# Publications

## Spin pumping through a topological insulator probed by x-ray detected ferromagnetic resonance

Journal of Magnetism and Magnetic Materials Elsevier 400 (2015) 178-183

AI Figueroa, A Baker, L Collins-McIntyre, T Hesjedal, G van der Laan

In the field of spintronics, the generation of a pure spin current (without macroscopic charge flow) through spin pumping of a ferromagnetic (FM) layer opens up the perspective of a new generation of dissipation-less devices. Microwave driven ferromagnetic resonance (FMR) can generate a pure spin current that enters adjacent layers, allowing for both magnetization reversal (through spin-transfer torque) and to probe spin coherence in non-magnetic materials. However, standard FMR is unable to probe multilayer dynamics directly, since the measurement averages over the contributions from the whole system. The synchrotron radiation-based technique of x-ray detected FMR (XFMR) offers an elegant solution to this drawback, giving access to element-, site-, and layer-specific dynamical measurements in heterostructures. In this work, we show how XFMR has provided unique information to understand spin pumping and spin transfer torque effects through a topological insulator (TI) layer in a pseudo-spin valve heterostructure. We demonstrate that TIs function as efficient spin sinks, while also allowing a limited dynamic coupling between ferromagnetic layers. These results shed new light on the spin dynamics of this novel class of materials, and suggest future directions for the development of room temperature TI-based spintronics.

## The magnetic ground state of two isostructual polymeric quantum magnets, [Cu(HF2)(pyrazine)SbF6 and [Co(HF2)(pyrazine)2]SbF6, investigated with neutron powder diffraction

Physical Review B American Physical Society 92 (2015) 134406-

J Brambleby, P Goddard, R Johnson, J Liu, D Kaminski, A Ardavan, AJ Steele, T Lancaster, P Manuel, PJ Baker, J Singleton, SG Schwalbe, PM Spurgeon, HE Tran, PK Peterson, JF Corbey, JL Manson, SJ Blundell

The magnetic ground state of two isostructural coordination polymers (i) the quasi two-dimensional S = 1/2 square-lattice antiferromagnet [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$; and (ii) a new compound [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, were examined with neutron powder diffraction measurements. We find the ordered moments of the Heisenberg S = 1/2 Cu(II) ions in [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 0.6(1)$\mu_{B}$, whilst the ordered moments for the Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 3.02(6)$\mu_{B}$. For Cu(II), this reduced moment indicates the presence of quantum fluctuations below the ordering temperature. We show from heat capacity and electron spin resonance measurements, that due to the crystal electric field splitting of the S = 3/2 Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, this isostructual polymer also behaves as an effective spin-half magnet at low temperatures. The Co moments in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ show strong easy-axis anisotropy, neutron diffraction data which do not support the presence of quantum fluctuations in the ground state and heat capacity data which are consistent with 2D or close to 3D spatial exchange anisotropy.

## Local Structure and Bonding of Transition Metal Dopants in Bi2Se3 Topological Insulator Thin Films

JOURNAL OF PHYSICAL CHEMISTRY C 119 (2015) 17344-17351

AI Figueroa, G van der Laan, LJ Collins-McIntyre, G Cibin, AJ Dent, T Hesjedal

## Theory of Electromagnons in CuO.

Physical review letters 114 (2015) 197201-

K Cao, F Giustino, PG Radaelli

We develop a theory of electromagnons in CuO by combining a symmetry analysis based on irreducible corepresentations, ab initio calculations, and simulations of spin dynamics using a model Hamiltonian and the Landau-Lifshitz-Gilbert equation. We show that the electromagnon measured in [Jones et al., Nat. Commun. 5, 3787 (2014)] with the electric field along the [101] direction originates from a magnetoelectric coupling mediated by Dzyaloshinskii-Moriya interactions and consists of a rigid rotation of the Cu spins around the axis defined by the electric field. Furthermore we predict the existence of a second electromagnon originating from exchange striction and coupled to electric fields along the [010] direction in the AF2 phase.

## 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).

## The contribution of Diamond Light Source to the study of strongly correlated electron systems and complex magnetic structures.

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 373 (2015)

We review some of the significant contributions to the field of strongly correlated materials and complex magnets, arising from experiments performed at the Diamond Light Source (Harwell Science and Innovation Campus, Didcot, UK) during the first few years of operation (2007-2014). We provide a comprehensive overview of Diamond research on topological insulators, multiferroics, complex oxides and magnetic nanostructures. Several experiments on ultrafast dynamics, magnetic imaging, photoemission electron microscopy, soft X-ray holography and resonant magnetic hard and soft X-ray scattering are described.

## Exchange spring switching in Er-doped DyFe2/YFe2 magnetic thin films

PHYSICAL REVIEW B 92 (2015) ARTN 104404

GBG Stenning, GJ Bowden, PAJ de Groot, G van der Laan, AI Figueroa, P Bencok, P Steadman, T Hesjedal

## Massive Dirac fermion observed in lanthanide-doped topological insulator thin films

Scientific Reports Nature Publishing Group 5 (2015) 15767

SE Harrison, LJ Collins-McIntyre, P Schönherr, A Vailionis, V Srot, PA van Aken, AJ Kellock, A Pushp, SSP Parkin, JS Harris, B Zhou, T Hesjedal, YL Chen

The breaking of time reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs), and thus the opening of a ‘Dirac-mass gap’ in the linearly dispersed Dirac surface state, is a prerequisite for unlocking exotic physical states. Introducing ferromagnetic long-range order by transition metal doping has been shown to break TRS. Here, we present the study of lanthanide (Ln) doped Bi2Te3, where the magnetic doping with high-moment lanthanides promises large energy gaps. Using molecular beam epitaxy, single-crystalline, rhombohedral thin films with Ln concentrations of up to ~35%, substituting on Bi sites, were achieved for Dy, Gd, and Ho doping. Angle-resolved photoemission spectroscopy shows the characteristic Dirac cone for Gd and Ho doping. In contrast, for Dy doping above a critical doping concentration, a gap opening is observed via the decreased spectral intensity at the Dirac point, indicating a topological quantum phase transition persisting up to room-temperature.

