Positive exchange bias in epitaxial permalloy/MgO integrated with Si (1 0 0)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).     

Calculations of (1 1 0) ferromagnetic surface modes: A method of solution

We present a theoretical method for the calculations of (1 1 0) ferromagnetic surface modes in the context of the Heisenberg model with only nearest neighbors interactions. It does not include either relaxation or reconstruction at the surface. The propagating and evanescent spin waves fields in the two-dimensional plane normal to the surface are investigated in this theoretical approach. Emphasis has been laid upon the advantage of handling each displacement field, characterized by its symmetry in particular. Analytic expressions for the bulk and surface equations of motion are also derived in a low-temperature spin wave approximation which enable us to analyze qualitatively the nature of the modes, by observing the response of the energy branches to the variation of structure factors and exchange parameters occurring on the surface. The results are used to calculate the localized modes of spin waves associated with the surface. Numerical examples of the localized modes are given and they are found to exhibit various effects due to the interplay between the bulk and surface modes. It is shown, that under certain circumstances, either one or two branches of surface spin waves may exist. Also the bulk spin fluctuation field as well as the magnetic surface waves, are in qualitative overall agreement with those obtained in the literature by using a Green’s function formalism.     

Tuning exchange bias in epitaxial Ni/MgO/TiN heterostructures integrated on Si(1 0 0)

Epitaxial Ni thin films are integrated with tunneling barrier MgO on Si(1 0 0) substrate. During pulsed laser deposition, early island-like structure transformed into uniform thin film with increasing number of laser pulses. This led to transitions in exchange bias from positive to negative and back to positive, which is ascribed to morphology associated residual strain. The Ni island structure has a coercive field as high as 3 times of that of the continuous film. The current work holds a tremendous promise in the realization of magnetic devices integrated with the Si-platform.     

Diffusion of a vacancy on Fe(1 0 0): A molecular-dynamics study

The diffusion of a surface vacancy on Fe(1 0 0) has been studied at various temperatures by means of molecular-dynamics simulations in conjunction with a many body potential in the context of the embedded atom method. This interatomic potential was recently constructed by fitting its parameters to both experimental and first-principles results. From the analysis of the vacancy jumps, three main diffusion mechanisms have been investigated. The first one corresponds to the migration of the vacancy on the surface layer (intra-layer jumps) by hopping to a neighboring site, while the two others involve the participation of an atom of the second layer (inter-layer jumps) as well. The temperature dependence of the associated diffusion coefficients follows an Arrhenius behavior, from which the migration energies and pre-exponential factors were deduced. It was found that one of the mechanisms corresponding to inter-layer jumps is energetically slightly favored over the two other processes and in addition, its diffusivity is about six times higher than that of the two others. Our results show that the contribution of vacancy diffusion to mass transport is important.     

Influence of ageing on H-induced ferromagnetism in Zn1 − xMxO (M = Co, Fe, Mn)

Many dilute magnetic semiconductors, when annealed in hydrogen atmosphere, have been reported to show a giant ferromagnetism. The H-induced lattice defects, mainly the oxygen vacancies, have been suggested to mediate this ferromagnetic coupling. We have also observed huge magnetic induction in paramagnetic Co and Fe-doped ZnO systems, upon hydrogenation; however, the re-heating caused this magnetism to vanish quickly. In the present work, we have furthered these studies on their longevity aspect i.e. the ageing effect. Alarmingly, the magnetic properties of hydrogenated samples stored in dry atmosphere degrade gradually with ageing. On the other hand, the 2% Mn doped ZnO, which depicts only a weak ferromagnetism, and no enhancement in magnetization upon hydrogenation, exhibits no such ageing effect.     

Microstructure and magnetic properties of (0 0 1)-oriented L10 FePt films: Role of Ag underlayer and Fe/Pt ratio

FePt multilayer films were deposited on Si(1 0 0) substrate with thermally grown SiO₂ film and sputtered Ag underlayer at room temperature by dc magnetron sputtering and subsequently annealing in vacuum. Experimental results suggest that proper thickness of Ag underlayer and slightly rich of Fe content can effectively induce the (0 0 1) texture of FePt films. A Fe_57.4Pt_42.6 thin film on the 8 nm Ag underlayer exhibits a large perpendicular coercivity of 7.6 kOe with magnetic remanence close to 1.     

Numerical approach to the study of the stressed (0 0 1)NiO/(1 1 1)Ni coatings

This work is devoted to the study of the thermomechanical behaviour of the oxide films formed during the high-temperature oxidation of loaded (1 1 1)Ni single-crystalline nickel samples. The NiO/Ni system is submitted to a high-temperature creep in the oxygen atmosphere. First experiments performed with this system have shown, several deformation modes (diffusional creep, gliding at the interfaces, etc.) in the oxide film and the appearance of periodic cracks perpendicular to the loading direction. The inter-crack distance varies strongly with the load amplitude. In order to clarify these phenomena, we developed two numerical models. Both models take into account the oxide growth phenomenon and consequently the epitaxial strain due to the lattice misfit between the film and the substrate. A simplified 1D model of the oxide layer evolution and a complete 3D finite element model. The results obtained for this model (residual stress and strain and their distribution in the film along the direction perpendicular to the substrate) are used in the 3D model as initial data for the simulation of successive cracks that appear during external loading. For that simulation, we associate remeshing techniques and field mapping. Using the proposed approach, we study, namely, the dissymmetrical coating growth. We analyse also the effect of the crack generation during the loading on the mechanical fields along the metal/oxide interface.     

