Publications by Thorsten Hesjedal

Vapour-liquid-solid growth of ternary Bi2Se2Te nanowires.

Nanoscale research letters 9 (2014) 127-

P Schönherr, LJ Collins-McIntyre, S Zhang, P Kusch, S Reich, T Giles, D Daisenberger, D Prabhakaran, T Hesjedal

: High-density growth of single-crystalline Bi2Se2Te nanowires was achieved via the vapour-liquid-solid process. The stoichiometry of samples grown at various substrate temperatures is precisely determined based on energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy on individual nanowires. We discuss the growth mechanism and present insights into the catalyst-precursor interaction.

Three dimensional magnetic abacus memory.

Scientific reports 4 (2014) 6109-

S Zhang, J Zhang, AA Baker, S Wang, G Yu, T Hesjedal

Stacking nonvolatile memory cells into a three-dimensional matrix represents a powerful solution for the future of magnetic memory. However, it is technologically challenging to access the data in the storage medium if large numbers of bits are stacked on top of each other. Here we introduce a new type of multilevel, nonvolatile magnetic memory concept, the magnetic abacus. Instead of storing information in individual magnetic layers, thereby having to read out each magnetic layer separately, the magnetic abacus adopts a new encoding scheme. It is inspired by the idea of second quantisation, dealing with the memory state of the entire stack simultaneously. Direct read operations are implemented by measuring the artificially engineered 'quantised' Hall voltage, each representing a count of the spin-up and spin-down layers in the stack. This new memory system further allows for both flexible scaling of the system and fast communication among cells. The magnetic abacus provides a promising approach for future nonvolatile 3D magnetic random access memory.

Catalyst-free growth of Bi2Te3 nanostructures by molecular beam epitaxy

Applied Physics Letters American Institute of Physics Inc. 105 (2014)

SE Harrison, P Schönherr, Y Huo, JS Harris, T Hesjedal

We present the catalyst-free growth of binary Bi2Te3 topological insulator nanostructures on c-plane sapphire substrates by molecular beam epitaxy. Dense arrays of single-crystalline nanostructures, growing along the [110] direction, are obtained for substrate temperatures ranging from ∼180°C to 260°C. The growth rate and shape of the nanostructures are highly temperature-dependent. The microscopic study of the nanostructures and their relationship to the underlying Bi2Te3 thin film gives an insight into the growth mechanism.

Study of Gd-doped Bi2Te3 thin films: Molecular beam epitaxy growth and magnetic properties

Journal of Applied Physics 115 (2014) 2

SE Harrison, LJ Collins-McIntyre, S Li, AA Baker, LR Shelford, Y Huo, A Pushp, SSP Parkin, JS Harris, E Arenholz, G van der Laan, T Hesjedal

Preparation of layered thin film samples for angle-resolved photoemission spectroscopy

Applied Physics Letters American Institute of Physics Inc. 105 (2014)

SE Harrison, B Zhou, Y Huo, A Pushp, AJ Kellock, SSP Parkin, JS Harris, Y Chen, T Hesjedal

Materials with layered van der Waals crystal structures are exciting research topics in condensed matter physics and materials science due to outstanding physical properties associated with their strong two dimensional nature. Prominent examples include bismuth tritelluride and triselenide topological insulators (TIs), which are characterized by a bulk bandgap and pairwise counter-propagating spin-polarized electronic surface states. Angle-resolved photoemission spectroscopy (ARPES) of ex-situ grown thin film samples has been limited by the lack of suitable surface preparation techniques. We demonstrate the shortcomings of previously successful conventional surface preparation techniques when applied to ternary TI systems which are susceptible to severe oxidation. We show that in-situ cleaving is a simple and effective technique for preparation of clean surfaces on ex-situ grown thin films for high quality ARPES measurements. The method presented here is universally applicable to other layered van der Waals systems as well.

Controlling spin-dependent tunneling by bandgap tuning in epitaxial rocksalt MgZnO films

Scientific Reports Nature Publishing Group (2014)

D Li, S-G Wang, RCC Ward, T Hesjedal, X-G Zhang, A Kohn, E Amsellem, G Yang, J Liu, J Jiang, H-X Wei, X Han

Widespread application of magnetic tunnel junctions (MTJs) for information storage has so far been limited by the complicated interplay between tunnel magnetoresistance (TMR) ratio and the product of resistance and junction area (RA). An intricate connection exists between TMR ratio, $RA$ value and the bandgap and crystal structure of the barrier, a connection that must be unravelled to optimise device performance and enable further applications to be developed. Here, we demonstrate a novel method to tailor the bandgap of an ultrathin, epitaxial Zn-doped MgO tunnel barrier with rocksalt structure. This structure is attractive due to its good Δ_1 spin filtering effect, and we show that MTJs based on tunable MgZnO barriers allow effective balancing of TMR ratio and $RA$ value. In this way spin-dependent transport properties can be controlled, a key challenge for the development of spintronic devices.