## Effect of interfacial structures on spin dependent tunneling in epitaxial L1(0)-FePt/MgO/FePt perpendicular magnetic tunnel junctions

JOURNAL OF APPLIED PHYSICS 117 (2015) ARTN 083904

G Yang, DL Li, SG Wang, QL Ma, SH Liang, HX Wei, XF Han, T Hesjedal, RCC Ward, A Kohn, A Elkayam, N Tal, X-G Zhang

## Magnetic reversal in Dy-doped DyFe2/YFe2 superlattice films

PHYSICAL REVIEW B 91 (2015) ARTN 094403

GBG Stenning, GJ Bowden, PAJ de Groot, G van der Laan, AI Figueroa, P Bencok, P Steadman, T Hesjedal

## Topological computation based on direct magnetic logic communication

Scientific Reports Nature Publishing Group 5 (2015)

S Zhang, AA Baker, S Komineas, T Hesjedal

Non-uniform magnetic domains with non-trivial topology, such as vortices and skyrmions, are proposed as superior state variables for nonvolatile information storage. So far, the possibility of logic operations using topological objects has not been considered. Here, we demonstrate numerically that the topology of the system plays a significant role for its dynamics, using the example of vortex-antivortex pairs in a planar ferromagnetic film. Utilising the dynamical properties and geometrical confinement, direct logic communication between the topological memory carriers is realised. This way, no additional magnetic-to-electrical conversion is required. More importantly, the information carriers can spontaneously travel up to ~300 nm, for which no spin-polarised current is required. The derived logic scheme enables topological spintronics, which can be integrated into large-scale memory and logic networks capable of complex computations.

## 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.

## Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations

PHYSICAL REVIEW B 91 (2015) ARTN 121105

SF Blake, MD Watson, A McCollam, S Kasahara, RD Johnson, A Narayanan, GL Pascut, K Haule, V Kiryukhin, T Yamashita, D Watanabe, T Shibauchi, Y Matsuda, AI Coldea

## Local electronic and structural environment of transition metal doped Bi2Se3 topological insulator thin films

Journal of Physical Chemistry C (2015)

AI Figuorea, G van der Laan, Collins-McIntyre, G Cibin, AJ Dent, T Hesjedal

Transition metal (TM) doped topological insulators have been the focus of many recent studies since they exhibit exotic quantum and magneto-electric effects, and offer the prospect of potential applications in spintronic devices. Here we report a systematic study of the local electronic and structural environment using x-ray absorption fine structure (XAFS) in TM (= Cr, Mn, and Fe) doped Bi2Se3 thin films grown by molecular beam epitaxy. Analysis of the TM K-edge XAFS reveals a divalent character for Cr, Mn, and Fe when substituting Bi in the films, despite the trivalent character of the Bi. All dopants occupy octahedral sites in the Bi2Se3 lattice, which agrees with substitutional incorporation onto the Bi sites. With the incorporation of TM dopants a local structural relaxation of the Bi2Se3 lattice is observed, which strengthens the covalent character of the TM–Se bond. The presence of additional phases and interstitial incorporation for the Mn and Fe dopants is also observed, even at low concentrations.

## Angular control of a hybrid magnetic metamolecule using anisotropic FeCo

Physical Review Applied American Physical Society 4 (2015) 054015

SA Gregory, LC Maple, GBG Stenning, T Hesjedal, G van der Laan, GJ Bowden

By coupling magnetic elements to metamaterials, hybrid metamolecules can be created with useful properties such as photon-magnon mode mixing. Here, we present results for a split-ring resonator (SRR) placed in close proximity to a thin crystalline film of magnetically hard FeCo. Eddy-current shielding is suppressed by patterning the FeCo into 100-μm disks. At the ferromagnetic resonance (FMR) condition of FeCo, photon-magnon coupling strengths of 5% are observed. Altogether, three distinct features are presented and discussed: (i) remanent magnets allow FMR to be performed in a near-zero field, partially eliminating the need for applied fields; (ii) the anisotropic FMR permits angular control over hybrid SRR and FMR resonances; and (iii) the in-plane and out-of-plane magnetization of FeCo opens the door to “magnetically configurable metamaterials” in real time. Finally, a special study is presented of how best to excite the numerous transverse magnetic and electric modes of the SRR by using near-field excitation from a coplanar waveguide.

## Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

AIP ADVANCES 5 (2015) ARTN 077117

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

## 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.

## Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures

ADVANCED ELECTRONIC MATERIALS 1 (2015) ARTN 1400054

S Zhang, AA Baker, J-Y Zhang, G Yu, S Wang, T Hesjedal

## Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices

APPLIED PHYSICS LETTERS 106 (2015) ARTN 173502

J Beilsten-Edmands, GE Eperon, RD Johnson, HJ Snaith, PG Radaelli

## 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.