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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Hf{sub 3}Fe{sub 4}Sn{sub 4} and Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x}: Two stannide intermetallics with low-dimensional iron sublattices

This article reports two new Hf-rich intermetallics synthesized using Sn flux: Hf{sub 3}Fe{sub 4}Sn{sub 4} and Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x}. Hf{sub 3}Fe{sub 4}Sn{sub 4} adopts an ordered variant the Hf{sub 3}Cu{sub 8} structure type in orthorhombic space group Pnma with unit cell edges of a=8.1143(5) Å, b=8.8466(5) Å, and c=10.6069(6) Å. Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x}, on the other hand, adopts a new structure type in Cmc2{sub 1} with unit cell edges of a=5.6458(3) Å, b=35.796(2) Å, and c=8.88725(9) Å for x=0. It exhibits a small amount of phase width in which Sn substitutes on one of the Fe sites. Both structures are fully three-dimensional and are characterized by pseudo one- and two-dimensional networks of Fe–Fe homoatomic bonding. Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x} exhibits antiferromagnetic order at T{sub N}=46(2) K and its electrical transport behavior indicates that it is a normal metal with phonon-dictated resistivity. Hf{sub 3}Fe{sub 4}Sn{sub 4} is also an antiferromagnet with a rather high ordering temperature of T{sub N}=373(5) K. Single crystal resistivity measurements indicate that Hf{sub 3}Fe{sub 4}Sn{sub 4} behaves as a Fermi liquid at low temperatures, indicating strong electron correlation. - Graphical abstract: Slightly different growth conditions in Sn flux produce two new intermetallic compounds: Hf{sub 3}Fe{sub 4}Sn{sub 4} and Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x}. - Highlights: • Single crystals of both Hf{sub 3}Fe{sub 4}Sn{sub 4} and Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x} were grown using Sn flux. • The crystal structures were determined using single crystal X-ray diffraction. • The Fe moments in Hf{sub 3}Fe{sub 4}Sn{sub 4} display AFM order below T{sub N}=373 K. • The Fe moments in Hf{sub 9}Fe{sub 4−x}Sn{sub 10+x} display AFM order below T{sub N}=46 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Low temperature synthesis of LnOF rare-earth oxyfluorides through reaction of the oxides with PTFE

Highlights: Black-Right-Pointing-Pointer Low temperature synthesis of LnOF rare-earth oxyfluorides from Ln{sub 2}O{sub 3} and PTFE (CF{sub 2}). Black-Right-Pointing-Pointer Rhombohedral LnOF is the major phase and forms as nanocrystals, 29-103 nm. Black-Right-Pointing-Pointer Expected lanthanide contraction observed in lattice parameters and bond lengths. Black-Right-Pointing-Pointer TbOF orders antiferromagnetically at 10 K and has a metamagnetic transition at 1.8 T. Black-Right-Pointing-Pointer GdOF orders antiferromagnetically at 5 K, other LnOF are paramagnetic. -- Abstract: A low temperature solid-state synthesis route, employing polytetrafluoroethylene (PTFE) and the rare-earth oxides, for the formation of the LnOF rare-earth oxyfluorides (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), is reported. With the exception of LaOF, which forms in a tetragonal variant, rhomobohedral LnOF is found to be the major product of the reaction. In the case of PrOF, a transition from the rhombohedral to the cubic fluorite phase is observed on heating in air to 500 Degree-Sign C. X-ray diffraction shows the expected lanthanide contraction in the lattice parameters and bond lengths. Magnetic susceptibility measurements show antiferromagnetic-like ordering in TbOF, T{sub m} = 10 K, with a metamagnetic transition at a field {mu}{sub 0}H{sub t} = 1.8 T at 2 K. An antiferromagnetic transition, T{sub N} = 4 K, is observed in GdOF. Paramagnetic behavior is observed above 2 K in PrOF, NdOF, DyOF, HoOF and ErOF. The magnetic susceptibility of EuOF is characteristic of Van Vleck paramagnetism.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Chemical pressure effects on structural, dielectric and magnetic properties of solid solutions Mn{sub 3−x}Co{sub x}TeO{sub 6}

