Steven R. Spurgeon

Magnetic properties of Co2C and Co3C nanoparticles and their assemblies

Nano-composite material consisting of Co2C and Co3C nanoparticles has recently been shown to exhibit unusually large coercivities and energy products. Experimental studies that can delineate the properties of individual phases have been undertaken and provide information on how the coercivities and the energy product change with the size and composition of the nanoparticles. The studies indicate that while both phases are magnetic, the Co3C has higher magnetization and coercivity compared to Co2C. Through first principles electronic structure studies using a GGA+U functional, we provide insight on the role of C intercalation on enhancing the magnetic anisotropy of the individual phases.

Thickness-dependent crossover from charge- to strain-mediated magnetoelectric coupling in ferromagnetic/piezoelectric oxide heterostructures.

Magnetoelectric oxide heterostructures are proposed active layers for spintronic memory and logic devices, where information is conveyed through spin transport in the solid state. Incomplete theories of the coupling between local strain, charge, and magnetic order have limited their deployment into new information and communication technologies. In this study, we report direct, local measurements of strain- and charge-mediated magnetization changes in the La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 system using spatially resolved characterization techniques in both real and reciprocal space. Polarized neutron reflectometry reveals a graded magnetization that results from both local structural distortions and interfacial screening of bound surface charge from the adjacent ferroelectric. Density functional theory calculations support the experimental observation that strain locally suppresses the magnetization through a change in the Mn-eg orbital polarization. We suggest that this local coupling and magnetization suppression may be tuned by controlling the manganite and ferroelectric layer thicknesses, with direct implications for device applications.

Polarization screening-induced magnetic phase gradients at complex oxide interfaces

Thin-film oxide heterostructures show great potential for use in spintronic memories, where electronic charge and spin are coupled to transport information. Here we use a La0.7Sr0.3MnO3 (LSMO)/PbZr0.2Ti0.8O3 (PZT) model system to explore how local variations in electronic and magnetic phases mediate this coupling. We present direct, local measurements of valence, ferroelectric polarization and magnetization, from which we map the phases at the LSMO/PZT interface. We combine these experimental results with electronic structure calculations to elucidate the microscopic interactions governing the interfacial response of this system. We observe a magnetic asymmetry at the LSMO/PZT interface that depends on the local PZT polarization and gives rise to gradients in local magnetic moments; this is associated with a metal-insulator transition at the interface, which results in significantly different charge-transfer screening lengths. This study establishes a framework to understand the fundamental asymmetries of magnetoelectric coupling in oxide heterostructures.

Influence of LaFeO3 Surface Termination on Water Reactivity

The polarity of oxide surfaces can dramatically impact their surface reactivity, in particular, with polar molecules such as water. The surface species that result from this interaction change the oxide electronic structure and chemical reactivity in applications such as photoelectrochemistry but are challenging to probe experimentally. Here, we report a detailed study of the surface chemistry and electronic structure of the perovskite LaFeO3 in humid conditions using ambient-pressure X-ray photoelectron spectroscopy. Comparing the two possible terminations of the polar (001)-oriented surface, we find that the LaO-terminated surface is more reactive toward water, forming hydroxyl species and adsorbing molecular water at lower relative humidity than its FeO2-terminated counterpart. However, the FeO2-terminated surface forms more hydroxyl species during water adsorption at higher humidity, suggesting that adsorbate-adsorbate interactions may impact reactivity. Our results demonstrate how the termination of a complex oxide can dramatically impact its reactivity, providing insight that can aid in the design of catalyst materials.

Exchange bias and bistable magneto-resistance states in amorphous TbFeCo thin films

Amorphous TbFeCo thin films sputter deposited at room temperature on thermally oxidized Si substrate are found to exhibit strong perpendicular magnetic anisotropy. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy mapping have revealed two nanoscale amorphous phases with different Tb atomic percentages distributed within the amorphous film. Exchange bias accompanied by bistable magneto-resistance states has been uncovered near room temperature by magnetization and magneto-transport measurements. The exchange anisotropy originates from the exchange interaction between the ferrimagnetic and ferromagnetic components corresponding to the two amorphous phases. This study provides a platform for exchange bias and magneto-resistance switching using single-layer amorphous ferrimagnetic thin films that require no epitaxial growth.

