Publications by Thorsten Hesjedal


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


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.


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

JOURNAL OF APPLIED PHYSICS 115 (2014) ARTN 17D140

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


Engineering of Bi2Se3 nanowires by laser cutting

EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS 66 (2014) ARTN 10401

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


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

Applied Physics Letters 104 (2014) 253103

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

We present a comparative study of TiO2-based and Au catalysts for the physical vapor deposition of (Bi1− x Sb x )2Se3 topological insulator nanowires. The standard Au nanoparticle catalyst was compared to five TiO2 nanoparticle based catalysts (anatase, rutile, P-25, high surface area anatase, and TiO2 supported Au particles). The use of Au nanoparticles seriously harms the properties of nanowires, thereby limiting their application. In contrast, TiO2 based catalysts lead to the residue-free growth of nanowires with a higher degree of crystallinity. Homogeneous nanowire ensembles are achieved with the mixed phase P-25 catalyst, and a possible growth mechanism is proposed.


Three dimensional magnetic abacus memory

Scientific Reports Nature Publishing Group 4 (2014)

SL Zhang, J Zhang, A 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.


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 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.


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.


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.


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.


Magnetic susceptibility of n-type GaAs

Semiconductor Science and Technology IOP 27 (2012) 055018

T Hesjedal, U Kretzer, A Ney


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.


Magnetic reversal in a YFe₂ dominated DyFe₂/YFe₂ multilayer film

Applied Physics Letters 101 (2012) 072412

GBG Stenning, GJ Bowden, SA Gregory, J-ML Beaujour, PAJ de Groot, G van der Laan, LR Shelford, P Bencok, P Steadman, AN Dobrynin, T Hesjedal


Micromagnetic Investigation of the S-State Reconfigurable Logic Element

IEEE Transactions on Magnetics (2012)

L Hu, T Hesjedal


Transverse magnetic exchange springs in a DyFe2/YFe2 superlattice

Physical Review B: Condensed Matter and Materials Physics 86 (2012) 174420

GBG Stenning, GJ Bowden, SA Gregory, PAJ de Groot, G van der Laan, LR Shelford, P Bencok, P Steadman, AN Dobrynin, T Hesjedal

Using a history-dependent method, it is possible to prepare magnetic superlattices, consisting of alternating hard and soft layers, in transverse exchange spring states. The procedure, which involves both physical rotation and magnetization routines, is illustrated using a (110)-oriented [DyFe2(60 Å)/YFe2(240 Å)]15 multilayer film. In small applied fields, it is shown that the magnetic response of a transverse magnetic exchange spring is reversible. However, in fields of up to 14 T, the Dy moments are pulled up out of their local in-plane [00¯1] minimum into an out-of-plane [100] (or equivalent [010]) axis. The reversible transverse exchange spring state is then lost. Thereafter, the magnetic loop is characterized by an irreversible out-of-plane magnetic exchange spring state.


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


Electronic structure of Fe and Co magnetic adatoms on Bi 2Te 3 surfaces

Physical Review B - Condensed Matter and Materials Physics 86 (2012)

LR Shelford, T Hesjedal, L Collins-Mcintyre, SS Dhesi, F MacCherozzi, G Van Der Laan

Magnetic doping of topological insulators (TIs) is a prerequisite for their application as spin-based devices. Using x-ray magnetic circular dichroism (XMCD) we investigate the influence of an ultralow coverage (∼0.5% of a monolayer) of magnetic atoms on a TI substrate. For Fe and Co adatoms on Bi 2Te 3 at ∼1.5 K we find an orbital-to-spin magnetic moment ratio of ∼0.45. The magnetization curve of the Fe atoms recorded by XMCD is in quantitative agreement with a paramagnetic behavior with no indication of long-range magnetic order. The spectral shape of the XMCD indicates that the adatoms are weakly hybridized with the substrate and form narrowband states. The results show that the adatoms are not capable of breaking time-reversal symmetry. © 2012 American Physical Society.


Micromagnetic analysis of unusual, V-shaped domain transitions in MnAs nanowires

Journal of Magnetism and Magnetic Materials 323 (2011) 1840-1845

R Engel-Herbert, T Hesjedal

V-shaped domain transitions in αMnAs nanowires were investigated by micromagnetic simulations. These rather unusual domain patterns are commonly observed experimentally by surface-sensitive magnetic imaging techniques. It has been speculated that the accompanying inclined domain walls in MnAs are the result of either an exchange biasing effect between ferromagnetic αMnAs wires and antiferromagnetic βMnAs wires or possibly due to competing exchange mechanisms in MnAs. Here we present evidence that these domain features are in fact transitions between three-dimensional flux-closure domains of opposite chirality and can therefore rule out the involvement of an antiferromagnetic biasing effect or anisotropic exchange. The formation of the energetically unfavorable V-shaped domain transitions is discussed in the context of the magneto-structural phase transition of the sample. © 2011 Elsevier B.V.


Development of an electronic nose sensing platform for undergraduate education in nanotechnology

European Journal of Physics 32 (2011) 675-686

DV Russo, MJ Burek, RM Iutzi, JA Mracek, T Hesjedal

The teaching of the different aspects of a sensor system, with a focus on the involved nanotechnology, is a challenging, yet important task. We present the development of an electronic nose system that utilizes a nanoscale amperometric sensing mechanism for gas mixtures. The fabrication of the system makes use of a basic microfabrication facility, as well as an undergraduate chemistry laboratory for material synthesis and preparation. The sensing device consists of an array of cross-reactive sensors composed of metal-oxide semiconducting nanoparticles. Each sensor in the array produces a unique response in the presence of a target gas, allowing the sensor to determine the identity and concentration of multiple gases in a mixture. The educational aspects include microheater simulation and fabrication, design and fabrication of interdigitated electrodes, development of interfacing circuitry and software, development and calibration of a sensory array, sol-gel processing of nanoparticle films and their characterization, and details of the fundamental chemical sensing mechanism. © 2011 IOP Publishing Ltd.