Publications


Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

Journal of Magnetism and Magnetic Materials 421 (2017) 428-439

A Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, MA Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

© 2016 Elsevier B.V.Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.


Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations

Journal of Magnetism and Magnetic Materials 421 (2017) 428

AA Baker, M Beg, G Ashton, M Albert, D Chernyshenko, W Wang, S Zhang, M-A Bisotti, M Franchin, CL Hu, R Stamps, T Hesjedal, H Fangohr

Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.


Room-temperature helimagnetism in FeGe thin films.

Sci Rep 7 (2017) 123-

SL Zhang, I Stasinopoulos, T Lancaster, F Xiao, A Bauer, F Rucker, AA Baker, AI Figueroa, Z Salman, FL Pratt, SJ Blundell, T Prokscha, A Suter, J Waizner, M Garst, D Grundler, G van der Laan, C Pfleiderer, T Hesjedal

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α intr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.


Direct experimental determination of the topological winding number of skyrmions in Cu2OSeO3.

Nat Commun 8 (2017) 14619-

SL Zhang, G van der Laan, T Hesjedal

The mathematical concept of topology has brought about significant advantages that allow for a fundamental understanding of the underlying physics of a system. In magnetism, the topology of spin order manifests itself in the topological winding number which plays a pivotal role for the determination of the emergent properties of a system. However, the direct experimental determination of the topological winding number of a magnetically ordered system remains elusive. Here, we present a direct relationship between the topological winding number of the spin texture and the polarized resonant X-ray scattering process. This relationship provides a one-to-one correspondence between the measured scattering signal and the winding number. We demonstrate that the exact topological quantities of the skyrmion material Cu2OSeO3 can be directly experimentally determined this way. This technique has the potential to be applicable to a wide range of materials, allowing for a direct determination of their topological properties.


Temperature evolution of topological surface states in Bi<inf>2</inf>Se<inf>3</inf> thin films studied using terahertz spectroscopy

Proceedings of SPIE - The International Society for Optical Engineering 10103 (2017)

VS Kamboj, A Singh, HE Beere, T Hesjedal, CHW Barnes, DA Ritchie

© 2017 SPIE.We have measured the terahertz (THz) conductance of a 23 quintuple layer thick film of bismuth selenide (Bi2Se3) and found signatures for topological surface states (TSSs) below 50 K. We provide evidence for a topological phase transition as a function of lattice temperature by optical means. In this work, we used THz time-domain spectroscopy (THz-TDS) to measure the optical conductance of Bi2Se3, revealing metallic behavior at temperatures below 50 K. We measure the THz conductance of Bi2Se3 as 10 e2/h at 4 K, indicative of a surface dominated response. Furthermore, the THz conductance spectra reveal characteristic features at ∼1.9 THz attributed to the optical phonon mode, which is weakly visible at low temperatures but which becomes more prominent with increasing temperature. These results present a first look at the temperature-dependent behavior of TSSs in Bi2Se3 and the capability to selectively identify and address them using THz spectroscopy.


X-ray magnetic circular dichroism study of Dy-doped Bi<inf>2</inf>Te<inf>3</inf> topological insulator thin films

Journal of Magnetism and Magnetic Materials 422 (2017) 93-99

AI Figueroa, AA Baker, SE Harrison, K Kummer, G van der Laan, T Hesjedal

© 2016 The AuthorsMagnetic doping of topological insulators (TIs) is crucial for unlocking novel quantum phenomena, paving the way for spintronics applications. Recently, we have shown that doping with rare earth ions introduces large magnetic moments and allows for high doping concentrations without the loss of crystal quality, however no long range magnetic order was observed. In Dy-doped Bi2Te3 we found a band gap opening above a critical doping concentration, despite the paramagnetic bulk behavior. Here, we present a surface-sensitive x-ray magnetic circular dichroism (XMCD) study of an in situ cleaved film in the cleanest possible environment. The Dy M4,5 absorption spectra measured with circularly polarized x-rays are fitted using multiplet calculations to obtain the effective magnetic moment. Arrott–Noakes plots, measured by the Dy M5 XMCD as a function of field at low temperatures, give a negative transition temperature. The evaporation of a ferromagnetic Co thin film did not introduce ferromagnetic ordering of the Dy dopants either; instead a lowering of the transition temperature was observed, pointing towards an antiferromagnetic ordering scenario. This result shows that there is a competition between the magnetic exchange interaction and the Zeeman interaction. The latter favors the Co and Dy magnetic moments to be both aligned along the direction of the applied magnetic field, while the exchange interaction is minimized if the Dy and Co atoms are antiferromagnetically coupled, as in zero applied field.


