AFM Structure Research Articles

Blast furnace slag-Mg(OH)2 cements activated by sodium carbonate.

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge.

Formation of gapless Z 2 spin liquid phase manganites in the (Sm1–y Gd y )0.55Sr0.45MnO3 system in zero magnetic field: Topological phase transitions to states with low and high density of 2D-vortex pairs induced by the magnetic field

The evolution of the ground state of the manganese spin ensemble in the (Sm1–y Gd y )0.55Sr0.45MnO3 in the case of isovalent substitution of rare-earth samarium ions with large radii with gadolinium ions with significantly smaller radii is studied. The measured temperature dependences of the ac magnetic susceptibility and the field dependences of the dc magnetizations are analyzed using the Heisenberg–Kitaev model describing the transition from the ordered spin state with classical isotropic AFM exchange to the frustrated spin state with quantum highly anisotropic FM exchange. A continuous transition from the 3D ferromagnetic state of manganese spins in the initial sample with y = 0 to zigzag AFM ordering of CE-type spins in ab planes for y = 0.5, coexisting in samples with y = 0.5, 0.6, and 0.7 at temperatures below T N ≅ 48.5 K with a disordered phase such as a quantum Griffiths phase is identified. As the gadolinium concentration further increases, the CE-type zigzag AFM structure is molten, which leads to the appearance of an unusual phase in Gd0.55Sr0.45MnO3 in the temperature range close to the absolute zero. This phase has characteristic features of a gapless Z 2 quantum spin liquid in zero external magnetic field. The step changes in the magnetization isotherms measured at 4.2 K in the field range of ±75 kOe are explained by quantum phase transitions of the Z 2 spin liquid to a phase with topological order in weak magnetic fields and a polarized phase in strong fields. The significant difference between critical fields and magnetization jumps in isotherms indicates the existence of hysteretic phenomena in quantum spin liquid magnetization–demagnetization processes caused by the difference between localization–delocalization of 2D vortex pairs induced by a magnetic field in a quantum spin liquid with disorder.

The Dipole Moment Inversion Effects in Self-Assembled Nanodielectrics for Organic Transistors

We compare and contrast the properties of hybrid organic–inorganic self-assembled nanodielectrics (SANDs) based on alternating layers of solution-processed ZrOx and either of two phosphonic acid-functionalized azastilbazolium π-units having opposite dipolar orientations. Conventional Zr-SAND and new inverted IZr-SAND are characterized by Kevin probe, optical spectroscopy, capacitance–voltage measurements, AFM, X-ray reflectivity, and electronic structure computation. The molecular dipolar orientation affects thin-film transistor (TFT) threshold and turn-on voltages for devices based on either p-channel pentacene or n-channel copper perfluorophthalocyanine. Specifically, Zr-SAND shifts the threshold and turn-on voltages to more positive values, whereas IZr-SAND shifts them in the opposite direction. Capping these SANDs with −SiMe3 groups enhances the effect, affording a 1.3 V difference in turn-on voltage for IZr-SAND vs Zr-SAND-gated organic TFTs. Such tunability should facilitate the engineering of more ...