Magnetic ordering in Cr-doped Bi2Se3 thin films

EPL 107 (2014) ARTN 57009

LJ Collins-McIntyre, SE Harrison, P Schoenherr, N-J Steinke, CJ Kinane, TR Charlton, D Alba-Veneroa, A Pushp, AJ Kellock, SSP Parkin, JS Harris, S Langridge, G van der Laan, T Hesjedal

Magnetic Cr doping of Bi2Se3: Evidence for divalent Cr from x-ray spectroscopy

Physical Review B - Condensed Matter and Materials Physics American Physical Society 90 (2014)

AI Figueroa, G Van Der Laan, LJ Collins-Mcintyre, S-L Zhang, AA Baker, SE Harrison, P Schönherr, G Cibin, T Hesjedal

Ferromagnetically doped topological insulators with broken time-reversal symmetry are a prerequisite for observing the quantum anomalous Hall effect. Cr-doped (Bi,Sb)2(Se,Te)3 is the most successful materials system so far, as it combines ferromagnetic ordering with acceptable levels of additional bulk doping. Here, we report a study of the local electronic structure of Cr dopants in epitaxially grown Bi2Se3 thin films. Contrary to the established view that the Cr dopant is trivalent because it substitutionally replaces Bi3+, we find instead that Cr is divalent. This is evidenced by the energy positions of the Cr K and L2,3 absorption edges relative to reference samples. The extended x-ray absorption fine structure at the K edge shows that the Cr dopants substitute on octahedral sites with the surrounding Se ions contracted by Δd=-0.36 Å, in agreement with recent band structure calculations. Comparison of the Cr L2,3 x-ray magnetic circular dichroism at T=5 K with multiplet calculations gives a spin moment of 3.64 μB/Crbulk, which is close to the saturation moment for Cr2+ d4. The reduced Cr oxidation state in doped Bi2Se3 is ascribed to the formation of a covalent bond between Cr d(eg) and Se p orbitals, which is favored by the contraction of the Cr-Se distances.

Modelling ferromagnetic resonance in magnetic multilayers: Exchange coupling and demagnetisation-driven effects


AA Baker, CS Davies, AI Figueroa, LR Shelford, G van der Laan, T Hesjedal

Comparison of Au and TiO2 based catalysts for the synthesis of chalcogenide nanowires


P Schoenherr, D Prabhakaran, W Jones, N Dimitratos, M Bowker, T Hesjedal

Engineering of Bi2Se3 nanowires by laser cutting


P Schoenherr, AA Baker, P Kusch, S Reich, T Hesjedal

Transverse field muon-spin rotation signature of the skyrmion lattice phase in Cu2OSeO3

ArXiv (2014)

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, T Hesjedal, SJ Clark, PD Hatton, MC Hatnean, DS Keeble, G Balakrishnan

We present the results of transverse field (TF) muon-spin rotation (muSR) measurements on Cu2OSeO3, which has a skyrmion lattice phase. We are able to identify that phase via its characteristic TF muSR signal and distinguish it from the other magnetic phases of the material. Dipole field simulations support our interpretation and reveal TF muSR, which shows the skyrmion lattice to be static on the muon timescale, to be a promising tool for the investigation of skyrmion materials and the determination of their phase diagrams.

Study of the structural, electric and magnetic properties of Mn-doped Bi 2 Te 3 single crystals

New Journal of Physics 15 (2013) 10

MD Watson, LJ Collins-McIntyre, LR Shelford, AI Coldea, D Prabhakaran, SC Speller, T Mousavi, CRM Grovenor, Z Salman, SR Giblin, GVD Laan, T Hesjedal

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi 2 Te 3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi 2 Te 3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with T C varying between 9 and 13 K (bulk values) and a saturation moment that reaches4.4(5) μ B per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab -plane. The electrical transport data show an anomaly around T C that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n -doped with carrier concentrations of ∼ 0.5–3.0 × 10 20 cm −3 . X-ray magnetic circular dichroism (XMCD) at the Mn L 2,3 edge at 1.8 K reveals a large spin magnetic moment of4.3(3) μ B /Mn, and a small orbital magnetic moment of0.18(2) μ B /Mn. The results also indicate a ground state of mixed d 4 –d 5 –d 6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga 1− x Mn x As. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c -axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.

