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Full-Text Articles in Physics

Half-Metallic Co-Based Quaternary Heusler Alloys For Spintronics: Defect- And Pressure-Induced Transitions And Properties, Enamullah, D. D. Johnson, K. G. Suresh, Aftab Alam Nov 2016

Half-Metallic Co-Based Quaternary Heusler Alloys For Spintronics: Defect- And Pressure-Induced Transitions And Properties, Enamullah, D. D. Johnson, K. G. Suresh, Aftab Alam

Ames Laboratory Publications

Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite ...


Origin Of Magnetic Anisotropy In Doped Ce2co17 Alloys, Liqin Ke, D. A. Kukusta, Duane D. Johnson Oct 2016

Origin Of Magnetic Anisotropy In Doped Ce2co17 Alloys, Liqin Ke, D. A. Kukusta, Duane D. Johnson

Ames Laboratory Publications

Magnetocrystalline anisotropy (MCA) in doped Ce2Co17 and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the ...


Metallic Layer-By-Layer Photonic Crystals For Linearly-Polarized Thermal Emission And Thermophotovoltaic Device Including Same, Jae-Hwang Lee, Kai-Ming Ho, Kristen P. Constant Jul 2016

Metallic Layer-By-Layer Photonic Crystals For Linearly-Polarized Thermal Emission And Thermophotovoltaic Device Including Same, Jae-Hwang Lee, Kai-Ming Ho, Kristen P. Constant

Iowa State University Patents

Metallic thermal emitters consisting of two layers of differently structured nickel gratings on a homogeneous nickel layer are fabricated by soft lithography and studied for polarized thermal radiation. A thermal emitter in combination with a sub-wavelength grating shows a high extinction ratio, with a maximum value close to 5, in a wide mid-infrared range from 3.2 to 7.8 μm, as well as high emissivity up to 0.65 at a wavelength of 3.7 μm. All measurements show good agreement with theoretical predictions. Numerical simulations reveal that a high electric field exists within the localized air space surrounded ...


Titanium Α−Ω Phase Transformation Pathway And A Predicted Metastable Structure, Nikolai A. Zarkevich, Duane D. Johnson Jan 2016

Titanium Α−Ω Phase Transformation Pathway And A Predicted Metastable Structure, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

As titanium is a highly utilized metal for structural lightweighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band method employing two climbing images combined with density functional theory DFT + U methods for accurate energetics, we detail the pressure-induced α (ductile) to ω (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path and discover a metastable body-centered orthorhombic structure, with stable phonons, a lower density than the end-point phases, and decreasing stability with increasing pressure.