Anisotropic magnetic switching along hard [1 1 0]-type axes in Er-doped DyFe<inf>2</inf>/YFe<inf>2</inf> thin films

Journal of Magnetism and Magnetic Materials 439 (2017) 287-293

GBG Stenning, GJ Bowden, G van der Laan, AI Figueroa, P Bencok, P Steadman, T Hesjedal

© 2017 Elsevier B.V.Epitaxial-grown DyFe2/YFe2 multilayer thin films form an ideal model system for the study of magnetic exchange springs. Here the DyFe2 (YFe2) layers are magnetically hard (soft). In the presence of a magnetic field, exchange springs form in the YFe2 layers. Recently, it has been demonstrated that placing small amounts of Er into the centre of the YFe2 springs generates substantial changes in magnetic behavior. In particular, (i) the number of exchange-spring states is increased dramatically, (ii) the resulting domain-wall states cannot simply be described as either Néel or Bloch walls, (iii) the Er and Dy magnetic loops are strikingly different, and (iv) it is possible to engineer Er-induced magnetic exchange-spring collapse. Here, results are presented for Er-doped (1 1 0)-oriented DyFe2 (60 Å/YFe2(240 Å)15 multilayer films, at 100 K in fields of up to 12 T. In particular, we contrast magnetic loops for fields applied along seemingly equivalent hard-magnetic [1 1 0]-type axes. MBE-grown cubic Laves thin films offer the unique feature of allowing to apply the magnetic field along (i) a hard out-of-plane [1 1 0]-axis (the growth axis) and (ii) a similar hard in-plane [1¯10]-axis. Differences are found and attributed to the competition between the crystal-field interaction at the Er site and the long-range dipole-dipole interaction. In particular, the out-of-plane [1 1 0] Er results show the existence of a new magnetic exchange spring state, which would be very difficult to identify without the aid of element-specific technique of X-ray magnetic circular dichroism (XMCD).


Magnetic proximity-coupling to Cr-doped Sb₂Te₃ thin films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2017)

LB Duffy, AI Figueroa, L Gladczuk, NJ Steinke, K Kummer, G van der Laan, T Hesjedal

Using soft x-ray absorption spectroscopy we determined the chemical and magnetic properties of the magnetic topological insulator (MTI) Cr:Sb2Te3. X-ray magnetic circular dichroism (XMCD) at the Cr L2,3, Te M4,5, and Sb M4,5 edges shows that the Te 5p moment is aligned antiparallel to both the Cr 3d and Sb 5p moments, which is characteristic for carrier-mediated ferromagnetic coupling. Comparison of the Cr L2,3 spectra with multiplet calculations indicates a hybridized Cr state, consistent with the carrier-mediated coupling scenario. We studied the enhancement of the Curie temperature, TC, of the MTI thin film through the magnetic proximity effect. Arrott plots, measured using the Cr L3 XMCD, show a TC ≈ 87 K for the as-cleaved film. After deposition of a thin layer of ferromagnetic Co onto the surface, the TC increases to ∼93 K, while the Co and Cr moments are parallel. This increase in TC is unexpectedly small compared to similar systems reported earlier. The XMCD spectra demonstrate that the Co/MTI interface remains intact, i.e., no reaction between Co and the MTI takes place. Our results are a useful starting point for refining the physical models of Cr-doped Sb2Te3, which is required for making use of them in device applications.


Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

Physical Review B 94 (2016)

AI Figueroa, SL Zhang, AA Baker, R Chalasani, A Kohn, SC Speller, D Gianolio, C Pfleiderer, G Van Der Laan, T Hesjedal

© 2016 authors. Published by the American Physical Society.We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500 Å) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature Tc assumes a thickness-independent enhanced value of ≥43 K as compared with that of bulk MnSi, where Tc≈29K. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111] direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi - except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.


