward, the in situ synthesis of superconductor-TI structures has been challenging. Here, we present a systematic study of the growth of the s-wave superconductor Sn on the TI Bi2Te3 by physical vapor transport. If Sn does not enter the Bi2Te3 lattice as a dopant, two types of structures are formed: Sn nanoparticles, that cover Bi2Te3 plates and belts in a cloud-like shape, and thin Sn layers on Bi2Te3 plates, that appear in puddle-like recessions. These heterostructures have potential applications as novel quantum devices.	Aug 2017
I05-ARPES	Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(1 1 1) Niels B. M. Schröter , Matthew Watson , Liam B. Duffy , Moritz Hoesch , Yulin Chen , Thorsten Hesjedal , Timur Kim 2d Materials 4 DOI: 10.1088/2053-1583/aa7963 Diamond Proposal Number(s): [12799] Open Access Show Abstract ▼ Abstract: After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we report the discovery of multiple Dirac-like electronic bands in ultrathin Ge films grown on Au(1 1 1) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the films, we are able to observe the evolution of their electronic structure when passing through the monolayer limit. Our discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a promising platform for exploring exotic topological phenomena and enabling potential applications.	Jul 2017
B18-Core EXAFS	Magnetic proximity coupling to Cr-doped Sb2Te3 thin films L. B. Duffy , A. I. Figueroa , L. Gladczuk , N.-j. Steinke , K Kummer , G. Van Der Laan , Thorsten Hesjedal Physical Review B 95, 224422 DOI: 10.1103/PhysRevB.95.224422 Diamond Proposal Number(s): [15702] Open Access Show Abstract ▼ 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.	Jun 2017
I10-Beamline for Advanced Dichroism	Anisotropic magnetic switching along hard [110]-type axes in Er-doped DyFe 2 /YFe 2 thin films G. B. G. Stenning , G. J. Bowden , G. Van Der Laan , A. I. Figueroa , P. Bencok , P. Steadman , T. Hesjedal Journal Of Magnetism And Magnetic Materials DOI: 10.1016/j.jmmm.2017.05.007 Diamond Proposal Number(s): [8213, 13758] Show Abstract ▼ Abstract: Epitaxial-grown DyFe2/YFe2 multilayer thin films form an ideal model system for the study of magnetic exchange springs. Here the DyFe2 (YFe2) layers are magnetically hard (soft). In the presence of a magnetic field, exchange springs form in the YFe2 layers. Recently, it has been demonstrated that placing small amounts of Er into the centre of the YFe2 springs generates substantial changes in magnetic behavior. In particular, (i) the number of exchange-spring states is increased dramatically, (ii) the resulting domain-wall states cannot simply be described as either Néel or Bloch walls, (iii) the Er and Dy magnetic loops are strikingly different, and (iv) it is possible to engineer Er-induced magnetic exchange-spring collapse. Here, results are presented for Er-doped (110)-oriented DyFe2(60Å)/YFe2(240Å)15(60Å)/YFe2(240Å)15 multilayer films, at 100 K in fields of up to 12 T. In particular, we contrast magnetic loops for fields applied along seemingly equivalent hard-magnetic [110]-type axes. MBE-grown cubic Laves thin films offer the unique feature of allowing to apply the magnetic field along (i) a hard out-of-plane [110]-axis (the growth axis) and (ii ) a similar hard in-plane [View the MathML source1¯10]-axis. Differences are found and attributed to the competition between the crystal-field interaction at the Er site and the long-range dipole-dipole interaction. In particular, the out-of-plane [110] Er results show the existence of a new magnetic exchange spring state, which would be very difficult to identify without the aid of element-specific technique of x-ray magnetic circular dichroism (XMCD).	May 2017
I10-Beamline for Advanced Dichroism	Room-temperature helimagnetism in FeGe thin films S. L. Zhang , I. Stasinopoulos , T. Lancaster , F. Xiao , Andreas Bauer , F. Rucker , A. A. Baker , A. I. Figueroa , Z. Salman , F. L. Pratt , S. J. Blundell , T. Prokscha , A. Suter , J. Waizner , M. Garst , D. Grundler , G. Van Der Laan , C. Pfleiderer , T. Hesjedal Scientific Reports 7 DOI: 10.1038/s41598-017-00201-z Diamond Proposal Number(s): [8703, 9595] Open Access Show Abstract ▼ Abstract: Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain αintr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.	Mar 2017