Highlights: • Mn{sub 3}TeO{sub 6} and Co{sub 3}TeO{sub 6} are antiferromagnets with corundum related structures. • The structural and magnetic properties of Mn{sub 3−x}Co{sub x}TeO{sub 6} ceramics were investigated. • All compounds adopt the trigonal structure of Mn{sub 3}TeO{sub 6} up to at least x = 2.4. • The antiferromagnetic transition temperature monotonously increases with x. • NPD data evidences the magnetic structure of the Mn{sub 3−x}Co{sub x}TeO{sub 6} ceramics. - Abstract: The effects of Co{sup 2+} doping on the structural, magnetic and dielectric properties of the multiferroic frustrated antiferromagnet Mn{sub 3}TeO{sub 6} have been investigated. Ceramic samples of the solid solution series Mn{sub 3−x}Co{sub x}TeO{sub 6} were prepared by a solid-state reaction route. X-ray and neutron powder diffraction and electron microscopy techniques were combined with calorimetric, dielectric and magnetic measurements to investigate the dependence of the crystal structure and physical properties on temperature and composition. It is shown that the compounds with x ≤ 2.4 adopt the trigonal corundum-related structure of pure Mn{sub 3}TeO{sub 6} (space group R3{sup ¯}) in the temperature range 5–295 K and that the lattice parameters a and c and the unit-cell volume V decrease linearly with increasing Co{sup 2+} concentration. The low-temperature magnetic susceptibility and heat capacity data evidence the antiferromagnetic ordering of all samples. The Neel temperature linearly increases with Co{sup 2+} concentration x. Curie–Weiss fits of the high temperature susceptibility indicate that the magnetic frustration decreases with x. The derived magnetic structure of Mn{sub 3}TeO{sub 6} can be described as an incommensurately modulated magnetic spin state with k = [0, 0, k{sub z}] and an elliptical spin-spiral order of spins within the chains of MnO{sub 6} octahedra. With increasing Co{sup 2+} concentration the propagation vector k{sub z} changes from 0.453 (x = 0) to 0.516 (x = 2.4). The magnetic anisotropy changes as well, leading to a reorientation of the spiral-basal plane. A possible coexistence of long-range order of electrical dipoles and magnetic moments in Mn{sub 3−x}Co{sub x}TeO{sub 6} is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Low dimensional magnetism in MnNb{sub 2−x}V{sub x}O{sub 6}

Highlights: • Magnetic structure is reported from neutron diffraction. • We examine changes of Néel temperature versus V content. • Composition dependence of intra and inter chain exchange parameters are determined. • The temperature dependence of the Mn magnetic moment is determined for MnNb{sub 1.9}V{sub 0.1}O{sub 6}. - Abstract: The structural and magnetic properties of MnNb{sub 2−x}V{sub x}O{sub 6} samples were studied. These materials belong to the AB{sub 2}O{sub 6} class, which presents low-dimensional magnetism. Curie-Weiss fitting gives a negative Weiss temperature that is a characteristic of dominant antiferromagnetic interactions in paramagnetic region. Rietveld refinements of neutron diffraction data show that magnetic moments are aligned antiferromagnetically within the zig-zag chains running along the c axis. Unlike other ANb{sub 2}O{sub 6} type systems crystallizing also in Pbcn space group and presenting ferromagnetic Ising chains, MnNb{sub 2−x}V{sub x}O{sub 6} samples exhibit antiferromagnetic type order along the chains. At 1.5 K, the Mn atoms are found to carry an ordered magnetic moment of about 4.3 μ{sub B}. The Mn chains are antiferromagnetic and do not present significant anisotropy. These characteristics and the high spin allowed us to model the system as weakly interacting classical Heisenberg chains, estimating the values of both intra- and interchain exchange constants evolution with x content.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Neutron diffraction study of the magnetic-field-induced transition in Mn{sub 3}GaC

The antiperovskite Mn{sub 3}GaC undergoes an isostructural cubic–cubic first order transition from a low-temperature, large-cell-volume antiferromagnetic state to a high-temperature, small-cell-volume ferromagnetic state at around 160 K. The transition can also be induced by applying a magnetic field. We study here the isothermal magnetic-field-evolution of the transition as ferromagnetism is stabilized at the expense of antiferromagnetism. We make use of the presence of the two distinct cell volumes of the two magnetic states as a probe to observe by neutron diffraction the evolution of the transition, as the external magnetic field carries the system from the antiferromagnetic to the ferromagnetic state. We show that the large-volume antiferromagnetic and the small-volume ferromagnetic states coexist in the temperature range of the transition. The ferromagnetic state is progressively stabilized as the field increases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of capping material on interfacial ferromagnetism in FeRh thin films

