# Publications

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

PHYSICAL REVIEW B 99 (2019) ARTN 104424

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

## Rare Earth Doping of Topological Insulators: A Brief Review of Thin Film and Heterostructure Systems (Phys. Status Solidi A 8∕2019)

Wiley (2019)

THORSTEN Hesjedal

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

Journal of Materials Chemistry C Royal Society of Chemistry 7 (2019) 8350-8356

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

<p>Double perovskite crystals such as Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> are expected to overcome the limitation of classic hybrid organic–inorganic perovskite crystals related to the presence of lead and the lack of structural stability. Perovskites are ionic crystals in which the carriers are expected to strongly couple to lattice vibrations. In this work we demonstrate that the photoluminescence (PL) emission in Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is strongly influenced by the strong electron–phonon coupling. Combining photoluminescence excitation (PLE) and Raman spectroscopy we show that the PL emission is related to a color center rather than a band-to-band transition. The broadening and the Stokes shift of the PL emission from Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is well explained using a Franck–Condon model with a Huang–Rhys factor of <em>S</em> = 11.7 indicating a strong electron–phonon interaction in this material.</p>

## 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 CoTi$_2$O$_5$

Physical Review B American Physical Society 99 (2019) 064403-

F Kirschner, R Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, S Blundell

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

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

Journal of Magnetism and Magnetic Materials Elsevier 473 (2018) 470-476

AA Baker, AI Figueroa, T Hesjedal, G Van Der Laan

We studied the magnetodynamics of ferromagnetic films coupling across a topological insulator (TI) Bi2Se3 layer using ferromagnetic resonance (FMR). TIs have attracted much attention across the physics community as they hold the potential for dissipationless carrier transport, extremely high spin-orbit torques, and are host to novel quantum effects. To investigate the coupling between the ferromagnetic (FM) layers, vector network analyzer (VNA)-FMR measurements of the resonance linewidth were performed as a function of bias field angle. By bringing the resonances of the two FM layers into close proximity, it was possible to observe antidamping torques that lead to a narrowing of linewidth, a characteristic of spin pumping. The element- and hence layer-specific technique of x-ray detected ferromagnetic resonance (XFMR) was used to circumvent the difficulty of obtaining accurate fits to the two overlapping resonances in close proximity. Our results confirm that the interaction across the TI is a dynamic exchange mediated by spin pumping, as opposed to a self-coupling of the surface state or similar, more unconventional mechanisms.

## Structural and optical properties of Cs2AgBiBr6 double perovskite

ACS Energy Letters American Chemical Society 4 (2018) 299-305

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

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs2AgBiBr6, which has emerged as a promising candidate for photovoltaic devices. On the basis of single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at Ts ≈ 122 K between the room-temperature cubic structure (space group Fm3̅m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy Eex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi−Ag−Br. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains.

## Cr2Te3 thin films for integration in magnetic topological insulator heterostructures

Scientific Reports Nature 9 (2019) 10793

DM Burn, L Duffy, R Fujita, S 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 xray 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 Crdoped 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) Wiley 216 (2019) 1800726-

T Hesjedal

Magnetic topological insulators (MTIs) are a novel materials class in which a topologically nontrivial electronic band structure coexists with long‐range ferromagnetic order. The ferromagnetic ground state can break time‐reversal symmetry, opening a gap in the topological surface states whose size is dependent on the magnitude of the magnetic moment. Doping with rare earth ions is one way to introduce higher magnetic moments into a material, however, in Bi2Te3 bulk crystals, the solubility limit is only a few percent. Using molecular beam epitaxy for the growth of doped (Sb,Bi)2(Se,Te)3 TI thin films, high doping concentrations can be achieved while preserving their high crystalline quality. The growth, structural, electronic, and magnetic properties of Dy, Ho, and Gd doped TI thin films will be reviewed. Indeed, high magnetic moments can be introduced into the TIs, which are, however, not ferromagnetically ordered. By making use of interfacial effects, magnetic long‐range order in Dy doped Bi2Te3, proximity‐coupled to the MTI Cr:Sb2Te3, has been achieved. Clearly, engineered MTI heterostructures offer new possibilities that combine the advantageous properties of different layers, and thus provide an ideal materials platform enabling the observation new quantum effects at higher temperatures.

## Skyrmions in anisotropic magnetic fields: strain and defect driven dynamics

MRS Advances Cambridge University Press 4 (2019) 643-650

R Brearton, S Zhang, MW Olszewski, G Van Der Laan, CJO Reichardt, C Reichardt, Eskildsen, 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 Nature Research 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 ܣଵ௨ ଶ 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 Three-Dimensional Magnetic Biskyrmion in MnNiGa Bulk Crystals

XY Li, S Zhang, H Li, D Alba Venero, JS White, R Cubitt, QZ Huang, G van der Laan, WH Wang, T Hesjedal, FW Wang

## 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 Wiley 31 (2019) 1807683

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

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.

## Oriented 3D magnetic biskyrmions in MnNiGa bulk crystals

Advanced Materials Wiley 31 (2019) 1900264

X Li, S Zhang, H Li, DA Venero, JS White, R Cubitt, Q Huang, J Chen, L He, GVD 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.

