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).     

Integration of epitaxial permalloy on Si (1 0 0) through domain matching epitaxy paradigm

This paper addresses epitaxial integration of magnetic materials with Si (1 0 0) based solid state devices. Epitaxial Ni_82.5Fe_17.5 (permalloy, Py) thin films have been synthesized by pulsed laser deposition (PLD) on Si (1 0 0) using MgO/TiN as a template buffer. This epitaxial growth of these large lattice misfit systems was achieved through domain matching epitaxy (DME). The in-plane XRD pattern and selective area electron diffraction (SAED) results clearly indicate cube-on-cube epitaxial alignment. The bright field TEM image of Py/MgO/TiN/Si (1 0 0) heterostructure infers a Py layer thickness of ∼30 nm, with a well aligned island (150–200 nm) structure that is consistent with Volmer–Weber type growth. Magnetization data collected at 4 K and 300 K indicates that the easy axis of the magnetization lies in the plane of the Py. In addition, we have observed an intrinsic positive exchange bias (PEB) field of ∼104 Oe, where the magnetic hysteresis loop is shifted toward the positive field axis under zero field cooling conditions.     

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.     

First-principles study of the stability and the electronic structure of NiO/MgO interface

First-principles full potential linearized augmented plane wave method (FP-LAPW) based on density functional theory has been performed to study the stability and the electronic structure of NiO/MgO interface. The surface energy, strain energy and the separation energy are calculated and discussed. The results reveal that NiO(0 0 1)/MgO(0 0 1) interface is more stable than NiO(1 1 1)/MgO(1 1 1) interface. Also examined were the electronic structure and the atomic spin magnetic moment of the NiO/MgO interface. It is found that the interface system significantly affects the electronic structure and magnetic properties of the oxide/oxide interface.     

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.     

First principles study of interface structure and electronic property of Au/SrTiO3(0 0 1)

First principles method is carried out to study the interface structure and electronic property of nine probable Au/SrTiO₃(0 0 1) models. It is revealed that besides O or Ti vacancies having to exist in the interface, the interface energy and bonding nature are strongly dependent on the atomic configurations at interface. Such interfacial configurations in which each Au atom in the upper Au layer bonds with each O atom in the TiO₂-terminated SrTiO₃(0 0 1) under layer would be preferable to be formed. From the analysis of the electronic structure of the defective Au/SrTiO₃(0 0 1) interfaces, the interactions between Au and various vacancies on SrTiO₃(0 0 1) interface are very different.     

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00 at.%) films on GaSe (0 0 0 1) lamellas and amorphous quartz substrates, obtained by annealing, at 700 K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27 eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00 ? 2.70 eV. Al doping induces an additional recombination level at 1.13 eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22 eV and 0.26 eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10 ? 1.40 eV, are created.     

Microstructure and transport properties of [0 0 1]-tilt bicrystal grain boundaries in iron pnictide superconductor,cobalt-doped BaFe2As2

Relationships between microstructure and transport properties of bicrystal grain boundary (BGB) junctions were studied in cobalt-doped BaFe₂As₂ (BaFe₂As₂:Co) epitaxial films grown on [0 0 1]-tilt bicrystal substrates of MgO and (La, Sr)(Al, Ta)O₃ with misorientation angles θ_GB = 3–45°. The θ_GB of BaFe₂As₂:Co BGBs were exactly transferred from those of the bicrystal substrates. No segregation of impurities was detected at the BGB junction interfaces, and the chemical compositions of the BGBs were uniform and the same as those in the bulk film regions. A transition from a strongly-coupled GB behavior to a weak-link behavior was observed in current density–voltage characteristics under self-field around θ_GB ～ 9°. The critical current density decreased from (1.2–1.6) × 10⁶ A/cm² of the intragrain transport to (0.7–1.1) × 10⁵ A/cm² of θ_GB = 45° because supercurrent becomes more governed by Josephson current with increasing θ_GB.     

Effect of divalent metal hydroxide solubility product on the size of ferrite nanoparticles

We study the effect of divalent metal hydroxide solubility product on the size and magnetic properties of nanoparticles formed during co-precipitation. We synthesized ferrite nanoparticles by varying the solubility product from 10^− 13 to 10^− 17 by using different divalent cations of Mn, Co, Fe and Zn, where the average particle size decreased from 29.1 to 8.9 nm. The Mn, Co and Fe ferrites were magnetic in nature with saturation magnetization of 44.6, 47.38 and 56.19 emu/g respectively, whereas the Zn ferrite was paramagnetic. The increase in particle size observed with increasing solubility product of divalent metal hydroxide is in agreement with the nucleation theory.     

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.     

Magnetic moments and exchange interaction in Sm(Co, Fe)5 from first-principles

The electronic structure, magnetization and exchange interaction in Sm(Co_1?xFe_x)₅ with x = 0–1 are studied from a first-principles density functional calculation. The dependence of the magnetization on Fe content shows a 3d-like Slater–Pauling relationship in these alloys. As the Fe content x increases from 0 to 1.0, the magnetization increases from 7.8 μ_B to 10.6 μ_B (x = 0.8) and then decreases to 10.0 μ_B (x = 1). The effective exchange interaction parameters show a peak value around x = 0.6, which is ascribed to the exchange parameters between Fe and Co being larger than those for Co–Co and Fe–Fe pairs. The estimated T_C from the calculated exchange parameters range between 890 K and 1357 K in Sm(Co_1?xFe_x)₅ using a multi-sublattices mean field model.     

