Publications by Thorsten Hesjedal

Correction to Step-Flow Growth of Bi 2 Te 3 Nanobelts

Crystal Growth & Design 17 (2017) 1438-1438

P Schönherr, T Tilbury, H Wang, AA Haghighirad, V Srot, PA van Aken, T Hesjedal

Thermoelectric measurement of a single, TiO₂-catalyzed Bi₂Te₃ nanowire

Proceedings MDPI 1 (2017) 311-

HS Moosavi, D Kojda, M Kockert, P Schoenherr, T Hesjedal, SF Fischer, M Kroener, P Woias

We report on the functionality of our Thermoelectric Nanowire Characterization Platform (TNCP). As a proof of concept of our design, we present a set of experimental results obtained from the characterization of a single Bi2Te3 nanowire, allowing for the determination of the nanowire’s electrical conductivity and Seebeck coefficient.

Topological insulators: Engineered heterostructures

Nature Materials Nature Publishing Group 16 (2016) 3–4-

T Hesjedal, Y Chen

The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena. In particular, the coupling between topological insulators and antiferromagnets enables magnetic and electronic structural engineering.

Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

Journal of Magnetism and Magnetic Materials Elsevier 421 (2016) 428-439

AA Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.

Room-temperature helimagnetism in FeGe thin films

Scientific Reports Nature Publishing Group 7 (2017) 123

AI Figueroa, SJ Blundell, FL Pratt, Z Salman, T Prokscha, A Suter, J Waizner, D Grundler, M Garst, G van der Laan, C Pfleiderer, T Hesjedal, S Zhang, I Stasinopoulos, F Xiao, T Lancaster, A Bauer, AA Baker, F Rucker

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 278K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray spectroscopic 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 310K 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.

Direct experimental determination of the topological winding number of skyrmions in Cu2OSeO3

Nature Communications Springer Nature 8 (2017) 14619

SL Zhang, G van der Laan, T Hesjedal

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.

Temperature evolution of topological surface states in bismuth selenide thin films studied using terahertz spectroscopy

Proceedings of SPIE SPIE 10103D (2017)

VS Kamboj, A Singh, T Hesjedal, HE Beere, CHW Barnes, DA Ritchie

We have measured the terahertz (THz) conductance of a 23 quintuple layer thick film of bismuth selenide (Bi2Se3) and found signatures for topological surface states (TSSs) below 50 K. We provide evidence for a topological phase transition as a function of lattice temperature by optical means. In this work, we used THz time-domain spectroscopy (THz-TDS) to measure the optical conductance of Bi2Se3, revealing metallic behavior at temperatures below 50 K. We measure the THz conductance of Bi2Se3 as 10 e2/h at 4 K, indicative of a surface dominated response. Furthermore, the THz conductance spectra reveal characteristic features at ~1.9 THz attributed to the optical phonon mode, which is weakly visible at low temperatures but which becomes more prominent with increasing temperature. These results present a first look at the temperature-dependent behavior of TSSs in Bi2Se3 and the capability to selectively identify and address them using THz spectroscopy.

X-ray magnetic circular dichroism study of Dy-doped Bi2Te3 topological insulator thin films

Journal of Magnetism and Magnetic Materials Elsevier 422 (2016) 93-99

AA Baker, SE Harrison, AI Figueroa, K Kummer, G van der Laan, T Hesjedal

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 lm in the cleanest possible environment. The Dy M4;5 absorption spectra measured with circularly polarized x-rays are tied using multiplet calculations to obtain the e ective magnetic moment. Arnott-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 lm 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 eld, while the exchange interaction is minimized if the Dy and Co atoms are antiferromagnetically coupled, as in zero applied field.

Van der Waals epitaxy between the highly lattice mismatched Cu doped FeSe and Bi₂Te₃

NPG Asia Materials Springer Nature 9 (2017) e402-

VK Lazarov, A Ghasemi, D Kepaptsoglou, PL Galindo, T Hesjedal, Q Ramasse

We present a structural and density functional theory study of FexCu1-xSe within the three-dimensional topological insulator Bi2Te3. The FexCu1-xSe inclusions are single-crystalline and epitaxially oriented with respect to the Bi2Te3 thin film. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy show an atomically-sharp FexCu1-xSe/Bi2Te3 interface. The FexCu1-xSe /Bi2Te3 interface is determined by Se-Te bonds and no misfit dislocations are observed, despite the different lattice symmetries and large lattice mismatch of ∼ 19%. First-principle calculations show that the large strain at the FexCu1-xSe /Bi2Te3 interface can be accommodated via van der Waals-like bonding between Se and Te atoms.

Anisotropic magnetic switching along hard [110]-type axes in Er-doped DyFe 2 /YFe 2 thin films

Journal of Magnetism and Magnetic Materials Elsevier 439 (2017) 287–293-

GBG Stenning, GJ Bowden, T Hesjedal, G van der Laan, P Bencok, AI Figueroa, P Steadman

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 (60 Å/YFe2(240 Å)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 [110]-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).

