Engineering topics

Harmon, Bruce N.

Publications and source records attributed to Harmon, Bruce N..

3 records

DOE OSTI2017

Electronic structure and magnetic properties in T 2 AlB 2 ( T = Fe, Mn, Cr, Co, and Ni) and their alloys

In this study, the electronic structure and intrinsic magnetic properties of Fe 2 AlB 2 -related compounds and their alloys have been investigated using density functional theory. For Fe 2 AlB 2 , the crystallographic a axis is the easiest axis, which agrees with experiments. The magnetic ground state of Mn 2 AlB 2 is found to be ferromagnetic in the basal ab plane, but antiferromagnetic along the c axis. All 3d dopings considered decrease the magnetization and Curie temperature in Fe 2 AlB 2 . Electron doping with Co or Ni has a stronger effect on the decreasing of Curie temperature in Fe 2 AlB 2 than hole doping with Mn or Cr. However, a larger amount of Mn doping on Fe 2 AlB 2 promotes the ferromagnetic to antiferromagnetic transition. A very anisotropic magnetoelastic effect is found in Fe 2 AlB 2 : the magnetization has a much stronger dependence on the lattice parameter c than on a or b, which is explained by electronic-structure features near the Fermi level. Dopings of other elements on B and Al sites are also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Physical properties of metallic antiferromagneticCaCo1.86As2 single crystals

X-ray powder diffraction (XRD), magnetic susceptibility χ, isothermal magnetization M, heat capacity Cp, and in-plane electrical resistivity ρ measurements as a function of temperature T and magnetic field H are presented for CaCo1.86As2 single crystals. The electronic structure is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of CaCo1.86As2 and by full-potential linearized augmented-plane-wave calculations for the supercell Ca8Co15As16 (CaCo1.88As2). Our XRD crystal structure refinement is consistent with the previous combined refinement of x-ray and neutron powder diffraction data showing a collapsed-tetragonal ThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to the composition CaCo1.86As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420 (2013)]. The anisotropic χ(T) data are consistent with the magnetic neutron diffraction data of Quirianale et al. that demonstrate the presence of A-type collinear antiferromagnetic order below the Néel temperature TN=52(1) K with the easy axis being the tetragonal c axis. However, no clear evidence from the ρ(T) and Cp(T) data for a magnetic transition at TN is observed. A metallic ground state is demonstrated from the band calculations and the ρ(T), Cp(T), and ARPES data, and spin-polarized calculations indicate a competition between the A-type AFM and FM ground states. The Cp(T) data exhibit a large Sommerfield electronic coefficient reflecting a large density of states at the Fermi energy D(EF) that is enhanced compared with the band structure calculation where the bare D(EF) arises from Co 3d bands. At 1.8 K, the M(H) data for H∥c exhibit a well-defined first-order spin-flop transition at an applied field of 3.5 T. The small ordered moment of ≈0.3μB/Co obtained from the M(H) data at low T, the large exchange enhancement of χ and the lack of a self-consistent interpretation of the χ(T) and M(H,T) data in terms of a local moment Heisenberg model together indicate that the magnetism of CaCo1.86As2 is itinerant.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Dramatic changes in the electronic structure upon transition to the collapsed tetragonal phase in CaFe 2 As 2

We use angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of CaFe 2 As 2 in the collapsed tetragonal (CT) phase. This unusual phase of iron arsenic high-temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the postgrowth thermal treatment of single crystals, we were able to stabilize the CT phase at ambient pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase, along with an apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.

36 MATERIALS SCIENCE↗