Browsing by Author "Nagarajan, V"

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    Element-specific depth profile of magnetism and stoichiometry at the La0.67Sr0.33MnO3/BiFeO3 interface
    (American Physical Society, 2014-07-11) Bertinshaw, J; Brück, S; Lott, D; Fritzsche, H; Khaydukov, Y; Soltwedel, O; Keller, T; Goering, E; Audehm, P; Cortie, DL; Hutchison, WD; Ramasse, QM; Arredondo, M; Maran, R; Nagarajan, V; Klose, F; Ulrich, C
    Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3/multiferroic BiFeO3 bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers [P. Yu et al., Phys. Rev. Lett. 105, 027201 (2010)]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment.© 2014, American Physical Society.
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    Neutron study of magnetic phase transition in SrCoO3 thin films
    (Australian Institute of Physics, 2020-02-04) Yick, S; Peroz, MF; Nagarajan, V; Klose, F; Seidel, J; Ulrich, C
    Transition metal oxides represent a wide set of materials with a broad range of functionalities which can be tuned by the careful choice of parameters such as strain, oxygen content, and applied electric or magnetic fields. When the material exhibits more than one primary ferroic ordering- ferromagnetism, ferroelectricity, ferroelasticity or ferrotoridicity in the same phase, it becomes multiferroic. Such class of materials are of immense technological interest as magnetic and electric transitions can be driven through external factors. This opens new avenues for fundamental research and technical applications in spintronic or magnonic devices. Here, we present results we obtained from neutron-based techniques to investigate the magnetic properties of SrCoO3 and similar thin films. SrCoO3 provides a particularly interesting system for these investigations. Lee and Rabe have simulated the effect of strain and have predicted that the magnetic state can be tuned through compressive or tensile strain with a ferromagnetic-antiferromagnetic phase transition [1,2]. Such a phase transition would be accompanied by a metal-to-insulator phase transition and a transition to a ferroelectric polarized state. By using different substrates, we investigated the effect different epitaxial strain has on SrCoO3 thin films. Previously, our neutron diffraction experiments on these 40 nm thin films have confirmed the predicted but hitherto unobserved phase transition from ferromagnetism to G-type antiferromagnetism when the film was grown on SrTiO3 and DyScO3 substrate respectively [3]. As such, SrCoO3 would constitute a new class of multiferroic material where magnetic and electric polarizations can be driven through external strain. This tunability makes them ideal candidate materials for use in developing novel information and energy technologies.
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    Strain-induced magnetic phase transition in SrCoO3 thin films
    (Australian Institute of Physics, 2015-02-06) Callori, SJ; Hu, S; Bertinshaw, J; Yue, ZJ; Danilkin, SA; Wang, XL; Nagarajan, V; Klose, F; Seidel, J; Ulrich, C
    Transition metal oxides represent a wide set of materials with a broad range of functionalities, including superconductivity, magnetism, and ferroelectricity, which can be tuned by the careful choice of parameters such as strain, oxygen content, and applied electric or magnetic fields. This tunability makes them ideal candidate materials for use in developing novel information and energy technologies. SrCoO3 provides a particularly interesting system for these investigations. Lee and Rabe have simulated the effect of strain and have predicted that the magnetic state can be tuned through compressive or tensile strain with a ferromagnetic-antiferromagnetic phase transition. Such a phase transition would be accompanied by a metal-to-insulator phase transition and a transition to a ferroelectric polarised state. We have achieved large in-plane tensile strain in SrCoO3 thin films through the proper choice of substrate and our neutron diffraction experiments on only 40 nm thick films have indeed confirmed the transition from a ferromagnetic to an antiferromagnetic ground state, as theoretically predicted. As such, SrCoO3 would constitute a new class of multiferroic material where magnetic and electric polarisations can be driven through external strain.
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    Strain-induced magnetic phase transition in SrCoO3−δ thin films
    (American Physical Society, 2015-04-10) Callori, SJ; Hu, S; Bertinshaw, J; Yue, ZJ; Danilkin, SA; Wang, XL; Nagarajan, V; Klose, F; Seidel, J; Ulrich, C
    It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO3−δ (δ<0.2) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic-to-antiferromagnetic phase transition in SrCoO3−δ films grown on DyScO3 substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO3 substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Neél temperatures between TN∼135±10K and ∼325±10K, depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO3−δ thin films under large epitaxial strain. © 2015 American Physical Society.
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    Studying multiferroic BiFeO3 and ferromagnetic La0.67Sr0.33MnO3 tunnel junctions with Raman spectroscopy and neutron scattering techniques
    (Australian Institute of Physics, 2011-02-02) Bertinshaw, J; Saerbeck, T; Nelson, A; James, M; Nagarajan, V; Klose, F; Ulrich, C
    Bismuth Ferrite (BiFeO3 or BFO) is a prominent multiferroic material candidate for industrial implementation as it is among one of the rare cases where ferroelectric polarisation and magnetic order coexist at room temperature [1]. We have investigated its potential in functional thin film heterostructures, where it is possible the interplay between FE and FM at the interface between layers can enable controllable magnetoelectric coupling, allowing for the control of the magnetic polarisation through applied electric fields and vice-versa [2]. Epitaxial (001) BiFeO3 / La0.67Sr0.33MnO3 (LSMO) multiferroic tunnel junctions have been grown by pulsed laser deposition at the University of NSW [3]. These trilayer systems layer: 40nm of LSMO, 10nm of BFO, and 40nm of LSMO on a SrTiO3 substrate, with a RMS roughness of not more than one unit cell. We have found initial experimental evidence of a correlation between the spin polarisation of the FM LSMO layers and the FE polarisation of the BiFeO3 layer through flips in the domain structure through a number of electrical resistance based experimental techniques [3]. We plan to combine results from Raman spectroscopy conducted at the UNSW with polarised neutron reflectometry on PLATYPUS and inelastic neutron scattering on TAIPAN at the Bragg Institute, ANSTO to perform a detailed analysis of: the magnetisation reversal process in the LSMO contact layers, the interplay (exchange bias) between the BFO AFM and LSMO FM parameters, the magnetic depth profile of the heterostructure, in particular the interface regions, and the effect of switching the electric polarisation of the BiFeO3 layer on the domain wall structure, and therefore on the magnetic structure of the entire thin film system.