Browsing by Author "Valanoor, N"
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- ItemCanted magnetism in modulated thin-film superlattices Oliver(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Paull, O; Wong., J; Sando, D; Lee, WT; Holt, SA; Valanoor, NInteractions at interfaces of magnetic materials such as giant magnetoresistance (GMR) form the physical foundation of many technological devices in today’s market. Although GMR technologies are now somewhat dated, interactions at the interface between layers of oxide materials continue to demonstrate attractive mechanisms for technology applications due to the wide variety of available oxide materials as well as their ability to be well lattice-matched to form heterostructures. Magnetic thin-film superlattices made of 15 repetitions of La2/3Sr1/3MnO3 (LSMO) and LaNiO3 (LNO) have been shown to exhibit a magnetic exchange interaction in LNO that is dependent upon its thickness n (where n is the number of unit cells of LNO) between LSMO layers [1]. At n = 1, the LSMO layers are ferromagnetically aligned with eachother. At n = 3 however, the LSMO layers are canted by 110◦ with respect to one another. This canting is reportedly driven by an emergent c-axis spin-helix in LNO that arises due to charge transfer at the interfaces between LNO and LSMO [2]. We have fabricated superlattices of similar quality to Ref. [1] using Reflected High Energy Electron Diffraction (RHEED) assisted Pulsed Laser Deposition (PLD), where RHEED is used to ensure unit-cell precision in layer thicknesses. These superlattices have been designed so they exhibit n = 3 and n = 1 interactions within the same superlattice in a modulated pattern. This modulation of the effective LNO exchange interaction in these samples creates a toy model of coupled magnetic layers to explore. The modulated design of these samples create “frustrated” environments where the effective exchange interaction is different on each side of an LSMO layer within the superlattice. We utilise polarised neutron reflectometry using PLAYPUS to reveal the complex magnetic profile of these modulated superlattices by measuring the non-spin-flip (R++, R−−), and spin-flip (R+−, R−+) cross sections. Presented are the current progress and prospects in the fitting process of these complex datasets. © The authors.
- ItemComplex magnetic structure in strained nanoscale bismuth ferrite thin films(Australian Institute of Physics, 2016-02-02) Ulrich, C; Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheng, J; Danilkin, SA; Hu, S; Siedel, J; Valanoor, NMultiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. Bismuth ferrite (BiFeO3) is a rare exemption where both order parameters coexist far beyond room temperature, making it the ideal candidate for technological applications. In particular, multiferroic thin films are the most promising pathway for spintronics applications. Therefore we have investigated BiFeO3 thin films by neutron diffraction. At present, the underlying physics of the magnetoelectric coupling is not fully understood and competing theories exist with partly conflicting predictions. For example, the existence of spin cycloid is a mandatory requirement to establish a direct magnetoelectric coupling. Thus far internal strain in epitaxially grown films has limited the stability of the spin cycloid for BiFeO3 films with less than 300 nm thickness, causing the spin cycloid to collapses to a collinear G-type antiferromagnetic structure. Our neutron diffraction experiments have demonstrated that we were able to realize a spin cycloid in films of just 100 nm thickness through improved electrostatic and epitaxial constraints. This underlines the importance of the correct mechanical and electrical boundary conditions required to achieve emergent spin properties in mutiferroic thin film systems. The discovery of a large scale uniform cycloid in thin film BiFeO3 opens new avenues for fundamental research and technical applications that exploit the spin cycloid in spintronic or magnonic devices.
- ItemDirect evidence for the spin cycloid in strained nanoscale bismuth ferrite thin films(Australian Institute of Physics, 2017-01-31) Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheung, J; Dainlkin, SA; Lee, WT; Hu, S; Seidel, J; Valanoor, N; Ulrich, CMultiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. Bismuth ferrite (BiFeO3) is a rare exemption where both order parameters exist far beyond room temperature, making it the ideal candidate for technological applications. In particular, magnonic devices that utilize electric control of spin waves mediated by complex spin textures are an emerging direction in spintronics research. To realize magnonic devices, a robust long-range spin cycloid with well known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (100 nm) thin BiFeO3 films has not been accomplished. Here, we demonstrate cycloidal spin order in 100 nm BiFeO3 thin films through the careful choice of crystallographic orientation, and control of the electrostatic and strain boundary conditions during growth [1]. Neutron diffraction, in conjunction with X-ray diffraction, reveals an incommensurate spin cycloid with a unique [112] propagation direction. While this direction is different from bulk BiFeO3, the cycloid length and Néel temperature remain equivalent to bulk single crystals. The discovery of a large scale uniform cycloid in thin film BiFeO3 opens new avenues for fundamental research and technical applications that exploit the spin cycloid in spintronic or magnonic devices.
