Browsing by Author "Causer, GL"

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    Depth control of ferromagnetism in FePt3 films by ion-irradiation
    (International Conference on Neutron Scattering, 2017-07-12) Causer, GL; Cortie, DL; Zhu, HL; Ionescu, M; Mankey, GJ; Klose, F
    The roadmap which outlines storage technology of magnetic hard disk drives predicts storage densities above 5 Tb/in2 to be realised by isolated, individually addressable ferromagnetic (FM) bits of <10 nm in lateral dimension. In principle, artificially patterned structures of this type can be manufactured by x-ray, ion-and electron-beam lithography. However, there may be alternative solutions for obtaining these regular, nanoscale patterns of isolated FM dots. Our proposal is to locally transform a non-magnetic layer into a pattern of geometrically defined FM islands. Such a phase transition could be initiated by locally changing some physical parameter of the layer, such as its strain state or chemical composition leading to ferromagnetism. Here, we present a chemical order (paramagnetic) to chemical disorder (FM) phase transition stimulated by He+ irradiation of a FePt3 thin film. This talk will present preliminary work focussing on depth profiling the ion-beam induced FM order. By controlling the energy (15 keV) and fluence (2x1016 ions/cm2) of the ion-beam, we show ferromagnetism can be locally induced into the upper-half volume of the initially chemically well-ordered 280 nm FePt3 film. Polarised neutron reflectometry was used to investigate the depth dependence of the layer averaged ion-beam induced FM moment within the thin film. Data analysis of the Kiessig fringes observed in the reflectivity post-irradiation suggest the FM / nonmagnetic interface is atomically sharp. The resulting bilayer structure was found to be homogenous in chemical composition but heterogeneous in both chemical and magnetic orders.
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    Depth-profiling magnetic interfaces formed intrinsically in FePt3 by ion-beams
    (American Physical Society, 2018-03-07) Causer, GL; Cortie, DL; Zhu, HL; Ionescu, M; Mankey, GJ; Wang, XL; Klose, F
    Using ion-beams to locally modify material properties is rapidly gaining momentum as a technique of choice for the fabrication of magnetic nano-elements because the method provides the capability to nano-engineer in 3D, which is important for many future spintronic technologies. The precise definition of the resulting element shape is crucial for device functionality. In this work, the intrinsic sharpness of a magnetic interface formed by nano-machining FePt3 films using He+ irradiation is investigated. Through careful selection of the irradiating ion’s energy and fluence, ferromagnetism is locally induced into a fractional volume of a paramagnetic (PM) FePt3 film by modifying the chemical order parameter. Using a combination of magnetometry, transmission electron microscopy and polarised neutron reflectometry it is demonstrated that the interface over which the PM to ferromagnetic modulation occurs is confined to a few atomic monolayers only. Using density functional theory, the mechanism for the ion-beam induced magnetic transition is elucidated and shown to be caused by an intermixing of Fe and Pt atoms in anti-site defects above a threshold density.
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    Hydrogen-driven switching of the magnetic surface anisotropy at the Co/Pd interface
    (American Physical Society, 2019-03-05) Causer, GL; Kostylev, M; Cortie, DL; Wang, XL; Klose, F
    Heterostructures exhibiting perpendicular magnetic anisotropy (PMA) have proven to be indispensable within the magnetic recording industry. By exploiting the hydrogen-induced modifications to PMA which occur exclusively at the ferromagnetic/Pd interface, an opportunity exists to expand the potential applications of PMA-based heterostructures into the realm of hydrogen sensing using ferromagnetic resonance (FMR) - an electron-spin based technology. Here, we present an interface-resolved in-operando study of a Co/Pd film which features tailorable PMA in the presence of hydrogen gas. We combine polarized neutron reflectometry with in-situ FMR to explore the nanoscopic interactions of hydrogen at the Co/Pd interface which affects the spin-resonance condition during hydrogen cycling. Key experimental data and theoretical modelling reveal that the interfacial PMA of the Co/Pd film suppresses non-reversibly upon primary exposure to hydrogen gas – highlighting a potential avenue for spintronics-based hydrogen sensing.
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    Magnetic interface phenomena in nano-architectures and their applications
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Causer, GL; Cortie, DL; Zhu, HL; Kostylev, M; Ionescu, M; Mankey, GJ; Wang, XL; Klose, F
    Interfaces between heterostructure components in nanoscale films play important roles in communicating low-dimensional phenomena and act as anchor points for the direct control and tunability of device performance. In this talk I will give an overview of our group’s recent investigations into the occurrence of magnetic interface phenomena in low-dimensional thin-film systems which have conceivable utility in future condensed-matter technologies. First, the magnetic interface quality of an FePt3 nano-magnet formed via ion-induced chemical disorder will be analysed [1]. Here, neutron and electron measurements used in combination with density functional theory calculations reveal a rather counterintuitive result which could prove beneficial towards the development of ultra-high density magnetic recording devices. In a second study, the layer-averaged static magnetisation and macroscopic magneto-dynamic behaviours of a Co/Pd bilayer during hydrogen-gas cycling are analysed. To perform this characterisation, we first had to develop and commission an original sample environment which innovatively combines polarised neutron reflectometry and microwave spectroscopy [2]. The Co/Pd interface is found to feature tailorable magnetic surface anisotropy in the presence of hydrogen gas – the mechanism of which could act as a safety switch in next-generation vehicles powered by hydrogen.