Magnetic proximity-coupling to Cr-doped Sb₂Te₃ thin films

Physical Review B: Condensed Matter and Materials Physics American Physical Society 95 (2017) 224422-

LB Duffy, L Gladczuk, AI Figueroa, N-J Steinke, G van der Laan, K Kummer, T Hesjedal

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.

Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(1 1 1)

2D Materials 4 (2017) 031005

NBM Schröter, MD Watson, LB Duffy, M Hoesch, Y Chen, T Hesjedal, TK Kim

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 flms grown on Au(1 1 1) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the flms, 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.

Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

Physical Review B - Condensed Matter and Materials Physics American Physical Society (2016)

AA Baker, R Chalasani, A Kohn, SC Speller, D Gianolio, C Pfleiderer, G van der Laan, T Hesjedal, AI Figueroa, SL Zhang

We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500 A) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature Tc assumes a thickness-independent enhanced value of ≥43 K as compared with that of bulk MnSi, where Tc ≈ 29 K. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111]-direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi — except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.

Step-flow growth of Bi2Te3 nanobelts

Crystal Growth and Design American Chemical Society 16 (2016) 6961–6966-

P Schoenherr, T Tilbury, H Wang, AA Haghighirad, V Srot, T Hesjedal, P van Aken

Understanding the growth mechanism of nanostructures is key to tailoring their properties. Many compounds form nanowires following the vapor-liquid-solid (VLS) growth mechanism, and the growth of Bi2Te3 nanobelts was also explained following the VLS route. Here, we present another growth mechanism of Bi2Te3 nano- and sub-micron belts and ribbons. The samples were grown by physical vapor transport from Bi2Te3 precursor using TiO2 nanoparticles as catalyst, and analyzed by scanning electron microscopy and scanning transmission electron microscopy. The growth starts from a Te-rich cluster, and proceeds via a thin, tip-catalyzed primary layer growing in the [110] direction. The primary layer serves as a support for subsequent step-flow growth. The precursor predominantly absorbs on the substrate and reaches the belt by migration from the base to the tip. Terrace edges pose energy barriers that enhance the growth rate of secondary layers compared to the primary layer. Broadening of the sidewalls is commonly observed and leads to triangular voids that can even result in a branching of the growing belts. Step-flow growth of Bi2Te3 sub-micron belts is different from the spiral-like growth mode of Bi2Te3 thin films, and an important step towards the growth of layered topological insulator nanostructures.

Analytical STEM Study of Dy-doped Bi2Te3 Thin Films

European Microscopy Congress 2016: Proceedings, (Ed.). Wiley-VCH Verlag GmbH & Co. KGaA (2016) 1050-1051

V Srot, P Schönherr, B Bussmann, SE Harrison, PA van Aken, T Hesjedal

Spin pumping in magnetic trilayer structures with an MgO barrier

Scientific Reports Nature Publishing Group 6 (2016) 35582-

AA Baker, D Pingstone, AI Figueroa, VK Lazarov, G van der Laan, T Hesjedal

We present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4 nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element-specific technique of x-ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.

Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown CrxSb2−xTe3 thin films

Europhysics Letters European Physical Society (2016)

LB Duffy, N-J Steinke, A Singh, TR Charlton, CJ Kinane, LJ Collins-McIntyre, AJ Kellock, A Pushp, SSP Parkin, SN Holmes, CHW Barnes, G van der Laan, S Langridge, T Hesjedal

We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of x = 0:42 (in CrxSb2 xTe3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from ~2 μB to almost 3 μB per Cr ion. The transition temperature (Tc) depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase-pure films of x = 0:42, a Tc of 125 K was determined. Electric transport measurements find surface-dominated transport below ~10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.

Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3

Applied Physics Letters American Institute of Physics 109 (2016) 192406-

S Zhang, A Bauer, H Berger, C Pfleiderer, G van der Laan, T Hesjedal

Nanoscale chiral skyrmions in noncentrosymmetric helimagnets are promising binary state variables in highdensity, low-energy nonvolatile memory. Nevertheless, they normally appear in 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 fielddriven domain manipulation.

One-step SnO2 nanotree-growth

Chemistry - A European Journal Wiley 22 (2016) 13823–13825-

P Schoenherr, T Hesjedal

We present a comparison between Au, TiO2, and self-catalysed growth of SnO2 nanostructures using chemical vapour deposition. TiO2 enables growth of a nanonetwork of SnO2, whereas self-catalysed growth results in nanoclusters. Using Au catalyst, single-crystalline SnO2 nanowire trees can be grown in a one-step process. Two types of trees are identified that differ in size, presence of a catalytic tip, and degree of branching. The growth mechanism of these nanotrees is based on branch-splitting and self-seeding by the catalytic tip, facilitating at least three levels of branching, namely trunk, branch, and leaf.

Experimental and density functional study of Mn doped Bi₂Te₃ topological insulator

APL Materials American Institute of Physics 4 (2016) 126103-1

A Ghasemi, D Kepaptsoglou, AI Figueroa, GA Naydenov, PJ Hasnip, MIJ Probert, Q Ramasse, G van der Laan, T Hesjedal, V Lazarov

We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure show 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 der Waals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanningtransmission electron microscopy and EELS show 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.