Publications by Thorsten Hesjedal


Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance

NEW JOURNAL OF PHYSICS 17 (2015) ARTN 013019

GBG Stenning, LR Shelford, SA Cavill, F Hoffmann, M Haertinger, T Hesjedal, G Woltersdorf, GJ Bowden, SA Gregory, CH Back, PAJ de Groot, G van der Laan


Structural properties and growth mechanism of Cd3As2 nanowires

APPLIED PHYSICS LETTERS 106 (2015) ARTN 013115

P Schoenherr, T Hesjedal


Spin pumping in Ferromagnet-Topological Insulator-Ferromagnet Heterostructures.

Scientific reports 5 (2015) 7907-

AA Baker, AI Figueroa, LJ Collins-McIntyre, G van der Laan, T Hesjedal

Topological insulators (TIs) are enticing prospects for the future of spintronics due to their large spin-orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. However, a means to interact with and exploit the topological surface state remains elusive. Here, we report a study of spin pumping at the TI-ferromagnet interface, investigating spin transfer dynamics in a spin-valve like structure using element specific time-resolved x-ray magnetic circular dichroism, and ferromagnetic resonance. Gilbert damping increases approximately linearly with increasing TI thickness, indicating efficient behaviour as a spin sink. However, layer-resolved measurements suggest that a dynamic coupling is limited. These results shed new light on the spin dynamics of this novel material class, and suggest great potential for TIs in spintronic devices, through their novel magnetodynamics that persist even up to room temperature.


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


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.


A new topological insulator built from quasi one-dimensional atomic ribbons

Physica Status Solidi - Rapid Research Letters Wiley-VCH Verlag (2014)

P Scho nherr, S Zhang, Y Liu, P Kusch, S Reich, T Giles, D Daisenberger, D Prabhakaran, Y Chen, T Hesjedal

A novel topological insulator with orthorhombic crystal structure is demonstrated. It is characterized by quasi one-dimensional, conducting atomic chains instead of the layered, two-dimensional sheets known from the established Bi2(Se,Te)3 system. The Sb-doped Bi2Se3 nanowires are grown in a TiO2-catalyzed process by chemical vapor deposition. The binary Bi2Se3 is transformed from rhombohedral to orthorhombic by substituting Sb on ∼38% of the Bi sites. Pure Sb2Se3 is a topologically trivial band insulator with an orthorhombic crystal structure at ambient conditions, and it is known to transform into a topological insulator at high pressure. Angle-resolved photoemission spectroscopy shows a topological surface state, while Sb doping also tunes the Fermi level to reside in the bandgap.


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.


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


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

AIP ADVANCES 4 (2014) ARTN 127136

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


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


Controlled removal of amorphous Se capping layer from a topological insulator

APPLIED PHYSICS LETTERS 105 (2014) ARTN 241605

K Virwani, SE Harrison, A Pushp, T Topuria, E Delenia, P Rice, A Kellock, L Collins-McIntyre, J Harris, T Hesjedal, S Parkin


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


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


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


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.


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.


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


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.


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.