The role of the capping material in stabilizing a thin ferromagnetic layer at the interface between a FeRh film and cap in the nominally antiferromagnetic phase at room temperature was studied by x-ray magnetic circular dichroism in photoemission electron microscopy and polarized neutron reflectivity. These techniques were used to determine the presence or absence of interfacial ferromagnetism (FM) in films capped with different oxides and metals. Chemically stable oxide caps do not generate any interfacial FM while the effect of metallic caps depends on the element, showing that interfacial FM is due to metallic interdiffusion and the formation of a ternary alloy with a modified antiferromagnetic to ferromagnetic transition temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Mesoporous Co{sub 3}O{sub 4} nanostructured material synthesized by one-step soft-templating: A magnetic study

A combined magnetization and zero-field {sup 59}Co spin-echo nuclear magnetic resonance (NMR) study has been carried out on one member of a recently developed class of highly ordered mesoporous nanostructured materials, mesoporous Co{sub 3}O{sub 4} (designated UCT-8, University of Connecticut, mesoporous materials). The material was synthesized using one-step soft-templating by an inverse micelles packing approach. Characterization of UCT-8 by powder x-ray diffraction and electron microscopy reveals that the mesostructure consists of random close-packed Co{sub 3}O{sub 4} nanoparticles ≈ 12 nm in diameter. The N{sub 2} sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m{sup 2}/g BET surface area and a 7.7 nm BJH desorption pore diameter. The effect of heat treatment on the structure is discussed. The antiferromagnetic Co{sub 3}O{sub 4} nanoparticles have a Néel temperature T{sub N} = 27 K, somewhat lower than the bulk. A fit to the Curie-Weiss law over the temperature range 75 K ≤ T ≤ 300 K yields an effective magnetic moment of μ{sub eff} = 4.36 μ{sub B} for the Co{sup 2+} ions, indicative of some orbital contribution, and a Curie-Weiss temperature Θ = −93.5 K, consistent with antiferromagnetic ordering. The inter-sublattice and intra-sublattice exchange constants for the Co{sup 2+} ions are J{sub 1}/k{sub B} = (−)4.75 K and J{sub 2}/k{sub B} = (−)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below T{sub N} with shifts in the hysteresis loops, i.e., an exchange-bias effect. The {sup 59}Co NMR spectrum for UCT-8, which is attributed to Co{sup 2+} ions at the tetrahedral A sites, is asymmetrically broadened with a peak at ≈55 MHz (T = 4.2 K). Since there is cubic symmetry at the A-sites, the broadening is indicative of a magnetic field distribution due to the uncompensated surface spins. The spectrum is consistent with antiferromagnetically ordered particles that are nanometer in size and single domain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural, magnetic and thermal study of Cr doped iron-tungsten-oxygen system (Fe{sub 2}WO{sub 6})

The structural, magnetic and thermal properties of Cr doped polycrystalline Fe{sub 2}WO{sub 6} have been investigated. The Cr doped sample shows tetragonal space group of P4{sub 2}/mnm and crystallizes with inverse trirutile structure. All Cr doped samples shows antiferromagnetic-paramagnetic transition (T{sub N}), with increasing Cr percentage the T{sub N} is decreasing to low temperature and finally stable at Cr{sub 2}WO{sub 6} antiferromagnetic phase (43 K). The magnetic momentum increases with Cr doping and the strength of antiferromagnetic interaction is decreases due to the decreasing the percentage of half-filled Fe{sup 3+}(3d{sup 5}) e{sub g} orbitals. The heat capacity study on Cr doped samples shows fairly sharp λ-like peak near the ordering temperature indicates the existence of long range antiferromagnetic ordering in the present samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The effects of Co 3 O 4 on the structure and unusual magnetism of LaCoO 3

