Publications by Thorsten Hesjedal

Magnetic Skyrmions

MagNews (2020) 19-21

G van der Laan, T Hesjedal

Skyrmions getting an X-ray

MagNews UK Magnetics Society (2020) 22-22

S Zhang, T Hesjedal, G van der Laan

Proximity-induced odd-frequency superconductivity in a topological insulator

Physical Review Letters American Physical Society 125 (2020) 026802

J Krieger, A Pertsova, S Giblin, M Döbeli, T Prokscha, C Schneider, A Suter, T Hesjedal, A Balatsky, Z Salman

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi2Se3 proximity coupled to Nb. From depthresolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi2Se3 that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.

Mode-resolved detection of magnetization dynamics using x-ray diffractive ferromagnetic resonance

Nano Letters American Chemical Society (2019)

T Hesjedal, G Van Der Laan, Y Sun, Y Chai, S Zhang, D Burn, K Zhai

Collective spin excitations of ordered magnetic structures o er great potential for the development of novel spintronic devices. The present approach is to rely on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic x-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR (DFMR) technique builds on x-ray detected FMR (XFMR) that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.

Element- and Time-Resolved Measurements of Spin Dynamics Using X-ray Detected Ferromagnetic Resonance

Synchrotron Radiation News Informa UK Limited 33 (2020) 12-19

C Klewe, Q Li, M Yang, AT N’Diaye, DM Burn, T Hesjedal, AI Figueroa, C Hwang, J Li, RJ Hicken, P Shafer, E Arenholz, G van der Laan, Z Qiu

Depth-resolved magnetization dynamics revealed by x-ray reflectometry ferromagnetic resonance

Physical Review Letters American Physical Society (2020)

D Burn, S Zhang, G Yu, Y Guang, H Chen, X Qiu, G van der Laan, T Hesjedal

Magnetic multilayers offer diverse opportunities for the development of ultrafast functional devices through advanced interface and layer engineering. Nevertheless, a method for determining their dynamic properties as a function of depth throughout such stacks have remained elusive. By probing the ferromagnetic resonance (FMR) modes with element-selective soft x-ray resonant reflectivity, we gain access to the magnetization dynamics as a function of depth. Most notably, using reflectometry ferromagnetic resonance (RFMR), we find a phase lag between the coupled ferromagnetic layers in [CoFeB/MgO/Ta]4 multilayers, which is invisible to other techniques. RFMR enables the time- and layer-resolved probing of the complex magnetization dynamics of a wide range of functional magnetic heterostructures with absorption edges in the soft x-ray wavelength regime.

Magnetic skyrmion interactions in the micromagnetic framework

Physical Review B: Condensed Matter and Materials Physics American Physical Society 101 (2020) 134422

R Brearton, G van der Laan, T Hesjedal

Magnetic skyrmions are localized swirls of magnetization with a nontrivial topological winding number. This winding increases their robustness to superparamagnetism and gives rise to a myriad of novel dynamical properties, making them attractive as next-generation information carriers. Recently the equation of motion for a skyrmion was derived using the approach pioneered by Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently. This powerful technique suffers from the prerequisite that one must have a priori knowledge of the functional form of the interaction between a skyrmion and all other magnetic structures in its environment. Here we attempt to alleviate this problem by providing a simple analytic expression that can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian, using it to provide a correction to the interaction between a skyrmion and the boundary of its material. We also discuss a toy model of the radial profile of a skyrmion, which is accurate for a wide range of material parameters.

Direct observation of the energy gain underpinning ferromagnetic superexchange in the electronic structure of CrGeTe3

Physical Review B: Condensed Matter and Materials Physics American Physical Society 101 (2020) 205125

M Watson, I Markovic, F Mazzola, A Rajan, E Morales, D Burn, T Hesjedal, G van der Laan, S Mukherjee, T Kim, C Bigi, I Vobornik, M Hatnean, G Balakrishnan, P King

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

Robust perpendicular skyrmions and their surface-confinement

Nano Letters American Chemical Society (2020)

S Zhang, D Burn, N Jaouen, J-Y Chauleau, A Haghighirad, Y Wang, W Wang, G Van Der Laan, T Hesjedal

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension, and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic x-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field we find that a state of perpendicularly ordered skyrmions forms - in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined in a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface-guiding makes the robust state particular interesting for racetrack-like devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.

Electron beam lithography of magnetic skyrmions

Advanced Materials Wiley (2020)

Y Guang, Y Peng, Z Yan, Y Liu, J Zhang, X Zeng, S Zhang, S Zhang, DM Burn, N Jaouen, J Wei, H Xu, J Feng, C Fang, G van der Laan, T Hesjedal, B Cui, X Zhang, G Yu, X Han

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their non-trivial topology through single skyrmions, ordered, and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as the chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is key to advancing topological magnetism. Here, we present a new, yet general, approach to the ‘printing’ of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, we use a focused electron beam with a graphic pattern generator to ‘print’ skyrmions, which we refer to as skyrmion lithography. Our work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Kerr effect anomaly in magnetic topological insulator superlattices

Nanotechnology IOP Publishing 31 (2020) 434001

J Liu, A Singh, B Kuerbanjiang, C Barnes, T Hesjedal

We report the magneto-optical Kerr effect (MOKE) study of magnetic topological insulator superlattice films with alternating transition-metal and rare-earth doping. We observe an unexpected hump in the MOKE hysteresis loops upon magnetization reversal at low temperatures, reminiscent of the topological Hall effect(THE) reported in transport measurements. The THE is commonly associated with the existence of magnetic skyrmions, i.e., chiral spin textures originating from topological defects in real space. Here, the observation of the effect is tied to ferromagnetic ordering in the rare-earth-doped layers of the superlattice. Our study may provide a new approach for the non-invasive optical investigation of skyrmions in magnetic films, complementary to electrical transport measurements, where the topological Hall signal is often the only hint of non-trivial magnetization patterns.