High field induced magnetic transitions in the Y0.7Er0.3Fe2D4.2 deuteride

The influence of the partial Er for Y substitution on the crystal structure and magnetic properties of $\mathrm{YF}{\mathrm{e}}_{2}{\mathrm{D}}_{4.2}$ has been investigated by high field magnetization and neutron diffraction experiments. ${\mathrm{Y}}_{0.7}\mathrm{E}{\mathrm{r}}_{0.3}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{D}}_{4.2}$ compound crystallizes in the same monoclinic structure as $\mathrm{YF}{\mathrm{e}}_{2}{\mathrm{D}}_{4.2}$ described in $Pc\phantom{\rule{0.28em}{0ex}}({P}_{1}{c}_{1})$ space group with D atoms located in 18 different tetrahedral interstitial sites. A cell volume contraction of 0.6% is observed upon Er substitution, inducing large modification of the magnetic properties. Electronic effect of D insertion as well as lowering of crystal symmetry are important factors determining the magnetic properties of Fe sublattice, which evolves towards more delocalized behavior and modifying the Er-Fe exchange interactions. In the ground state, the Er and Fe moments are arranged ferrimagnetically within the plane perpendicular to the monoclinic $b$ axis and with average moments ${m}_{\mathrm{Er}}=6.4(3){\ensuremath{\mu}}_{B}{\mathrm{Er}}^{\ensuremath{-}1}$ and ${\mathrm{m}}_{\mathrm{Fe}}=2.0(1){\ensuremath{\mu}}_{B}{\mathrm{Fe}}^{\ensuremath{-}1}$ at 10 K. Upon heating, ${m}_{\mathrm{Er}}$ decreases progressively until ${T}_{\mathrm{Er}}=55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Between 55 K and 75 K, the Fe sublattice undergoes a first-order ferromagnetic-antiferromagnetic (FM-AFM) transition with a cell volume contraction due to the itinerant metamagnetic behavior of one Fe site. In the AFM structure, ${m}_{\mathrm{Fe}}$ decreases until the N\'eel temperature ${T}_{\mathrm{N}}=125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At high field, two different types of field induced transitions are observed. The Er moments become parallel to the Fe one and saturates to the $\mathrm{E}{\mathrm{r}}^{3+}$ free ion value, leading to an unusual field induced FM arrangement at a transition field ${B}_{\mathrm{Trans}}$ of only 78 kG below 30 K. Then above ${{T}_{\mathrm{M}}}_{0}=66\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, an AFM-FM transition of the Fe sublattice, accompanied by a cell volume increase is observed. ${B}_{\mathrm{Trans}}$ increases linearly versus temperature and with a larger $d{B}_{\mathrm{Trans}}/dT$ slope than for $\mathrm{YF}{\mathrm{e}}_{2}{\mathrm{D}}_{4.2}$. This has been explained by the additional contribution of Er induced moments above ${B}_{\mathrm{Trans}}$.

Uptake of chloride and carbonate by Mg-Al and Ca-Al layered double hydroxides in simulated pore solutions of alkali-activated slag cement

Chloride ingress and carbonation are major causes of degradation of reinforced concrete. To enable prediction of chloride ingress, and thus to improve the durability of structural alkali-activated slag cement (AAS) based concretes, it is necessary to understand the ionic interactions taking place between chlorides, carbonates, and the individual solid phases which comprise AAS. This study focused on two layered double hydroxides (LDH) representing those typically identified as reaction products in AAS: an Mg-Al hydrotalcite-like phase, and an AFm structure (strätlingite), in simulated AAS pore solutions. Surface adsorption and interlayer ion-exchange of chlorides occurred in both LDH phases; however, chloride uptake in hydrotalcite-group structures is governed by surface adsorption, while strätlingite shows the formation of a hydrocalumite-like phase and ion exchange. For both Ca-Al and Mg-Al LDHs, decreased chloride uptakes were observed from solutions with increased [CO32−]/[OH−] ratios, due to the formation of carbonate-containing hydrotalcite and decomposition of AFm phases, respectively.