Magnetic properties of gadolinium substituted Bi2Te3 thin films

Applied Physics Letters 102 (2013) 242412

S Li, SA Harrison, Y Huo, A Pushp, HT Yuan, B Zhou, AJ Kellock, SSP Parkin, Y-L Chen, T Hesjedal, JS Harris

Thin film GdBiTe3 has been proposed as a candidate material in which to observe the quantum anomalous Hall effect. As a thermal non-equilibrium deposition method, molecular beam epitaxy (MBE) has the ability to incorporate large amounts of Gd into Bi2Te3 crystal structures. High-quality rhombohedral (GdxBi1−x)2Te3 films with substitutional Gd concentrations of x ≤ 0.4 were grown by MBE. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact up to the highest Gd concentration. Magnetoresistance measurements show weak antilocalization, indicating strong spin orbit interaction. Magnetometry reveals that the films are paramagnetic with a magnetic moment of 6.93 μB per Gd3+ ion.

Structure of epitaxial L10-FePt/MgO perpendicular magnetic tunnel junctions

Applied Physics Letters 102 (2013) 062403

A Kohn, N Tal, A Elkayam, A Kovacs, D Li, S Wang, S Ghannadzadeh, T Hesjedal, RCC Ward

Perpendicular magnetic tunnel junctions (p-MTJs) with MgO barriers are interesting for high-density information-storage devices. Chemically ordered L10-FePt is a potential electrode due to its large perpendicular magnetocrystalline anisotropy. To-date, a single theoretical study on L10-FePt/MgO p-MTJ based on an idealized structure reported significant dependence of spin-dependent tunneling on interface structure. [Y. Taniguchi et al., IEEE Trans. Magn. 44, 2585 (2008).] We report a structural study of epitaxial L10-FePt(001)[110]//MgO(001)[110]//L10-FePt(001)[110] p-MTJs, focusing on the interfaces using aberration-corrected scanning transmission electron microscopy. Interfaces are semi-coherent, with oxygen atomic-columns of MgO located opposite to iron atomic-columns in L10-FePt. Up to three lattice planes show atomic-column steps, the origin of which is attributed to antiphase boundaries in L10-FePt.

Nonvolatile full adder based on a single multivalued Hall junction

SPIN World Scientific Publishing 3 (2013) 1350008

SL Zhang, LJ Collins-McIntyre, JY Zhang, SG Wang, GH Yu, T Hesjedal

Multivalued logic devices are promising candidates for achieving high-density, low-power memory and logic functionalities. We present a ferromagnetic multilayer Hall junction device with four distinct resistance - and thus logic - states. The states can be encoded as a quaternary bit and decoded into two binary bits. We demonstrate a nonvolatile full adder that is based on a single Hall junction, the extraordinary Hall balance. The device can be easily integrated into complex logic circuits for logic-in-memory architectures.

Extraordinary hall balance

Scientific Reports Nature Publishing Group 3 (2013) 2087

SL Zhang, Y Liu, LJ Collins-McIntyre, T Hesjedal, JY Zhang, SG Wang, GH Yu

Magnetoresistance (MR) effects are at the heart of modern information technology. However, future progress of giant and tunnelling MR based storage and logic devices is limited by the usable MR ratios of currently about 200% at room-temperature. Colossal MR structures, on the other hand, achieve their high MR ratios of up to 106% only at low temperatures and high magnetic fields. We introduce the extraordinary Hall balance (EHB) and demonstrate room-temperature MR ratios in excess of 31,000%. The new device concept exploits the extraordinary Hall effect in two separated ferromagnetic layers with perpendicular anisotropy in which the Hall voltages can be configured to be carefully balanced or tipped out of balance. Reprogrammable logic and memory is realised using a single EHB element.

Magnetic susceptibility of n-type GaAs

Semiconductor Science and Technology IOP 27 (2012) 055018

T Hesjedal, U Kretzer, A Ney

Interface Characterization of Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions

Journal of Nanoscience and Nanotechnology 12 (2012) 1006-1023

SG Wang, RCC Ward, T Hesjedal, XG Zhang, C Wang, A Kohn, QL Ma, J Zhang, HF Liu, XF Han

Methane chemical vapor deposition on transition metal/GaAs samples - A possible route to Haeckelite carbon nanotubes?

Surface and Interface Analysis 44 (2012) 456-465

MJ Burek, T Hesjedal

We present a systematic study of atmospheric chemical vapor deposition growth of carbon nanotubes (CNTs) on patterned, transition metal/GaAs samples employing methane as the carbon feedstock. Controlled CNT growth was found to occur from the exposed metal-semiconductor interface, rather than from the metal or semiconductor surfaces themselves. A fast sample loading system allowed for a minimization of the exposure to high temperatures, thereby preventing excessive sample damage. The optimum growth temperature for CrNi/GaAs interfaces is 700 °C (at a methane flow rate of 700 sccm). Possible growth scenarios involving the Ni-As-Ga system and its interaction with C is discussed. Raman spectroscopy of the CNTs revealed the presence of pentagon-heptagon defects. Closer analysis of the spectra points towards a mixture of so-called Haeckelite CNTs. © 2011 John Wiley & Sons, Ltd.