Publications by Thorsten Hesjedal


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.


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.


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

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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.


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.


Reciprocal space mapping of magnetic order in thick epitaxial MnSi films

arxiv (0)

B Wiedemann, A Chacon, SL Zhang, Y Khaydukov, T Hesjedal, O Soltwedel, T Keller, S Mühlbauer, T Adams, M Halder, C Pfleiderer, P Böni

We report grazing incidence small angle neutron scattering (GISANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit. Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a magnetic modulation perpendicular to the films under magnetic fields parallel and perpendicular to the film, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of field orientation, our data does not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct difference with bulk samples of MnSi of similar thickness under perpendicular field, in which a skyrmion lattice dominates the phase diagram. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell control the magnetic state, representing the main difference between the films and thin bulk samples.


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

arXiv:1511.04972v1 (0)

T Lancaster, F Xiao, IO Thomas, Z Salman, SJ Blundell, FL Pratt, T Prokscha, SJ Clark, 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.


Messungen an Interdigitalwandlern mit dem akustischen Kraftmikroskop

Fortschritte der Akustik - DAGA95 DEGA (1995) 731-734

T Hesjedal, E Chilla, HJ Fröhlich


Scanning acoustic force microscope detection of SAWs

Proceedings of the IEEE Ultrasonics Symposium 1 (1994) 363-366

E Chilla, T Hesjedal, HJ Froehlich

We present a novel method for the investigation of surface acoustic wave (SAW) fields with nanometer resolution by a scanning acoustic force microscope (SAFM). The detection of ultrasound by a force microscope is connected with the nonlinear dependence of the force on the distance between the tip and the surface. Due to this nonlinearity there is an effective shift of the mean position of the tip if one modulates the gap distance by a propagating SAW. Furthermore, the surface charges influence the tip deflection. By operating the SAFM in the dynamic mode the wave amplitude as well as the charge distribution can be measured at common surface structures. The measurements were carried out by a contact and a non-contact mode scanning force microscope (SFM). The SAWs were excited by an interdigital transducer (IDT) on a YZ-LiNbO3 crystal cut at 30.75 MHz (contact) and 39.5 MHz (non-contact). The amplitude of the SAW was modulated by a low frequency (5 Hz and 50 kHz). The low frequency oscillation of the average of the tip-to-surface distance was measured by lock-in technique in dependence on the amplitude and the frequency of the SAW. The measurements of the amplitude of the SAW and the charge distribution were carried out within the IDT.


Epitaxial Heusler Alloys on III-V Semiconductors

John Wiley & Sons, Ltd (0)

T Hesjedal, KH Ploog


Magnetic skyrmion interactions in the micromagnetic framework

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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.


Diameter-independent skyrmion Hall angle observed in chiral magnetic multilayers

Nature Communications Nature Research (part of Springer Nature) (0)

K Zeissler, S Finizio, C Barton, A Huxtable, J Massey, J Raabe, A Sadovnikov, S Nikitov, R Brearton, T Hesjedal, G van der Laan, M Rosamond, E Linfield, G Burnell, C Marrows


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


Calculation and experimental verification of the acoustic stress at GHz frequencies in SAW resonators

Proc. 33rd European Microwave Conference (0)

F Kubat, W Ruile, T Hesjedal, J Stotz, U Roesler, L Reindl


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

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ME Harmon, MX Tang, T Hesjedal, CW Frank


Coherent transfer of spin angular momentum by evanescent spin waves within antiferromagnetic NiO

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DG Newman, M Dabrowski, A Frisk, T Nakano, ZQ Qiu, THORSTEN Hesjedal, E Arenholz, RJ Hicken, P Shafer, GVD Laan, DM Burn, C Klewe, M Yang, Q Li

Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz frequency within antiferromagnetic NiO at room temperature. Using element-specific and phase-resolved x-ray ferromagnetic resonance, we probe the injection and transmission of ac spin current, and demonstrate that insertion of a few nanometre thick epitaxial NiO(001) layer between a ferromagnet and non-magnet can even enhance the flow of spin current. Our results pave the way towards coherent control of the phase and amplitude of spin currents at the nanoscale, and enable the realization of spin-logic devices and spin current amplifiers that operate at GHz and THz frequencies.


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

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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.

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