Publications by Thorsten Hesjedal
Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance
NEW JOURNAL OF PHYSICS 17 (2015) ARTN 013019
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
Local Structure and Bonding of Transition Metal Dopants in Bi<inf>2</inf>Se<inf>3</inf> Topological Insulator Thin Films
Journal of Physical Chemistry C 119 (2015) 17344-17351
© 2015 American Chemical Society. 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 Bi<inf>2</inf>Se<inf>3</inf> 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 Bi<inf>2</inf>Se<inf>3</inf> lattice, which agrees with substitutional incorporation onto the Bi sites. With the incorporation of TM dopants a local structural relaxation of the Bi<inf>2</inf>Se<inf>3</inf> 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.
The Review of scientific instruments 86 (2015) 043901-
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).
Phys Rev B. Solid State 91 (2015) 224408
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.
APPLIED PHYSICS LETTERS 106 (2015) ARTN 013115
AIP Advances 5 (2015)
© 2015 Author(s). We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, m<inf>l</inf>/m<inf>s</inf>. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states.
Scientific reports 5 (2015) 7907-
Topological insulators (TIs) are enticing prospects for the future of spintronics due to their large spin-orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. However, a means to interact with and exploit the topological surface state remains elusive. Here, we report a study of spin pumping at the TI-ferromagnet interface, investigating spin transfer dynamics in a spin-valve like structure using element specific time-resolved x-ray magnetic circular dichroism, and ferromagnetic resonance. Gilbert damping increases approximately linearly with increasing TI thickness, indicating efficient behaviour as a spin sink. However, layer-resolved measurements suggest that a dynamic coupling is limited. These results shed new light on the spin dynamics of this novel material class, and suggest great potential for TIs in spintronic devices, through their novel magnetodynamics that persist even up to room temperature.
physica status solidi (b) 252 (2015) 1334-1338
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.
Physica Status Solidi - Rapid Research Letters 9 (2015) 130-135
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. A novel topological insulator with orthorhombic crystal structure is demonstrated. It is characterized by quasi one-dimensional, conducting atomic chains instead of the layered, two-dimensional sheets known from the established Bi2(Se,Te)3 system. The Sb-doped Bi2Se3 nanowires are grown in a TiO2-catalyzed process by chemical vapor deposition. The binary Bi2Se3 is transformed from rhombohedral to orthorhombic by substituting Sb on ∼38% of the Bi sites. Pure Sb2Se3 is a topologically trivial band insulator with an orthorhombic crystal structure at ambient conditions, and it is known to transform into a topological insulator at high pressure. Angle-resolved photoemission spectroscopy shows a topological surface state, while Sb doping also tunes the Fermi level to reside in the bandgap.
Journal of physics. Condensed matter : an Institute of Physics journal 27 (2015) 245602-
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.
Local electronic and structural environment of transition metal doped Bi2Se3 topological insulator thin films
Journal of Physical Chemistry C (2015)
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.
Modelling ferromagnetic resonance in magnetic multilayers: Exchange coupling and demagnetisation-driven effects
JOURNAL OF APPLIED PHYSICS 115 (2014) ARTN 17D140
Journal of Applied Physics 115 (2014) 2
Scientific reports 4 (2014) 6109-
Stacking nonvolatile memory cells into a three-dimensional matrix represents a powerful solution for the future of magnetic memory. However, it is technologically challenging to access the data in the storage medium if large numbers of bits are stacked on top of each other. Here we introduce a new type of multilevel, nonvolatile magnetic memory concept, the magnetic abacus. Instead of storing information in individual magnetic layers, thereby having to read out each magnetic layer separately, the magnetic abacus adopts a new encoding scheme. It is inspired by the idea of second quantisation, dealing with the memory state of the entire stack simultaneously. Direct read operations are implemented by measuring the artificially engineered 'quantised' Hall voltage, each representing a count of the spin-up and spin-down layers in the stack. This new memory system further allows for both flexible scaling of the system and fast communication among cells. The magnetic abacus provides a promising approach for future nonvolatile 3D magnetic random access memory.
APPLIED PHYSICS LETTERS 104 (2014) ARTN 253103
EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS 66 (2014) ARTN 10401
Nanoscale research letters 9 (2014) 127-
: High-density growth of single-crystalline Bi2Se2Te nanowires was achieved via the vapour-liquid-solid process. The stoichiometry of samples grown at various substrate temperatures is precisely determined based on energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy on individual nanowires. We discuss the growth mechanism and present insights into the catalyst-precursor interaction.
APPLIED PHYSICS LETTERS 105 (2014) ARTN 241605