Magnetic and dielectric properties on sol–gel combustion synthesis of Pb(Zr0.52,Ti0.43X0.05)O3 (X = Fe,Ni, and Co) nanoparticles

Pure and X-doped (X = Fe, Ni, and Co) PZT nanoparticles (PZT-NPs) were synthesized by a sol–gel combustion method. The structural characterization of the obtained pure and doped PZT-NPs was carried out by X-ray diffraction (XRD). The crystallite size of the prepared samples was then evaluated by the size strain plot (SSP) method using the XRD results. The average particle sizes of 30–40 nm were measured for pure and doped PZT-NPs by a particle size analyzer. For dielectric measurements, the synthesized powders were pressed into pellets and then sintered at 1250 °C for 2 h. The magnetic properties of the synthesized doped PZT-NPs were studied by a vibration sample magnetometer (VSM) and ferromagnetic behavior was observed.     

Dehydrogenation properties of epitaxial (1 0 0) MgH2/TiH2 multilayers – A DFT study

The effect of structural changes in MgH₂ induced by contact with TiH₂ on the thermodynamics of hydrogen cycling has been studied using DFT calculations. Models of epitaxial (1 0 0) MgH₂/TiH₂ multilayers with different layer thicknesses and Mg:Ti ratios have been designed. The hydrogen desorption energy from the MgH₂ layer is found to depend on the thickness of the MgH₂ layers and the Mg:Ti ratio because the MgH₂ structure tends to change from fluorite to rutile with increasing MgH₂ thickness and Mg:Ti ratio. Three types of hydrogen can be distinguished: hydrogen in MgH₂ layer with desorption energy of 0.44 eV/H₂, hydrogen in MgH₂/TiH₂ interface with desorption energy of 1.19 eV/H₂, hydrogen in TiH₂ layer with desorption energy of 0.95 eV/H₂. The desorption energy of hydrogen desorption from the fluorite-like MgH₂ is 0.23 eV/H₂ lower than the bulk rutile MgH₂. The structural deformations as well as the interfacial energy differences between the partial hydrogenated and hydrogenated states have been found to be responsible for the tuned hydrogen desorption energy.     

MgO:Dy3 + nanophosphor: Self ignition route,characterization and its photoluminescence properties

For the first time series of MgO phosphors doped with different concentrations of Dy^3 + (1–9 mol%) were prepared by solution combustion method using glycine as a fuel. The final products were well characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The powder X-ray diffraction patterns of the as-formed product show single cubic phase. The crystallite size estimated using Scherrer's method was found to be in the range 5–15 nm and the same was confirmed by transmission electron microscopy result. Photoluminescence properties of Dy^3 + (1–9 mol%) doped MgO for near ultra violet excitation (325 nm) was studied in order to investigate the possibility of its use in white light emitting diode applications. The emission spectra consists of intra 4f transitions of Dy^3 +, namely ⁴F_9/2 → ⁶H_15/2 (483 nm), and ⁴F_9/2 → ⁶H_13/2 (573 nm). Further, the emission at 573 nm shows strong yellow emission and can be applied to the yellow emission of phosphor for the application for near ultraviolet excitation. The intensity of yellow emission was attributed to intrinsic defects, especially oxygen-vacancies, which could assist the energy transfer from the MgO host to the Dy^3 + ions. The Commission International De I-Eclairage chromaticity co-ordinates were calculated from emission spectra, the values (x,y) were very close to the National Television System Committee standard value of white emission. Therefore, the present phosphor was highly useful for display applications.     

Co1−xZnxFe2O4 (0 ≤ x ≤ 1) nanocrystalline solid solution prepared by the polyol method: Characterization and magnetic properties

Highly crystalline stoichiometric Co_1?xZn_xFe₂O₄ (0 ≤ x ≤ 1) nanoparticles were successfully synthesized by the polyol process. X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), transmission electron microscopy (TEM), infrared spectroscopy (IR), zero-field ⁵⁷Fe Mössbauer spectrometry and magnetic measurements using a SQUID magnetometer were employed to investigate the effect of the substitution of Zn²⁺ ions for Co²⁺ ones on the structure, and the magnetic properties of the cobalt ferrite, CoFe₂O₄. The unit cell parameter almost increases linearly with increasing Zn concentration, x, following Vegard's law. The red and blue shifts observed for the metal-oxygen ν₁ and ν₂ IR vibration bands, respectively, were consistent with the preferential entrance of Zn²⁺ ions in tetrahedral sites. Besides, detailed magnetic investigation in correlation with the cation distribution has been reported. All the particles exhibit superparamagnetic behaviour at room temperature. In addition, the magnetic characteristics (blocking temperature, saturation magnetization, coercivity, Curie temperature) clearly depend on the chemical composition and cation distribution. Both the blocking temperature and Curie temperature decrease drastically with Zn composition, x, increase. Further, the saturation magnetization follows an almost bulk-like behaviour with values notably larger than that of the bulk, mainly attributed to cation distribution deviation.     

