Publications


The science and art of seeing: Scope: monograph. Level: general reader

Contemporary Physics (2015)

SJ Blundell


Engineering helimagnetism in MnSi thin films

AIP ADVANCES 6 (2016) ARTN 015217

SL Zhang, R Chalasani, AA Baker, N-J Steinke, AI Figueroa, A Kohn, G van der Laan, T Hesjedal


Spin pumping in magnetic trilayer structures with an MgO barrier.

Scientific reports 6 (2016) 35582-

AA Baker, AI Figueroa, D Pingstone, VK Lazarov, G van der Laan, T Hesjedal

We present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4 nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element-specific technique of x-ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.


Evidence for unidirectional nematic bond ordering in FeSe

Physical Review B - Condensed Matter and Materials Physics American Physical Society (2016)

MD Watson, TK Kim, LC Rhodes, M Eschrig, M Hoesch, AA Haghighirad, AI Coldea

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.


Coherent Magnetoelastic Domains in Multiferroic BiFeO_{3} Films.

Physical review letters 117 (2016) 177601-

N Waterfield Price, RD Johnson, W Saenrang, F Maccherozzi, SS Dhesi, A Bombardi, FP Chmiel, C-B Eom, PG Radaelli

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of nonresonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO_{3} films support submicron antiferromagnetic domains, which are magnetoelastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO_{3} film devices via epitaxial strain.


Modeling the angle-dependent magnetoresistance oscillations of Fermi surfaces with hexagonal symmetry

PHYSICAL REVIEW B 93 (2016) ARTN 245105

JCA Prentice, AI Coldea


Ab initio cycloidal and chiral magnetoelectric responses in Cr2O3

PHYSICAL REVIEW B 94 (2016) ARTN 100405

N Tillack, JR Yates, PG Radaelli


Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

Scientific Reports 6 (2016)

S Friedemann, H Chang, MB Gamża, P Reiss, X Chen, P Alireza, WA Coniglio, D Graf, S Tozer, FM Grosche


Ultrasmooth organic-inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells.

Nature communications 6 (2015) 6142-

W Zhang, M Saliba, DT Moore, SK Pathak, MT Hörantner, T Stergiopoulos, SD Stranks, GE Eperon, JA Alexander-Webber, A Abate, A Sadhanala, S Yao, Y Chen, RH Friend, LA Estroff, U Wiesner, HJ Snaith

To date, there have been a plethora of reports on different means to fabricate organic-inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (lead acetate) instead of lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry.


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

SCIENTIFIC REPORTS 5 (2015) ARTN 14475

J Alberto Rodriguez-Velamazan, O Fabelo, A Millan, J Campo, RD Johnson, L Chapon


Surface acoustic wave devices on bulk ZnO crystals at low temperature

APPLIED PHYSICS LETTERS 106 (2015) ARTN 063509

EB Magnusson, BH Williams, R Manenti, M-S Nam, A Nersisyan, MJ Peterer, A Ardavan, PJ Leek


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).


Evidence of quantum dimer excitations in Sr3Ir2O7

PHYSICAL REVIEW B 92 (2015) ARTN 024405

MM Sala, V Schnells, S Boseggia, L Simonelli, A Al-Zein, JG Vale, L Paolasini, EC Hunter, RS Perry, D Prabhakaran, AT Boothroyd, M Krisch, G Monaco, HM Ronnow, DF McMorrow, F Mila


Robustness of superconductivity to structural disorder in Sr-0.3(NH2)(y)(NH3)(1-y)Fe2Se2

PHYSICAL REVIEW B 92 (2015) ARTN 134517

FR Foronda, S Ghannadzadeh, SJ Sedlmaier, JD Wright, K Burns, SJ Cassidy, PA Goddard, T Lancaster, SJ Clarke, SJ Blundell


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.


Inverse order-disorder transition of charge stripes

PHYSICAL REVIEW B 92 (2015) ARTN 205114

S-H Lee, Y-C Lai, C-H Du, AF Siegenfeld, Y-J Kao, PD Hatton, D Prabhakaran, Y Su, D-J Huang


Crystal field states of Tb3+ in the pyrochlore spin liquid Tb2Ti2O7 from neutron spectroscopy

PHYSICAL REVIEW B 91 (2015) ARTN 224430

AJ Princep, HC Walker, DT Adroja, D Prabhakaran, AT Boothroyd


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.


Sc2NiMnO6: A Double-Perovskite with a Magnetodielectric Response Driven by Multiple Magnetic Orders.

Inorganic chemistry 54 (2015) 8012-8021

W Yi, AJ Princep, Y Guo, RD Johnson, D Khalyavin, P Manuel, A Senyshyn, IA Presniakov, AV Sobolev, Y Matsushita, M Tanaka, AA Belik, AT Boothroyd

Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.35281 Å, c = 7.34496 Å, and β = 90.7915°), exhibiting sequential magnetic transitions at T1 = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector k1 = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k1 = (1/2, 0, 1/2) and k2 = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k1 is associated with the Mn sublattice and k2 with the Ni sublattice, suggesting that Mn-Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchange-striction between the two magnetic sublattices belonging to k1 and k2. The behavior of Sc2NiMnO6 demonstrates 3d double-perovskites with small A-site cations as a promising avenue in which to search for magnetoelectric materials.

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