Microstructural and magnetic properties of CoPt nanowires

In this work, the magnetic and microstructural properties of CoPt nanowires are presented as a function of the electrolyte pH and current density during electrodeposition into anodized alumina templates. CoPt nanowires of high aspect ratio have been prepared using electrolyte pH values in the range from 2 to 6. The as-made samples exhibit a face centered cubic (fcc) structure with soft magnetic properties which transform into the face centered tetragonal (fct) L10 phase after thermal treatment. Different pH values of the electrolyte during electrodeposition lead to significantly different microstructures and, therefore, different magnetic properties. The CoPt nanowires prepared at high pH value are composed of fcc nanorods of about 25 nm in length. Thermal annealing of these samples leads to a preferred (0 0 1) orientation (along the direction perpendicular to the direction of nanowires) which increases with annealing time. On the other hand, the CoPt nanowires prepared at lower pH value are composed of uniform fcc nanograins with the size ∼2−3 nm. Magnetization curves for the later sample are virtually identical in both directions indicating an isotropic behavior.     

Dual behaviors of magnetic CoxFe1−x (0≤x≤1) nanowires embedded in nanoporous with different diameters

Co_xFe_1−x nanowire arrays with various diameters and different composition were fabricated by ac electrodeposition using porous alumina template. Coercivity along the easy axis reaches to a maximum at 2330 Oe, for Co_xFe_1−x nanowires containing about 40 at% Co. The crystalline structure of the nanowires was concentration-independent and shows a bcc structure. The critical diameter for transition from coherent rotation to curling mode is 35 nm for CoFe containing less than 40 at% Co while it is 30 nm for those with more than 40 at% Co. Optimizing the magnetic properties of CoFe with different Co content was seen to be dependent on the diameter of nanowires. For 25 nm diameter, the optimum was shown below 50 at% Co while it was seen above 50 at% for nanowires with 50 nm diameter. The angular dependence of the coercivity with nanowires diameter were also studied.     

Structure and magnetic properties of CoxPb1−x nanowire arrays

Ordered Co_xPb_1−x nanowire arrays embedded in the porous anodic aluminum oxide (AAO) template have been fabricated by electrodeposition. XRD experiments prove that neither hexagonal-close-packed (hcp) nor face-centered-cubic (fcc) Co peaks are detected when the Co component (x) is below 0.91. The coercivity (H_c) and squareness (M_r/M_s) are found to increase with ferromagnetic Co component and the maximum value is at the position x=1 (pure Co nanowires). Annealing effects cause H_c and M_r/M_s increase, which surpasses the pure Co nanowires in the 0.2<x<0.6 at the annealing temperature of 700 °C. Microstructure change during annealing process is proposed to explain the magnetic properties change of samples.     

Texture development and magnetic properties of [ZrO2/CoPt]n/Ag nanocomposite films

The L1₀ CoPt films with (0 0 1) preferred orientation are achieved by fabricating on the glass substrates and post annealing at 600° C for 30 min. The preferred orientation of [ZrO₂/CoPt]_n/Ag films dependence of the Ag underlayer thickness, ZrO₂ and CoPt interlayer thickness is investigated. A large perpendicular magnetic anisotropy and a nearly perfect L1₀ CoPt (0 0 1) texture are obtained in the [ZrO₂ (3 nm)/CoPt (5 nm)]₃/Ag (10 nm) film. The existence of ZrO₂ plays an important role in reducing the intergranular interactions and in determining the size of CoPt grains. Magnetic reversal in textured CoPt films are close to a Stoner-Wolfarth rotation.     

Magnetism of surface alloys of the type MxN1−x/Rh(1 1 1)

We present a density functional theory study on the magnetic properties of two-dimensional surface alloys of the type M_xN_1−x (M=Fe, Co and Ni; N=Pt, Au, Ag, Cd and Pb) on Rh(1 1 1) for x=0.0, 0.25, 0.33, 0.5, 0.67, 0.75 and 1.0, in two types of geometric arrangements—striped phases or linear-chain type, and non-striped phases or mixed checkerboard type. Many pairs among these are bulk-immiscible but show mixing on the surface. We find that the trend in the magnetic moment of surface alloys of N with a given M follows the number of valence electrons in N: the higher the number of valence electrons, the lower the magnetic moment. Overlayer atoms when put on hcp sites show higher moment compared to fcc sites. In general, for a given composition x, linear-chain type structures show a reduced magnetic moment compared to checkerboard type structures. We find that Pb, when alloyed with magnetic elements (Fe, Co and Ni), has a lowering effect on their magnetic moments.     

Synthesis, structural and transport properties of nanocrystalline La1−xBaxMnO3 (0.0≤x≤0.3) powders

Nanocrystalline La_1−xBa_xMnO₃ (0.0≤x≤0.3) manganites have been prepared by a simple and instantaneous solution combustion method, which is a low temperature initiated synthetic route to obtain fine-grained powders with relatively high surface area. The phase purity and crystal structure of the combustion products are carried out by powder X-ray diffraction. The as-made nanopowders are in cubic phase. On calcination to 900 °C, barium doped manganites retain cubic phase, whereas barium free manganite transformed to rhombohedral phase. The scanning electron microscope (SEM) results revealed that the combustion-derived compounds are agglomerated with fine primary particles. The doped manganites have surface area in the range 24-44 m²/g. The surface area of the manganites increases with barium content, whereas it decreases on calcination. Both undoped and doped lanthanum manganites show two active IR vibrational modes at 400 and 600 cm⁻¹. The low temperature resistivity measurements have been carried out by four-probe method down to 77 K. All the samples exhibit metal-insulator behaviour and metal-insulator transition temperature (T_M-I) in the range 184-228 K and it is interesting to note that, as the barium content increases the T_M-I shifts to lower temperature side. The maximum T_M-I of 228 K is observed for La_0.9Ba_0.1MnO₃ sample.     