(3 0 1) and (1 0 1) RuO2 twins on nanostructural rutile TiO2 template

The paper deals with a statistical study on the thermal degradation of some old paper supports. The experimental data permit establishment of some regression models, which describe the dependence between the half time and the temperature at which the paper support is kept and, respectively, between the exposure time of the paper supports to a certain temperature and the absorbency value of a characteristic peak, representative for the paper support.     

A single vacancy diffusion near a Fe (1 1 0) surface: A molecular dynamics study

Both the formation energies, the intra-layer and inter-layer diffuse migration energies of a single vacancy in the first nine layers of the Fe (1 1 0) surface have been calculated by means of molecular dynamics in conjunction with the many-body potential of the embedded-atom method (EAM). The results show that the effect of the surface is only down to the fourth layer for the formation energy of a single vacancy. However, the surface presence affects the migration energy down to the seventh layer. It is easier for a vacancy in the first layer to form and to migrate than in the other layers and the bulk. For the inter-layer migration, a vacancy in each of the first six layers is favorable to migrate into the corresponding upper layer, especially for the first three layers.     

Anharmonic effects on Be(0 0 0 1): A molecular dynamics study

Using molecular dynamics simulations and a modified analytic embedded atom method (MAEAM), the anharmonic effects of Be(0 0 0 1) surface have been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer separation, mean square vibrational displacement, phonon frequencies and phonon line width, and layer structure factor are calculated. The obtained results for temperature dependence of interlayer separation and mean square displacement show that the anharmonic effects are small in the temperature range from 0 K to 1100 K. The calculated layer order parameters indicate that Be(0 0 0 1) surface loses its long-range translational order, but do not premelt up to 50 K below the bulk melting point. The surface disordering may result from strongly contracted c/a ratio of Be.     

Microwave-assisted synthesis: A fast and efficient route to produce LaMO3 (M = Al, Cr, Mn, Fe, Co) perovskite materials

A series of lanthanum perovskites, LaMO₃ (M = Al, Cr, Mn, Fe, Co), having important technological applications, have been successfully prepared by a very fast, inexpensive, reproducible, environment-friendly method: the microwave irradiation of the corresponding mixtures of nitrates. Worth to note, the microwave source is a domestic microwave oven. In some cases the reaction takes place in a single step, while sometimes further annealings are necessary. For doped materials the method has to be combined with others such as sol-gel. Usually, nanopowders are produced which yield high density pellets after sintering. Rietveld analysis, oxygen stoichiometry, microstructure and magnetic measurements are presented.     

90°coupling in (Fe/Cr/Fe)AFM/Cr/Fe system epitaxially grown on GaAs(0 0 1)

Single-crystalline (Fe/Cr/Fe)_AFM/Cr/Fe structures were epitaxially grown on atomically flat GaAs(0 0 1). Choosing the same thickness of the antiferromagnetically (AFM) coupled Fe layers in the bottom (Fe/Cr/Fe)_AFM structure, their net magnetization is balanced to zero, in particular up to a spin-flop transition when the field is applied along the [1 1 0] direction. For the Cr thicknesses at which the top Fe layer is weakly magnetically coupled to the bottom (Fe/Cr/Fe)_AFM structure, at low fields, the magneto-optical Kerr effect and/or SQUID signal from the sample corresponds to the top Fe layer only. An influence of the Cr spin structure on the magnetization reversal in the Fe layer is reported. In particular, a strong increase of coercivity (by a factor of 12) is found at low temperatures. A 90° coupling is detected which affects the minor loops measured along the [−1 1 0] and [1 0 0] directions.     

Electromechanical properties of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 crystals under uniaxial stress

The electric field and the stress induced strain and polarization responses of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN–0.32PT) relaxor ferroelectric single crystals have been systematically investigated by experimental study. The responses of [0 0 1] oriented single crystal to stress cycle and electric field are explained by polarization rotation and phase transformations mechanism. However, the responses of [0 1 1] and [1 1 1] oriented single crystal should be explained by domain switching. The differences of strain and polarization between the minimum and maximal values of electric field in [0 0 1] orientation firstly increase then decrease with enhancing of compressive stress.     

A comparative study of Co thin film deposited on GaAs (1 0 0) and glass substrates

The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity (49.0 G), lower saturation magnetization (3.65 × 10⁻⁴) and resistivity (8 μΩ cm) values as compared to that on glass substrate (22 G, 4.77 × 10⁻⁴ and 18 μΩ cm). The grazing incidence X-ray reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates.     

