Publications


Magnetic structure and spin-flop transition in the A -site columnar-ordered quadruple perovskite TmMn3O6

Physical Review B American Physical Society (APS) 99 (2019) 104424

AM Vibhakar, DD Khalyavin, P Manuel, L Zhang, K Yamaura, PG Radaelli, AA Belik, RD Johnson


Magnetic order and enhanced exchange in the quasi-one-dimensional molecule-based antiferromagnet Cu(NO3)2(pyz)3.

Physical chemistry chemical physics : PCCP 21 (2019) 1014-1018

BM Huddart, J Brambleby, T Lancaster, PA Goddard, F Xiao, SJ Blundell, FL Pratt, J Singleton, P Macchi, R Scatena, AM Barton, JL Manson

The quasi-one-dimensional molecule-based Heisenberg antiferromagnet Cu(NO3)2(pyz)3 has an intrachain coupling J = 13.7(1) K () and exhibits a state of long-range magnetic order below TN = 0.105(1) K. The ratio of interchain to intrachain coupling is estimated to be |J'/J| = 3.3 × 10-3, demonstrating a high degree of isolation for the Cu chains.


Paramagnon dispersion in beta-FeSe observed by Fe L-edge resonant inelastic x-ray scattering

PHYSICAL REVIEW B 99 (2019) ARTN 014505

MC Rahn, K Kummer, NB Brookes, AA Haghighirad, K Gilmore, AT Boothroyd


Local magnetism, magnetic order and spin freezing in the 'nonmetallic metal' FeCrAs.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 285803-285803

BM Huddart, MT Birch, FL Pratt, SJ Blundell, DG Porter, SJ Clark, W Wu, SR Julian, PD Hatton, T Lancaster

We present the results of x-ray scattering and muon-spin relaxation ([Formula: see text]SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120° periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. [Formula: see text]SR measurements indicate a magnetically ordered phase throughout the bulk of the material below [Formula: see text] K. There are signs of fluctuating magnetism in a narrow range of temperatures above [Formula: see text] involving low-energy excitations, while at temperatures well below [Formula: see text] behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.


Strain Engineering a Multiferroic Monodomain in Thin-Film BiFeO3

PHYSICAL REVIEW APPLIED 11 (2019) ARTN 024035

NW Price, AM Vibhakar, RD Johnson, J Schad, W Saenrang, A Bombardi, FP Chmiel, CB Eom, PG Radaelli


Antidamping torques from simultaneous resonances in ferromagnet-topological insulator-ferromagnet heterostructures

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 473 (2019) 470-476

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


Structural and Optical Properties of Cs2AgBiBr6 Double Perovskite

ACS ENERGY LETTERS 4 (2019) 299-305

L Schade, AD Wright, RD Johnson, M Dollmann, B Wenger, PK Nayak, D Prabhakaran, LM Herz, R Nicholas, HJ Snaith, PG Radaelli


Rare Earth Doping of Topological Insulators: A Brief Review of Thin Film and Heterostructure Systems

PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 216 (2019) ARTN 1800726

T Hesjedal


Spin-charge-lattice coupling in quasi-one-dimensional Ising spin chain CoNb2O6.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 195802-

M Nandi, D Prabhakaran, P Mandal

Magnetization, magnetostriction and dielectric constant measurements are performed on single crystals of quasi-one-dimensional Ising spin chain CoNb2O6 at temperatures below and above the antiferromagnetic phase transition. Field-induced magnetic transitions are clearly reflected in magnetodielectric and magnetostriction data. Sharp anomalies are observed around the critical fields of antiferromagnetic to ferrimagnetic and ferrimagnetic to saturated-paramagnetic transition in both magnetodielectric and magnetostriction experiments. Detailed analysis of temperature and field dependence of dielectric constant and magnetostriction suggests that spins are coupled with lattice as well as charges in CoNb2O6. Below the antiferromagnetic transition temperature, the overall resemblance in anomalies, observed in various physical parameters such as magnetization, dielectric constant, magnetostriction and magnetic entropy change gives a deeper insight about the influence of spin configuration on these parameters in CoNb2O6.


