Publications by Thorsten Hesjedal


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

Scientific Reports Nature Publishing Group 4 (2014) 1-8

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

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.


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

Physica Status Solidi - Rapid Research Letters 9 (2015) 130-135

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

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 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.


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


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

Physica Status Solidi - Rapid Research Letters Wiley 9 (2015) 130–135-

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

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.


An ultra-compact, high-throughput molecular beam epitaxy growth system.

The Review of scientific instruments 86 (2015) 043901-

AA Baker, W Braun, G Gassler, S Rembold, A Fischer, T Hesjedal

We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO (001).


Magnetic proximity-enhanced Curie temperature of Cr-doped Bi2Se3 thin films

Physical Review B American Physical Society 92 (2015) 094420

A Baker, AI Figuorea, K Kummer, LJ Collins-McIntyre, T Hesjedal, G van der Laan

We report a study on the transition temperature, T_C, of a Cr-doped Bi2Se3 topological insulator thin film, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics applications. Arrott plots, obtained by Cr L_2,3 x-ray magnetic circular dichroism as a function of field at various low temperatures, give a T_C ~7 K. This is similar to the bulk value of the sample, which means that there is no indication the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature to ~19 K. X-ray absorption spectroscopy shows that Cr enters the Bi2Se3 host matrix in a divalent state, and is unchanged by the Co deposition.


Effect of interfacial structures on spin dependent tunneling in epitaxial L1(0)-FePt/MgO/FePt perpendicular magnetic tunnel junctions

JOURNAL OF APPLIED PHYSICS 117 (2015) ARTN 083904

G Yang, DL Li, SG Wang, QL Ma, SH Liang, HX Wei, XF Han, T Hesjedal, RCC Ward, A Kohn, A Elkayam, N Tal, X-G Zhang


Spin pumping in ferromagnet-topological insulator-ferromagnet heterostructures

Scientific Reports Nature Publishing Group 5 (2014) 7907-

A Baker, AI Figueroa, L 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 approximate ly 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 magneto dynamics that persist even up to room temperature.


Magnetic reversal in Dy-doped DyFe2/YFe2 superlattice films

PHYSICAL REVIEW B 91 (2015) ARTN 094403

GBG Stenning, GJ Bowden, PAJ de Groot, G van der Laan, AI Figueroa, P Bencok, P Steadman, T Hesjedal


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

Phys Rev B. Solid State 91 (2015) 224408

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

We present the results of transverse field (TF) muon-spin rotation (μ+SR) measurements on Cu2OSeO3, which has a skyrmion-lattice (SL) phase. We measure the response of the TF μ+SR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale τ>100 ns.


Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

AIP ADVANCES 5 (2015) ARTN 077117

AA Baker, AI Figueroa, G van der Laan, T Hesjedal


Structural properties and growth mechanism of Cd3As2 nanowires

APPLIED PHYSICS LETTERS 106 (2015) ARTN 013115

P Schoenherr, T Hesjedal


Growth of Bi2Se3 and Bi2Te3 on amorphous fused silica by MBE

physica status solidi (b) 252 (2015) 1334-1338

LJ Collins-McIntyre, W Wang, B Zhou, Speller, Chen, T Hesjedal

Topological insulator (TI) thin films of Bi2Se3 and Bi2Te3 have been successfully grown on amorphous fused silica (vitreous SiO2) substrates by molecular beam epitaxy. We find that such growth is possible and investigations by X-ray diffraction reveal good crystalline quality with a high degree of order along the caxis. Atomic force microscopy, electron backscatter diffraction and X-ray reflectivity are used to study the surface morphology and structural film parameters. Angle-resolved photoemission spectroscopy studies confirm the existence of a topological surface state. This work shows that TI films can be grown on amorphous substrates, while maintaining the topological surface state despite the lack of in-plane rotational order of the domains. The growth on fused silica presents a promising route to detailed thermoelectric measurements of TI films, free from unwanted thermal, electrical, and piezoelectric influences from the substrate.


Study of Dy-doped Bi₂Te₃: thin film growth and magnetic properties.

Journal of physics. Condensed matter : an Institute of Physics journal 27 (2015) 245602-

SE Harrison, LJ Collins-McIntyre, S-L Zhang, AA Baker, AI Figueroa, AJ Kellock, A Pushp, SSP Parkin, JS Harris, G van der Laan, T Hesjedal

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential device applications. Here, we report the successful growth of high-quality (Dy(x)Bi(1-x))2Te3 thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive magnetisation studies using superconducting quantum interference device magnetometry find paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, μeff, is strongly doping concentration-dependent and reduces from ∼12.6 μ(B) Dy(-1) for x = 0.023 to ∼4.3 μ(B) Dy(-1) for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Dy M4,5 edge are employed to provide a deeper insight into the magnetic nature of the Dy(3+)-doped films. XMCD, measured in surface-sensitive total-electron-yield detection, gives μ(eff )= 4.2 μ(B) Dy(-1). The large measured moments make Dy-doped films interesting TI systems in which the TRS may be broken via the proximity effect due to an adjacent ferromagnetic insulator.


Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures

ADVANCED ELECTRONIC MATERIALS 1 (2015) ARTN 1400054

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


Cover Picture: A new topological insulator built from quasi one-dimensional atomic ribbons (Phys. Status Solidi RRL 2/2015)

physica status solidi (RRL) - Rapid Research Letters Wiley 9 (2015) n/a-n/a

P Schönherr, S Zhang, Y 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

Physical Review A American Physical Society 91 (2015) 224408

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

We present the results of transverse field (TF) muon-spin rotation (μ+SR) measurements on Cu2OSeO3, which has a skyrmion-lattice (SL) phase. We measure the response of the TF μ+SR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale τ>100 ns.


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


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

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