Structural stabilities, electronic structures and lithium deintercalation in LixMSiO4 (M = Mn, Fe, Co, Ni): A GGA and GGA + U study

Dilithium-orthosilicate oxides Li₂MSiO₄ (M denotes transition metals) have been one of the focuses in the field of new cathode materials for Li-ion batteries recently, due to their possible high capacities and probabilities achieving by experiment. Using the density functional theory within both the generalized gradient approximation (GGA) and GGA + U frameworks, the structural stabilities, electronic structures and delithiation process for the dilithium-orthosilicate oxides Li₂MSiO₄ (M = Mn, Fe, Co, Ni) are systematically investigated. Within the GGA + U approach, LiMSiO₄ is shown to be a stable non-stoichiometric structure, while the compound Li_1.5MSiO₄ are unstable relative to a two-phase form containing Li₂MSiO₄ and LiMSiO₄, which is consistent with the experimental voltage profiles. For Li_0.5MSiO₄, though the formation energies are negative for Mn-system and Ni-system, the absolute values are so small that they would be likely to also undergo phase separation at room temperature. The average deintercalation voltages calculated by the GGA + U scheme are in good agreement with the available experimental data. Furthermore, the possibility of the exchange of two electrons per M in Li₂MSiO₄ is also discussed based on the calculated results.     

Adsorption of His-tagged peptide to Ni,Cu and Au (1 0 0) surfaces: Molecular dynamics simulation

Molecular dynamics (MD) simulations are performed to study the interaction of His-tagged peptide with three different metal surfaces in explicit water. The equilibrium properties are analyzed by using pair correlation functions (PCF) to give an insight into the behavior of the peptide adsorption to metal surfaces in water solvent. The intermolecular interactions between peptide residues and the metal surfaces are evaluated. By pulling the peptide away from the peptide in the presence of solvent water, peeling forces are obtained and reveal the binding strength of peptide adsorption on nickel, copper and gold. From the analysis of the dynamics properties of the peptide interaction with the metal surfaces, it is shown that the affinity of peptide to Ni surface is the strongest, while on Cu and Au the affinity is a little weaker. In MD simulations including metals, the His-tagged region interacts with the substrate to an extent greater than the other regions. The work presented here reveals various interactions between His-tagged peptide and Ni/Cu/Au surfaces. The interesting affinities and dynamical properties of the peptide are also derived. The results give predictions for the structure of His-tagged peptide adsorbing on three different metal surfaces and show the different affinities between them, which assist the understanding of how peptides behave on metal surfaces and of how designers select amino sequences in molecule devices design.     

Synthesis of xLi2MnO3·(1 − x)LiMO2 (M = Cr, Mn, Co, Ni) nanocomposites and their electrochemical properties

A series of 0.4Li₂MnO₃·0.6LiMO₂ (M = Ni_1/3Co_1/3Mn_1/3 and Ni_1/3Cr_1/3Mn_1/3) cathode materials are prepared by a co-precipitation method with subsequent quenching. Crystal structures of samples are investigated by X-ray diffraction and electron diffraction, which show a co-existence of rhombohedral and monoclinic structures indicating nanocomposite characteristics of the sample of 0.4Li₂MnO₃·0.6Li Ni_1/3Cr_1/3Mn_1/3O₂. The average particle size distributions of the powders are analyzed to be an order 400 and 100 nm. The 0.4Li₂MnO₃·0.6LiMO₂ (M = Ni_1/3Co_1/3Mn_1/3 and Ni_1/3Cr_1/3Mn_1/3) electrodes, which consist of a well balanced partial phases of rhombohedral and monoclinic can deliver a high reversible capacity of 220-230 mAh/g during an extended cycling.     

Synthesis and properties of A6B2(OH)16Cl2·4H2O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) materials for environmental applications

Double layered hydroxide materials of composition A₆B₂(OH)₁₆Cl₂·4H₂O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) were synthesized by chemical precipitation at 60 °C. Different levels of crystallinity and ordering degree were observed depending upon the chemical environment or the combination between divalent and trivalent cations. The results from high-resolution transmission electron microscopy revealed that nanostructured layered samples were obtained with interplanar spacing compatible with previous literature. Raman scattering was employed to investigate the complex band structure observed, particularly the lattice vibrations at lower frequencies, which is intimately correlated to the cationic radius of both divalent and trivalent ions. The results showed that strongly coordinated water and chloride ions besides highly structured hydroxide layers have a direct influence on the stability of the hydrotalcites. It was observed that transition and decomposition temperatures varied largely for different chemical compositions.     

Microstructural properties of Co thin films grown on p-GaAs (1 0 0) Substrates

Microstructural properties of Co thin films grown on p-GaAs (1 0 0) substrates at room temperature by ion beam-assisted deposition were investigated. An atomic force microscopy image showed that the root mean square of the average surface roughness of the Co film was 32.2 Å, and X-ray diffraction and selected area diffraction pattern measurements showed that Co film layers grown on GaAs (1 0 0) substrates were polycrystalline. A bright-field transmission electron microscopy image showed that the Co/p-GaAs (1 0 0) heterointerface grown at room temperature was sudden. These results provide important information on the microstructural properties for Co thin films grown on p-GaAs (1 0 0) substrates at room temperature.