Publications by Thorsten Hesjedal


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

Scientific reports 4 (2014) 7277-

DL Li, QL Ma, SG Wang, RC Ward, T Hesjedal, XG Zhang, A Kohn, E Amsellem, G Yang, JL Liu, J Jiang, HX Wei, XF 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 Δ1 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.


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.


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

APPLIED PHYSICS LETTERS 105 (2014) ARTN 121608

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


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


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


Catalyst-free growth of Bi2Te3 nanostructures by molecular beam epitaxy

APPLIED PHYSICS LETTERS 105 (2014) ARTN 153114

SE Harrison, P Schoenherr, Y Huo, JS Harris, T 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


Magnetization Dynamics in an Exchange-Coupled NiFe/CoFe Bilayer

New J. Phys. 00 (2014) 000000-000000

GBG Stenning, LR Shelford, SA Cavill, F Hoffmann, M Haertinger, T Hesjedal, G Woltersdorf, GJ Bowden, SA Gregory, LC Maple, CH Back, PAJD Groot, GVD Laan


Controlled of amorphous Se capping layer from a topological insulator

Appl. Phys. Lett. 00 (2014) 000000-000000

K Virwani, SE Harrison, A Pushp, T Topuria, E Delenia, P Rice, A Kellock, LJ Collins-McIntyre, JS Harris, T Hesjedal, SSP Parkin


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.


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

APPLIED PHYSICS LETTERS 104 (2014) ARTN 253103

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


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


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

PHYSICAL REVIEW B 90 (2014) ARTN 134402

AI Figueroa, G van der Laan, LJ Collins-McIntyre, S-L Zhang, AA Baker, SE Harrison, P Schoenherr, G Cibin, T Hesjedal


A New Topological Insulator Built From Quasi One-Dimensional Atomic Ribbons

Phys. Stat. Sol. RRL 00 (2014) 000000-000000

P Schoenherr, SL Zhang, YQ Liu, P Kusch, S Reich, T Giles, D Daisenberger, D Prabhakaran, Y Chen, 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.


X-ray magnetic spectroscopy of MBE-grown Mn-doped Bi2Se3 thin films

AIP Advances 00 (2014) 000000-000000

LJ Collins-McIntyre, MD Watson, AA Baker, SL Zhang, AI Coldea, SE Harrison, A Pushp, AJ Kellock, SSP Parkin, GVD Laan, AT Hesjedal


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.


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.