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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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    Magnetic ordering in superconducting sandwiches
    (Australian Nuclear Science and Technology Organisation, 2021-11-24) Chan, A; van der Heijden, NJ; Causer, GL; Söhnel, T; Simpson, MC; Rule, KC; Lee, WTH; Bernhard, C; Mallett, BPP
    Our cuprate-manganite ‘superconducting sandwich’ multilayers exhibit a highly unusual magnetic-field induced insulating-to-superconducting transition (IST), contrary to the commonly held understanding that magnetic fields are detrimental to superconductivity [1, 2]. This new behaviour is a result of the specific magnetic and electronic properties of the manganite coupling with the high-Tc cuprate (YBa2Cu3O7-δ, YBCO). Due to the specific manganite composition, Nd0.65(Ca0.7Sr0.3)0.35MnO3 (NCSMO), we hypothesize the behaviour to originate from CE-type antiferromagnetic ordering as well as charge and orbital ordering [3]. The magnetic data presented here will focus on polarized neutron reflectometry (PNR) and elastic neutron scattering on a YBCO-NCSMO trilayer and superlattice. The model that best described the PNR data for the trilayer had antiparallel moments at the YBCO-NCSMO interfaces. In the superlattice, the direction of moments at NCSMO interfaces were found to alternate with film depth whose long-ranged ordering was broken below 35 K in a 1 T applied field. The stability of the AFM order in the superlattice was further supported by a robustness of magnetic in-plane half-order elastic scattering peaks at 9 T. This evidences the interplay of magnetism and superconductivity that play a role in realizing the IST effect in our superconducting sandwiches. © The Authors
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    Magnetic ordering in superconducting sandwiches
    (Australian Institute of Physics, 2020-02-04) Chan, A; van der Heijden, NJ; Söhnel, T; Simpson, MC; Rule, KC; Causer, GL; Lee, WT; Bernard, C; Mallett, BPP
    Our cuprate-manganite ‘superconducting sandwich’ multilayers exhibit a highly unusual magnetic-field induced insulating-to-superconducting transition, contrary to the commonly held understanding that magnetic fields are detrimental to superconductivity. This new behaviour is a result of the specific magnetic and electronic properties of the manganite coupling with the cuprate (YBa2Cu3O7-δ, YBCO). Due to the specific manganite composition, Nd0.65(Ca0.7Sr0.3)0.35MnO3 (NCSMO), we hypothesize the behaviour to originate from CE-type antiferromagnetic ordering as well as charge and orbital ordering. Zero-field cooled polarized neutron reflectometry (PNR) data in Fig 1(A) shows a sizable spin-flip (R+-) signal which may result from disordered ferromagnetic domains which sum to give a vanishing macroscopic magnetization. Initial elastic neutron scattering measurements performed on 100 nm thin film NCSMO display signatures of magnetic ordering (Fig 1(B)). Future neutron scattering measurements will look at the modification of magnetic order in a superlattice to better understand the relationship between NCSMO magnetization and our newly discovered insulating-to-superconducting transition.
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    Magnetic proximity effect in YBCO/STO/LCMO multilayers
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Paull, O; Causer, GL; Pan, AV; Klose, F
    Tailoring of the electronic properties of complex oxide heterostructures, thin films, and superlattices with atomically sharp interfaces is at the frontline of materials research at present [1, 2, 3]. Interfaces exhibit novel states that are not possible in bulk materials as a result of broken symmetry, induced strains, and modified exchange interactions. In this work we examined the interaction between a superconducting YBa2Cu3O7-δ thin filmlayer and a ferromagnetic La2/3Ca1/3MnO3 layer using polarised neutron reflectometry. The interaction at the interface between YBCO/LCMO multilayers has resulted in a variety of observed phenomena such as induced ferromagnetic moments in YBCO layers that are antiparallel to neighbouring LCMO layers, and a strong reduction in magnetization in LCMO near the interface. Prajapat et al. investigated the YBCO/LCMO interface using SrTiO3(STO) as an intermediate insulating layer, and reported that the magnetic depletion (MD) in LCMO near the interface is dependent on the critical temperature of YBCO and the thickness of the LCMO layer, indicating that the origin of the MD in LCMO is due to Cooper pairs tunneling through the STO interlayer across the interface [5]. Our work has verified the observation of the MD layer in LCMO near the interface as claimed in the past report, and additionally studied the effect of a magnetic field on this layer. Polarised neutron reflectometry measurements performed at ANSTO on the PLATYPUS reflectometer indicate that the effect of applying fields at 0.03 T and 1 T is to cause a small restoration of magnetization in this depleted layer. This result supports the mechanism proposed by Prajapat et al. of superconductivity-induced MD from tunnelling through STO.
