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Browsing by Author "Bake, A"

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Antiferromagnetic topological insulating state in Tb0.02Bi1.08Sb0.9Te2S single crystals
(American Physical Society (APS), 2023-03-13) Guo, L; Zhao, WY; Li, Q; Xu, M; Chen, L; Bake, A; Vu, THY; He, YH; Fang, Y; Cortie, DL; Mo, SK; Edmonds, MT; Wang, XL; Dong, S; Karel, J; Zheng, RK
Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantized anomalous Hall insulator, which has been observed in magnetic-topological-insulator-based devices. In this work, we report a successful doping of rare earth element Tb into Bi1.08Sb0.9Te2S topological insulator single crystals, in which the Tb moments are antiferromagnetically ordered below ∼10 K. Benefiting from the in-bulk-gap Fermi level, transport behavior dominant by the topological surface states is observed below ∼150 K. At low temperatures, strong Shubnikov-de Haas oscillations are observed, which exhibit 2D-like behavior. The topological insulator with long range magnetic ordering in rare earth doped Bi1.08Sb0.9Te2S single crystal provides an ideal platform for quantum transport studies and potential applications. ©2023 American Physical Society.
Creating thin magnetic layers at the surface of Sb2Te3 topological insulators using a low-energy chromium ion beam
(AIP Publishing, 2020-05-11) Cortie, DL; Zhao, WY; Yue, Z; Li, Z; Bake, A; Marenych, O; Pastuovic, Z; Nancarrow, M; Zhang, ZM; Qi, DC; Evans, PJ; Mitchell, DRG; Wang, XL
The surfaces of Sb2Te3 topological insulator crystals were implanted using a 40 keV chromium ion beam. To facilitate uniform doping, the Sb2Te3 was passivated with a thin TiO2 film before the implantation step. The resulting chemical structure was studied using atomic-resolution transmission electron microscopy. A fluence of 7 × 1015 ions/cm2 at 40 keV lead to amorphization of the Sb2Te3 surface, with chromium predominantly incorporated in the amorphous layer. Heating to 200 °C caused the amorphous region to recrystallize and led to the formation of a thin chromium-rich interfacial layer. Near-edge x-ray absorption spectroscopy indicates a uniform valence state of Cr3+ throughout, with no evidence of metallic clustering. High-temperature superparamagnetic behavior was detected up to 300 K, with an increased magnetic moment below 50 K. © 2020 Author(s).
Enhanced thermoelectric performance and mechanical strength of n-type BiTeSe materials produced via a composite strategy
(Elsevier, 2022-01) Yang, G; Sang, L; Mitchell, DRG; Yun, FF; See, KW; Ahmed, AJ; Sayyar, S; Bake, A; Liu, P; Chen, L; Yue, ZJ; Cortie, DL; Wang, XL
Zone-melted Bi2Te2.7Se0.3 (ZM BTS) alloys are typical n-type commercial thermoelectric (TE) materials and are utilized for refrigeration and power generation near room temperature. They usually suffer from poor mechanical performance, as well as having a low figure of merit (ZT). In this work, we report an effective composite strategy to improve both the TE and mechanical performance of n-type BTS materials by incorporating carbon microfibers. The introduction of carbon microfibers in BTS effectively reduces the lattice thermal conductivity due to phonon scattering at multi-scale boundaries and due to the large interfacial thermal resistance arising from phonon mismatch between the constituent phases. Simultaneously, it also gives rise to an enhancement of the electrical conductivity, which originates from the increased carrier density without significant limitation on its weighted mobility. Consequently, a high peak ZT of 1.1 at 400 K and an average ZTave value of 0.95 are achieved in the temperature range 300 ~ 550 K, yielding a calculated efficiency of η = 9%. Moreover, the BTS/carbon microfiber composites show superior compressive strength compared to a commercial ZM BTS sample. This improved strength is highly desirable for real-world TE applications. Our results demonstrate a novel way to produce high-performance TE materials, in which interfaces with large thermal resistance are used to achieve low thermal conductivity without significantly degrading the electrical properties of the materials. © 2021 Elsevier B.V.