- ItemFerroelastic domain switching fatigue in lead zirconate titanate ceramics(Elsevier, 2008-04) Pojprapai, S; Jones, JL; Studer, AJ; Russell, J; Valanoor, N; Hoffman, MThe influence of the frequency and amplitude of cyclic mechanical loading on soft, tetragonal lead zirconate titanate (PZT) ceramics was investigated via neutron diffraction. Intensity change in the {2 0 0} reflections provided quantitative measurements of domain switching behavior, domain texture and the strain resulting from domain switching. The results are explained using a viscoelasticity model. It was found that the magnitude of applied stress affects the level of strain accumulated, while its frequency affects the time taken for the strain to reach saturation. Furthermore, markedly different behaviors are exhibited by poled and unpoled samples. For samples loaded under identical conditions, the frequency effect is more pronounced in unpoled samples and the accumulated ferroelastic strain is greater in poled samples. © 2008, Elsevier Ltd.
- ItemNeutron studies of functional multiferroic BiFeO3 thin films(Australian Institute of Physics, 2012-02-01) Bertinshaw, G; Maran, R; Valanoor, N; Klose, F; Ulrich, CIn a multiferroic material, ferromagnetism (FM) and ferroelectricity (FE) coexist, presenting exciting opportunities for research into new phenomena and technological innovation. Bismuth Ferrite (BiFeO3) is among the rare cases where both properties exist at room temperature [1]. As such, its use in functional thin film heterostructures, like multiferroic tunnel junctions or exchange bias systems, which combine multiferroic (MF) and ferromagnetic layers, such as La0.67Sr0.33MnO3 (LaSrMnO3), is of particular interest [2]. In a series of resistivity based measurements [3] and neutron diffraction and polarised neutron reflectometry measurements of BiFeO3/LaSrMnO3 thin film bilayers we have investigated and found evidence of electromagnetic coupling between the layers. We use these results to provide deeper insight into the complex interplay between the orbital and spin degrees of freedom at the bilayer interface. Neutron experiments were performed at the Bragg Institute, ANSTO in Sydney, Australia, FRM-II in Munich, Germany and NRC-CNBC in Chalk River, Canada.
- ItemPolarised neutron diffraction study of the spin cycloid in strained nanoscale bismuth ferrite thin films(Australian Institute of Physics, 2017-01-31) Lee, WT; Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheung, J; Danilkin, SA; Hu, S; Seidel, J; Valanoor, N; Ulrich, CPolarised neutron scattering is capable of separating magnetic structure from chemical structure. Here we report an experiment using the newly available capability at ANSTO, namely polarised neutron diffraction using polarised 3He neutron spin-filters to obtain the detail magnetic structure in even highly complex magnetic materials. Magnonic devices that utilize electric control of spin waves mediated by complex spin textures are an emerging direction in spintronics research. Room-temperature multiferroic materials, such as BiFeO3, with a spin cycloidal structure would be ideal candidates for this purpose. In order to realise magnonic devices, a robust long-range spin cycloid with well-known direction is desired. Despite extensive investigation, the stabilization of a large scale uniform spin cycloid in nanoscale (100 nm) thin BiFeO3 films has not been accomplished. The polarized neutron diffraction experiment did confirm the existence of the spin cycloid in this BiFeO3 film, which is an important prerequisite for the multiferroic coupling.
- ItemStability and scaling behavior of the spin cycloid in BiFeO3 thin films(Australian Institute of Physics, 2018-01-30) Burns, SR; Sando, D; Bertinshaw, J; Russell, L; Xu, X; Maran, R; Callori, SJ; Ramash, V; Cheung, J; Danilkin, SA; Deng, G; Lee, WT; Hu, S; Bellaiche, L; Seidel, J; Valanoor, N; Ulrich, CMultiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. Bismuth ferrite (BiFeO3) is a rare exemption where both order parameters exist far beyond room temperature, making it the ideal candidate for technological applications. To realize magnonic devices, a robust longrange spin cycloid with well-known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (<300 nm) thin BiFeO3 films has not been accomplished. Using neutron diffraction we were able to demonstrate cycloidal spin order in 100 nm BiFeO3 thin films which became stable through the careful choice of crystallographic orientation and control of the electrostatic and strain boundary conditions during growth [1]. Furthermore, Co-doping, which has demonstrated to further stabilize the spin cycloid, did allow us to obtain spin cycloid order in films of just 50 nm thickness, i.e. films thinner than the cycloidal length of about 64 nm. Interestingly, in thin films the propagation direction of the spin cycloid has changed and shows a peculiar scaling behavior for thinnest films. We were able to support these observations by Monte Carlo theory based on a first-principles effective Hamiltonian method. Our results therefore offer new avenues for fundamental research and technical applications that exploit the spin cycloid in spintronic or magnonic devices.