GdRh2Si2: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy

The anisotropy of magnetic properties commonly is introduced in textbooks using the case of an antiferromagnetic system with Ising type anisotropy. This model presents huge anisotropic magnetization and a pronounced metamagnetic transition and is well-known and well-documented both, in experiments and theory. In contrast, the case of an antiferromagnetic $X$-$Y$ system with weak in-plane anisotropy is only poorly documented. We studied the anisotropic magnetization of the compound GdRh$_2$Si$_2$ and found that it is a perfect model system for such a weak-anisotropy setting because the Gd$^{3+}$ ions in GdRh$_2$Si$_2$ have a pure spin moment of S=7/2 which orders in a simple AFM structure with ${\bf Q} = (001)$. We observed experimentally in $M(B)$ a continuous spin-flop transition and domain effects for field applied along the $[100]$- and the $[110]$-direction, respectively. We applied a mean field model for the free energy to describe our data and combine it with an Ising chain model to account for domain effects. Our calculations reproduce the experimental data very well. In addition, we performed magnetic X-ray scattering and X-ray magnetic circular dichroism measurements, which confirm the AFM propagation vector to be ${\bf Q} = (001)$ and indicate the absence of polarization on the rhodium atoms.

The wasp-waisted hysteresis loop and exchange bias in multiferroic BaNiF4

Multiferroic BaNiF4 has been fabricated by hydrothermal method. The bifurcation between zero field cooling (ZFC) and field cooling (FC) temperature dependent magnetization (M-T) curves starts at 150 K, indicating the 2D antiferromagnetic (FM) transition. A further upturn of magnetization has been observed below 68 K in FC M-T curve, corresponding to the emergence of 3D AFM structure. Wasp-waisted hysteresis loop was observed under 130 K, which is explained by that the AFM aligned net spins from the canting of neighboring AFM spins due to Dzyaloshinskii-Moriya interaction can be easily aligned by the magnetic field. Exchange bias effect was detected below 70 K, which has been interpreted by the magnetization pinned by the spontaneous polarization through magnetoelectric coupling.

Magnetic-field-induced irreversible antiferromagnetic–ferromagnetic phase transition around room temperature in as-cast Sm–Co based SmCo7−xSix alloys

A magnetic-field-induced irreversible metamagnetic phase transition from antiferro- to ferromagnetism, which leads to an anomalous initial-magnetization curve lying outside the magnetic hysteresis loop, is reported in arc-melted SmCo7−xSix alloys. The transition temperatures are near room temperature, much higher than other compounds with similar initial curves. Detailed investigation shows that this phenomenon is dependent on temperature, magnetic field and Si content and shows some interesting characteristics. It is suggested that varying interactions between the Sm and Co layers in the crystal are responsible for the formation of a metastable AFM structure, which induces the anomalous phenomenon in as-cast alloys. The random occupation of 3g sites by Si and Co atoms also has an effect on this phenomenon.

Theoretical Study of Cubic and Orthorhombic Nd1–xSrxMnO3 as a Potential Solid Oxide Fuel Cell Cathode

AbstractIn this paper, the atomic and electronic structures of cubic and orthorhombic Nd1–xSrxMnO3 are investigated using the projector augmented‐wave (PAW) methods within the spin‐polarization generalized gradient approximation (GGA+U), where U is on‐site Coulomb interaction correction. The optimized structure parameters of both cubic and orthorhombic bulk phases are obtained. The difference between the AFM and FM structures for NdMnO3 is very small, indicating a small magneto‐elastic effect. In Nd1–xSrxMnO3, the pseudo‐cubic lattice constant decreases on increasing Sr doping due to the oxidation of Mn3+ cations to the smaller Mn4+ cations. The result of the total density of states shows the majority spin without gap and the minority spin with 4.8 × 10−19 J gap, indicating a half‐metallic ground state for NdMnO3 in GGA+U treatment. The Bader effective charges in both cubic and orthorhombic phases are analyzed. The oxygen vacancy formation energy of Nd1–xSrxMnO3 becomes smaller as the Sr doping concentration increases. The oxygen vacancies can be formed more easily on Nd0.5Sr0.5MnO3 than other systems. These results suggest that Nd0.5Sr0.5MnO3 could be a promising candidate for application in SOFC.