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    Magnetic proximity effect in YBCO/STO/LCMO multilayers
    (Australian Institute of Physics, 2017-01-31) Paull, O; Causer, GL; Pan, AV; Klose, F
    Tailoring of the electronic properties of complex oxide heterostructures, thin films, and superlattices with atomically sharp interfaces is at the frontline of materials research at present [1, 2, 3]. Interfaces exhibit novel states that are not possible in bulk materials as a result of broken symmetry, induced strains, and modified exchange interactions. In this work we examined the interaction between a superconducting YBa2Cu3O7-δ thin film layer and a ferromagnetic La2/3Ca1/3MnO3 layer using polarised neutron reflectometry. The interaction at the interface between YBCO/LCMO multilayers has resulted in a variety of observed phenomena such as induced ferromagnetic moments in YBCO layers that are antiparallel to neighbouring LCMO layers, and a strong reduction in magnetization in LCMO near the interface. Prajapat et al. investigated the YBCO/LCMO interface using SrTiO3 (STO) as an intermediate insulating layer, and reported that the magnetic depletion (MD) in LCMO near the interface is dependent on the critical temperature of YBCO and the thickness of the LCMO layer, and claimed that the origin of the MD in LCMO is due to Cooper pairs tunneling through the STO interlayer across the interface [5]. Our work has verified the observation of the MD layer in LCMO near the interface as claimed in the past report, and additionally studied the effect of a magnetic field on this layer. Polarised neutron reflectometry measurements performed at ANSTO on the PLATYPUS reflectometer indicate that the effect of applying fields at 0.03 T and 1 T is to cause a small restoration of magnetization in this depleted layer. This result supports the mechanism proposed by Prajapat et al. of a superconductivity-induced MD from tunneling through STO.
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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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    Opportunity for neutron scattering in spintronic thin film materials science
    (Institute of Electrical and Electronics Engineers (IEEE), 2016-08-01) Klose, F; Causer, GL; Cortie, DL
    Basic science has revealed novel magnetic materials or magnetic effects which, in principle, show promise to be deployed in future magnetic electronics or storage devices. A particularly promising area is spintronic materials. However, many of the most promising materials only work under extreme conditions such as very low temperatures or large magnetic fields. The big challenge is to understand the physics of these often artificially structured and engineered materials at the atomic or nanometer level and to make them work at room-temperature. In order to characterise new magnetic properties which are often caused by interface or finite size effects, neutron scattering techniques such as polarised neutron reflectometry (PNR), SANS and diffraction are very powerful tools. These techniques have the capability for characterising the magnetic structures of artificially layered films from the micron down to the sub-nanometre scale. Despite still being a relatively “slow” magnetic measurement technique, neutron scattering experiments are often the key to explaining the underlying physics as they provide information that is only accessible using the unique combination of properties provided by the neutron particle. This talk will review the current state-of-the-art and present striking examples which illustrate the usefulness of neutron quantum beams in magnetic thin film and spintronics research. The first example is a spin-polarised neutron reflectometry study on Co implanted TiO2 which demonstrates that giant magnetic moments up to 2.9μB per cobalt, not seen before in rutile or anatase, emerge from lightly Co doped regions of the amorphous TiO2 matrix [1]. The second example is a PNR study on ion beam irradiated FePt3 [2]. Here, the ion irradiation transforms AFM chemically ordered FePt3 into a chemically disordered state from which a distinct exchange bias effect emerges. © 2016 IEEE
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    Polarized neutron reflectometry of epitaxial Fe[0.25 + x] Pt[0.75 – x] layers.
    (Institute of Electrical and Electronics Engineers (IEEE), 2018-04-27) Mankey, GJ; Causer, GL; Cortie, DL; Wang, X; Zhu, H; Lonescu, M; Klose, F
    Epitaxial Fe[0.25 + x]Pt[0.75 - x] layers can be either antiferromagnetic (AF) or ferromagnetic (FM) depending on the degree of chemical ordering controlled by the deposition temperature. Our neutron diffraction studies were the first to study AF phase transitions in these thin films [1] and we have also shown using PNR that a mixed AF-FM film is exchange biased with itself [2]. In AF-FM exchange-biased superlattices with a modulated chemical order parameter, PNR shows the magnetization can be modulated through the film thickness with no composition modulations [3]. Our recent results reveal that He+ ion bombardment and annealing can be applied toward controlling magnetic phases in epitaxial Fe [0.25] Pt [0.75] layers [4]. © Copyright 2024 IEEE - All rights reserved, including rights for text and data mining and training of artificial intelligence and similar technologies.
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    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
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    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