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

Proceedings of SPIE Society of Photo-Optical Instrumentation Engineers 10917 (2019)

VS Kamboj, A Singh, L Jakob, L 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 IOP Publishing 21 (2019) 073063

GJ Bowden, G Van Der Laan, T Hesjedal, RJ Hicken

In 1878, the Dutch physicist Hendrik Antoon Lorentz first addressed the calculation of the local electric field at an atomic site in a ferroelectric material, generated by all the other electric dipoles within the sample. This calculation, which applies equally well to ferromagnets, is taught in Universities around the World. Here we demonstrate that the Lorentz concept can be used to speed up calculations of the local dipolar field in square, circular, and elliptical shaped monolayers and thin films, not only at the center of the film, but across the sample. Calculations show that long elliptical and rectangular films should exhibit the narrowest ferromagnetic resonance linewidth. In addition, discrete dipole calculations show that the Lorentz cavity field (u 0M/3) does not hold in tetragonal films. Depending on the ratio (b/a), the local field can be either less/greater than (u 0M/3): an observation that has implications for ferromagnetic resonance. 3D simple cubic (SC) systems are also examined. For example, while most texts discuss the Lorentz cavity field in terms of a Lorentz sphere, the Lorentz cavity field still holds when a Lorentz sphere is replaced by a the Lorentz cube, but only in cubic SC, FCC and BCC systems. Finally, while the primary emphasis is on the discrete dipole-dipole interaction, contact is made with the continuum model. For example, in the continuous SC dipole model, just one monolayer is required to generate the Lorentz cavity field. This is in marked contrast to the discrete dipole model, where a minimum of five adjacent monolayers is required.

## Systematic study of ferromagnetism in CrxSb2-xTe3 topological insulator thin films using electrical and optical techniques

Scientific Reports Springer Nature 8 (2018) 17024

A Singh, V Kamboj, J Liu, J Llandro, L Duffy, SP Senanayak, HE Beere, A Ionescu, DA Ritchie, T Hesjedal, CHW Barnes

Ferromagnetic ordering in a topological insulator can break time-reversal symmetry, realizing dissipationless electronic states in the absence of a magnetic field. The control of the magnetic state is of great importance for future device applications. We provide a detailed systematic study of the magnetic state in highly doped CrxSb2−xTe3 thin films using electrical transport, magneto-optic Kerr effect measurements and terahertz time domain spectroscopy, and also report an efficient electric gating of ferromagnetic order using the electrolyte ionic liquid [DEME][TFSI]. Upon increasing the Cr concentration from x = 0.15 to 0.76, the Curie temperature (Tc) was observed to increase by ~5 times to 176 K. In addition, it was possible to modify the magnetic moment by up to 50% with a gate bias variation of just ±3 V, which corresponds to an increase in carrier density by 50%. Further analysis on a sample with x = 0.76 exhibits a clear insulator-metal transition at Tc, indicating the consistency between the electrical and optical measurements. The direct correlation obtained between the carrier density and ferromagnetism - in both electrostatic and chemical doping - using optical and electrical means strongly suggests a carrier-mediated Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling scenario. Our low-voltage means of manipulating ferromagnetism, and consistency in optical and electrical measurements provides a way to realize exotic quantum states for spintronic and low energy magneto-electronic device applications.

## Helical magnetism in Sr-doped CaMn7O12 films

PHYSICAL REVIEW B 98 (2018) ARTN 224419

A Huon, AM Vibhakar, AJ Grutter, JA Borchers, S Disseler, Y Liu, W Tian, F Orlandi, P Manuel, DD Khalyavin, Y Sharma, A Herklotz, HN Lee, MR Fitzsimmons, RD Johnson, SJ May

## Proposal for a micromagnetic standard problem for materials with Dzyaloshinskii-Moriya interaction

New Journal of Physics Institute of Physics 20 (2018) 113015-

D Cortes-Ortuno, M Beg, V Nehruji, L Breth, R Pepper, T Kluyver, G Downing, T Hesjedal, P Hatton, T Lancaster, R Hertel, O Hovorka, H Fabgohr

Understanding the role of the Dzyaloshinskii-Moriya interaction (DMI) for the formation of helimagnetic order, as well as the emergence of skyrmions in magnetic systems that lack inversion symmetry, has found increasing interest due to the significant potential for novel spin based technologies. Candidate materials to host skyrmions include those belonging to the B20 group such as FeGe, known for stabilising Bloch-like skyrmions, interfacial systems such as cobalt multilayers or Pd/Fe bilayers on top of Ir(111), known for stabilising N´eel-like skyrmions, and, recently, alloys with a crystallographic symmetry where anti-skyrmions are stabilised. Micromagnetic simulations have become a standard approach to aid the design and optimisation of spintronic and magnetic nanodevices and are also applied to the modelling of device applications which make use of skyrmions. Several public domain micromagnetic simulation packages such as OOMMF, MuMax3 and Fidimag already offer implementations of different DMI terms. It is therefore highly desirable to propose a so-called micromagnetic standard problem that would allow one to benchmark and test the different software packages in a similar way as is done for ferromagnetic materials without DMI. Here, we provide a sequence of well-defined and increasingly complex computational problems for magnetic materials with DMI. Our test problems include 1D, 2D and 3D domains, spin wave dynamics in the presence of DMI, and validation of the analytical and numerical solutions including uniform magnetisation, edge tilting, spin waves and skyrmion formation. This set of problems can be used by developers and users of new micromagnetic simulation codes for testing and validation and hence establishing scientific credibility.