Search

Insight into the nucleation, growth and phase transformations of calcium sulphate could improve the performance of construction materials, reduce scaling in industrial processes and aid understanding of its formation in the natural environment. Recent studies have suggested that the calcium sulphate pseudo polymorph, gypsum (CaSO4·2H2O) can form in aqueous solution via a bassanite (CaSO4·0.5H2O) intermediate. Some in situ experimental work has also suggested that the transformation of bassanite to gypsum can occur through an oriented assembly mechanism. In this work, we have exploited liquid cell transmission electron microscopy (LCTEM) to study the transformation of bassanite to gypsum in an undersaturated aqueous solution of calcium sulphate. This was benchmarked against cryogenic TEM (cryo‐TEM) studies to validate internally the data obtained from the two microscopy techniques. When coupled with Raman spectroscopy, the real‐time data generated by LCTEM, and structural data obtained from cryo‐TEM show that bassanite can transform to gypsum via more than one pathway, the predominant one being dissolution/reprecipitation. Comparisons between LCTEM and cryo‐TEM also show that the transformation is slower within the confined region of the liquid cell as compared to a bulk solution. This work highlights the important role of a correlated microscopy approach for the study of dynamic processes such as crystallisation from solution if we are to extract true mechanistic understanding. [ABSTRACT FROM AUTHOR]

Annealing is necessary to achieve giant tunneling magnetoresistance (TMR) values in sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). In this study three complementary techniques were used to study the resulting changes in junction microstructure. The as-deposited TMR was modest, 5%, but rose to 101% after annealing at 325 °C for 1 h, corresponding to the tunneling spin polarization rising from 16% to 58%. Soft x-ray resonant magnetic scattering showed a roughening of the magnetic interfaces of the MTJ free layer, confirmed by transmission electron microscopy, which also showed the changes in the CoFeB and MgO to a lattice-matched polycrystalline form. [ABSTRACT FROM AUTHOR]

Chemically ordered B2 FeRh exhibits a remarkable antiferromagnetic-ferromagnetic phase transition that is first order. It thus shows phase coexistence, usually by proceeding though nucleation at random defect sites followed by propagation of phase boundary domain walls. The transition occurs at a temperature that can be varied by doping other metals onto the Rh site. We have taken advantage of this to yield control over the transition process by preparing an epilayer with oppositely directed doping gradients of Pd and Ir throughout its height, yielding a gradual transition that occurs between 350 K and 500 K. As the sample is heated, a horizontal antiferromagnetic-ferromagnetic phase boundary domain wall moves gradually up through the layer, its position controlled by the temperature. This mobile magnetic domain wall affects the magnetisation and resistivity of the layer in a way that can be controlled, and hence exploited, for novel device applications. [ABSTRACT FROM AUTHOR]

Synchrotron X-ray diffraction was used to study the phase transformation processes during the magnetostructural transition in a B2-ordered FeRh (001)-oriented epilayer grown on MgO by sputtering. Out-of-plane lattice constant measurements within the hysteretic regime of the transition reveal a microstructure consistent with the coexistence of lattice-expanded and contracted phases in spatially distinct regions. It was found that the phase separation is more pronounced during cooling than heating. Furthermore, whilst lattice-expanded domains that span the height of the film can be undercooled by several kelvins, there is no equivalent superheating. This asymmetry between the cooling and heating processes in FeRh is consistent with the difference in the kinetics of generic freezing and melting transitions. [ABSTRACT FROM AUTHOR]

The C–S–H present in water and alkali activated hardened pastes of neat white Portland cement (WPC) and blends of WPC with 30% fly ash hydrated at 85°C has been characterised. The chemical composition of C–S–H could not be determined by SEM–EDX because of intermixing with other phases; TEM–EDX was necessary. The mean Ca/(Al–Si) atomic ratio of C–S–H was lower in the water activated blend than in the neat cement paste due to the reaction of the fly ash. The fibrillar morphology of outer product C–S–H in the water activated neat cement paste was coarser than that formed at lower temperatures. The mean length of the aluminosilicate anions in the C–S–H was much higher than in C–S–H formed at lower temperatures and was higher in the blended pastes than with neat cement. The composition–structure data are discussed in terms of models for the nanostructure of C–S–H. The lattice parameter a inferred from compositional data for the hydrogarnet phase in the water activated blend was in good agreement with the value measured directly by XRD. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the effects of age on calcium silicate hydrate (C–S–H) present in two cement pastes: one in which the C–S–H at a younger age had a high Ca/Si ratio and another that had a low ratio. The study involved characterising a 20 year old neat ordinary Portland cement (OPC) paste (high Ca/Si) and a blend of the OPC with 90% ground granulated blast furnace slag (GGBS; low Ca/Si); the results are compared with data on the same mixes at 14 months old. The morphology of the outer product (Op) C–S–H in the blend was essentially the same as at 14 months, whereas that in the neat OPC paste appeared to be finer. The mean aluminosilicate chain length of the C–S–H was considerably longer in both pastes than in similar systems at younger ages. The chemical composition of the C–S–H in both pastes was significantly different than at 14 months: in the neat OPC paste, the Ca/Si ratio of the outer product C–S–H had increased with age, whereas that of the inner product C–S–H had decreased; the Ca/Si ratio of Op C–S–H in the blended paste had decreased with age. An entirely tobermorite based nanostructural model can account for the data for the C–S–H in the blend, whereas the data for the neat paste require some jennite or calcium hydroxide based structure; the distribution of the TEM–EDX analyses favours the T/CH model. [ABSTRACT FROM AUTHOR]

