Browsing by Author "Callori, SJ"

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    90° magnetic coupling in a NiFe/FeMn/biased NiFe spin valve investigated by polarised neutron reflectometry
    (Australian Institute of Physics, 2014-02-05) Callori, SJ; Zhu, T; Klose, F
    We have used the PLATYPUS reflectometer at ANSTO to perform polarised neutron reflectometry in order to investigate 90° magnetic coupling in a Ni81Fe19/Fe50Mn50/biased Ni81Fe19 spin valve system. Spin valves play an important role in current and developing technological systems, such as spintronics devices or magnetoresistive sensors. For the later usage, perpendicular coupling in a spin valve structure leads to a desired linear, reversible resistance response to an applied magnetic field. The spin valve presented here consists of both free and exchange biased ferromagnetic Ni81Fe19 layers, the later of which is pinned by an antiferromagnetic Ir25Mn75 layer at low applied magnetic fields. The free Ni81Fe19 may be magnetically reversed under low fields, and standard magnetometry measurements on similar systems have suggested perpendicular orientation of the free and biased magnetisations at zero field. Magnetometry measurements, however, are only capable of providing information about the magnetisation within a sample along the direction of the applied field. In contrast, polarised neutron reflectometry (PNR) is capable of resolving the in-plane magnetisation vectors both along and perpendicular to the applied magnetic field as function of layer depth. Here, PNR was used to obtain magnetic vector depth profiles of the spin valve at several applied fields, including low fields near the switching point of the free Ni81Fe19 layer. At these fields a large spin-flip signal was observed in the free layer, indicating magnetisation aligned perpendicular to the external field applied along the pinned layer magnetisation. Both the non-spin flip and spin-flip signals were also tracked around the free layer hysteresis loops and can be used to map the evolution of the free Ni81Fe19 layer during magnetic reversal.
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    Complex 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, N
    Multiferroic 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.
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    The magnetic interfacial properties of an exchange biased nanocrystalline Ni80Fe20/α-Fe2O3 bilayer studied by polarized neutron reflectometry and Monte Carlo simulation
    (Institute of Physics, 2019-11-22) Causer, GL; Cortie, DL; Callori, SJ; Manna, PK; van Lierop, J; Lee, YJ; Wang, XL; Lin, KW; Klose, K
    The strength of exchange bias can be influenced by interface roughness and antiferromagnetic morphology. Here, we studied the interface profile of an exchange biased, nanocrystalline Ni80Fe20/α-Fe2O3 bilayer. Magnetometry determined the bilayer's exchange bias is observed below a blocking temperature of 75 K. Polarized neutron reflectometry measurements revealed the Ni80Fe20 layer was fully saturated to yield a net-moment of 0.95 μB/atom, while the majority of the Fe2O3 layer exhibited zero net-magnetization with the exception of the interfacial region with an uncompensated moment between 0.5 and 1.0 μB/Fe2O3. Monte Carlo simulations of a ferromagnetic/antiferromagnetic bilayer incorporating a granular antiferromagnet indicate that an extrinsic uncompensated moment of ∼1.0 μB/Fe2O3 can arise from grain boundary disorder. The size of the modeled moment is equivalent to the experimental value, and comparable with previous calculations. Furthermore, unlike intrinsic uncompensated spins, it is found that the disorder-induced moment in the granular antiferromagnet is not destroyed by interface roughness. © 2019 The Japan Society of Applied Physics
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    Magneto-electronic hydrogen gas sensing
    (Australian Institute of Physics, 2017-01-31) Causer, GL; Leung, C; Callori, SJ; Metaxas, P; Klose, F; Kostylev, M
    Hydrogen (H2) as an energy carrier and associated H2 technologies such as fuel cells are establishing themselves as key players in the current green energy revolution. To address safety issues associated with H2, robust hydrogen gas sensors are required. We report on a superior method of using magneto-electronics to detect the presence of H2. Exploiting the strong affinity of Pd to reversibly absorb and chemically bind H2, resulting in the formation of PdH which expands the Pd lattice by up to 3%, our prototype device is based on the modification of magnetic, structural and electronic properties that occur upon hydrogenation of a Pd layer in a Pd/Co bilayer film. As H2 is absorbed by the Pd lattice, modifications to the perpendicular magnetic anisotropy (PMA) of interfacial Co moments result, leading to a variation of the ferromagnetic resonance (FMR) response of the Co layer. We report on data obtained from the first in-situ FMR polarised neutron reflectometry (PNR) measurement performed on the time-of-flight neutron reflectometer PLATYPUS at ANSTO. Here we simultaneously probed hydrogen depth profiles within Pd as a function of external H2 partial pressure (HPP), and correlated these against hydrogen induced changes to the FMR signal in the ferromagnetic layer. Decreases in the FMR field in excess of 30 Oe were observed upon H2 absorption, as a result of weakening PMA strength due to changes in interfacial electronic properties. In addition, we systematically investigated the relationship between Pd layer thickness and H2 concentration in the Pd layer in the presence of 3.5% HPP, and found that the uptake of hydrogen is severely hindered by post-deposition annealing. Although the annealing process served to remove lattice dislocations which could otherwise be occupied by H2, it led to more repeatable magnetic behaviors of the materials when measured over several H2 absorption/desorption cycles.
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    Polarised 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, C
    Polarised 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.
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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.