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.


Three dimensional magnetic abacus memory

Scientific Reports Nature Publishing Group 4 (2014) 6109-

S 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.


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


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


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

Europhysics Letters: a letters journal exploring the frontiers of physics European Physical Society 107 (2014) 57009-57009

L Collins-Mcintyre, SE Harrison, P Schoenherr, N-J Steinke, C Kinane, T Charlton, D Alba-Veneroa, A Phusp, A Kellock, SSP Parkin, JS Harris, S Langridge, G van der Laan, T Hesjedal

We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. High-resolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be ${\sim}2.1\ \mu_\text{B}$ per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of ${\sim}10\ \text{mT}$ . The ferromagnetic transition temperature was determined to be $8.5\ \text{K}$ analyzing the magnetization-temperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.


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


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.


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


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


Nonvolatile full adder based on a single multivalued Hall junction

SPIN World Scientific Publishing (2013)

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.


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 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.


Extraordinary hall balance

Scientific Reports 3 (2013)

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. PACS numbers: 85.75.Nn,85.70.Kh,72.15.Gd,75.60.Ej.


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.


Extraordinary hall balance

Scientific Reports 3 (2013) ---

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


Extraordinary hall balance

Scientific Reports 3 (2013) ---

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


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.


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

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