Exchange bias in magnetic topological insulator superlattices

Nano Letters American Chemical Society (2020)

J Liu, A Singh, T Hesjedal, Et al.

Magnetic doping and proximity coupling can open a band gap in a topological insulator (TI) and give rise to dissipationless quantum conduction phenomena. Here, by combining these two approaches, we demonstrate a novel TI superlattice structure that is alternately doped with transition and rare earth elements. An unexpected exchange bias effect is unambiguously confirmed in the superlattice with a large exchange bias field using magneto-transport and magneto-optical techniques. Further, the Curie temperature of the Cr-doped layers in the superlattice is found to increase by 60 K compared to a Cr-doped single-layer film. This result is supported by density-functional-theory calculations, which indicate the presence of antiferromagnetic ordering in Dy:Bi2Te3 induced by proximity coupling to Cr:Sb2Te3 at the interface. This work provides a new pathway to realizing the quantum anomalous Hall effect at elevated temperatures and axion insulator state at zero magnetic field by interface engineering in TI heterostructures.

Coherent transfer of spin angular momentum by evanescent spin #Waves within Antiferromagnetic NiO

Physical Review Letters American Physical Society 124 (2020) 217201

M Dąbrowski, N Takafumi, D Burn, A Frisk, D Newman, C Klewe, Q Li, M Yang, P Shafer, E Arenholz, T Hesjedal, G van der Laan, Z Qiu, R Hicken

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

Tailoring Hybrid Anomalous Hall Response in Engineered Magnetic Topological Insulator Heterostructures

Nano Letters: a journal dedicated to nanoscience and nanotechnology American Chemical Society (2020)

P Chen, Y Zhang, Q Yao, F Tian, L Li, Z Qi, X Liu, L Liao, C Song, J Wang, J Xia, G Li, DM Burn, G van der Laan, T HESJEDAL, S ZHANG, X Kou

Engineering the anomalous Hall effect (AHE) in the emerging magnetic topological insulators (MTIs) has great potentials for quantum information processing and spintronics applications. In this letter, we synthesize the epitaxial Bi2Te3/MnTe magnetic heterostructures and observe pronounced AHE signals from both layers combined together. The evolution of the resulting hybrid AHE intensity with the top Bi2Te3 layer thickness manifests the presence of an intrinsic ferromagnetic phase induced by the topological surface states at the heterolayer-interface. More importantly, by doping the Bi2Te3 layer with Sb, we are able to manipulate the sign of the Berry phase-associated AHE component. Our results demonstrate the un-paralleled advantages of MTI heterostructures over magnetically doped TI counterparts, in which the tunability of the AHE response can be greatly enhanced. This in turn unveils a new avenue for MTI heterostructure-based multifunctional applications.

Magnetic profile of proximity-coupled (Dy,Bi)2Te3/(Cr,Sb)2Te3 topological insulator heterostructures

Physical Review B: Condensed matter and materials physics American Physical Society 100 (2019) 054402

L Duffy, NJ Steinke, DB Burn, A Frisk, L Lari, B Kuerbanjiang, VK Lazarov, G Van Der Laan, S Langridge, T Hesjedal

<p>Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena building on time-reversal symmetry broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment Dy:Bi<sub>2</sub>Te<sub>3</sub>with high transition temperature Cr:Sb<sub>2</sub>Te<sub>3</sub> in a superlattice, and found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the Dy:Bi<sub>2</sub>Te<sub>3</sub> layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin film stack or is surface sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry (PNR), which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface-resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.</p>

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 30 (2019) 125201

J Liu, A Singh, J Llandro, L Duffy, Stanton, Holmes, MJ Applegate, 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.

Helical magnetic ordering in thin FeGe membranes

PHYSICAL REVIEW B 100 (2019) ARTN 184403

DM Burn, SL Zhang, S Wang, HF Du, G van der Laan, T Hesjedal

Tailoring the topological surface state in ultrathin α -Sn(111) films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2019)

VA Rogalev, F Reis, F Adler, M Bauernfeind, J Erhardt, L Dudy, LB Duffy, THORSTEN Hesjedal, M Hoesch, G Bihlmayer, R Claessen, J Schäfer, G Bihlmayer, J Schäfer, A Kowalewski, Scholz, THORSTEN Hesjedal, L Duffy, M Bauernfeind, VA Rogalev, J Erhardt, M Hoesch, F Adler, L Dudy

We report on the electronic structure of α -Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α -Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.

Tailoring the topological surface state in ultrathin alpha -Sn(111) films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2019)

VA Rogalev, F Reis, F Adler, M Bauernfeind, J Erhardt, L Dudy, LB Duffy, T Hesjedal, M Hoesch, G Bihlmayer, J Schaefer, R Claessen

We report on the electronic structure of α-Sn films in the low thickness regime grown on InSb(111)A. High-resolution angle-resolved photoemission (ARPES), enhanced at low photon energies, allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The cross-over to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α-Sn films where the hybridization gap in TSS coexists with the topologically non-trivial electronic structure which must result in a presence of 1D helical edge states.

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.