Publications


Reversible hydrogen control of antiferromagnetic anisotropy in α-Fe<sub>2</sub>O<sub>3</sub>.

Nature communications 12 (2021) 1668-

H Jani, J Linghu, S Hooda, RV Chopdekar, C Li, GJ Omar, S Prakash, Y Du, P Yang, A Banas, K Banas, S Ghosh, S Ojha, GR Umapathy, D Kanjilal, A Ariando, SJ Pennycook, E Arenholz, PG Radaelli, JMD Coey, YP Feng, T Venkatesan

Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel proof-of-principle ionic approach to control the spin reorientation (Morin) transition reversibly in the common antiferromagnetic insulator α-Fe<sub>2</sub>O<sub>3</sub> (haematite) - now an emerging spintronic material that hosts topological antiferromagnetic spin-textures and long magnon-diffusion lengths. We use a low-temperature catalytic-spillover process involving the post-growth incorporation or removal of hydrogen from α-Fe<sub>2</sub>O<sub>3</sub> thin films. Hydrogenation drives pronounced changes in its magnetic anisotropy, Néel vector orientation and canted magnetism via electron injection and local distortions. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate reversible control of the room-temperature spin-state by doping/expelling hydrogen in Rh-substituted α-Fe<sub>2</sub>O<sub>3</sub>.


Probing resonating valence bond states in artificial quantum magnets

Nature Communications Springer Nature 12 (2021) 993

K Yang, S-H Phark, Y Bae, T Esat, A Ardavan, P Willke, A Heinrich, C Lutz

Designing and characterizing the many-body behaviors of quantum materials represents a prominent challenge for understanding strongly correlated physics and quantum information processing. We constructed artificial quantum magnets on a surface by using spin-1/2 atoms in a scanning tunneling microscope (STM). These coupled spins feature strong quantum fluctuations due to antiferromagnetic exchange interactions between neighboring atoms. To characterize the resulting collective magnetic states and their energy levels, we performed electron spin resonance on individual atoms within each quantum magnet. This gives atomic-scale access to properties of the exotic quantum many-body states, such as a finite-size realization of a resonating valence bond state. The tunable atomic-scale magnetic field from the STM tip allows us to further characterize and engineer the quantum states. These results open a new avenue to designing and exploring quantum magnets at the atomic scale for applications in spintronics and quantum simulations.


Detailed crystallographic analysis of the ice V to ice XIII hydrogen-ordering phase transition.

The Journal of chemical physics 154 (2021) 134504-

CG Salzmann, A Rosu-Finsen, Z Sharif, PG Radaelli, JL Finney

Ice V is a structurally highly complex material with 28 water molecules in its monoclinic unit cell. It is classified as a hydrogen-disordered phase of ice. Yet, some of its hydrogen-bonded water molecules display significant orientational order. Upon cooling pure ice V, additional orientational ordering cannot be achieved on the experimental time scale. Doping with hydrochloric acid has been shown to be most effective in enabling the phase transition of ice V to its hydrogen-ordered counterpart ice XIII. Here, we present a detailed crystallographic study of this phase transition investigating the effects of hydrochloric and hydrofluoric acid as well as lithium and potassium hydroxide doping. The magnitudes of the stepwise changes in the lattice constants during the phase transition are found to be more sensitive indicators for the extent of hydrogen order in ice XIII than the appearance of new Bragg peaks. Hydrofluoric acid and lithium hydroxide doping enable similar ordering processes as hydrochloric acid but with slower kinetics. The various possible space groups and ordered configurations of ice XIII are examined systematically, and the previously determined P2<sub>1</sub>/a structure is confirmed. Interestingly, the partial hydrogen order already present in ice V is found to perpetuate into ice XIII, and these ordering processes are found to be independent of pressure. Overall, the hydrogen ordering goes along with a small increase in volume, which appears to be the origin of the slower hydrogen-ordering kinetics under pressure. Heating pressure-quenched samples at ambient pressure revealed low-temperature "transient ordering" features in both diffraction and calorimetry.


