Publications


First-principles study of structurally modulated multiferroic CaMn7O12

PHYSICAL REVIEW B 91 (2015) ARTN 064422

K Cao, RD Johnson, N Perks, F Giustino, PG Radaelli


Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound.

Scientific reports 5 (2015) 14475-

J Alberto Rodríguez-Velamazán, Ó Fabelo, Á Millán, J Campo, RD Johnson, L Chapon

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2 K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.


New constraints on electron-beam induced halogen migration in apatite

AMERICAN MINERALOGIST 100 (2015) 281-293

MJ Stock, MCS Humphreys, VC Smith, RD Johnson, DM Pyle, EIMF


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

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

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


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.


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.


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.


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

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

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


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


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


Unconventional magnetic order on the hyperhoneycomb Kitaev lattice in beta-Li2IrO3: Full solution via magnetic resonant x-ray diffraction

PHYSICAL REVIEW B 90 (2014) ARTN 205116

A Biffin, RD Johnson, S Choi, F Freund, S Manni, A Bombardi, P Manuel, P Gegenwart, R Coldea


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


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


Noncoplanar and counterrotating incommensurate magnetic order stabilized by Kitaev interactions in γ-Li(2)IrO(3).

Physical review letters 113 (2014) 197201-

A Biffin, RD Johnson, I Kimchi, R Morris, A Bombardi, JG Analytis, A Vishwanath, R Coldea

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here, we explore the magnetism of γ-Li(2)IrO(3), which has the topology of a three-dimensional Kitaev lattice of interconnected Ir honeycombs. Using magnetic resonant x-ray diffraction, we find a complex, yet highly symmetric incommensurate magnetic structure with noncoplanar and counterrotating Ir moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally accounts for all key features of the observed magnetic structure. Our results provide strong evidence that γ-Li(2)IrO(3) realizes a spin Hamiltonian with dominant Kitaev interactions.

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