An analysis of variations in the Auger parameters of vapour deposited Mg-V alloys with nominal compositions 0.48, 2.95, 8.9 and 15 at.% V has been undertaken in order to investigate the electronic changes that take place upon alloying Mg with V. The presence of vanadium in the Mg-V system required certain assumptions to be made in the calculations as a result of the electronic configuration of the valence band of V. Charge transfer from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. The application of Thomas and Weightman's approach is shown to have advantages over the use of core ionization energy shifts in combination with the potential model. With the use of the Auger parameter there is no need for explicit energy referencing of the core levels and, moreover, in the improved model the Madelung or surface dipole contributions are not required unlike in the potential model. An attempt has also been made to understand the metallurgical significance of charge transfer in terms of the tendency for ordering or disordering of solid solutions. [ABSTRACT FROM AUTHOR]

Evidence of boron diffusion into the MgO barrier of a CoFeB/MgO based magnetic tunnel junction has been identified using analytical scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy. Structures were deposited by DC/RF-magnetron sputtering, where defective, sub-stoichiometric MgO barriers degrading device performance have been previously mitigated against by deposition of thin Mg layers prior to MgO deposition. We show that despite the protection offered by the Mg layer, disorder in the MgO barrier is still evident by STEM analysis and is a consequence of the oxidation of the Co40Fe40B20 surface during MgO deposition. Evidence of boron diffusion from CoFeB into the MgO barrier in the as-deposited and annealed structure is also presented, which in the as-deposited case we suggest results from the defective structures at the barrier interfaces. Annealing at 375 °C results in the presence of B in the trigonal coordination of [BO3]3- in the MgO barrier and partial crystallization of the top electrode (we presume there is also some boron diffusion into the Ta capping layer). The bottom electrode, however, fails to crystallize and much of the boron is retained in this thicker electrode. A higher annealing temperature or lower initial boron content is required to crystallize the bottom electrode. [ABSTRACT FROM AUTHOR]

It is common to find a thin (∼0.5 nm) layer of Mg deposited prior to the MgO tunnel barrier in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions, due to the improved device performance that results. However, despite their common usage, the reasons why such layers are effective are unclear. We use structures that model the lower electrode of such devices to show that a suitably thick Mg insert layer enhances the crystal quality of both MgO and CoFe(B), permits interfacial oxides to reduce back to a metallic ferromagnetic state, and hence improves magnetic switching of the CoFe(B) electrode, properties which are inextricably linked to device performance. [ABSTRACT FROM AUTHOR]

The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Emzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of kB T at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies. [ABSTRACT FROM AUTHOR]

Yttrium iron garnet has a very high Verdet constant, is transparent in the infrared and is an insulating ferrimagnet leading to its use in optical and magneto-optical applications. Its high Q-factor has been exploited to make resonators and filters in microwave devices, but it also has the lowest magnetic damping of any known material. In this article we describe the structural and magnetic properties of single crystal thin-film YIG where the temperature dependence of the magnetisation reveals a decrease in the low temperature region. In order to understand this complex material we bring a large number of structural and magnetic techniques to bear on the same samples. Through a comprehensive analysis we show that at the substrate -YIG interface, an interdiffusion zone of only 4-6 nm exists. Due to the interdiffusion of Y from the YIG and Gd from the substrate, an addition magnetic layer is formed at the interface whose properties are crucially important in samples with a thickness of YIG less than 200 nm.

Electron energy loss spectroscopy (EELS) is a powerful tool for understanding the chemical structure of materials down to the atomic level, but challenges remain in accurately and quantitatively modelling the response. We compare comprehensive theoretical density functional theory (DFT) calculations of 1s core-level EEL K-edge spectra of pure, B-doped and N-doped graphene with and without a core-hole to previously published atomic-resolution experimental electron microscopy data. The ground state approximation is found in this specific system to perform consistently better than the frozen core-hole approximation. The impact of including or excluding a core-hole on the resultant theoretical band structures, densities of states, electron densities and EEL spectra were all thoroughly examined and compared. It is concluded that the frozen core-hole approximation exaggerates the effects of the core-hole in graphene and should be discarded in favour of the ground state approximation. These results are interpreted as an indicator of the overriding need for theorists to embrace many-body effects in the pursuit of accuracy in theoretical spectroscopy instead of a system-tailored approach whose approximations are selected empirically.