Increased phase coherence length in a porous topological insulator
(American Physical Society (APS), 2023-06-15) Nguyen, A; Akhgar, G; Cortie, DL; Bake, A; Pastuovic, Z; Zhao, W; Liu, C; Chen, YH; Suzuki, K; Fuhrer, MS; Culcer, D; Hamilton, AR; Edmonds, MT; Karel, J
The surface area of Bi2Te3 thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surfacelike. Weak antilocalization was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics. ©2023 American Physical Society
Interplay between thermal and magnetic properties of polymer nanocomposites with superparamagnetic Fe3O4 nanoparticles
(Elsevier, 2023-08-01) Rezoanur Rahman, M; Bake, A; Jumlat Ahmed, AI; Islam, SMKN; Wu, L; Khakbaz, HS; FitzGerald, S; Chalifour, A; Livesey, KL; Knott, JC; Innis, PC; Beirne, S; Cortie, DL
Magnetic nanoparticles embedded in polymer matrices have excellent potential for multifunctional applications like magnetic remote heating, controlled drug delivery, hyperthermia, and thermally functionalized biomedical devices. A solvent-based processing method was developed to produce magnetic composites consisting of magnetite (Fe3O4) superparamagnetic nanoparticles embedded in a biomedical-grade polyurethane (ChronoFlex® C). The particles had a log-normal size distribution spanning from 4−16 nm, with a mean-size of 9.5 ± 2 nm. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy with elemental mapping were used to assess the phase purity, surface morphology, particle size, and homogeneity of the resulting nanocomposite. The magnetic properties of composites with 7–13 wt% of Fe3O4 were studied between 5 and 300 K using vibrating sample magnetometry. Room temperature magnetic attraction was observed, with a saturation magnetization of up to 5 emu/g and a low coercive field (Hc < 50 Oe), where the non-zero coercive field was attributed to a small fraction of larger particles that are ferromagnetic at room temperature. Field-cooled and zero-field-cooled magnetometry data were fitted to a numerical model to determine the superparamagnetic mean blocking temperature (TB = 90 K) of the embedded magnetite particles, and an effective magnetic anisotropy of 6×105 erg/cm3. Using an AC magnetic field operating at 85 kHz, we demonstrate that remote heating of the base polyurethane material is greatly enhanced by compositing with Fe3O4 nanoparticles, leading to temperatures up to 45 °C within 18 min for composites submerged in water. This work demonstrates the fundamental principles of a custom-designed thermomagnetic polymer composite that could be used in applications, including medical and heat management. © 2023 Published by Elsevier B.V.
Iron oxide-palladium core-shell nanospheres for ferromagnetic resonance-based hydrogen gas sensing
(Elsevier, 2022-02-08) Khan, S; Lawler, NB; Bake, A; Rahman, R; Cortie, DL; Iyer, KS; Martyniuk, M; Kostylev, M
Interfaces of ferromagnetic transition metals such as Iron, Cobalt, and Nickel with non-magnetic palladium are of interest due to their unique magnetic and spintronic properties. These interfaces enable ferromagnetic resonance (FMR) based sensing of hydrogen gas. In the present work, we synthesized Fe3O4–Pd core-shell nanospheres via a one-pot synthesis method using the thermal decomposition of Fe3+ acetylacetonate in the presence of a reducing agent to produce the Fe3O4 core, followed by the reduction of a Pd2+ precursor to form the pure Pd shell. We found that our in-situ synthesized core-shell nanostructure is magnetically active and shows excellent H2 gas sensing properties. The effect of reversible hydrogen gas absorption on the magnetism of Fe3O4–Pd core-shell nanospheres was investigated. The hydrogen-induced ferromagnetic-resonance (FMR) peak shift amounted to 30% of the peak linewidth for the virgin state of the sample. In addition, in the presence of hydrogen gas, we observed a fully reversible decrease in the FMR peak linewidth by about two times. This was accompanied by a nearly doubling of the FMR peak height. Response and recovery times of about 2 and 50 s, respectively, were extracted from the measurements. All the data was collected using a mix of just 3% hydrogen in a nitrogen carrier gas. © 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd.