In this work, bulk La w CoO 3 particles with w = 1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La 2 O 3 and Co 3 O 4 . The resulting particles are characterized as LaCoO3 crystals interfaced with a crystalline Co 3 O 4 phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO 3 phase resulting from the Co 3 O 4 content, but magnetization data indicate that the amount of Co3O4 strongly affects the ferromagnetic ordering at the interfaces below $T_C$ ≈ 89 K. In addition to ferromagnetic long-range order, LaCoO 3 exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe ≤ H ≤ 5 kOe is fit to a combination of a power law $((T – T_C)/T_C)^β$ behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie–Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent $\textit{β}$ = 0.63 ± 0.02 is consistent with 2D (surface) ordering. Increased Co 3 O 4 correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below $\textit{T}$ ≈ 40 K is a consequence of the lattice reaching a critical rhombahedral distortion as $\textit{T}$ is decreased for core regions far from the Co 3 O 4 interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetic structure and resistivity minimum in GdCuAs 2

The electrical resistivity of GdCuAs 2 single crystals exhibits an anomalous Kondo-like resistivity minimum above the antiferromagnetic ordering temperature T N 1 approximate to 10.6 K, which is unusual for a highly localized 4f-moment (Gd 3+ ) system. Using x-ray resonant magnetic scattering, we determined the magnetic structure of GdCuAs 2 , where Gd moments are antiferromagnetically aligned along the crystallographic a axis and in the (+ + --) arrangement in the c direction and ferromagnetically arranged in the b direction. The antiferromagnetic order appears first at q = (δ, 0, 0.5) below 10 K, with an incommensurate modulation along the a axis and locks into a commensurate position at q = (1/3, 0, 0.5) below T N 2 approximate to 6 K. Our high-resolution x-ray diffraction measurements show a two-peak structure at Q = (2, 0, 6) above the resistivity minimum, suggesting a lower-symmetry crystal structure than the reported tetragonal structure, and the Q = (2, 0, 6) peak becomes a sharp one-peak structure below the resistivity minimum, implying a magnetoelastic coupling above T N . In conclusion, our findings suggest a complex interplay between the crystal structure and antiferromagnetic structure through a magnetoelastic coupling, associated with the anomalous resistivity minimum above T N 1 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Large Spin-Orbit Torque in 𝑎-Plane 𝛼−Fe 2 ⁢O 3 /Pt Bilayers

Realization of efficient spin-orbit torque switching of the Neel vector in insulating antiferromagnets is a challenge, often complicated by spurious effects. Quantifying the spin-orbit torques in antiferromagnet or heavy metal heterostructures is an important first step toward this goal. Here, we employ magneto-optic techniques to study damping-like spin-orbit torque (DL-SOT) in a-plane α-Fe 2 O 3 (hematite) with a Pt spin-orbit overlayer. We find that the DL-SOT efficiency is 2 orders of magnitude larger than reported in c- and r-plane hematite/Pt using harmonic Hall techniques. The large magnitude of DL-SOT is supported by direct imaging of current-induced motion of antiferromagnetic domains that happens at moderate current densities. Furthermore, our study introduces a new method for quantifying spin-orbit torque in antiferromagnets with a small canted moment and identifies a-plane α-Fe 2 O 3 as a promising candidate to realize efficient SOT switching.

Antiferromagnetism↗
DOE OSTI2022

Direct Visualization of Surface Spin-Flip Transition in MnBi 4 Te 7

Here, we report direct visualization of spin-flip transition of the surface layer in antiferromagnet MnBi 4 Te 7 , a natural superlattice of alternating MnBi 2 Te 4 and Bi 2 Te 3 layers, using cryogenic magnetic force microscopy (MFM). The observation of magnetic contrast across domain walls and step edges confirms that the antiferromagnetic order persists to the surface layers. The magnetic field dependence of the MFM images reveals that the surface magnetic layer undergoes a first-order spin-flip transition at a magnetic field that is lower than the bulk transition, in excellent agreement with a revised Mills model. Our analysis suggests no reduction of the order parameter in the surface magnetic layer, implying robust ferromagnetism in the single-layer limit. The direct visualization of surface spin-flip transition not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultrathin films of MnBi 4 Te 7 and other natural superlattice topological magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic structure of triangular lattice compound Tb2Ni0.90Si2.94

