Publications


Real-Space Observation of Skyrmionium in a Ferromagnet-Magnetic Topological Insulator Heterostructure.

Nano letters ACS 18 (2018) 1057-1063

S Zhang, F Kronast, G van der Laan, T Hesjedal

The combination of topological insulators, i.e., bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically-protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.


Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials

Physical review B: Condensed matter and materials physics American Physical Society (2018)

F Giustino, K Cao, P Radaelli


Magneto-orbital ordering in the divalent A-site quadruple perovskite manganites AMn7O12(A=Sr, Cd, and Pb)

Physical Review B American Physical Society 96 (2017) 054448-

R Johnson, DD Khalyavin, P Manuel, PG Radaelli, IS Glazkova, N Terada, AA Belik

<p>Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn<sub>7</sub>O<sub>12</sub>, CdMn<sub>7</sub>O<sub>12</sub>, and PbMn<sub>7</sub>O<sub>12</sub> in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn<sub>7</sub>O<sub>12</sub>. They all feature a higher temperature lock-in phase with <i>commensurate</i> magneto-orbital coupling, and a delocked, multi-<b>k</b> magnetic ground state where <i>incommensurate</i> magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn<sub>7</sub>O<sub>12</sub> represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn<sub>7</sub>O<sub>12</sub>. Furthermore, we show that the model is universal to the <i>A</i><sup<2+< sup=""> quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the <i>A</i>-site ionic radius.</sup<2+<></p>


Probing the topological surface state in Bi₂Se₃ thin films using temperature-dependent terahertz spectroscopy

ACS Photonics American Chemical Society 4 (2017) 2711-2718

VS Kamboj, A Singh, T Ferrus, HE Beere, L Duffy, T Hesjedal, C Barnes, DA Ritchie

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (&lt; 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state.


Codoping of Sb 2 Te 3 thin films with V and Cr

Physical Review Materials American Physical Society 1 (2017) 064409

LB Duffy, AI Figueroa, G van der Laan, T Hesjedal

Magnetically doped topological insulators (TIs) are key to realizing the quantum anomalous Hall (QAH) effect, with the prospect of enabling dissipationless electronic devices in the future. Doping of the well-established three-dimensional TIs of the (Bi,Sb)2(Se,Te)3 family with the transition metals Cr and V is now an established approach for observing the QAH state at very low temperatures. While the magnetic transition temperatures of these materials is on the order of 10’s of K, full quantization of the QAH state is achieved below ∼100 mK, governed by the size of the magnetic gap and thus the out-of-plane magnetic moment. In an attempt to raise the size of the magnetic moment and transition temperature, we carried out a structural and magnetic investigation of codoped (V,Cr):Sb2Te3 thin films. Starting from singly doped Cr:Sb2Te3 films, free of secondary phases and with a transition temperature of ∼72 K, we introduced increasing fractions of V and found a doubling of the transition temperature, while the magnetic moment decreases. In order to separate the properties and contributions of the two transition metals in the complex doping scenario independently, we employed spectroscopic x-ray techniques. Surprisingly, already small amounts of V lead to the formation of the secondary phase Cr2Te3. No V was detectable in the Sb2Te3 matrix. Instead, it acts as a surfactant and can be found in the near-surface layers at the end of the growth. Our study highlights the importance of x-ray-based studies for the doping of van der Waals systems, for which the optimization of magnetic moment or transition temperature alone is not necessarily a good strategy.


Electrical switching of magnetic polarity in a multiferroic BiFeO3 device at room temperature

Physical Review Applied American Physical Society 8 (2017) 014033

N Waterfield Price, RD Johnson, W Saenrang, A Bombardi, FP Chmiel, CB Eom, PG Radaelli