Crystallographic, optical, and electronic properties of the Cs2AgBi1-xInxBr6 double perovskite: understanding the fundamental photovoltaic efficiency challenges

ACS Energy Letters American Chemical Society 6 (2021) 1073-1081

L Schade, S Mahesh, G Volonakis, M Zacharias, B Wenger, F Schmidt, S Vajjala Kesava, D Prabhakaran, M Abdi-Jalebi, M Lenz, F Giustino, G Longo, P Radaelli, H Snaith

We present a crystallographic and optoelectronic study of the double perovskite Cs2AgBi1–xInxBr6. From structural characterization we determine that the indium cation shrinks the lattice and shifts the cubic-to-tetragonal phase transition point to lower temperatures. The absorption onset is shifted to shorter wavelengths upon increasing the indium content, leading to wider band gaps, which we rationalize through first-principles band structure calculations. Despite the unfavorable band gap shift, we observe an enhancement in the steady-state photoluminescence intensity, and n-i-p photovoltaic devices present short-circuit current greater than that of neat Cs2AgBiBr6 devices. In order to evaluate the prospects of this material as a solar absorber, we combine accurate absorption measurements with thermodynamic modeling and identify the fundamental limitations of this system. Provided radiative efficiency can be increased and the choice of charge extraction layers are specifically improved, this material could prove to be a useful wide band gap solar absorber.


Coherent electric field manipulation of Fe3+-spins in PbTiO3

Science Advances American Association for the Advancement of Science 7 (2021) eabf8103

J Liu, V Laguta, K Inzani, W Huang, S Das, R Chatterjee, E Sheridan, S Griffin, A Ardavan, R Ramesh

Magnetoelectrics, materials which exhibit coupling between magnetic and electric degrees of freedom, not only offer a rich environment for studying the fundamental materials physics of spin-charge coupling, but also present opportunities for future information technology paradigms. We present results of electric field manipulation of spins in a ferroelectric medium using dilute Fe3+-doped PbTiO3 as a model system. Combining first-principles calculations and electron paramagnetic resonance (EPR), we show that the Fe3+ spins are preferentially aligned perpendicular to the ferroelectric polar axis, which we can manipulate using an electric field. We also demonstrate coherent control of the phase of spin superpositions by applying electric field pulses during time-resolved EPR measurements. Our results suggest a new pathway towards the manipulation of spins for quantum and classical spintronics.


Effect of sodium bicarbonate solution on methyltrimethoxysilane-derived silica aerogels dried at ambient pressure

FRONTIERS OF CHEMICAL SCIENCE AND ENGINEERING (2021)

Y Liu, X Han, B Kuerbanjiang, VK Lazarov, L Siller


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.


Halide segregation in mixed-halide perovskites: influence of a-site cations

ACS Energy Letters American Chemical Society 6 (2021) 799-808

A Knight, AJ Borchert, RDJ Oliver, J Patel, PG Radaelli, H Snaith, MB Johnston, LM Herz

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. We report evidence for low-barrier ionic pathways in MAPb(Br0.5I0.5)3, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.


Spin-current mediated exchange coupling in MgO-based magnetic tunnel junctions

Physical Review B: Condensed Matter and Materials Physics American Physical Society 103 (2021) 064416

L Gladczuk, L Gladczuk, P Dluzewski, K Lasek, P Aleshkevych, D Burn, G van der Laan, T Hesjedal

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L3 x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping.


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.


Creation of a Chiral Bobber Lattice in Helimagnet-Multilayer Heterostructures

PHYSICAL REVIEW LETTERS 126 (2021) ARTN 017204

K Ran, Y Liu, Y Guang, DM Burn, G van der Laan, T Hesjedal, H Du, G Yu, S Zhang


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

&#xA9; 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


Polarizing an antiferromagnet by optical engineering of the crystal field (June, 10.1038/s41567-020-0936-3, 2020)

NATURE PHYSICS 16 (2020) 1238-1238

AS Disa, M Fechner, TF Nova, B Liu, M Foerst, D Prabhakaran, PG Radaelli, A Cavalleri


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.


Magnetic order and disorder in a quasi-two-dimensional quantum Heisenberg antiferromagnet with randomized exchange

PHYSICAL REVIEW B 102 (2020) ARTN 174429

F Xiao, WJA Blackmore, BM Huddart, M Gomilsek, TJ Hicken, C Baines, PJ Baker, FL Pratt, SJ Blundell, H Lu, J Singleton, D Gawryluk, MM Turnbull, KW Kramer, PA Goddard, T Lancaster


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.


Unveiling the ultrafast optoelectronic properties of 3D Dirac semi-metal CdAs

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2020-November (2020) 389-

JL Boland, CQ Xia, DA Damry, P Schonherr, T Hesjedal, LM Herz, MB Johnston

We employ ultrafast optical-pump terahertz-probe spectroscopy and ultrafast THz emission spectroscopy to investigate the ultrafast charge carrier dynamics in the 3D Dirac semi-metal CdAs. We extract the temperature-dependent electron mobility (16,000cmVs at 5K) for CdAs nanowire ensemble. We also demonstrate strong THz emission from both CdAs single crystal and nanowires, whose polarity depends strongly on incident angle and pump polarisation.

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