Non-collinear magnetic structures of TbCoO3 and DyCoO3

The orthoperovskites TbCoO3 and DyCoO3 with Co3+ in a non-magnetic low-spin state have been investigated by neutron diffraction down to 0.25 K. Magnetic ordering is evidenced below TN = 3.3 K and 3.6 K, respectively, and the ordered arrangements are of canted type, AxGy for TbCoO3 and GxAy for DyCoO3 in Bertaut's notation. The experiments are confronted with the first-principle calculations of the crystal field and magnetism of Tb3+ and Dy3+ ions, located in the Pbnm structure on sites of Cs point symmetry. Both these ions exhibit an Ising behavior, which originates in the lowest energy levels, in particular in accidental doublet of non-Kramers Tb3+ (4f8 configuration) and in ground Kramers doublet of Dy3+ (4f9) and it is the actual reason for the non-collinear AFM structures. Very good agreement between the experiment and theory is found. For comparison, calculations of the crystal field and magnetism for other systems with Kramers ions, NdCoO3 and SmCoO3, are also included.

Phospholipids Induce Conformational Changes of SecA to Form Membrane-Specific Domains: AFM Structures and Implication on Protein-Conducting Channels

SecA, an essential component of the Sec machinery, exists in a soluble and a membrane form in Escherichia coli. Previous studies have shown that the soluble SecA transforms into pore structures when it interacts with liposomes, and integrates into membranes containing SecYEG in two forms: SecAS and SecAM; the latter exemplified by two tryptic membrane-specific domains, an N-terminal domain (N39) and a middle M48 domain (M48). The formation of these lipid-specific domains was further investigated. The N39 and M48 domains are induced only when SecA interacts with anionic liposomes. Additionally, the N-terminus, not the C-terminus of SecA is required for inducing such conformational changes. Proteolytic treatment and sequence analyses showed that liposome-embedded SecA yields the same M48 and N39 domains as does the membrane-embedded SecA. Studies with chemical extraction and resistance to trypsin have also shown that these proteoliposome-embedded SecA fragments exhibit the same stability and characteristics as their membrane-embedded SecA equivalents. Furthermore, the cloned lipid-specific domains N39 and M48, but not N68 or C34, are able to form partial, but imperfect ring-like structures when they interact with phospholipids. These ring-like structures are characteristic of a SecA pore-structure, suggesting that these domains contribute part of the SecA-dependent protein-conducting channel. We, therefore, propose a model in which SecA alone is capable of forming a lipid-specific, asymmetric dimer that is able to function as a viable protein-conducting channel in the membrane, without any requirement for SecYEG.

<i>Ab Initio</i> Electronic Structure Studies of CuO Multiferroics

We have performed detailed structural, electronic and magnetic properties of high - TC multiferroic CuO using first principles density functional theory. The total energy results revealed that AFM is the most stable magnetic ground state of CuO. The DOS and electronic band structure calculations show that in the absence of on-site Coulomb interaction (U), AFM structure of CuO heads to a metallic state. However, upon incorporating U in the calculations, a band gap of 1.2 eV is recovered. Furthermore, the Born effective charges calculated on Cu does not show any anomalous character.This suggests that the polarization seen in CuO could be attributed to the spin induced AFM ordering effect.

Influence of calcium nitrate and nitrite on the constitution of AFm and AFt cement hydrates

Layered double hydroxides corresponding to Ca4Al2(NO3)2(OH)12·4H2O and Ca4Al2(NO2)2(OH)12·4H2O (nitrate and nitrite AFm phases, respectively) were synthesised and characterised using x-ray diffraction, thermogravimetry, Fourier transform infrared spectroscopy and scanning electron microscopy. Solubility and regions of stability, including thermodynamic data, are presented. Ionic speciations at various temperatures were calculated. The water content of nitrate AFm was found to be sensitive to humidity conditions, unlike nitrite AFm whose water content seems to be independent of the moisture conditions over a wide range of pH2O. Nitrates and nitrites readily displace hydroxide, sulfate and carbonate from the AFm structures so AFm is mainly responsible for storing nitrite and nitrate in cement solids. A comprehensive picture of the phase relations of AFm phases and the binding power for nitrite/nitrate is given for pH∼12 and a temperature of 25°C. The presence of nitrate and nitrite alter the AFm/AFt balance and affect the specific volume of paste solids. Implications for the distribution of nitrate and nitrite in cements undergoing alteration are discussed.

