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7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Exchange-Biased Quantum Anomalous Hall Effect

The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr-doped (Bi,Sb) 2 Te 3 (CBST) grown on an uncompensated antiferromagnetic insulator Al-doped Cr 2 O 3 . Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al-Cr 2 O 3 surface spins fixing interfacial magnetic moments perpendicular to the film plane. Further, the interfacial coupling results in an exchange-biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al-Cr 2 O 3 layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH-based spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Strengthening of nanocrystalline Al using grain boundary solute additions: Effects of thermal annealing and ion irradiation

Strengthening of nanocrystalline Al by grain boundary solute additions was investigated for a series of dilute aluminum alloys, Al-Sc, Al-Sb, Al-Cr, and Al-W with grain sizes in the range of 50–200 nm. Thermal annealing of the alloys at low temperatures led to alloy softening, but with negligible change in the grain size. The re- duction in strength can be attributed to the loss of solute in the grain boundaries arising from grain boundary diffusion and precipitation. Annealing at higher temperatures led to grain growth, but with little additional loss of strength, a result of precipitation hardening. The Al-Sc and Al-Sb alloys were additionally subjected to ion irradiation at various temperatures. Furthermore, these studies revealed that annealed samples regained their hardness due to solute redistribution by ion beam mixing. Alloy strength was independent of grain size between 50 and 150 nms. Irradiation-induced segregation of Sb to grain boundaries in Al-Sb further enhanced strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Dynamic Observation of Dendritic Quasicrystal Growth upon Laser-Induced Solid-State Transformation

In this paper, we report the laser-induced solid-state transformation between a periodic “approximant” and quasicrystal in the Al-Cr system during rapid quenching. Dynamic transmission electron microscopy allows us to capture in situ the dendritic growth of the metastable quasicrystals. The formation of dendrites during solid-state transformation is a rare phenomenon, which we attribute to the structural similarity between the two intermetallics. Through ab initio molecular dynamics simulations, we identify the dominant structural motif to be a 13-atom icosahedral cluster transcending the phases of matter.

36 MATERIALS SCIENCE↗
NASA NTRS1984

The effects of Cr, Co, Al, Mo and Ta on the cyclic oxidation behavior of a prototype cast Ni-base superalloy based on a 2(5) composite statistically designed experiment

A series of cast Ni-base superalloys were systematically varied at selected levels of Co, Cr, Mo, Ta, and Al. The elemental levels varied were Mo, 0 to 4 percent; Cr, 6 to 18 percent; Co, 0 to 20 percent, Ta, 0 to 8 percent; and Al, 3.25 to 6.25 percent. The cyclic oxidation resistance was determined from specific weight change data as a function of time for 1 hr cycles in static air at 1100 C. The significant terms in decreasing order of their importance were Al, Ta, Cr2, Al-Cr, Cr-Co, Co2, Al-Mo, Cr-Mo, Al-Al, and Mo-Ta. The Al term alone accounted for close to 82 percent of the explained variability. The estimating equation showed that the Al level was the most important and should be at its 6.25 wt % maximum value. The Mo and Ta levels should also be at their maximum 4 and 8 wt % respectively. The cobalt composition should be as low as possible, i.e., 0 wt%. The Cr level optimum varies depending on the other 4 levels. The X-ray diffaction results indicate the most protective scales are alumina/aluminate spinel stabilizized with a tri-rutile oxide high in Ta and Mo.

Barrett, C. A.↗
DOE OSTI2020

Materials Data on AlCr2 by Materials Project

Cr2Al crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Cr is bonded in a 9-coordinate geometry to four equivalent Cr and five equivalent Al atoms. All Cr–Cr bond lengths are 2.38 Å. There are four shorter (2.63 Å) and one longer (2.73 Å) Cr–Al bond lengths. Al is bonded in a 8-coordinate geometry to ten equivalent Cr and four equivalent Al atoms. All Al–Al bond lengths are 2.96 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Al8Cr5 by Materials Project

Al8Cr5 is Bergman Structure: Mg32(Al,Zn)49 Bergman-derived structured and crystallizes in the trigonal R3m space group. The structure is three-dimensional. there are four inequivalent Cr sites. In the first Cr site, Cr is bonded in a 12-coordinate geometry to nine Cr and three equivalent Al atoms. There are a spread of Cr–Cr bond distances ranging from 2.30–2.71 Å. All Cr–Al bond lengths are 2.61 Å. In the second Cr site, Cr is bonded in a 12-coordinate geometry to six Cr and six Al atoms. There are three shorter (2.58 Å) and two longer (2.81 Å) Cr–Cr bond lengths. There are a spread of Cr–Al bond distances ranging from 2.51–2.72 Å. In the third Cr site, Cr is bonded in a 12-coordinate geometry to five Cr and seven Al atoms. Both Cr–Cr bond lengths are 2.82 Å. There are a spread of Cr–Al bond distances ranging from 2.55–2.64 Å. In the fourth Cr site, Cr is bonded in a 12-coordinate geometry to four Cr and eight Al atoms. There are a spread of Cr–Al bond distances ranging from 2.60–2.88 Å. There are five inequivalent Al sites. In the first Al site, Al is bonded in a distorted cuboctahedral geometry to three equivalent Cr and nine Al atoms. There are a spread of Al–Al bond distances ranging from 2.68–2.77 Å. In the second Al site, Al is bonded in a 1-coordinate geometry to four Cr and nine Al atoms. There are a spread of Al–Al bond distances ranging from 2.62–3.04 Å. In the third Al site, Al is bonded in a 12-coordinate geometry to four Cr and eight Al atoms. There are a spread of Al–Al bond distances ranging from 2.71–2.86 Å. In the fourth Al site, Al is bonded in a 11-coordinate geometry to three Cr and eight Al atoms. Both Al–Al bond lengths are 2.92 Å. In the fifth Al site, Al is bonded in a 12-coordinate geometry to five Cr and six Al atoms. Both Al–Al bond lengths are 2.79 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Al3Cr by Materials Project

CrAl3 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Cr is bonded to twelve Al atoms to form CrAl12 cuboctahedra that share corners with four equivalent CrAl12 cuboctahedra, corners with eight equivalent AlAl8Cr4 cuboctahedra, edges with eight equivalent CrAl12 cuboctahedra, edges with sixteen equivalent AlAl8Cr4 cuboctahedra, faces with four equivalent CrAl12 cuboctahedra, and faces with fourteen AlAl8Cr4 cuboctahedra. There are four shorter (2.67 Å) and eight longer (2.77 Å) Cr–Al bond lengths. There are two inequivalent Al sites. In the first Al site, Al is bonded to four equivalent Cr and eight Al atoms to form AlAl8Cr4 cuboctahedra that share corners with twelve equivalent AlAl8Cr4 cuboctahedra, edges with eight equivalent CrAl12 cuboctahedra, edges with sixteen AlAl8Cr4 cuboctahedra, faces with four equivalent CrAl12 cuboctahedra, and faces with fourteen AlAl8Cr4 cuboctahedra. There are four shorter (2.67 Å) and four longer (2.77 Å) Al–Al bond lengths. In the second Al site, Al is bonded to four equivalent Cr and eight equivalent Al atoms to form AlAl8Cr4 cuboctahedra that share corners with four equivalent AlAl8Cr4 cuboctahedra, corners with eight equivalent CrAl12 cuboctahedra, edges with twenty-four AlAl8Cr4 cuboctahedra, faces with six equivalent CrAl12 cuboctahedra, and faces with twelve AlAl8Cr4 cuboctahedra.

36 MATERIALS SCIENCE↗