Publications


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


Temperature-induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi0.9Sm0.1FeO3 driven by f-d induced magnetic anisotropy

Physical Review B - Condensed Matter and Materials Physics American Physical Society 95 (2017) 054420-

R Johnson, PA McClarty, DD Khalyavin, P Manuel, P Svedlindh, CS Knee

In multiferroic BiFeO3 a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that substituting samarium for 10% of the bismuth ions increases the periodicity of the room-temperature cycloid, and upon cooling to below ∼15 K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from f-d exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order - a result of general significance regarding the stability of noncollinear magnetic structures in the presence of multiple magnetic sublattices.


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.


Room-temperature helimagnetism in FeGe thin films

Scientific Reports Nature Publishing Group 7 (2017) 123

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

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.


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, HE Beere, T Hesjedal, 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

AI Figueroa, AA Baker, SE Harrison, 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.


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)

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

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.


Coupled commensurate charge density wave and lattice distortion in Na2Ti2Pn2O (Pn = As,Sb) determined by x-ray diffraction and angle-resolved photoemission spectroscopy

Physical Review B American Physical Society 94 (2016)

NR Davies, RD Johnson, AJ Princep, LA Gannon, JZ Ma, T Qian, P Richard, H Li, M Shi, H Nowell, PJ Baker, YG Shi, H Ding, J Luo, YF Guo, A Boothroyd

We report single-crystal x-ray-diffraction measurements on Na2Ti2Pn2O (Pn = As,Sb) which reveal a charge superstructure that appears below the density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements we establish that the associated distortion mode can be described by two propagation vectors q1 = (1/2,0,l) and q2 = (0,1/2,l) with l = 0 (Sb) or l = 1/2 (As) and primarily involves in-plane displacements of the Ti atoms perpendicular to the Ti-O bonds.We also present angle-resolved photoemission spectroscopy measurements, which show band folding and backbending consistent with a density wave with the samewave-vectors q1 and q2 associated with Fermi-surface nesting, and muon-spin relaxation data, which show no indication of spin density wave order. The results provide direct evidence for phonon-assisted charge density wave order in Na2Ti2Pn2O and fully characterize a proximate ordered phase that could compete with superconductivity in doped BaTi2Sb2O.


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, P van Aken, T Hesjedal

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, AI Figueroa, D Pingstone, 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.


Coherent magnetoelastic domains in multiferroic films

Physical Review Letters American Physical Society 117 (2016) 177601-

N Waterfield Price, RD Johnson, W Saenrang, F Maccherozzi, SS Dhesi, A Bombardi, FP Chmiel, C-B Eom, P Radaelli

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of non-resonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO3 films support sub-micron antiferromagnetic domains, which are magneto-elastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO3 film devices via epitaxial strain.


Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO2

Physical Review B American Physical Society (2016)

J Beilsten-Edmands, SJ Magorrian, FR Foronda, D Prabhakaran, P Radaelli, RD Johnson

We investigate polarization memory effects in single-crystal CuFeO2, which has a magnetically induced ferroelectric phase at low temperatures and applied B fields between 7.5 and 13 T. Following electrical poling of the ferroelectric phase, we find that the nonpolar collinear antiferromagnetic ground state at B=0 T retains a strong memory of the polarization magnitude and direction, such that upon reentering the ferroelectric phase a net polarization of comparable magnitude to the initial polarization is recovered in the absence of external bias. This memory effect is very robust: in pulsed-magnetic-field measurements, several pulses into the ferroelectric phase with reverse bias are required to switch the polarization direction, with significant switching only seen after the system is driven out of the ferroelectric phase and ground state either magnetically (by application of B>13 T) or thermally. The memory effect is also largely insensitive to the magnetoelastic domain composition, since no change in the memory effect is observed for a sample driven into a single-domain state by application of stress in the [110] direction. On the basis of Monte Carlo simulations of the ground-state spin configurations, we propose that the memory effect is due to the existence of helical domain walls within the nonpolar collinear antiferromagnetic ground state, which would retain the helicity of the polar phase for certain magnetothermal histories.


Magnetostriction-driven ground-state stabilization in 2H perovskites

Physical Review B American Physical Society 94 (2016) 134404-

DG Porter, MS Senn, DD Khalyavin, A Cortese, N Waterfield-Price, P Radaelli, P Manuel, H-C zur-Loye, C Mazzoli, A Bombardi

The magnetic ground state of Sr3ARuO6, with A=(Li,Na), is studied using neutron diffraction, resonant x-ray scattering, and laboratory characterization measurements of high-quality crystals. Combining these results allows us to observe the onset of long-range magnetic order and distinguish the symmetrically allowed magnetic models, identifying in-plane antiferromagnetic moments and a small ferromagnetic component along the c axis. While the existence of magnetic domains masks the particular in-plane direction of the moments, it has been possible to elucidate the ground state using symmetry considerations. We find that due to the lack of local anisotropy, antisymmetric exchange interactions control the magnetic order, first through structural distortions that couple to in-plane antiferromagnetic moments and second through a high-order magnetoelastic coupling that lifts the degeneracy of the in-plane moments. The symmetry considerations used to rationalize the magnetic ground state are very general and will apply to many systems in this family, such as Ca3ARuO6, with A=(Li,Na), and Ca3LiOsO6 whose magnetic ground states are still not completely understood.


Single crystal growth from separated educts and its application to lithium transition-metal oxides

Scientific Reports Nature Publishing Group 6 (2016) 35362

F Freund, SC Williams, R Johnson, R Coldea, P Gegenwart, A Jesche

Thorough mixing of the starting materials is the first step of a crystal growth procedure. This holds true for almost any standard technique, whereas the intentional separation of educts is considered to be restricted to a very limited number of cases. Here we show that single crystals of α-Li2IrO3 can be grown from separated educts in an open crucible in air. Elemental lithium and iridium are oxidized and transported over a distance of typically one centimeter. In contrast to classical vapor transport, the process is essentially isothermal and a temperature gradient of minor importance. Single crystals grow from an exposed condensation point placed in between the educts. The method has also been applied to the growth of Li2RuO3, Li2PtO3 and β-Li2IrO3. A successful use of this simple and low cost technique for various other materials is anticipated.


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.


Ab initio cycloidal and chiral magnetoelectric responses in Cr2 O3

Physical Review B - Condensed Matter and Materials Physics American Physical Society 94 (2016) 100405-

N Tillack, JR Yates, P Radaelli

We present a thorough density functional theory study of the magneto-electric (ME) effect in Cr2O3. The spin-lattice ME tensor α was determined in the low-field and spin flop (SF) phases, using the method of dynamical magnetic charges, and found to be the sum of three distinct components. Two of them, a large relativistic "cycloidal" term and a small longitudinal term, are independent on the spin orientation. The third, only active in the SF phases is also of relativistic origin and arises from magnetic-field-induced chirality, leading to a non-toroidal ME response.


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.


Detailed crystallographic analysis of the ice VI to ice XV hydrogen ordering phase transition

Journal of Chemical Physics American Institute of Physics 145 (2016) 204501-

CG Salzmann, B Slater, P Radaelli, JL Finney, JJ Shephard, M Rosillo-Lopez, J Hindley

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new computational approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data and we argue that only structural models that are capable of describing full hydrogen order should be used.


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.

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