Lamellae preparation for atomic-resolution STEM imaging from ion-beam-sensitive topological insulator crystals
(AIP Publishing, 2022-04-06) Bake, A; Zhao, WY; Mitchell, DRG; Wang, XL; Nancarrow, M; Cortie, DL
Good specimen quality is a key factor in achieving successful scanning transmission electron microscope analysis. Thin and damage-free specimens are prerequisites for obtaining atomic-resolution imaging. Topological insulator single crystals and thin films in the chalcogenide family such as Sb2Te3 are sensitive to electron and ion beams. It is, therefore, challenging to prepare a lamella suitable for high-resolution imaging from these topological insulator materials using standard focused ion-beam instruments. We have developed a modified method to fabricate thin focused ion-beam (FIB) lamellae with minimal ion-beam damage and artefacts. The technique described in the current study enables the reliable preparation of high-quality transmission electron microscope (TEM) specimens necessary for studying ultra-thin surface regions. We have successfully demonstrated that the careful selection of FIB milling parameters at each stage minimizes the damage layer without the need for post-treatment. © 2022 Author(s). Published under an exclusive license by the AVS.
Magnetotransport and Berry phase tuning in Gd-doped Bi2Se3 topological insulator single crystals
(American Physical Society, 2022-05-01) Chen, L; Li, SS; Zhao, W; Bake, A; Cortie, DL; Wang, XL; Karel, J; Li, H; Zheng, RK
The Berry phase is an important concept in solids, correlated to the band topology, axion electrodynamics, and potential applications of topological materials. Here, we investigate the magnetotransport and Berry phase of rare earth element Gd-doped Bi2Se3 (Gd:Bi2Se3) topological insulators (TIs) at low temperatures and high magnetic fields. Gd:Bi2Se3 single crystals show Shubnikov-de Haas (SdH) oscillations with nontrivial Berry phase, while Bi2Se3 single crystals show zero Berry phase in SdH oscillations. A fitting of the temperature-dependent magnetization curves using the Curie-Weiss law reveals that the Gd dopants in the crystals show paramagnetism in the 3-300 K region, indicating that the origin of the Berry phase is not long-range magnetic ordering. Moreover, Gd doping has limited influence on the quantum oscillation parameters (e.g., frequency of oscillation, area of Fermi surface, effective electron mass, and Fermi wave vectors) but has a significant impact on the Hall mobility, carrier density, and band topology. Our results demonstrate that Gd doping can tune the Berry phase of TIs effectively, which may pave the way for the future realization of many predicted exotic transport phenomena of topological origin. ©2022 American Physical Society
Optical and magnetic properties of cobalt doped TiN thin films grown by RF/DC magnetron sputtering
(Elsevier, 2022-05) Abualgassem, E; Maarouf, M; Bake, A; Cortie, DL; Alam, K; Haider, MB
Here we report the study of cobalt doped titanium nitride thin films grown by reactive co-sputtering of cobalt and titanium targets in the presence of argon and nitrogen gases. Thin films were grown with different cobalt concentrations and analyzed by atomic force microscopy, spectrophotometry, X-ray photoelectron spectroscopy, X-ray reflectometry and vibrating sample magnetometry. We have found that the films readily oxidize after exposure to the atmosphere and form a cobalt doped titanium oxynitride. The variable cobalt concentration in the films affects the optical, chemical and magnetic properties. The bandgap of the films generally becomes narrower at higher cobalt concentrations. Cobalt is found to be in a mixture of the high spin + 2 and low spin + 3 oxidation states. At the lower cobalt concentrations, the magnetization is low and it is temperature-independent. Whereas at higher cobalt concentrations, the magnetic response becomes temperature-dependent with a mix of paramagnetism and superparamagnetism at 3–5 K. © 2022 Elsevier B.V.
Significant reduction in thermal conductivity and improved thermopower of electron‐doped Ba1–xLaxTiO3 with nanostructured rectangular pores
(Wiley, 2021-04) Ahmed, AlJ; Cortie, DL; Yun, FF; Rahman, Y; Islam, KN; Bake, A; Konstantinov, K; Hossain, SA; Alowasheeir, A; Yamauchi, Y; Wang, X
Electron‐doped BaTiO3 is a less studied n‐type metal oxide thermoelectric material. In this work, the electrical conductivity of BaTiO3 samples has been improved by introducing La to yield an n‐type Ba1–xLaxTiO3 semiconducting material. Density functional theory calculations show that the optimal electron‐doping occurs at x = 0.2, and this is also confirmed experimentally. To improve the thermoelectric properties further, nanostructured cuboidal pores are introduced into the bulk Ba1–xLaxTiO3 using F127 surfactant micelles for a chemical templating process, followed by spark plasma sintering. Interestingly, transmission electron microscopy images and X‐ray powder diffraction analysis confirms that our fabricated samples are cubic BaTiO3 perovskite phase with the nanostructured rectangular‐prism pores of >4 nm. Scanning electron microscopy images show that all the samples have similar grain boundaries and uniform La doping, which suggests that the large reduction in the lattice thermal conductivity in the F127‐treated samples arises primarily from the pore distribution, which introduces anisotropic phonon scattering within the unique nanoarchitecture. The sample with 20 at% La doping and nanopores also shows a thermopower that is doubled compared to the related sample without porosity. Together with the lattice thermal conductivity, enables a significant improvement in figure of merit, zT compared to the other samples. © 2021 Wiley-VCH GmbH.
