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  1. Abstract

    In an effort to reconcile the various interpretations for the cation components of the 2p3/2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p3/2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p3/2core levels of NiFe2O4thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles NixCo1−xFe2O4(x= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe2O4thin film. Surface-to-bulk core level shifts are also observed in CoFe2O4and NiCo2O4thin films but not as significantly as in NiFe2O4thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles.

     
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    Free, publicly-accessible full text available April 12, 2025
  2. Free, publicly-accessible full text available August 9, 2024
  3. NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion.

     
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  4. This paper describes the 3D printing of a ternary composite of polydimethylsiloxane (PDMS) and nanoparticles of iron oxide and barium titanate. The composite was printed using a commercially available 3D printer. Thermal curing of the composite during printing allowed for overall low process times of a few minutes. Scanning electron microscopy indicated uniform composite layers. The resulting composite films showed ferromagnetic behaviour, and applicability in magnetic actuation and piezoelectric energy harvesting.

     
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  5. Abstract

    Electrical modulation of magnetic states in single-phase multiferroic materials, using domain-wall magnetoelectric (ME) coupling, can be enhanced substantially by controlling the population density of the ferroelectric (FE) domain walls during polarization switching. In this work, we investigate the domain-wall ME coupling in multiferroic h-YbFeO3thin films, in which the FE domain walls induce clamped antiferromagnetic (AFM) domain walls with reduced magnetization magnitude. Simulation according to the phenomenological theory indicates that the domain-wall ME effect is dramatically enhanced when the separation between the FE domain walls shrinks below the characteristic width of the clamped AFM domain walls during the ferroelectric switching. Experimentally, we show that while the magnetization magnitude remains same for both the positive and the negative saturation polarization states, there is evidence of magnetization reduction at the coercive voltages. These results suggest that the domain-wall ME effect is viable for electrical control of magnetization.

     
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  6. The ferrimagnetic inverse spinel NiCo 2 O 4 has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo 2 O 4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo 2 O 4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo 2 O 4 films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo 2 O 4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo 2 O 4 thin films for nanoscale spintronic applications. 
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  7. Multiferroic materials are an interesting functional material family combining two ferroic orderings, e.g. , ferroelectric and ferromagnetic orderings, or ferroelectric and antiferromagnetic orderings, and find various device applications, such as spintronics, multiferroic tunnel junctions, etc. Coupling multiferroic materials with plasmonic nanostructures offers great potential for optical-based switching in these devices. Here, we report a novel nanocomposite system consisting of layered Bi 1.25 AlMnO 3.25 (BAMO) as a multiferroic matrix and well dispersed plasmonic Au nanoparticles (NPs) and demonstrate that the Au nanoparticle morphology and the nanocomposite properties can be effectively tuned. Specifically, the Au particle size can be tuned from 6.82 nm to 31.59 nm and the 6.82 nm one presents the optimum ferroelectric and ferromagnetic properties and plasmonic properties. Besides the room temperature multiferroic properties, the BAMO-Au nanocomposite system presents other unique functionalities including localized surface plasmon resonance (LSPR), hyperbolicity in the visible region, and magneto-optical coupling, which can all be effectively tailored through morphology tuning. This study demonstrates the feasibility of coupling single phase multiferroic oxides with plasmonic metals for complex nanocomposite designs towards optically switchable spintronics and other memory devices. 
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