Pressure induced reactions amongst calcium aluminate hydrate phases

The compressibilities of two AFm phases (strätlingite and calcium hemicarboaluminate hydrate) and hydrogarnet were obtained up to 5 GPa by using synchrotron high-pressure X-ray powder diffraction with a diamond anvil cell. The AFm phases show abrupt volume contraction regardless of the molecular size of the pressure-transmitting media. This volume discontinuity could be associated to a structural transition or to the movement of the weakly bound interlayer water molecules in the AFm structure. The experimental results seem to indicate that the pressure-induced dehydration is the dominant mechanism especially with hygroscopic pressure medium. The Birch–Murnaghan equation of state was used to compute the bulk modulus of the minerals. Due to the discontinuity in the pressure–volume diagram, a two stage bulk modulus of each AFm phase was calculated. The abnormal volume compressibility for the AFm phases caused a significant change to their bulk modulus. The reliability of this experiment is verified by comparing the bulk modulus of hydrogarnet with previous studies.

Impact of chloride on the mineralogy of hydrated Portland cement systems

Chloride ion is in part bound into ordinary Portland cement paste and modifies its mineralogy. To understand this a literature review of its impacts has been made and new experimental data were obtained. Phase pure preparations of Friedel's salt, Ca 4Al 2(Cl) 1.95(OH) 12.05·4H 2O, and Kuzel's salt, Ca 4Al 2(Cl)(SO 4) 0.5(OH) 12·6H 2O, were synthesized and their solubilities were measured at 5, 25, 55 and 85 °C. After equilibration, solid phases were analysed by X-ray diffraction while the aqueous solutions were analysed by atomic absorption spectroscopy and ion chromatography. The solid solutions and interactions of Friedel's salt with other AFm phases were determined at 25 °C experimentally and by calculations. In hydrated cements, anion sites in AFm are potentially occupied by OH, SO 4 and CO 3 ions whereas Cl may be introduced under service conditions. Chloride readily displaces hydroxide, sulfate and carbonate in the AFm structures. A comprehensive picture of phase relations of AFm phases and their binding capacity for chloride is provided for pH ∼ 12 and 25 °C. The role of chloride in AFt formation and its relevance to corrosion of embedded steel are discussed in terms of calculated aqueous [Cl −]/[OH −] molar ratios.

Magnetic Phase Diagram and Magnetization Dynamics of Frustrated Spin-Chain Compounds

The magnetic phase diagram of Ising spin chains packed into the frustrated triangular lattice is discussed. A structure of a low-temperature phase depends strongly on interactions between the next-to-nearest-neighbor chains because they lift the degeneracy of the triangular AFM Ising model. That is why, a variety of low-temperature phases is observed in CsCoCl3, Ca3Co2O6, and Sr5Rh4O12. On the contrary, the high-temperature phase (honeycomb AFM structure) is unique. The frustrated Ising chain systems demonstrate an unusual and very slow magnetization dynamics. A model of the magnetization dynamics similar to the Glauber theory is developed.