I10-Beamline for Advanced Dichroism	Direct experimental determination of the topological winding number of skyrmions in Cu2OSeO3 S. L. Zhang , G. Van Der Laan , T. Hesjedal Nature Communications 8 DOI: 10.1038/ncomms14619 Diamond Proposal Number(s): [11784, 12958] Open Access Show Abstract ▼ Abstract: The mathematical concept of topology has brought about significant advantages that allow for a fundamental understanding of the underlying physics of a system. In magnetism, the topology of spin order manifests itself in the topological winding number which plays a pivotal role for the determination of the emergent properties of a system. However, the direct experimental determination of the topological winding number of a magnetically ordered system remains elusive. Here, we present a direct relationship between the topological winding number of the spin texture and the polarized resonant X-ray scattering process. This relationship provides a one-to-one correspondence between the measured scattering signal and the winding number. We demonstrate that the exact topological quantities of the skyrmion material Cu2OSeO3 can be directly experimentally determined this way. This technique has the potential to be applicable to a wide range of materials, allowing for a direct determination of their topological properties.	Feb 2017
B18-Core EXAFS	Experimental and density functional study of Mn doped Bi2Te3 topological insulator A. Ghasemi , D. Kepaptsoglou , A. I. Figueroa , G. A. Naydenov , P. J. Hasnip , M. I. J. Probert , Q. Ramasse , G. Van Der Laan , Thorsten Hesjedal , V. K. Lazarov Apl Materials 4 DOI: 10.1063/1.4971354 Diamond Proposal Number(s): [9178] Open Access Show Abstract ▼ Abstract: We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure shows that Mn has valency of nominally 2+. Extended x-ray absorption fine structure spectroscopy in combination with electron energy loss spectroscopy (EELS) shows that Mn is a substitutional dopant of Bi and Te and also resides in the van derWaals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanning transmission electron microscopy and EELS shows that Mn substitution of Te occurs in film regions with increased Mn concentration. First-principles calculations show that the Mn dopants favor octahedral sites and are ferromagnetically coupled.	Dec 2016
I10-Beamline for Advanced Dichroism	Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3 S. L. Zhang , A. Bauer , H. Berger , C. Pfleiderer , G. Van Der Laan , T. Hesjedal Applied Physics Letters 109, 192406 DOI: 10.1063/1.4967499 Diamond Proposal Number(s): [12958, 11784] Open Access Show Abstract ▼ Abstract: Nanoscale chiral skyrmions in noncentrosymmetric helimagnets are promising binary state variables in high-density, low-energy nonvolatile memory. Skyrmions are ubiquitous as an ordered, single-domain lattice phase, which makes it difficult to write information unless they are spatially broken up into smaller units, each representing a bit. Thus, the formation and manipulation of skyrmion lattice domains is a prerequisite for memory applications. Here, using an imaging technique based on resonant magnetic x-ray diffraction, we demonstrate the mapping and manipulation of skyrmion lattice domains in Cu2OSeO3. The material is particularly interesting for applications owing to its insulating nature, allowing for electric field-driven domain manipulation.	Nov 2016
I10-Beamline for Advanced Dichroism	X-ray magnetic circular dichroism study of Dy-doped Bi2Te3 topological insulator thin films A. I. Figueroa , A. A. Baker , S. E. Harrison , K. Kummer , G. Van Der Laan , T. Hesjedal Journal Of Magnetism And Magnetic Materials DOI: 10.1016/j.jmmm.2016.08.063 Diamond Proposal Number(s): [10207] Open Access Show Abstract ▼ Abstract: Magnetic doping of topological insulators (TIs) is crucial for unlocking novel quantum phenomena, paving the way for spintronics applications. Recently, we have shown that doping with rare earth ions introduces large magnetic moments and allows for high doping concentrations without the loss of crystal quality, however no long range magnetic order was observed. In Dy-doped Bi2Te3 we found a band gap opening above a critical doping concentration, despite the paramagnetic bulk behavior. Here, we present a surface-sensitive x-ray magnetic circular dichroism (XMCD) study of an in-situ cleaved film in the cleanest possible environment. The Dy M4,5 absorption spectra measured with circularly polarized x-rays are fitted using multiplet calculations to obtain the effective magnetic moment. Arrott-Noakes plots, measured by the Dy M5 XMCD as a function of field at low temperatures, give a negative transition temperature. The evaporation of a ferromagnetic Co thin film did not introduce ferromagnetic ordering of the Dy dopants either; instead a lowering of the transition temperature was observed, pointing towards an antiferromagnetic ordering scenario. This result shows that there is a competition between the magnetic exchange interaction and the Zeeman interaction. The latter favors the Co and Dy magnetic moments to be both aligned along the direction of the applied magnetic field, while the exchange interaction is minimized if the Dy and Co atoms are antiferromagnetically coupled, as in zero applied field.	Nov 2016

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