The effects of core-level broadening in determining band alignment at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction

Chemical effects at the surface and interface can broaden core-level spectra in X-ray photoemission for thin-film heterojunctions, as can electronic charge redistributions. We explore these effects and their influence on the measurement of valence and conduction band offsets at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction. We observe a clear broadening in Ge 3d and Sr 3d core-level X-ray photoelectron spectra relative to those of clean, bulk Ge(001), and homoepitaxial SrTiO3(001), respectively. Angle-resolved measurements indicate that this broadening is driven primarily by chemical shifts associated with surface hydroxylation, with built-in potentials playing only a minor role. The impact of these two interpretations on the valence band offset is significant on the scale of transport energetics, amounting to a difference of 0.2 eV.

A study of the effect of iron island morphology and interface oxidation on the magnetic hysteresis of Fe-MgO (001) thin film composites

Fe-MgO tunnel junctions have received much attention for their use in hard drive read heads and other spintronic applications. The system is particularly interesting because of its magnetoresistive behavior and the abundance and low cost of its constituent elements. However, many questions remain about how the structure and chemistry of the Fe-MgO interface mediates magnetic behavior. In this study, we report on transmission electron microscopy, electron energy loss spectroscopy, and magnetic characterization of Fe-MgO composite films with various morphologies. We explore relationships between film morphology, intermixing, and the resulting effects on magnetic structure. We find the presence of oxidation at the Fe-MgO interface, with a detrimental impact on the saturation magnetization of the composite. We also observe changes in coercivity and magnetocrystalline anisotropy with film morphology and thickness. These results will inform the design of MgO-based tunnel junctions and improve our understanding ...

Measurement Error in Atomic-Scale Scanning Transmission Electron Microscopy—Energy-Dispersive X-Ray Spectroscopy (STEM-EDS) Mapping of a Model Oxide Interface

With the development of affordable aberration-correctors, analytical scanning transmission electron microscopy (STEM) studies of complex interfaces can now be conducted at high spatial resolution at laboratories worldwide. Energy-dispersive X-ray spectroscopy (STEM-EDS) in particular has grown in popularity, since it enables elemental mapping over a wide range of ionization energies. However, the interpretation of atomically-resolved data is greatly complicated by beam-sample interactions that are often overlooked by novice users. Here we describe the practical factors—namely, sample thickness and the choice of ionization edge—that affect the quantification of a model perovskite oxide interface. Our measurements of the same sample in regions of different thickness indicate that interface profiles can vary by as much as 2–5 unit cells, depending on the spectral feature. This finding is supported by multislice simulations, which reveal that on-axis maps of even perfectly abrupt interfaces exhibit significant delocalization. Quantification of thicker samples is further complicated by channeling to heavier sites across the interface, as well as an increased signal background. We show that extreme care must be taken to prepare samples to minimize channeling effects and argue that it may not be possible to extract atomically-resolved information from many chemical maps.With the development of affordable aberration correctors, analytical scanning transmission electron microscopy (STEM) studies of complex interfaces can now be conducted at high spatial resolution at laboratories worldwide. Energy-dispersive X-ray spectroscopy (EDS) in particular has grown in popularity, as it enables elemental mapping over a wide range of ionization energies. However, the interpretation of atomically resolved data is greatly complicated by beam-sample interactions that are often overlooked by novice users. Here we describe the practical factors-namely, sample thickness and the choice of ionization edge-that affect the quantification of a model perovskite oxide interface. Our measurements of the same sample, in regions of different thickness, indicate that interface profiles can vary by as much as 2-5 unit cells, depending on the spectral feature. This finding is supported by multislice simulations, which reveal that on-axis maps of even perfectly abrupt interfaces exhibit significant delocalization. Quantification of thicker samples is further complicated by channeling to heavier sites across the interface, as well as an increased signal background. We show that extreme care must be taken to prepare samples to minimize channeling effects and argue that it may not be possible to extract atomically resolved information from many chemical maps.

Interface-Induced Polarization in SrTiO3-LaCrO3 Superlattices

Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated considerable interest because they possess a range of desirable properties for functional devices. In this work, emergent polarization in superlattices of SrTiO

Built‐In Potential in Fe2O3‐Cr2O3 Superlattices for Improved Photoexcited Carrier Separation

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