Structure and magnetism of ultra-small cobalt particles assembled at titania surfaces by ion beam synthesis
(Elsevier, 2021-12) Bake, A; Rezoanur Rahman, M; Evans, PJ; Cortie, MB; Nancarrow, M; Abrudan, R; Radu, F; Khaydukov, Y; Causer, GL; Callori, SJ; Livesey, KL; Mitchell, DRG; Pastuovic, Z; Wang, XL; Cortie, DL
Metallic cobalt nanoparticles offer attractive magnetic properties but are vulnerable to oxidation, which suppresses their magnetization. In this article, we report the use of ion beam synthesis to produce ultra-small, oxidation-resistant, cobalt nanoparticles embedded within substoichiometric TiO2-δ thin films. Using high fluence implantation of cobalt at 20–60 keV, the particles were assembled with an average size of 1.5 ± 1 nm. The geometry and structure of the nanoparticles were studied using scanning transmission electron microscopy. Near-edge X-ray fluorescence spectroscopy on the L2,3 Co edges confirms that the majority of the particles beneath the surface are metallic, unoxidised cobalt. Further evidence of the metallic nature of the small particles is provided via their high magnetization and superparamagnetic response between 3 and 300 K with a low blocking temperature of 4.5 K. The magnetic properties were studied using a combination of vibrating sample magnetometry, element-resolved X-ray magnetic circular dichroism, and depth-resolved polarised neutron reflectometry. These techniques provide a unified picture of the magnetic metallic Co particles. We argue, based on these experimental observations and thermodynamic calculations, that the cobalt is protected against oxidation beneath the surface of titania owing to the enthalpic stability of TiO2 over CoO which inhibits solid state reactions. Crown Copyright © 2021 Published by Elsevier B.V.
Sub-3nm cobalt nanoparticles embedded in titania glass via ion implantation studied by polarised neutron reflectometry
(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Bake, A; Pastuovic, Z; Cortie, DL; Mitchell, DRG; Wang, XL
Synthesis of stable metallic cobalt particles at the nanoscale is challenging using chemical methods due to their unstable nature and their tendency to oxidise. Nevertheless, ultra-small metallic cobalt nanoparticles offer unusual electronic and magnetic properties including large spin moments and magnetic anisotropy[1] [2]. Here we report the use of ion beam implantation to synthesis stable metallic cobalt nanoparticles embedded within TiO2-x thin films. Scanning transmission electron microscopy (STEM) images revealed that the Co nanoclusters were around 1-3 nm in size. The blocking temperature of Co nanoparticles is estimated to be TB = 6.5 K from the field-cooled and zero-field-cooled magnetization curves. Polarised neutron reflectometry (PNR) measurements reveal superparamagnetic behaviour with a strong temperature dependency due to the small size of the nanoclusters. The nuclear scattering length density, together with near-edge X-ray absorption spectroscopy (NEXAFS) show that the magnetic cobalt clusters are metallic and unoxidized below the TiO2-x surface, whereas near the surface the clusters are prone to form cobalt oxides in a magnetic dead layer that extends over a few nanometers. Our results show that ion beam synthesis can produce ultra-small particles that are not possible with other methods, and these particles survive for long periods without much oxidation.