Transverse and longitudinal spin-fluctuations in INVAR Fe0.65Ni0.35.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 025802-

JR Stewart, SR Giblin, D Honecker, P Fouquet, D Prabhakaran, JW Taylor

The presence of spin-fluctuations deep within the ordered state of ferromagnetic [Formula: see text] alloy [Formula: see text] has long been suspected but seldom directly observed. Inhomogeneities of one type or another have been cited as important in stabilizing [Formula: see text] behaviour-either longitudinal spin-fluctuations associated with the [Formula: see text]-state (local environment) model or transverse magnetisation arising from non-collinear spin structures. In this study we employ small-angle neutron scattering with neutron polarization analysis to distinguish between the two possibilities. Surprisingly we in fact find evidence of dominant but uncorrelated longitudinal spin-fluctuations coexisting with transverse magnetisation which exists in short-range clusters of size ~[Formula: see text]. This finding supports recent first principles calculations of [Formula: see text] in which both longitudinal spin-fluctuations and magnetic short-range order are identified as important ingredients in reproducing the equilibrium [Formula: see text] lattice.


Skyrmions in anisotropic magnetic fields: strain and defect driven dynamics

MRS Advances Cambridge University Press (CUP) (2019)

R Brearton, MW Olszewski, S ZHANG, MR Eskildsen, C Reichardt, CJO Reichardt, G van der Laan, T HESJEDAL

Magnetic skyrmions are particle-like, topologically protected magnetization entities that are promising candidates for information carriers in racetrack-memory schemes. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Recently, we demonstrated experimentally that chiral skyrmions in Cu2OSeO3 can be effectively manipulated by a magnetic field gradient, leading to a collective rotation of the skyrmion lattice with well-defined dynamics in a radial field gradient. Here, we employ a skyrmion particle model to numerically study the effects of resultant shear forces on the structure of the skyrmion lattice. We demonstrate that anisotropic peak broadening in experimentally observed diffraction patterns can be attributed to extended linear regions in the magnetic field profile. We show that topological (5-7) defects emerge to protect the six-fold symmetry of the lattice under the application of local shear forces, further enhancing the stability of proposed magnetic field driven devices.


Spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet α-Li2IrO3

Physical review B: Condensed matter and materials physics American Physical Society (0)

S Choi, S Manni, J Singleton, CV Topping, T Lancaster, SJ Blundell, DT Adroja, V Zapf, P Gegenwart, R Coldea

The layered honeycomb iridate alpha-Li2IrO3 displays an incommensurate magnetic structure with counterrotating moments on nearest-neighbor sites, proposed to be stabilized by strongly-frustrated anisotropic Kitaev interactions between spin-orbit entangled Ir4+ magnetic moments. Here we report powder inelastic neutron scattering measurements that observe sharply dispersive low-energy magnetic excitations centered at the magnetic ordering wavevector, attributed to Goldstone excitations of the incommensurate order, as well as an additional intense mode above a gap Delta~2.3 meV. Zero-field muon-spin relaxation measurements show clear oscillations in the muon polarization below the Neel temperature T_N ~ 15 K with a time-dependent profile consistent with bulk incommensurate long-range magnetism. Pulsed field magnetization measurements observe that only about half the saturation magnetization value is reached at the maximum field of 64 T. A clear anomaly near 25 T indicates a transition to a phase with reduced susceptibility. The transition field has a Zeeman energy comparable to the zero-field gapped mode, suggesting gap suppression as a possible mechanism for the field-induced transition.


The effect of substrate and surface plasmons on symmetry breaking at the substrate interface of the topological insulator Bi2Te3.

