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Hartshorne, Taheri, and colleagues publish in Ultramicroscopy

Posted in News on June 12th, 2014 by admin – Comments Off

Matthew Hartshorne, Dr. Taheri, and colleagues’ latest paper, titled “Specimen preparation for correlating transmission electron microscopy and atom probe tomography of mesoscale features,” has been published online in Ultramicroscopy.

From the abstract: “Atom-probe tomography (APT) provides atomic-scale spatial and compositional resolution that is ideally suited for the analysis of grain boundaries. The small sample volume analyzed in APT presents, however, a challenge for capturing mesoscale features, such as grain boundaries. A new site-specific method utilizing transmission electron microscopy (TEM) for the precise selection and isolation of mesoscale microstructural features in a focused-ion-beam (FIB) microscope lift-out sample, from below the original surface of the bulk sample, for targeted preparation of an APT microtip by FIB-SEM microscopy is presented. This methodology is demonstrated for the targeted extraction of a prior austenite grain boundary in a martensitic steel alloy; it can, however, be easily applied to other mesoscale features, such as heterophase interfaces, precipitates, and the tips of cracks.”

The accepted proof of the paper can be accessed here.

More information is available on the Elsevier website.

Spurgeon defends doctoral thesis

Posted in News on June 3rd, 2014 by admin – Comments Off

Steven Spurgeon has successfully defended his doctoral thesis, titled, “Correlating Interfacial Structure and Magnetism in Thin Film Oxide Heterostructures Using Transmission Electron Microscopy and Neutron Reflectometry.”

Winkler, Jablonski, Taheri and colleagues publish in Nano Letters

Posted in News on May 12th, 2014 by admin – Comments Off

Christopher Winkler, Michael Jablonski, Dr. Taheri, and colleagues’ latest paper, titled “Real-time observation of local strain effects on non-volatile ferroelectric memory storage mechanisms,” has been published online in the Nano Letters.

From the abstract: “We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics towards a Nucleation Limited Switching model.”

The accepted proof of the paper can be accessed here.

More information is available on the ACS website.

Vetterick defends doctoral thesis

Posted in News on May 9th, 2014 by admin – Comments Off

Vetterick Defense

Greg Vetterick has successfully defended his doctoral thesis on the “Radiation Tolerance Mechanisms in Nanocrystalline BCC Iron!”

Winkler defends doctoral thesis

Posted in News on March 14th, 2014 by admin – Comments Off

Winkler Defense

Christopher Winkler has successfully defended his doctoral thesis on the “Characterization of the Dynamic Behavior of Bismuth Ferrite Thin Films Under DC Bias Using In Situ Transmission Electron Microscopy.” Chris has accepted a senior research scientist position at Virginia Tech.

For more information, see the ICTAS Nanoscale Characterization and Fabrication Laboratory website.

Barr, Vetterick, Taheri, and colleagues publish in Acta Materialia

Posted in News on January 27th, 2014 by admin – Comments Off

Christopher Barr, Greg Vetterick, Dr. Taheri, and colleagues’ latest paper, titled “Anisotropic radiation-induced segregation in 316L austenitic stainless steel with grain boundary character” has been published online in the Acta Materialia.

From the abstract: “Radiation-induced segregation (RIS) and subsequent depletion of chromium along grain boundaries has been shown to be an important factor in irradiation-assisted stress corrosion cracking in austenitic face-centered cubic (fcc)-based alloys used for nuclear energy systems. A full understanding of RIS requires examination of the effect of the grain boundary character on the segregation process. Understanding how specific grain boundary structures respond under irradiation would assist in developing or designing alloys that are more efficient at removing point defects, or reducing the overall rate of deleterious Cr segregation. This study shows that solute segregation is dependent not only on grain boundary misorientation, but also on the grain boundary plane, as highlighted by markedly different segregation behavior for the Σ3 incoherent and coherent grain boundaries. . .”

The accepted proof of the paper can be accessed here.

More information is available on the Springer website.

Scotto D’Antuono, Taheri, and colleagues publish in Scripta Materialia

Posted in News on January 13th, 2014 by admin – Comments Off

Daniel Scotto D’Antuono, Dr. Taheri, and colleagues’ latest paper, titled “Grain Boundary Misorientation Dependence of β Phase Precipitation in an Al-Mg Alloy” has been published online in the Scripta Materialia.

From the abstract: “Precipitation of β phase (Al3Mg2) in an Al-Mg alloy is investigated by in situ heating and precession diffraction-based orientation imaging in a transmission electron microscope. Initial β formation occurs more readily at low angle grain boundaries rather than at high angle boundaries, however larger precipitates are found at high angle boundaries. We propose a mechanism for β formation based on boundary energy and grain boundary free volume. These results advance the understanding of Β phase formation, aiding future corrosion prevention.”

The accepted proof of the paper can be accessed here.

More information is available on the Elsevier website.

Spurgeon, Sloppy, Taheri and colleagues publish in ACS Nano

Posted in News on December 8th, 2013 by admin – Comments Off

Steven Spurgeon, Dr. Jennifer Sloppy, Dr. Taheri, and colleagues’ latest paper, titled “Thickness-Dependent Crossover from Charge- to Strain-Mediated Magnetoelectric Coupling in Ferromagnetic / Piezoelectric Oxide Heterostructures” has been published online in the ACS Nano.

From the abstract: “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.”

The uncorrected proof of the paper can be accessed on the ACS website: ACS Nano.

Barr and Spurgeon win MRS awards

Posted in News on December 7th, 2013 by admin – Comments Off

Christopher Barr and Steven Spurgeon recently won awards for their research at the 2013 Fall MRS Meeting in Boston.

Barr’s poster, titled “Anisotropic Behavior of Radiation Induced Segregation with Grain Boundary Character and Grain Size in Model Fcc Ni-Cr Alloys,” was selected for a silver poster award in Symposium ZZ — Nanostructured Materials in Extreme Environments. The award consists of a certificate and a $100 honorarium.

Spurgeon was invited to present a talk, titled “Thickness-Dependent Crossover from Charge- to Strain-Mediated Magnetoelectric Coupling in Ferromagnetic/Piezoelectric Oxide Heterostructures,” and was awarded a gold graduate student research award. This prestigious award is given to only eight of 240 applicants and consists of a plaque and $400 honorarium.

For more information about the award, visit MRS Fall 2013 Meeting Awards.

Ghassemi, Lang, Taheri, and colleagues publish in Journal of Applied Physics

Posted in News on August 13th, 2013 by admin – Comments Off

Dr. Ghassemi, Andrew Lang, Dr. Taheri, and colleagues’ latest paper, titled “Evolution of strain in aluminum gallium nitride/gallium nitride high electron mobility transistors under on-state bias” has been published online in the Journal of Applied Physics.

From the abstract: “On-state degradation of AlGaN/GaN high electron mobility transistors (HEMTs) was quantified as a function of defect generation and strain evolution using high-resolution transmission electron microscopy. Observation of devices under on-state stress conditions elucidated defect formation mechanisms, which is known to be caused by hot electrons. Geometric phase analysis indicated a 25% decrease of the in-plane tensile strain in the AlGaN barrier after extended bias. Changes in sheet polarization charge density were estimated based on observations of the defect formation and resulting strain relaxation. We propose three regimes of degradation during HEMT device operation, and suggest the presence of a critical point at which defects become stable or permanent.”

The paper can be viewed directly here.

The paper can also be accessed on the AIP website: AIP Publishing.