Publications by Thorsten Hesjedal

On the temperature dependence of spin pumping in ferromagnet-topological insulator-ferromagnet spin valves

Results in Physics 6 (2016) 293-294

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

© 2016 The Authors.Topological insulators (TIs) have a large potential for spintronic devices owing to their spin-polarized, counter-propagating surface states. Recently, we have investigated spin pumping in a ferromagnet-TI-ferromagnet structure at room temperature. Here, we present the temperature-dependent measurement of spin pumping down to 10 K, which shows no variation with temperature.

Atomic-level structural and chemical analysis of Cr-doped Bi2Se3 thin films.

Scientific reports 6 (2016) 26549-

A Ghasemi, D Kepaptsoglou, LJ Collins-McIntyre, Q Ramasse, T Hesjedal, VK Lazarov

We present a study of the structure and chemical composition of the Cr-doped 3D topological insulator Bi2Se3. Single-crystalline thin films were grown by molecular beam epitaxy on Al2O3 (0001), and their structural and chemical properties determined on an atomic level by aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. A regular quintuple layer stacking of the Bi2Se3 film is found, with the exception of the first several atomic layers in the initial growth. The spectroscopy data gives direct evidence that Cr is preferentially substituting for Bi in the Bi2Se3 host. We also show that Cr has a tendency to segregate at internal grain boundaries of the Bi2Se3 film.

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


AI Figueroa, AA Baker, LJ Collins-McIntyre, T Hesjedal, G van der Laan

Magnetic ordering in Ho-doped Bi<inf>2</inf>Te<inf>3</inf> topological insulator

Physica Status Solidi - Rapid Research Letters (2016)

AI Figueroa, SE Harrison, LJ Collins-Mcintyre, G van der Laan, T Hesjedal

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.We investigate the magnetic properties of Ho-doped Bi2Te3 thin films grown by molecular beam epitaxy. Analysis of the polarized X-ray absorption spectra at the Ho M5 absorption edge gives an effective 4f magnetic moment which is ∼45% of the Hund's rule ground state value. X-ray magnetic circular dichroism (XMCD) shows no significant anisotropy, which suggests that the reduced spin moment is not due to the crystal field effects, but rather the presence of non-magnetic or antiferromagnetic Ho sites. Extrapolating the temperature dependence of the XMCD measured in total electron yield and fluorescence yield mode in a field of 7 T gives a Curie-Weiss temperature of θ CW ≈ -30 K, which suggests antiferromagnetic ordering, in contrast to the paramagnetic behavior observed with SQUID magnetometry. From the anomaly of the XMCD signal at low temperatures, a Néel temperature TN between 10 K and 25 K is estimated.

Anisotropic Absorption of Pure Spin Currents.

Physical review letters 116 (2016) 047201-

AA Baker, AI Figueroa, CJ Love, SA Cavill, T Hesjedal, G van der Laan

Spin transfer in magnetic multilayers offers the possibility of ultrafast, low-power device operation. We report a study of spin pumping in spin valves, demonstrating that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter, α, in the spin sink layer. Using lab- and synchrotron-based ferromagnetic resonance, we show that an in-plane variation of damping in a crystalline Co_{50}Fe_{50} layer leads to an anisotropic α in a polycrystalline Ni_{81}Fe_{19} layer. This anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. These results offer a valuable insight into the transmission and absorption of spin currents, and a mechanism by which enhanced spin torques and angular control may be realized for next-generation spintronic devices.

Oxidation Effects in Rare Earth Doped Topological Insulator Thin Films.

