56 research outputs found
Precipitating Ordered Skyrmion Lattices from Helical Spaghetti
Magnetic skyrmions have been the focus of intense research due to their
potential applications in ultra-high density data and logic technologies, as
well as for the unique physics arising from their antisymmetric exchange term
and topological protections. In this work we prepare a chiral jammed state in
chemically disordered (Fe, Co)Si consisting of a combination of
randomly-oriented magnetic helices, labyrinth domains, rotationally disordered
skyrmion lattices and/or isolated skyrmions. Using small angle neutron
scattering, (SANS) we demonstrate a symmetry-breaking magnetic field sequence
which disentangles the jammed state, resulting in an ordered, oriented skyrmion
lattice. The same field sequence was performed on a sample of powdered Cu2OSeO3
and again yields an ordered, oriented skyrmion lattice, despite relatively
non-interacting nature of the grains. Micromagnetic simulations confirm the
promotion of a preferred skyrmion lattice orientation after field treatment,
independent of the initial configuration, suggesting this effect may be
universally applicable. Energetics extracted from the simulations suggest that
approaching a magnetic hard axis causes the moments to diverge away from the
magnetic field, increasing the Dzyaloshinskii-Moriya energy, followed
subsequently by a lattice re-orientation. The ability to facilitate an emergent
ordered magnetic lattice with long-range orientation in a variety of materials
despite overwhelming internal disorder enables the study of skyrmions even in
imperfect powdered or polycrystalline systems and greatly improves the ability
to rapidly screen candidate skyrmion materials
Correlation between microstructure and magnetotransport in organic semiconductor spin valve structures
We have studied magnetotransport in organic-inorganic hybrid multilayer
junctions. In these devices, the organic semiconductor (OSC) Alq
(tris(8-hydroxyquinoline) aluminum) formed a spacer layer between ferromagnetic
(FM) Co and Fe layers. The thickness of the Alq layer was in the range of
50-150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current
perpendicular to plane geometry, and these effects persisted up to room
temperature. The devices' microstructure was studied by X-ray reflectometry,
Auger electron spectroscopy and polarized neutron reflectometry (PNR). The
films show well-defined layers with modest average chemical roughness (3-5 nm)
at the interface between the Alq and the surrounding FM layers.
Reflectometry shows that larger MR effects are associated with smaller
FM/Alq interface width (both chemical and magnetic) and a magnetically dead
layer at the Alq/Fe interface. The PNR data also show that the Co layer,
which was deposited on top of the Alq, adopts a multi-domain magnetic
structure at low field and a perfect anti-parallel state is not obtained. The
origins of the observed MR are discussed and attributed to spin coherent
transport. A lower bound for the spin diffusion length in Alq was estimated
as nm at 80 K. However, the subtle correlations between
microstructure and magnetotransport indicate the importance of interfacial
effects in these systems.Comment: 21 pages, 11 figures and 2 table
Polarization-analyzed small-angle neutron scattering. II. Mathematical angular analysis
Polarization-analyzed small-angle neutron scattering (SANS) is a powerful tool for the study of magnetic morphology with directional sensitivity. Building upon polarized scattering theory, this article presents simplified procedures for the reduction of longitudinally polarized SANS into terms of the three mutually orthogonal magnetic scattering contributions plus a structural contribution. Special emphasis is given to the treatment of anisotropic systems. The meaning and significance of scattering interferences between nuclear and magnetic scattering and between the scattering from magnetic moments projected onto distinct orthogonal axes are discussed in detail. Concise tables summarize the algorithms derived for the most commonly encountered conditions. These tables are designed to be used as a reference in the challenging task of extracting the full wealth of information available from polarization-analyzed SANS
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Interfacial-Redox-Induced Tuning of Superconductivity in YBa2Cu3O7-δ.
Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa2Cu3O7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation
Ionic Tuning of Cobaltites at the Nanoscale
Control of materials through custom design of ionic distributions represents
a powerful new approach to develop future technologies ranging from spintronic
logic and memory devices to energy storage. Perovskites have shown particular
promise for ionic devices due to their high ion mobility and sensitivity to
chemical stoichiometry. In this work, we demonstrate a solid-state approach to
control of ionic distributions in (La,Sr)CoO thin films. Depositing a Gd
capping layer on the perovskite film, oxygen is controllably extracted from the
structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate
with the oxygen extraction, the Co valence state and saturation magnetization
show a smooth continuous variation. In contrast, magnetoresistance measurements
show no-change in the magnetic anisotropy and a rapid increase in the
resistivity over the same range of oxygen stoichiometry. These results suggest
significant phase separation, with metallic ferromagnetic regions and
oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated
networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering,
including transformation to a brownmillerite crystal structure. The unexpected
transformation to the brownmillerite phase at ambient temperature is further
confirmed by high-resolution scanning transmission electron microscopy which
shows significant structural - and correspondingly chemical - phase separation.
This work demonstrates room-temperature ionic control of magnetism, electrical
resistivity, and crystalline structure in a 36 nm thick film, presenting new
opportunities for ionic devices that leverage multiple material
functionalities
Particle Moment Canting in CoFe2O4 Nanoparticles
Polarization-analyzed small-angle neutron scattering methods are used to determine the spin morphology in high crystalline anisotropy, 11 nm diameter CoFe2O4 nanoparticle assemblies with randomly oriented easy axes. In moderate to high magnetic fields, the nanoparticles adopt a uniformly canted structure, rather than forming domains, shells, or other arrangements. The observed canting angles agree quantitatively with those predicted from an energy model dominated by Zeeman and anisotropy competition, with implications for the technological use of such nanoparticles
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