AlB 2 -type ternary intermetallic compound Tb 2 Ni 0.90 Si 2.94 (space group P 6=mmm, hP 3, No. 191) was reported to exhibit spin freezing behaviour of the ferromagnetic clusters present in the system below T f = 9:9 K, along with the presence of spatially limited antiferromagnetic phase. In this work, on the basis of variable temperature zero-field neutron diffraction measurements, we have shown that the antiferromagnetic phase transition occurs for the compound below T N ~13 K. Neutron diffraction study indicates ab-plane non-collinear sine-modulated antiferromagnetic ordering of the system with wave vectors of k 1 = [±1/6, ±1/6, 0] and k 2 = [±1/3, ±1/3, 0] down to 1.7 K. The weak and diffuse nature of the magnetic Bragg peaks along with limited coherence length further confirm the short-range nature of the antiferromagnetic phase in this compound.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic interactions in praseodymium ruthenate Pr{sub 3}RuO{sub 7} with fluorite-related structure

Solid solutions Pr{sub 3}(Ru{sub 1-x}Ta{sub x})O{sub 7} (0≤x≤1.0) and (Pr{sub 1-x}Y{sub x}){sub 3}RuO{sub 7} (0≤x≤0.7) were obtained as a single phase compound. They crystallize in an orthorhombic superstructure derived from that of the cubic fluorite with space group Cmcm. The results of the Rietveld analysis for X-ray diffraction profiles of Pr{sub 3}(Ru{sub 1-x}Ta{sub x})O{sub 7} showed that Ru and Ta atoms are randomly situated at the six-coordinate 4b site. For (Pr{sub 1-x}Y{sub x}){sub 3}RuO{sub 7}, with increasing the concentration of Y ions (x value), the smaller Y ions occupy selectively the seven-coordinate 8g site rather than the eight-coordinate 4a site. Through magnetic susceptibility measurements for Pr{sub 3}(Ru{sub 1-x}Ta{sub x})O{sub 7}, the antiferromagnetic transition temperatures decrease linearly with increasing x value, and at x=0.75 no magnetic ordering was found down to 1.8 K, indicating the magnetic interaction is not one-dimensional, but three-dimensional. On the other hand, the antiferromagnetic transition temperature for (Pr{sub 1-x}Y{sub x}){sub 3}RuO{sub 7} decreases with increasing x value, but above x≥0.50 it becomes constant (~12 K). This result indicates that Pr{sup 3+} ions at the seven-coordinate site greatly contribute to the antiferromagnetic interactions observed in (Pr{sub 1-x}Y{sub x}){sub 3}RuO{sub 7}. Density functional calculations of Pr{sub 3}RuO{sub 7} demonstrate that the electronic structure gives insulating character and that oxygen 2p orbitals hybridize strongly with Ru 4d orbitals in the valence band (VB). Near the top of VB, the Pr 4 f orbitals at the seven-coordinated site also show a weak hybridization with the O(1) 2p orbitals. The Ru-O(1)-Pr superexchange pathway take part in three-dimensional magnetic interaction and play an important role in an enhancement of long-range magnetic ordering. - Graphical abstract: The spin densities and the spin polarization of Pr{sub 3}RuO{sub 7} are shown. Significant spin polarization is seen on the magnetic Pr and Ru ions, but there is also some on the O(1), (3) ligands of Ru. - Highlights: • New fluorite-related quaternary praseodymium ruthenates were prepared. • Pr{sub 3}RuO{sub 7} shows an antiferromagnetic transition at 55 K. • The Ru-O-Pr superexchange interactions are three-dimensional.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Structure and property correlations in FeS