Step-Flow Growth of Bi<inf>2</inf>Te<inf>3</inf> Nanobelts

Crystal Growth and Design 16 (2016) 6961-6966

P Schönherr, T Tilbury, H Wang, AA Haghighirad, V Srot, PA Van Aken, T Hesjedal

© 2016 American Chemical Society.Understanding the growth mechanism of nanostructures is key to tailoring their properties. Many compounds form nanowires following the vapor-liquid-solid (VLS) growth mechanism, and the growth of Bi2Te3 nanobelts was also explained following the VLS route. Here, we present another growth mechanism of Bi2Te3 nano- and submicron belts and ribbons. The samples were grown by physical vapor transport from Bi2Te3 precursors using TiO2 nanoparticles as a catalyst and analyzed by scanning electron microscopy and scanning transmission electron microscopy. The growth starts from a Te-rich cluster and proceeds via a thin, tip-catalyzed primary layer growing in the [110] direction. The primary layer serves as a support for subsequent step-flow growth. The precursor predominantly absorbs on the substrate and reaches the belt by migration from the base to the tip. Terrace edges pose energy barriers that enhance the growth rate of secondary layers compared to the primary layer. Broadening of the sidewalls is commonly observed and leads to triangular voids that can even result in a branching of the growing belts. Step-flow growth of Bi2Te3 submicron belts is different from the spiral-like growth mode of Bi2Te3 thin films and an important step toward the growth of layered topological insulator nanostructures.


Anisotropic Absorption of Pure Spin Currents.

Physical review letters 116 (2016) 047201-

AA Baker, AI Figueroa, CJ Love, SA Cavill, T Hesjedal, G van der Laan

Spin transfer in magnetic multilayers offers the possibility of ultrafast, low-power device operation. We report a study of spin pumping in spin valves, demonstrating that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter, α, in the spin sink layer. Using lab- and synchrotron-based ferromagnetic resonance, we show that an in-plane variation of damping in a crystalline Co_{50}Fe_{50} layer leads to an anisotropic α in a polycrystalline Ni_{81}Fe_{19} layer. This anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. These results offer a valuable insight into the transmission and absorption of spin currents, and a mechanism by which enhanced spin torques and angular control may be realized for next-generation spintronic devices.


Engineering helimagnetism in MnSi thin films

AIP ADVANCES 6 (2016) ARTN 015217

SL Zhang, R Chalasani, AA Baker, N-J Steinke, AI Figueroa, A Kohn, G van der Laan, T Hesjedal


Spin pumping in magnetic trilayer structures with an MgO barrier.

Sci Rep 6 (2016) 35582-

AA Baker, AI Figueroa, D Pingstone, VK Lazarov, G van der Laan, T Hesjedal

We present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4 nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element-specific technique of x-ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.


Organic Transistors: Universal Magnetic Hall Circuit Based on Paired Spin Heterostructures (Adv. Electron. Mater. 6/2015)

Advanced Electronic Materials 1 (2016)

S Zhang, AA Baker, JY Zhang, G Yu, S Wang, T Hesjedal


Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown Cr<inf>x</inf>Sb<inf>2-x</inf>Te<inf>3</inf> thin films

EPL 115 (2016)

LJ Collins-Mcintyre, LB Duffy, A Singh, NJ Steinke, CJ Kinane, TR Charlton, A Pushp, AJ Kellock, SSP Parkin, SN Holmes, CHW Barnes, G Van Der Laan, S Langridge, T Hesjedal

© EPLA, 2016.We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of (in CrxSb2-xTe3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from to almost per Cr ion. The transition temperature depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase-pure films of X = 0.42 ,a Tcof 125 K was determined. Electric transport measurements find surface-dominated transport below ∼10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.