Scientific reports 6 (2016) 22935-

AI Figueroa, G van der Laan, SE Harrison, G Cibin, T Hesjedal

The breaking of time-reversal symmetry (TRS) in topological insulators is a prerequisite for unlocking their exotic properties and for observing the quantum anomalous Hall effect (QAHE). The incorporation of dopants which exhibit magnetic long-range order is the most promising approach for TRS-breaking. REBiTe3, wherein 50% of the Bi is substitutionally replaced by a RE atom (RE = Gd, Dy, and Ho), is a predicted QAHE system. Despite the low solubility of REs in bulk crystals of a few %, highly doped thin films have been demonstrated, which are free of secondary phases and of high crystalline quality. Here we study the effects of exposure to atmosphere of rare earth-doped Bi2(Se, Te)3 thin films using x-ray absorption spectroscopy. We demonstrate that these RE dopants are all trivalent and effectively substitute for Bi(3+) in the Bi2(Se, Te)3 matrix. We find an unexpected high degree of sample oxidation for the most highly doped samples, which is not restricted to the surface of the films. In the low-doping limit, the RE-doped films mostly show surface oxidation, which can be prevented by surface passivation, encapsulation, or in-situ cleaving to recover the topological surface state.

Free-standing millimetre-long Bi₂Te₃ sub-micron belts catalysed by TiO₂ nanoparticles

Nanoscale Research Letters SpringerOpen (2016)

Schoenherr, FY Zhang, D Kojda, R Mitdank, M Albrecht, SF Fischer, T Hesjedal

Physical vapour deposition (PVD) is used to grow millimetre-long Bi2Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross-section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20×105 S⋅m−1 indicates an excellent sample quality.

Multidomain Skyrmion Lattice State in Cu2OSeO3.

Nano letters 16 (2016) 3285-3291

SL Zhang, A Bauer, DM Burn, P Milde, E Neuber, LM Eng, H Berger, C Pfleiderer, G van der Laan, T Hesjedal

Magnetic skyrmions in chiral magnets are nanoscale, topologically protected magnetization swirls that are promising candidates for spintronics memory carriers. Therefore, observing and manipulating the skyrmion state on the surface level of the materials are of great importance for future applications. Here, we report a controlled way of creating a multidomain skyrmion state near the surface of a Cu2OSeO3 single crystal, observed by soft resonant elastic X-ray scattering. This technique is an ideal tool to probe the magnetic order at the L3 edge of 3d metal compounds giving an average depth sensitivity of ∼50 nm. The single-domain 6-fold-symmetric skyrmion lattice can be broken up into domains, overcoming the propagation directions imposed by the cubic anisotropy by applying the magnetic field in directions deviating from the major cubic axes. Our findings open the door to a new way to manipulate and engineer the skyrmion state locally on the surface or on the level of individual skyrmions, which will enable applications in the future.

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

Physical Review B - Condensed Matter and Materials Physics 93 (2016)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, FL Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

© 2016 American Physical Society.We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40-nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around T≈40 and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.

The magneto-Hall difference and the planar extraordinary Hall balance

AIP Advances 6 (2016)

SL Zhang, T Hesjedal

© 2016 Author(s).The extraordinary Hall balance (EHB) is a general device concept that harnesses the net extraordinary Hall effect (EHE) arising from two independent magnetic layers, which are electrically in parallel. Different EHB behavior can be achieved by tuning the strength and type of interlayer coupling, i.e., ferromagnetic or antiferromagnetic of varying strength, allowing for logic and memory applications. The physics of the EHE in such a multilayered systems, especially the interface-induced effect, will be discussed. A discrepancy between the magnetization and the Hall effect, called the magneto-Hall difference (MHD) is found, which is not expected in conventional EHE systems. By taking advantage of the MHD effect, and by optimizing the materials structure, magnetoresistance ratios in excess of 40,000% can be achieved at room-temperature. We present a new design, the planar EHB, which has the potential to achieve significantly larger magnetoresistance ratios.

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

Physical Review B: Condensed Matter and Materials Physics American Physical Society 93 (2016) 140412(R)

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, F Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

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

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, SL Zhang, AA Baker, AI Figueroa, AJ Kellock, A Pushp, SS 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.

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


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

Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures


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

Angular Control of a Hybrid Magnetic Metamolecule Using Anisotropic FeCo


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

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).

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

PHYSICAL REVIEW B 91 (2015) ARTN 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, MC Hatnean, DS Keeble, G Balakrishnan

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.

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.