Search NASA⌕ Search

Engineering topics

Yuan, S. J.

Publications and source records attributed to Yuan, S. J..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Evidence for a low-temperature magnetic ground state in double-perovskite iridates with I r 5 + ( 5 d 4 ) ions

Here, we report an unusual magnetic ground state in single-crystal, double-perovskite Ba 2 YIrO 6 and Sr-doped Ba 2 YIrO 6 with Ir 5+ (5d 4 ) ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin S=1 state, or a singlet J eff =0 state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund's rule coupling and inherently large electron hopping, which cannot stabilize the singlet J eff =0 ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Simultaneous metal-insulator and antiferromagnetic transitions in orthorhombic perovskite iridate Sr 0.94 Ir 0.78 O 2.68 single crystals

The orthorhombic perovskite SrIrO 3 is a semimetal, an intriguing exception in iridates where the strong spin-orbit interaction coupled with electron correlations tends to impose an insulating state. Here, we report results of our investigation of bulk single-crystal Sr 0.94 Ir 0.78 O 2.68 or Ir-deficient, orthorhombic perovskite SrIrO 3 . It retains the same crystal structure as stoichiometric SrIrO 3 but exhibits a sharp, simultaneous antiferromagnetic (AFM) and metal-insulator (MI) transition occurring in the basal-plane resistivity at 185 K. Above it, the basal-plane resistivity features an extended regime of almost linear temperature dependence up to 800 K but the strong electronic anisotropy renders an insulating behavior in the out-of-plane resistivity. The Hall resistivity undergoes an abrupt sign change and grows below 40 K, which along with the Sommerfeld constant of 20 mJ/mol K 2 suggests a multiband effect. All results including our first-principles calculations underscore a delicacy of the paramagnetic, metallic state in SrIrO 3 that is in close proximity to an AFM insulating state. The contrasting ground states in isostructural Sr 0.94 Ir 0.78 O 2.68 and SrIrO 3 illustrate a critical role of lattice distortions and Ir deficiency in rebalancing the ground state in the iridates. Finally, the concurrent AFM and MI transitions reveal a direct correlation between the magnetic transition and formation of an activation gap in the iridate, which is conspicuously absent in Sr 2 IrO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ground-state tuning of metal-insulator transition by compositional variations in BaIr 1–x Ru x O 3 (0 ≤ x ≤ 1)

Hexagonal BaIrO 3 is a magnetic insulator driven by the spin-orbit interaction (SOI), whereas BaRuO 3 is an enhanced paramagnetic metal. Here, our investigation of structural, magnetic, transport, and thermal properties reveals that substitution of Ru 4+ (4d 4 ) ions for Ir 4+ (5d 5 ) ions in BaIrO 3 reduces the magnitudes of the SOI and a monoclinic structural distortion and rebalances the competition between the SOI and the lattice degrees of freedom to render an evolution from a magnetic insulting state to a robust metallic state. The central findings of this paper are as follows: (1) light Ru doping (0 < x ≤ 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature T N and the electrical resistivity, and (2) heavier Ru doping (0.41 ≤ x ≤ 0.9) induces a robust metallic state without any long-range magnetic order. All results suggest a critical role of the lattice degrees of freedom in determining the ground state in the heavy transition-metal oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr 2 IrO 4

Sr 2 IrO 4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at T N = 240 K. We report results of a comprehensive study of single-crystal Sr 2 Ir 1-x Tb x O 4 (0≤x≤0.03). This study found that mere 3% (x=0.03) tetravalent Tb 4+ (4f 7 ) substituting for Ir 4+ (rather than Sr 2+ ) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x = 0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund’s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. Furthermore, this work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Evolution of magnetism in single-crystal C a 2 R u 1 - x I r x O 4 ( 0 ≤ x ≤ 0.65 )

In this paper, we report structural, magnetic, transport, and thermal properties of single-crystal Ca 2 Ru 1-x Ir x O 4 (0≤x≤0.65). Ca 2 RuO 4 is a structurally driven Mott insulator with a metal-insulator transition at T MI =357K, which is well separated from antiferromagnetic order at T N =110K. Substitution of a 5d element, Ir, for Ru enhances spin-orbit coupling and locking between the structural distortions and magnetic moment canting. Ir doping intensifies the distortion or rotation of Ru/IrO 6 octahedra and induces weak ferromagnetic behavior along the c axis. In particular, Ir doping suppresses T N but concurrently causes an additional magnetic ordering T N2 at a higher temperature up to 210 K for x=0.65. The effect of Ir doping sharply contrasts with that of 3d-element doping such as Cr, Mn, and Fe, which suppresses T N and induces unusual negative volume thermal expansion. Finally, the stark difference between 3d- and 5d-element doping underlines a strong magnetoelastic coupling inherent in the Ir-rich oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Coexisting charge and magnetic orders in the dimer-chain iridate Ba 5 AlIr 2 O 11

In this paper, we have synthesized and studied single-crystal Ba 5 AlIr 2 O 11 that features dimer chains of two inequivalent octahedra occupied by tetravalent Ir 4+ (5d 5 ) and pentavalent Ir 5+ (5d 4 ) ions, respectively. Ba 5 AlIr 2 O 11 is a Mott insulator that undergoes a subtle structural phase transition near T S =210K and a magnetic transition at T M =4.5K; the latter transition is surprisingly resistant to applied magnetic fields μ o H≤12T but more sensitive to modest applied pressure (dT M /dp ≈ +0.61K/GPa). All results indicate that the phase transition at T S signals an enhanced charge order that induces electrical dipoles and strong dielectric response near T S . It is clear that the strong covalency and spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state that is neither S=3/2 nor J=1/2, but rather lies in an “intermediate” regime between these two states. Finally, the novel behavior of Ba 5 AlIr 2 O 11 therefore provides unique insights into the physics of SOI along with strong covalency in competition with double-exchange interactions of comparable strength.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetic anisotropies in epitaxial Fe{sub 3}O{sub 4}/GaAs(100) patterned structures

Previous studies on epitaxial Fe{sub 3}O{sub 4} rings in the context of spin-transfer torque effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe{sub 3}O{sub 4} thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negligible. We demonstrate that in the Fe{sub 3}O{sub 4} planar wires patterned from our thin film, such a unique magnetic anisotropy system has been preserved, and relatively simple magnetic domain configurations compared to those previous reports can be obtained.

36 MATERIALS SCIENCE↗