Magnetic ordering in Ho-doped Bi<inf>2</inf>Te<inf>3</inf> topological insulator

Physica Status Solidi - Rapid Research Letters 10 (2016) 467-470

AI Figueroa, SE Harrison, LJ Collins-McIntyre, G van der Laan, T Hesjedal

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimWe investigate the magnetic properties of Ho-doped Bi2Te3 thin films grown by molecular beam epitaxy. Analysis of the polarized X-ray absorption spectra at the Ho M5 absorption edge gives an effective 4f magnetic moment which is ∼45% of the Hund's rule ground state value. X-ray magnetic circular dichroism (XMCD) shows no significant anisotropy, which suggests that the reduced spin moment is not due to the crystal field effects, but rather the presence of non-magnetic or antiferromagnetic Ho sites. Extrapolating the temperature dependence of the XMCD measured in total electron yield and fluorescence yield mode in a field of 7 T gives a Curie–Weiss temperature of ϑCW ≈ –30 K, which suggests antiferromagnetic ordering, in contrast to the paramagnetic behavior observed with SQUID magnetometry. From the anomaly of the XMCD signal at low temperatures, a Néel temperature TN between 10 K and 25 K is estimated. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim).


One-Step SnO2 Nanotree Growth.

Chemistry (Weinheim an der Bergstrasse, Germany) (2016)

P Schönherr, T Hesjedal

A comparison between Au, TiO2 and self-catalysed growth of SnO2 nanostructures using chemical vapour deposition is reported. TiO2 enables growth of a nanonetwork of SnO2 , whereas self-catalysed growth results in nanoclusters. Using Au catalyst, single-crystalline SnO2 nanowire trees can be grown in a one-step process. Two types of trees are identified that differ in size, presence of a catalytic tip, and degree of branching. The growth mechanism of these nanotrees is based on branch-splitting and self-seeding by the catalytic tip, facilitating at least three levels of branching, namely trunk, branch and leaf.


Oxidation Effects in Rare Earth Doped Topological Insulator Thin Films.

Scientific reports 6 (2016) 22935-

AI Figueroa, G van der Laan, SE Harrison, G Cibin, T Hesjedal

The breaking of time-reversal symmetry (TRS) in topological insulators is a prerequisite for unlocking their exotic properties and for observing the quantum anomalous Hall effect (QAHE). The incorporation of dopants which exhibit magnetic long-range order is the most promising approach for TRS-breaking. REBiTe3, wherein 50% of the Bi is substitutionally replaced by a RE atom (RE = Gd, Dy, and Ho), is a predicted QAHE system. Despite the low solubility of REs in bulk crystals of a few %, highly doped thin films have been demonstrated, which are free of secondary phases and of high crystalline quality. Here we study the effects of exposure to atmosphere of rare earth-doped Bi2(Se, Te)3 thin films using x-ray absorption spectroscopy. We demonstrate that these RE dopants are all trivalent and effectively substitute for Bi(3+) in the Bi2(Se, Te)3 matrix. We find an unexpected high degree of sample oxidation for the most highly doped samples, which is not restricted to the surface of the films. In the low-doping limit, the RE-doped films mostly show surface oxidation, which can be prevented by surface passivation, encapsulation, or in-situ cleaving to recover the topological surface state.


Multidomain Skyrmion Lattice State in Cu2OSeO3.

Nano letters 16 (2016) 3285-3291

SL Zhang, A Bauer, DM Burn, P Milde, E Neuber, LM Eng, H Berger, C Pfleiderer, G van der Laan, T Hesjedal

Magnetic skyrmions in chiral magnets are nanoscale, topologically protected magnetization swirls that are promising candidates for spintronics memory carriers. Therefore, observing and manipulating the skyrmion state on the surface level of the materials are of great importance for future applications. Here, we report a controlled way of creating a multidomain skyrmion state near the surface of a Cu2OSeO3 single crystal, observed by soft resonant elastic X-ray scattering. This technique is an ideal tool to probe the magnetic order at the L3 edge of 3d metal compounds giving an average depth sensitivity of ∼50 nm. The single-domain 6-fold-symmetric skyrmion lattice can be broken up into domains, overcoming the propagation directions imposed by the cubic anisotropy by applying the magnetic field in directions deviating from the major cubic axes. Our findings open the door to a new way to manipulate and engineer the skyrmion state locally on the surface or on the level of individual skyrmions, which will enable applications in the future.


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

Physical Review B 93 (2016)

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

© 2016 American Physical Society.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 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.

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