The epitaxial growth of (1 1 1) oriented monocrystalline Si film based on a 4:5 Si-to-SiC atomic lattice matching interface

Due to a huge lattice mismatch of about 20% theoretically existing between SiC and Si, it is difficult for growing monocrystalline Si/SiC heterojunction to realize the light control of SiC devices. However, based on a 4:5 Si-to-SiC atomic lattice matching interface structure, the monocrystalline Si films were epitaxially prepared on the 6H-SiC (0 0 0 1) substrate by hot-wall chemical vapor deposition in our work. The film was characterized by X-ray diffraction analysis with only (1 1 1) orientation occurring. The X-ray rocking curves illustrated good symmetry with a full width at half maximum of 0.4339° omega. A 4:5 Si-to-SiC atomic matching structure of the Si/6H-SiC interface clearly observed by the transmission electron microscope revealed the essence of growing the monocrystalline Si film on the SiC substrate.     

Characterization of M(H2PO4)2·2H2O (M = Mn,Co, Ni) and their in situ thermal decomposition by magnetic measurements

It has been established that M(H₂PO₄)₂·2H₂O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M²⁺ are preserved. From the data obtained it could be supposed that in M(H₂PO₄)₂·2H₂O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H₂PO₄)₂·2H₂O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M₂P₄O₁₂ (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H₂PO₄)₂·2H₂O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P2₁/n with Z = 2.     

Magnetic and electrical behaviour of La0.67Ba0.33Mn1 − xFexO3 perovskites

(La_0.67Ba_0.33)Mn_1 ? xFe_xO₃ manganites compounds have been prepared by doping up to 20% of Fe at the Mn site. As previously reported in the literature paramagnetic (PM) to ferromagnetic (FM) phase transition has been observed in materials with low Fe doping (≤ 10%). In our x = 0 and x = 0.05 compounds the Curie temperature (T_C) is close to room temperature. Above 10% Fe amount, specimens exhibit a glass magnetic behaviour with a spin- or cluster-like freezing process that can be related to a loss of ferromagnetic double exchange interaction. Below 10% of Fe³⁺ doping electrical-resistivity measurement shows metal – semiconductor transition with a maximum peak of resistivity (ρ_max) at a temperature T_P close to T_C. Above 10% of Fe³⁺ doping amount the materials exhibit only semiconductor behaviour. Both T_C and T_P decrease with doping rates with an increasing difference in temperature (T_P being lower than T_C). Results are consistent with a reduction of the number of available hopping sites for the Mn e_g(↑) electron due to substitution of Mn³⁺ by Fe³⁺ that suppress the double exchange (DE) interactions.     

Synthesis of GdCo1−xCuxO3−δ (x = 0, 0.15, 0.30) perovskites by ultrasonic spray pyrolysis

Single-phase GdCo_1−xCu_xO_3−δ (x = 0, 0.15, 0.30) perovskites were prepared by ultrasonic-assisted spray pyrolysis, at a reactor temperature of 950 °C. The starting aqueous solutions containing stoichiometric amounts of gadolinium, cobalt and copper nitrates, were atomized using an ultrasonic transducer, operated at a frequency of ∼1.7 MHz. The aerosol droplets were then transported by O₂ inside a preheated tube furnace, where solvent evaporation and chemical reaction take place. X-ray powder diffraction gave as a result pure GdCo_1−xCu_xO_3−δ (x = 0, 0.15, 0.30) perovskites; this technique also showed that the replacement of Co by Cu increased the unit cell volume. By field emission scanning electron microscopy and transmission electron microscopy, spherical-hollow particles were observed; a detailed inspection showed that the shells are formed by nanoparticles with an average particle size of 49 nm. The substitution of Co by Cu in members with x = 0.15 and 0.30, was confirmed by energy-dispersive X-ray microanalysis.     

Elastic behaviour of microwave hydrothermally synthesized nanocrystalline Mn1−x–Znx ferrites

Microwave-hydrothermal (M-H) method has been successfully used for synthesis of nanocrystalline Mn–Zn ferrites which are used for high frequency applications. The nanopowders were characterized using XRD and TEM. The particle size of the samples varies from ～20 nm to 25 nm. The powders were densified at 900 °C/30 min using microwave sintering method. The sintered ferrite samples were characterized using XRD, X-ray photoelectron spectroscopy (XPS), SEM and energy dispersive X-ray spectroscopy (EDS). The elastic behaviour and internal friction studies were carried out using composite piezoelectric oscillator method in the temperature range of 300–600 K. It was found that the anomalous behaviour observed in the temperature dependence of Young's modulus and internal friction disappears with the application of a magnetic field equal to the saturation field (900 mT) of the specimen under investigation. The observed anomalous behaviour in the vicinity of the Curie temperature was understood with the help of Landau's theory.