<p>We have directly imaged reversible electrical switching of the cycloidal rotation direction (magnetic polarity) in a (111)<sub>pc</sub>-BiFeO3 epitaxial-film device at room temperature by non-resonant x-ray magnetic scattering. Consistent with previous reports, fully relaxed (111)<sub>pc</sub>-BiFeO3 epitaxial films consisting of a single ferroelectric domain were found to comprise a sub-micron-scale mosaic of magneto-elastic domains, all sharing a common direction of the magnetic polarity, which was found to switch reversibly upon reversal of the ferroelectric polarization without any measurable change of the magneto-elastic domain population. A real-space polarimetry map of our device clearly distinguished between regions of the sample electrically addressed into the two magnetic states with a resolution of a few tens of micron. Contrary to the general belief that the magneto-electric coupling in BiFeO3 is weak, we find that electrical switching has a dramatic effect on the magnetic structure, with the magnetic moments rotating on average by 90 degrees at every cycle.</p>


Ultrahigh magnetic field spectroscopy reveals the band structure of the three-dimensional topological insulator Bi2Se3

PHYSICAL REVIEW B 96 (2017) ARTN 121111

A Miyata, Z Yang, A Surrente, O Drachenko, DK Maude, O Portugall, LB Duffy, T Hesjedal, P Plochocka, RJ Nicholas


Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures.

Nature communications 8 (2017) 1583-1583

W Saenrang, BA Davidson, F Maccherozzi, JP Podkaminer, J Irwin, RD Johnson, JW Freeland, J Íñiguez, JL Schad, K Reierson, JC Frederick, CAF Vaz, L Howald, TH Kim, S Ryu, MV Veenendaal, PG Radaelli, SS Dhesi, MS Rzchowski, CB Eom

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.


QDB: a new database of plasma chemistries and reactions

PLASMA SOURCES SCIENCE & TECHNOLOGY 26 (2017) ARTN 055014

J Tennyson, S Rahimi, C Hill, L Tse, A Vibhakar, D Akello-Egwel, DB Brown, A Dzarasova, JR Hamilton, D Jaksch, S Mohr, K Wren-Little, J Bruckmeier, A Agarwal, K Bartschat, A Bogaerts, J-P Booth, MJ Goeckner, K Hassouni, Y Itikawa, BJ Braams, E Krishnakumar, A Laricchiuta, NJ Mason, S Pandey, ZLJ Petrovic, Y-K Pu, A Ranjan, S Rauf, J Schulze, MM Turner, P Ventzek, JC Whitehead, J-S Yoon


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.


Synthesis of superconductor-topological insulator nanoribbon heterostructures

Nano World Scientific Publishing 12 (2017) 1750095

P Schönherr, F Zhang, V Srot, P van Aken, T Hesjedal

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.


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.


Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

PHYSICAL REVIEW B 95 (2017) ARTN 134435

J Brambleby, JL Manson, PA Goddard, MB Stone, RD Johnson, P Manuel, JA Villa, CM Brown, H Lu, S Chikara, V Zapf, SH Lapidus, R Scatena, P Macchi, Y-S Chen, L-C Wu, J Singleton


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

NPG Asia Materials Springer Nature 9 (2017) e402-

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

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.


Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft X-ray scattering

Physical Review B American Physical Society 96 (2017) 094401

SL Zhang, G van der Laan, T Hesjedal

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.


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, G van der Laan, AI Figueroa, P Bencok, P Steadman, T Hesjedal

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).


Perfect quintuple layer Bi₂Te₃ nanowires: Growth and thermoelectric properties

APL Materials American Institute of Physics 5 (2017) 086110-

P Schoenherr, D Kojda, V Srot, SF Fischer, PA van Aken, T Hesjedal

Bi2Te3 nanowires are promising candidates for thermoelectric applications. Vapor-liquid-solid growth of these nanowires is straightforward, but the traditional Au-catalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications.


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, AI Figueroa, L Gladczuk, N-J Steinke, K Kummer, G van der Laan, 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.


Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3

ADVANCED MATERIALS 29 (2017) UNSP 1602327

A Agbelele, D Sando, C Toulouse, C Paillard, RD Johnson, R Ruffer, AF Popkov, C Carretero, P Rovillain, J-M Le Breton, B Dkhil, M Cazayous, Y Gallais, M-A Measson, A Sacuto, P Manuel, AK Zvezdin, A Barthelemy, J Juraszek, M Bibes


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.

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