Low-temperature magnetic and transport properties of single-crystal CeCoGe

The low-temperature properties of single-crystal CeCoGe were investigated by specific heatC(T,H),magnetoresistivity ρ(T,H), and differential susceptibility measurementsχ(T,H). The zero-field low-temperature specific heat evolves asC = γT+βT3 = 42T+23.5T3 mJ mol−1 K−1. On comparingits γ = 42 mJ mol−1 K−1 with thatof LaCoGe (12 mJ mol−1 K−2) it is inferred that both 3d (Co) and 4f (Ce) orbitals contribute to the density of states at theFermi level. Assuming that its phonic contribution to the specific heat is similar to LaCoGe(β = 0.5 mJ mol−1 K−4), then the extra cubic term in the specific heat(23T3 mJ mol−1 K−1) must be due to magnon excitation within the antiferromagnetically ordered state,T<TN. On the other hand, the thermal evolution of the resistivity is found to be dominated bythe following scattering processes: magnon scattering operating within the ordered state atT<TN leadingto a T4 resistive contribution and a spin fluctuation process associated with the Cosubsystem giving rise to both a quadratic resistive term below 15 K and a saturatedresistive term at higher temperatures. The isothermal magnetoresistivity belowTN,ρ(T<TN,H), manifests a peak which is centered at the same critical field that appears in the magnetizationisotherms. This peak, together with the peak observed at a temperature 0.7 K belowTN, is attributed to a spin rearrangement of the AFM structure of the Ce sublattice.

Statistical characteristics of fluctuation of heights, surface roughness and fractal properties of Cu thin films

To investigate the surface roughness, and get some information about statistical behaviour of Cu thin films and knowledge about morphology and fractal characteristics of them, copper thin films have been deposited on glass substrate by dc magnetron sputtering method. In different deposition rate (0.4-2.5 nm/sec), effect of this parameter on surface roughness has been investigated. By AFM studies interface width (W or surface roughness) and structure function S2(l) as well as different moment of structure function Sq(l) and correlation length also have been investigated for various experimental conditions. Fractal properties of them have been studied too and height distribution satisfied Gaussian distribution function.

Evolution of iron speciation during hydration of C 4AF

It is now well accepted and demonstrated that calcium silicate, calcium aluminate and calcium sulfo aluminate (ettringite, AFm) phases exhibit a good capability to fix metals and metalloids. Unfortunately the role of minor phases and especially calcium-ferric aluminate phase, shorthand C 4AF is not well defined. In other systems like in soils or sediments iron phases play a key role in the fixation of pollutant. In cement sorption isotherms, indicated that various metals can be retained by the C 4AF hydrated products. Therefore the capabilities of those phase to retain heavy metal should not be neglected. Previous investigations have shown that the minerals formed during the hydration of C 4AF are similar to those formed from C3A (pure tri-calcium aluminate) under comparable conditions. Nevertheless no investigation was conducted at the molecular level and there is still a controversy whether Fe substitutes for Al in the hydrated minerals in whole or in part, or if it forms FeOOH clusters scattered throughout the matrix. In this context we have conducted XAS experiments using synchrotron radiation. It was found that the hydration of C 4AF forms C 3AH 6 (hydrogarnet) in which Fe randomly substitutes for Al as well as an amorphous FeOOH phase. Intermediate products like AFm (i.e., an ill organized lamellar phase) are also formed but rapidly evolve to C 3AH 6; iron does not seem to be incorporated in the AFm structure.

A High Precision AFM for Nanometrology of Large Area Micro-Structured Surfaces

This paper presents a high precision AFM for nanometrology of large area micro-structured surfaces. A PZT with a stroke of 100 microns is used as the Z-directional actuator for the AFM cantilever. Two capacitance-type displacement probes are aligned at two sides of the PZT along the movement direction. The displacement as well as the tilt motion of the PZT can be accurately measured and compensated for based on the probe outputs. It was confirmed that the tilt motion of the PZT was approximately 32 arcseconds over the 100 micron stroke. The sample is moved by two linear stages for scanning in the X- and Y-directions over an area of 50 mm x 40 mm. The angular error motions of the stages that influence the AFM accuracy are measured by an autocollimator for compensation. A piezo-resistive cantilever, which can output the atomic force signal by itself, was employed instead of the conventional optical force sensing device for compactness of the AFM structure. A large area sinusoidal metrology surface has been successfully measured by the developed high-precision AFM.