Top-down patterning of topological surface and edge states using a focused ion beam
(Springer Nature, 2023-03-27) Bake, A; Zhang, Q; Ho, CS; Causer, GL; Zhao, WY; Yue, ZJ; Nguyen, A; Akhgar, G; Karel, J; Mitchell, DRG; Pastuovic, Z; Lewis, RA; Cole, JH; Nancarrow, M; Wang, XL; Cortie, DL
The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ2 to pattern conducting channels. It is shown that modifying Sb2Te3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics. Open Access This article is licensed under a Creative Commons Attribution 4.0 © Crown Copyright 2023
Topological insulator VxBi1.08-x Sn0.02Sb0.9Te2S as a promising n-type thermoelectric material
(Elsevier, 2022-10) Chen, L; Zhao, WY; Li, M; Yang, G; Guo, L; Bake, A; Liu, P; Cortie, DL; Zheng, RK; Cheng, ZX; Wang, XL
As one of the most important n-type thermoelectric (TE) materials, Bi2Te3 has been studied for decades, with efforts to enhance the thermoelectric performance based on element doping, band engineering, etc. In this study, we report a novel bulk-insulating topological material system as a replacement for n-type Bi2Te3 materials: V doped Bi1.08Sn0.02Sb0.9Te2S (V:BSSTS). The V:BSSTS is a bulk insulator with robust metallic topological surface states. Furthermore, the bulk band gap can be tuned by the doping level of V, which is verified by magnetotransport measurements. Large linear magnetoresistance is observed in all samples. Excellent thermoelectric performance is obtained in the V:BSSTS samples, e.g., the highest figure of merit ZT of ~ 0.8 is achieved in the 2% V doped sample (denoted as V0.02) at 530 K. The high thermoelectric performance of V:BSSTS can be attributed to two synergistic effects: (1) the low conductive secondary phases Sb2S3, and V2S3 are believed to be important scattering centers for phonons, leading to lower lattice thermal conductivity; and (2) the electrical conductivity is increased due to the high-mobility topological surface states at the boundaries. In addition, by replacing one third of costly tellurium with abundant, low-cost, and less-toxic sulfur element, the newly produced BSSTS material is inexpensive but still has comparable TE performance to the traditional Bi2Te3-based materials, which offers a cheaper plan for the electronics and thermoelectric industries. Our results demonstrate that topological materials with unique band structures can provide a new platform in the search for new high performance TE materials. © 2022 Elsevier B.V.
Transport measurements in porous Bi2Te3 thin films
(American Physical Society, 2022-03-16) Akhgar, G; Nguyen, A; Cortie, DL; Bake, A; Zhao, WY; Liu, C; Fuhrer, MS; Culcer, D; Hamilton, AR; Edmonds, MT; Karel, J
Recent theoretical work has predicted the existence of disordered topological insulators , however, minimal experimental work has been conducted on disordered TIs. Here we used molecular-beam epitaxy (MBE) to grow Bi2Te3 thin films that were comprised of nanocrystals embedded in an amorphous matrix. Further disorder was introduced through Ne ion irradiation which produced porosity in the films. In this talk we will present magnetoresistance measurements on porous Bi2Te3, where weak anti-localisation (WAL) was observed. The magnetoresistance curves were fitted using a Dirac Fermion model specifically derived to model weak antilocalization in TIs. Our results also show that the temperature dependence of the phase coherence length in porous Bi2Te3, with an increased surface to volume ratio, exhibits 2D-like transport.
Ultra-small cobalt particles embedded in titania by ion beam synthesis: additional datasets including electron microscopy, neutron reflectometry, modelling outputs and particle size analysis
(Elsevier, 2022-02) Bake, A; Rahman, R; Evans, PJ; Cortie, MB; Nancarrow, M; Abrudan, R; Radu, F; Khaydukov, Y; Causer, GL; Livesey, KL; Callori, SJ; Mitchell, DRG; Pastuovic, Z; Wang, XL; Cortie, DL
This Data-in-brief article includes datasets of electron microscopy, polarised neutron reflectometry and magnetometry for ultra-small cobalt particles formed in titania thin films via ion beam synthesis. Raw data for polarised neutron reflectometry, magnetometry and the particle size distribution are included and made available on a public repository. Additional elemental maps from scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) are also presented. Data were obtained using the following types of equipment: the NREX and PLATYPUS polarised neutron reflectometers; a Quantum Design Physical Property Measurement System (14 T); a JEOL JSM-6490LV SEM, and a JEOL ARM-200F scanning transmission electron microscope (STEM). The data is provided as supporting evidence for the article in Applied Surface Science (A. Bake et al., Appl. Surf. Sci., vol. 570, p. 151068, 2021, DOI 10.1016/j.apsusc.2021.151068), where a full discussion is given. The additional supplementary reflectometry and modelling datasets are intended to assist future scientific software development of advanced fitting algorithms for magnetization gradients in thin films. Crown Copyright © 2021 - Open Access CC BY-NC-ND

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