Scientific reports 9 (2019) 6147-

M Wiesner, RH Roberts, J-F Lin, D Akinwande, T Hesjedal, LB Duffy, S Wang, Y Song, J Jenczyk, S Jurga, B Mroz

A pressing challenge in engineering devices with topological insulators (TIs) is that electron transport is dominated by the bulk conductance, and so dissipationless surface states account for only a small fraction of the conductance. Enhancing the surface-to-volume ratio is a common method to enhance the relative contribution of such states. In thin films with reduced thickness, the confinement results in symmetry-breaking and is critical for the experimental observation of topologically protected surface states. We employ micro-Raman and tip-enhanced Raman spectroscopy to examine three different mechanisms of symmetry breaking in Bi2Te3 TI thin films: surface plasmon generation, charge transfer, and application of a periodic strain potential. These mechanisms are facilitated by semiconducting and insulating substrates that modify the electronic and mechanical conditions at the sample surface and alter the long-range interactions between Bi2Te3 and the substrate. We confirm the symmetry breaking in Bi2Te3 via the emergence of the Raman-forbidden [Formula: see text] mode. Our results suggest that topological surface states can exist at the Bi2Te3/substrate interface, which is in a good agreement with previous theoretical results predicting the tunability of the vertical location of helical surface states in TI/substrate heterostructures.


Oriented 3D Magnetic Biskyrmions in MnNiGa Bulk Crystals.

Advanced materials (Deerfield Beach, Fla.) 31 (2019) e1900264-

X Li, S Zhang, H Li, DA Venero, JS White, R Cubitt, Q Huang, J Chen, L He, G van der Laan, W Wang, T Hesjedal, F Wang

A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee-bean-shaped objects are observed in real space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface-confined objects, or, analogous to skyrmions in noncentrosymmetric helimagnets, 3D tube-like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single- and polycrystalline-MnNiGa samples using small-angle neutron scattering. It is found that biskyrmions are not long-range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in-plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.


Unconventional Field-Induced Spin Gap in an S=1/2 Chiral Staggered Chain

PHYSICAL REVIEW LETTERS 122 (2019) ARTN 057207

J Liu, S Kittaka, RD Johnson, T Lancaster, J Singleton, T Sakakibara, Y Kohama, J van Tol, A Ardavan, BH Williams, SJ Blundell, ZE Manson, JL Manson, PA Goddard


Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

Nature npj Quantum Materials Springer Nature 4 (2019) 2

AI Coldea, SF Blake, S Kasahara, AA Haghighirad, MD Watson, W Knafo, ES Choi, A McCollam, P Reiss, T Yamashita, M Bruma, SC Speller, Y Matsuda, T Wolf, T Shibauchi, AJ Schofield


Anatomy of Skyrmionic Textures in Magnetic Multilayers.

Advanced materials (Deerfield Beach, Fla.) 31 (2019) e1807683-

W Li, I Bykova, S Zhang, G Yu, R Tomasello, M Carpentieri, Y Liu, Y Guang, J Gräfe, M Weigand, DM Burn, G van der Laan, T Hesjedal, Z Yan, J Feng, C Wan, J Wei, X Wang, X Zhang, H Xu, C Guo, H Wei, G Finocchio, X Han, G Schütz

Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real-space spin configuration. Here, two advanced X-ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO]n multilayers. First, by using ptychography, a high-resolution diffraction imaging technique, the 2D out-of-plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X-ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depth-dependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole-dipole interactions together with the external magnetic field play an important role in stabilizing sub-100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. This combined X-ray-based approach opens the door for in-depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.


Manipulating quantum materials with quantum light

PHYSICAL REVIEW B 99 (2019) ARTN 085116

M Kiffner, JR Coulthard, F Schlawin, A Ardavan, D Jaksch


Temperature dependence of the ferromagnetic response in CrxSb2-xTe3 topological insulator thin films investigated using terahertz spectroscopy and magneto-transport

TERAHERTZ, RF, MILLIMETER, AND SUBMILLIMETER-WAVE TECHNOLOGY AND APPLICATIONS XII 10917 (2019)

VS Kamboj, A Singh, L Jakob, LB Duffy, N Idros, SP Senanayak, A Ionescu, HE Beere, CHW Barnes, T Hesjedal, DA Ritchie


Real-Space Observation of Skyrmionium in a Ferromagnet-Magnetic Topological Insulator Heterostructure.

Nano letters ACS 18 (2018) 1057-1063

S Zhang, F Kronast, G van der Laan, T Hesjedal

The combination of topological insulators, i.e., bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically-protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.

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