Here, for iron-sulfide (FeS), we investigate the correlation between the structural details, including its dimensionality and composition, with its magnetic and superconducting properties. We compare, theoretically and experimentally, the two-dimensional (2D) layered tetragonal (“t-FeS”) phase with the 3D hexagonal ("h-FeS") phase. X-ray diffraction reveals iron-deficient chemical compositions of t-Fe 0.93(1) S and h-Fe 0.84(1) S that show no low-temperature structural transitions. First-principles calculations reveal a high sensitivity of the 2D structure to the electronic and magnetic properties, predicting marginal antiferromagnetic instability for our compound (sulfur height of z S =0.252) with an ordering energy of about 11 meV/Fe, while the 3D phase is magnetically stable. Experimentally, h-Fe 0.84 S orders magnetically well above room temperature, while t-Fe 0.93 S shows coexistence of antiferromagnetism at T N =116 and filamentary superconductivity below T c =4 K. Low temperature neutron diffraction data reveals antiferromagnetic commensurate ordering with wave vector k m = (0.25,0.25,0) and 0.46(2) µ B /Fe. Additionally, neutron scattering measurements were used to find the particle size and iron vacancy arrangement of t-FeS and h-FeS. The structure of iron sulfide has a delicate relationship with the superconducting transition; while our sample with a=3.6772(7) Å is a filamentary superconductor coexisting with an antiferromagnetic phase, previously reported samples with a>3.68 Å are bulk superconductors with no magnetism, and those with a ≈ 3.674 Å show magnetic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

An ab initio study of the adsorption of Eu 3+ , Pu 3+ , Am 3+ , and Cm 3+ hydroxide complexes on hematite (001) surface: Role of magnetism on adsorption

Inner-sphere hydroxide complexes of Eu 3+ , Pu 3+ , Am 3+ , and Cm 3+ adsorbed to antiferromagnetic and ferromagnetic hematite (001) surfaces were modeled by using DFT (GGA+U) calculations in order to compare the electronic properties, atomic structures, and adsorption energies between the trivalent actinide analog Eu 3+ and the corresponding actinides An 3+ . Eu 3+ forms a tridentate–binuclear surface complex on the hematite surface with an Eu–Fe distance of 3.4–3.8 Å. Eu 3+ is partially reduced upon adsorption, as indicated by the increase of original Eu 3+ spin value from 6 to ~7. Eu 3+ adsorption on the ferromagnetic surface is energetically more favorable than on the antiferromagnetic surface, while the opposite is the case for all actinides An 3+ . When retaining a similar adsorption configuration (e.g., coordination number = 6), An 3+ cations are adsorbed ~0.2 Å closer to the surface compared to Eu 3+ . Partial density of states analyses indicate ionic bonding between O ions on the hematite surface and the Eu 3+ /An 3+ adsorbates. An increasing number of bands above the Fermi energy were found when Eu 3+ and An 3+ were adsorbed to the ferromagnetic surface but not on the antiferromagnetic surface, indicating the ferromagnetic substrate becomes more semiconducting upon cation adsorption. Here, these results show that the spin configurations of the hematite (001) surface have a weak but distinct influence on the cation adsorption energy. Although the antiferromagnetic spin configuration of hematite is dominant in nature, metastable ferromagnetic domains on nanohematite grains may influence the energetics of certain cation adsorption reactions.

42 ENGINEERING↗
DOE OSTI2021

A Polar Magnetic and Insulating Double Corundum Oxide: Mn2MnSbO6 with Ordered Mn(II) and Mn(III) Ions

A new magnetic insulator Mn2MnSbO6 with a polar crystal structure and an ordered Mn2+ and Mn3+ arrangement was synthesized under a high pressure of 7.5 GPa and 1300 degrees C. The crystal structure of Mn2MnSbO6, investigated by synchrotron powder X-ray diffraction, was found to be isomorphous with that of Ni3TeO6-type, space group R3. The non-centrosymmetric structure was confirmed by the second-harmonic generation measurements. The X-ray absorption near-edge spectroscopy measurement confirmed the nominal oxidation states of Mn22+Mn3+SbO6. Magnetic measurements indicate that Mn2MnSbO6 orders antiferromagnetically below 44 K and undergoes a field-induced spin-flop transition at 5 K. First-principles calculations indicate an antiferromagnetic ground state with up/up/up/down/down/down (uuuddd) spin configuration of the six crystallographically unique Mn ions in the c-axis doubled magnetic structure. The density functional theory calculations also substantiate the experimentally observed charge ordering of the Mn2+/Mn3+ ions and the insulating behavior due to a bandgap of 0.52 eV. To the best of our knowledge, this is the first double corundum oxide containing Jahn-Teller active Mn3+ ions.

Feng, Hai L.↗