# Publications by Thorsten Hesjedal

## Diameter-independent skyrmion Hall angle in the plastic flow regime observed in chiral magnetic multilayers

arxiv.org (0)

K Zeissler, S Finizio, C Barton, A Huxtable, J Massey, J Raabe, AV Sadovnikov, SA Nikitov, R Brearton, T Hesjedal, GVD Laan, MC Rosamond, EH Linfield, G Burnell, CH Marrows

Magnetic skyrmions are topologically non-trivial nanoscale objects. Their topology, which originates in their chiral domain wall winding, governs their unique response to a motion-inducing force. When subjected to an electrical current, the chiral winding of the spin texture leads to a deflection of the skyrmion trajectory, characterized by an angle with respect to the applied force direction. This skyrmion Hall angle was believed to be skyrmion diameter-dependent. In contrast, our experimental study finds that within the plastic flow regime the skyrmion Hall angle is diameter-independent. At an average velocity of 6 $\pm$ 1 m/s the average skyrmion Hall angle was measured to be 9{\deg} $\pm$ 2{\deg}. In fact, in the plastic flow regime, the skyrmion dynamics is dominated by the local energy landscape such as materials defects and the local magnetic configuration.

## Optically and microwave induced magnetization precession in [Co/Pt]/NiFe exchange springs

ACS Applied Materials and Interfaces American Chemical Society (0)

M Dabrowski,, A Frisk, D Burn, D Newman, C Klewe, A N'Diaye, P Shafer, E Arenholz, G Bowden, T Hesjedal, G van der Laan, G Hrkac, R Hicken

Microwave and heat assisted magnetic recording are two competing technologies that have greatly increased the capacity of hard disk drives. The efficiency of the magnetic recording process can be further improved by employing non-collinear spin structures that combine perpendicular and in-plane magnetic anisotropy. Here, we investigate both microwave and optically excited magnetization dynamics in [Co/Pt]/NiFe exchange spring samples. The resulting canted magnetization within the nanoscale [Co/Pt]/NiFe interfacial region allows for optically stimulated magnetization precession to be observed for an extended magnetic field and frequency range. The results can be explained by formation of an imprinted domain structure, which locks the magnetization orientation and makes the structures more robust against external perturbations. Tuning the canted interfacial domain structure may provide greater control of optically excited magnetization reversal and optically generated spin currents, which are of paramount importance for future ultrafast magnetic recording and spintronic applications.

## Magnetic Order in 3D Topological Insulators - Wishful Thinking or Gateway to Emergent Quantum Effects?

Applied Physics Letters AIP Publishing (0)

AI Figueroa, T Hesjedal, N-J Steinke

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counter-propagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI, and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS), and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)2Te3, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or by proximity coupling to a magnetically ordered layer or substrate, in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)2(Se,Te)3 family, and try to answer the question whether these materials can live up to the hype surrounding them.

## Proximity-induced odd-frequency superconductivity in a topological insulator

arxiv (0)

JA Krieger, A Pertsova, SR Giblin, M Döbeli, T Prokscha, CW Schneider, A Suter, T Hesjedal, AV 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, Bi$_2$Se$_3$, proximity-coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ 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 an odd-frequency component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer (BCS) 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.

## Direct observation of the energy gain underpinning ferromagnetic superexchange in the electronic structure of CrGeTe$_3$

arxiv (0)

I Marković, F Mazzola, A Rajan, EA Morales, DM Burn, THORSTEN Hesjedal, GVD Laan, S Mukherjee, TK Kim, C Bigi, I Vobornik, G Balakrishnan, MC Hatnean, PDC King, G Balakrishnan, S Mukherjee, MC Hatnean, I Vobornik, GVD Laan, PDC King, C Bigi, A Rajan, TK Kim, I Marković

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe$_3$. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through ${T_\mathrm{C}}$. 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 ${e_g}$ orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr ${t_{2g}}$ states that carry the majority of the spin moment. The ${t_{2g}}$ 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.

## Magnetic skyrmion interactions in the micromagnetic framework

arxiv (0)

GVD Laan, R Brearton, T Hesjedal

Magnetic skyrmions are localized swirls of magnetization with a non-trivial 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 which can generate arbitrary repulsive interaction potentials from the micromagnetic Hamiltonian. We also discuss a toy model of the radial profile of a skyrmion which is accurate for a wide range of material parameters.

## Field and temperature dependence of the skyrmion lattice phase in chiral magnet membranes

Physical review B: Condensed matter and materials physics American Physical Society (0)

D Burn, S Wang, W Wang, G Van Der Laan, S Zhang, H Du, T Hesjedal

Magnetic skyrmions are nanosized magnetization whirls that exhibit topological robustness and nontrivial magnetoelectrical properties, such as emergent electromagnetism and intriguing spin dynamics in the microwave-frequency region. In chiral magnets, skyrmions are usually found at a pocket in the phase diagram in the vicinity of the ordering temperature, wherein they order in the form of a hexagonal skyrmion lattice (SkL). It is generally believed that this equilibrium SkL phase is a uniform, long-range-ordered magnetic structure with a well-defined lattice constant. Here, using high-resolution small angle resonant elastic x-ray scattering, we study the field- and temperature-dependence of the skyrmion lattice in FeGe and membranes. Indeed, shows the expected rigid skyrmion lattice, known from bulk samples, that is unaffected by tuning field and temperature within the phase pocket. In stark contrast, the lattice constant and skyrmion size in FeGe membranes undergo a continuous evolution within the skyrmion phase pocket, whereby the lattice constant changes by up to 15% and the magnetic scattering intensity varies significantly. Using micromagnetic modeling, it is found that for FeGe the competing energy terms contributing to the formation of the skyrmion lattice fully explain this breathing behavior. In contrast, for this stabilizing energy balance is less affected by the smaller field variation across the skyrmion pocket, leading to the observed rigid lattice structure.

