Publications


Electron spin as fingerprint for charge generation and transport in doped organic semiconductors

Journal of Materials Chemistry C Royal Society of Chemistry (2021)

A Privitera, P Warren, G Londi, P Kaienburg, J Liu, A Sperlich, AE Lauritzen, O Thimm, A Ardavan, D Beljonne, M Riede

We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two main paramagnetic species distinguished by two different g-tensors, which are assigned based on density functional theory calculations to the formation of a positive polaron on the host and a radical anion on the dopant. Close inspection of the EPR spectra shows that radical anions on the dopants couple in an antiferromagnetic manner at device-relevant doping concentrations, thereby suggesting the presence of dopant clustering, and that positive polarons on the molecular host move by polaron hopping with an activation energy of 5 meV. This activation energy is substantially smaller than that inferred from electrical conductivity measurements (∼233 meV), as the latter also includes a (major) contribution from charge-transfer state dissociation. It emerges from this study that probing the electron spin can provide rich information on the nature and dynamics of charge carriers generated upon doping molecular semiconductors, which could serve as a basis for the design of the next generation of dopant and host materials.


Canted standing spin-wave modes of Permalloy thin films observed by Ferromagnetic Resonance

New Journal of Physics IOP Publishing (2021)

M Dabrowski, RJ Hicken, A Frisk, DG Newman, AT N'Diaye, C Klewe, P Shafer, G van der Laan, T Hesjedal, G Bowden


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

AIP Advances American Institute of Physics 11 (2021) 15327

D 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 recently 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 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

AIP Advances AIP Publishing 11 (2021) 015108

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 Néel 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 Néel twisting is 7× 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.


Effects of magnetic dilution in the ferrimagnetic columnar ordered Sm2MnMnMn4-xTixO12 perovskites

PHYSICAL REVIEW B 102 (2020) ARTN 214428

AM Vibhakar, DD Khalyavin, P Manuel, R Liu, K Yamaura, AA Belik, RD Johnson

© 2020 American Physical Society. Powder neutron-diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the Sm2MnMnMn4-xTixO12 A-site columnar ordered quadruple perovskite manganites (x=1, x=2, and x=3). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27, 62, and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the x=1 and x=2 samples, which indicates a considerable departure from mean-field behavior. This result is corroborated by large reductions in the theoretical ground-state magnetic moments observed across the series, which indicate the presence of spin fluctuations and/or disorder. We show that long-range magnetic order in the x=3 sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if it is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally, we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.


Magnetically induced metal-insulator transition in Pb2CaOsO6

PHYSICAL REVIEW B 102 (2020) ARTN 214409

H Jacobsen, HL Feng, AJ Princep, MC Rahn, Y Guo, J Chen, Y Matsushita, Y Tsujimoto, M Nagao, D Khalyavin, P Manuel, CA Murray, C Donnerer, JG Vale, MM Sala, K Yamaura, AT Boothroyd


Controlling spin current polarization through non-collinear antiferromagnetism.

Nature communications 11 (2020) 4671-

T Nan, CX Quintela, J Irwin, G Gurung, DF Shao, J Gibbons, N Campbell, K Song, S-Y Choi, L Guo, RD Johnson, P Manuel, RV Chopdekar, I Hallsteinsen, T Tybell, PJ Ryan, J-W Kim, Y Choi, PG Radaelli, DC Ralph, EY Tsymbal, MS Rzchowski, CB Eom

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn<sub>3</sub>GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn<sub>3</sub>GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.


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

ACS Applied Materials and Interfaces American Chemical Society 12 (2020) 52116-52124

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.


Severe Dirac Mass Gap Suppression in Sb<sub>2</sub>Te<sub>3</sub>-Based Quantum Anomalous Hall Materials.

Nano letters 20 (2020) 8001-8007

YX Chong, X Liu, R Sharma, A Kostin, G Gu, K Fujita, JCS Davis, PO Sprau

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> to that of its nonmagnetic parent (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, to explore the cause. In (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb<sub>2</sub>Te<sub>3</sub>-based FMTI materials to very low temperatures.


Magnetic order in 3D topological insulators - wishful thinking or gateway to emergent quantum effects?

Applied Physics Letters AIP Publishing 117 (2020) 150502

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.


Glide symmetry breaking and Ising criticality in the quasi-1D magnet CoNb2O6

Proceedings of the National Academy of Sciences National Academy of Sciences 117 (2020) 25219-25224

M Fava, R Coldea, S Ashok Parameswaran

We construct a microscopic spin-exchange Hamiltonian for the quasi–one-dimensional (1D) Ising magnet CoNb2O6 that captures detailed and hitherto-unexplained aspects of its dynamic spin structure factor. We perform a symmetry analysis that recalls that an individual Ising chain in this material is buckled, with two sites in each unit cell related by a glide symmetry. Combining this with numerical simulations benchmarked against neutron scattering experiments, we argue that the single-chain Hamiltonian contains a staggered spin-exchange term. We further argue that the transverse-field–tuned quantum critical point in CoNb2O6 corresponds to breaking this glide symmetry, rather than an on-site Ising symmetry as previously believed. This gives a unified microscopic explanation of the dispersion of confined states in the ordered phase and quasiparticle breakdown in the polarized phase at high transverse field.


Magnetic skyrmions

MagNews UK Magnetics Society 2019 (2020) 19-21

G van der Laan, T Hesjedal


Skyrmions getting an X-ray

MagNews UK Magnetics Society 2019 (2020) 22-22

S Zhang, T Hesjedal, G van der Laan


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 &#x2018;printing&#x2019; 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 &#x2018;print&#x2019; 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.


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

Physical Review Letters American Physical Society 125 (2020) 137201

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.


Magnetically driven loss of centrosymmetry in metallic Pb2CoOsO6

PHYSICAL REVIEW B 102 (2020) ARTN 104410

AJ Princep, HL Feng, YF Guo, F Lang, HM Weng, P Manuel, D Khalyavin, A Senyshyn, MC Rahn, YH Yuan, Y Matsushita, SJ Blundell, K Yamaura, AT Boothroyd


Photo-molecular high temperature superconductivity

Physical Review X American Physical Society 10 (2020) 031028

M Buzzi, D Nicoletti, M Fechner, N Tancogne-Dejean, MA Sentef, A Georges, T Biesner, E Uykur, M Dressel, A Henderson, T Siegrist, JA Schlueter, K Miyagawa, K Kanoda, M-S Nam, A Ardavan, J Coulthard, J Tindall, F Schlawin, D Jaksch, A Cavalleri

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ−(BEDT−TTF)2 Cu[N(CN)2] Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T∗≃50K, far higher than the equilibrium transition temperature TC=12.5K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.


Information and Decoherence in a Muon-Fluorine Coupled System

PHYSICAL REVIEW LETTERS 125 (2020) 87201

J Wilkinson, S Blundell


Absolute crystal and magnetic chiralities in the langasite compound Ba3NbFe3Si2O14 determined by polarized neutron and x-ray scattering

Physical Review B American Physical Society 102 (2020) 54417

N Qureshi, A Bombardi, S Picozzi, P Barone, E Lelièvre-Berna, X Xu, C Stock, D McMorrow, A Hearmon, F Fabrizi, P Radaelli, S-W Cheong, L Chapon

We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.


Emergent helical texture of electric dipoles

Science American Association for the Advancement of Science 369 (2020) 680-684

D Khalyavin, R Johnson, F Orlandi, P Radaelli, P Manuel, AA Belik

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