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 profile of proximity-coupled (Dy,Bi)(2)Te-3/(Cr,Sb)(2)Te(3 )topological insulator heterostructures

PHYSICAL REVIEW B 100 (2019) ARTN 054402

LB Duffy, N-J Steinke, DM Burn, A Frisk, L Lari, B Kuerbanjiang, VK Lazarov, G van der Laan, S Langridge, T Hesjedal


Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6

JOURNAL OF MATERIALS CHEMISTRY C 7 (2019) 8350-8356

SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, RD Johnson, M Dollmann, PK Nayak, HJ Snaith, P Radaelli, R Kudrawiec, RJ Nicholas, P Plochocka, M Baranowski


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


Spin Jahn-Teller antiferromagnetism in CoTi2O5

PHYSICAL REVIEW B 99 (2019) ARTN 064403

FKK Kirschner, RD Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, SJ Blundell


A low-temperature Kerr effect microscope for the simultaneous magneto-optic and magneto-transport study of magnetic topological insulators

Measurement Science and Technology IOP Publishing (2019)

J Liu, A Singh, J Llandro, L Duffy, MR Stanton, SN Holmes, MJ Applegate, RT Phillips, T HESJEDAL, CHW Barnes

Magneto-optical Kerr effect (MOKE) microscopy is a surface-sensitive probe of magnetisation with micron-sized lateral resolution. Here, we present a low-temperature, focused polar MOKE microscope for the simultaneous magnetooptical and magneto-transport measurements, which has a temperature range of 1.6-300 K and is equipped with a magnet capable of delivering a field of up to 9 T. In this microscope, all optical components are integrated in a free-standing probe, allowing for the straightforward incorporation into many non-optical cryostat systems. Two-dimensional magnetisation scans on patterned ferromagnetic [CoFeB/Pt]n films demonstrate a magnetisation sensitivity of 10 µrad (Kerr angle) and a spatial resolution of 2.2 µm. The combination of optical and electrical measurements provides complementary temperature-dependent information, as demonstrated by the study of magnetic topological insulator thin films with out-of-plane magnetic anisotropy. Using this complementary approach, we study the effects of a secondary phase in Cr and V co-doped Sb2Te3 thin films, which show a combination of weak antilocalization and anisotropic magnetoresistance effects above 70 K. Our results highlight the virtue of MOKE and electrical transport to optimise exotic topological magnetic materials, paving the way for energy-efficient spintronic devices.


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


Cr2Te3 Thin Films for Integration in Magnetic Topological Insulator Heterostructures.

Scientific reports 9 (2019) 10793-

DM Burn, LB Duffy, R Fujita, SL Zhang, AI Figueroa, J Herrero-Martin, G van der Laan, T Hesjedal

Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.


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


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.


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


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.


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


Expanding the Lorentz concept in magnetism

NEW JOURNAL OF PHYSICS 21 (2019) ARTN 073063

GJ Bowden, G van der Laan, T Hesjedal, RJ Hicken


Spin-induced multiferroicity in the binary perovskite manganite Mn2O3.

Nature communications 9 (2018) 2996-

J Cong, K Zhai, Y Chai, D Shang, DD Khalyavin, RD Johnson, DP Kozlenko, SE Kichanov, AM Abakumov, AA Tsirlin, L Dubrovinsky, X Xu, Z Sheng, SV Ovsyannikov, Y Sun

The ABO3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn2O3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn2O3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.


Intrinsic Triple Order in A-site Columnar-Ordered Quadruple Perovskites: Proof of Concept.

Chemphyschem : a European journal of chemical physics and physical chemistry (2018)

AA Belik, DD Khalyavin, L Zhang, Y Matsushita, Y Katsuya, M Tanaka, RD Johnson, K Yamaura

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A2 A'A''B4 O12 quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy2 CuMnMn4 O12 (1) and Ho2 MnGaMn4 O12 (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho2 ]A [Mn]A' [Ga0.66 Mn0.34 ]A'' [Mn3.66 Ga0.34 ]B O12 and [Dy2 ]A [Cu0.73 Mn0.27 ]A' [Mn0.80 Dy0.20 ]A'' [Mn1.89 Cu0.11 ]B1 [Mn2 ]B2 O12 . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.


Topological surface state of α-Sn on InSb(001) as studied by photoemission

Physical review B: Condensed matter and materials physics American Physical Society 97 (2018) 075101

MR Scholz, L Dudy, F Reis, F Adler, J Aulbach, LJ Collins-McIntyre, LB Duffy, HF Yang, YL Chen, T Hesjedal, ZK Liu, M Hoesch, S Muff, JH Dil, J Schaefer, R Claessen

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