## The topological surface state of $α$-Sn on InSb(001) as studied by photoemission

arxiv Museu de Ciències Naturals de Barcelona (0)

MR Scholz, VA Rogalev, L Dudy, F Reis, F Adler, J Aulbach, LJ Collins-McIntyre, LB Duffy, HF Yang, YL Chen, T Hesjedal, ZK Liu, M Hoesch, S Muff, JH Dil, J Schäfer, R Claessen

We report on the electronic structure of the elemental topological semimetal $\alpha$-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional $p$-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.

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

arXiv:1511.04972v1 (0)

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

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 K 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.

## Three-dimensional micromagnetic domain structure of MnAs films on GaAs(001): Experimental imaging and simulations

PHYSICAL REVIEW B AMERICAN PHYSICAL SOC 75 (0) 9

R Engel-Herbert, T Hesjedal, DM Schaadt

The micromagnetic domain structure of MnAs films on GaAs(001) has been systematically investigated by micromagnetic imaging and simulations. The magnetic force microscopy (MFM) contrast resulting from the stray field of the simulated three-dimensional domain patterns was calculated and found to be in excellent agreement with MFM experiments. By combining three-dimensional stray-field imaging by MFM with surface sensitive probing and micromagnetic simulations, we were able to derive a consistent picture of the micromagnetic structure of MnAs. For example, the origin of the comblike contrast observed through MFM was identified as a metastable domain configuration exhibiting a cross-tie wall.

## Epitaxial Heusler Alloys on III-V Semiconductors

John Wiley & Sons, Ltd (0)

T Hesjedal, KH Ploog

## Magnetization dynamics in ordered spin structures revealed by diffractive and reflectometry ferromagnetic resonance

DM Burn, S ZHANG, G van der Laan, T HESJEDAL

Synchrotron radiation based techniques provide unique insight into both the element and time resolved magnetization behavior in magnetic spin systems. Here, we highlight the power of two recent developments, utilizing x-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz regime, both in diffraction and reflection configurations. Our newly developed diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) techniques provide novel ways to explore the dynamics of modern magnetic materials, thereby opening up new pathways for the development of spintronic devices. In this paper we provide an overview of these techniques, and discuss the new understanding they provide into in the magnetization dynamics in the chiral magnetic structure in Y-type hexaferrite and the depth dependence to the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer.

## Magnetization dynamics in ordered spin structures revealed by diffractive and reflectometry ferromagnetic resonance

DM Burn, S ZHANG, G van der Laan, T HESJEDAL

Synchrotron radiation based techniques provide unique insight into both the element and time resolved magnetization behavior in magnetic spin systems. Here, we highlight the power of two recent developments, utilizing x-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz regime, both in diffraction and reflection configurations. Our newly developed diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) techniques provide novel ways to explore the dynamics of modern magnetic materials, thereby opening up new pathways for the development of spintronic devices. In this paper we provide an overview of these techniques, and discuss the new understanding they provide into in the magnetization dynamics in the chiral magnetic structure in Y-type hexaferrite and the depth dependence to the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer.

## Depth profiling of 3D skyrmion lattices in a chiral magnet - A story with a twist

G van der Laan, S Zhang, T Hesjedal

From the perspective of surface science, only the topmost atomic layers usually exhibit physical properties that are different to those of the bulk material, whereas the deeper layers are assumed to be bulk-like and remain largely unexplored. Going beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of magnetic skyrmions below the surface of a bulk Cu2OSeO3 sample using the polarization dependence of resonant elastic x-ray scattering (REXS). While the bulk spin configuration showed the anticipated Bloch type structure, it was found that the skyrmion lattice changes to a N&#xB4;eel twisting (i.e., with a different helicity angle) at the surface within a distance of several hundred nm. The exact surface helicity angle and penetration length of this twist have been determined, revealing the detailed internal structure of the skyrmion tube. It was found that the experimental penetration length of the N&#xB4;eel twisting is 7&#xD7; longer than the theoretical value given by the ratio of J=D. This indicates that apart from the considered spin interactions, i.e., the Heisenberg exchange interaction J and the Dzyaloshinskii-Moriya interaction D, as well as the Zeeman interaction, other effects must play an important role. The findings suggest that the surface reconstruction of the skyrmion lattice is a universal phenomenon, stemming from the breaking of translational symmetry at the interface.

## Stress-induced semiautomatic wet chemical etching of MnAs nanodots on GaAs(001) substrates and their magnetic properties

Proc. 7th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructure (0)

Y Takagaki, E Wiebicke, T Hesjedal, L Daeweritz, KH Ploog

## A Simple and Novel Approach to Fabricating Microfluidic Components Actuated By Termoresponsive Hydrogels

(0)

ME Harmon, MX Tang, T Hesjedal, CW Frank