CoPt alloy grown on the WSe2(0 0 0 1) van der Waals surface

The structural and magnetic properties of 3-nm-thick CoPt alloys grown on WSe₂(0 0 0 1) at various temperature are investigated. Deposition at room temperature leads to the formation of a chemically disordered fcc CoPt alloy with [1 1 1] orientation. Growth at elevated temperatures induces L1₀ chemical order starting at 470 K accompanied with an increase in grain size and a change in grain morphology. As a consequence of the [1 1 1] growth direction, the CoPt grains can adopt one of the three possible variants of the L1₀ phase with tetragonal c-axis tilted from the normal to the film plane direction at 54°. The average long-range order parameter is found to be 0.35(±0.05) and does not change with the increase in the deposition temperature from 570 to 730 K. This behavior might be related to Se segregation towards the growing facets and surface disorder effects promoted by a high surface-to-volume ratio. Magnetic studies reveal a superparamagnetic behavior for the films grown at 570 and 730 K in agreement with the film morphology and degree of chemical order. The measurements at 10 K reveal the orientation of the easy axis of the magnetization lying basically in the film plane.     

Analysis of the Ge1−xCx films deposited by MFMST

Ge_1−xC_x films deposited by using a medium frequency magnetron sputtering technique (MFMST) were analyzed with X-ray photoelectron and Raman spectroscopy. The deposited Ge_1−xC_x films consist of C, Ge, GeC and GeO_y. The GeC content in the Ge_1−xC_x films linearly decreases, and the C content linearly increases with increasing deposition temperature from 150 to 350 °C. The GeC content decreases from 11.6% at a substrate bias of 250 V to a lowest value of 9.6% at 350 V, then increases again to 10.4% at 450 V. While the C content increases from 49.0% at the bias of 250 V to a largest value of 58.0% at 350 V and then maintains this level at 450 V. It is found that selecting a bias parameter seems more effective than deposition temperature if we want to obtain a higher content of GeC in the deposited films. In addition, a new method is presented in this paper to estimate the changes of GeC content in the Ge_1−xC_x films by observing the shifts of Ge-Ge LO phonon peak in Raman spectra for the Ge_1−xC_x films. The related mechanism is also discussed in this paper.     

Structural study of CoxGe100−x alloys produced by mechanical alloying

Two alloys of the Co-Ge system were produced by mechanical alloying starting from the elemental powders in the compositions Co₂₀Ge₈₀ and Co₄₀Ge₆₀. The crystalline structures of the Co_xGe_100−x (x=20, 40) alloys obtained were investigated using the X-ray diffraction (XRD) technique. The measured XRD patterns showed the presence of the peaks corresponding to the crystalline m-CoGe phase and also to the high pressure and temperature phase c-CoGe in the as-milled sample for Co₂₀Ge₈₀, although it was milled at room temperature and pressure. For Co₄₀Ge₆₀, the crystalline Co₃Ge₂ phase was obtained, and structural data for all phases were determined by means of a Rietveld refinement procedure. The thermal stability of the phases was investigated performing a heat treatment of the alloys at 450 °C for 6 h and, after that, new XRD measurements were collected and were also studied using a Rietveld refinement procedure. The m-CoGe and Co₃Ge₂ phases seem to be very stable, but the relative amount of c-CoGe decreases a little, indicating a less stable phase, which can be explained by the fact that it is produced usually under extreme conditions.     

Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.     

Structure, magneto-structural transitions and magnetocaloric properties in Ni50−xMn37+xIn13 melt spun ribbons

Melt spun Ni_50−xMn_37+xIn₁₃ (2≤x≤5) ribbons were investigated for the structure, microstructure, magneto-structural transitions and inverse magnetocaloric effect (IMCE) associated with the first-order martensitic phase transition. The influence of excess Mn in Ni site (or Ni/Mn content) on the martensite transition and the associated magnetic and magnetocaloric properties are discussed. It was found that with the increase in Mn content, the martensitic transition shifted from 325 to 240 K as x is varied from 2 to 4, and the austenite phase was stabilized at room temperature. The x=5 ribbon did not show the martensitic transition. For the x=3 ribbon, the structural and magnetic transitions are close together unlike in the x=4 ribbon in which they are far (∼60 K) apart. The zero field cooled and field cooled curves support the presence of exchange bias blocking temperature due to antiferromagnetic interactions in the ribbons. A large change in the magnetization between the martensite and austenite phases was observed for a small variation in the Ni/Mn content, which resulted in large IMCE. A large positive magnetic entropy change (ΔS_M) of 32 J/kg K at room temperature (∼ 300 K) for a field change of 5 T with a net refrigeration capacity of 64 J/kg was obtained in the Ni₄₇Mn₄₀In₁₃ ribbon.     

Hard magnetic properties of melt-spun Co82Zr18−xTix alloys

The phases and magnetic properties of Co-Zr-Ti melt-spun ribbons were studied by XRD analysis and magnetic measurements. The optimal magnetic properties of Ms=59.0 emu/g, Mr=4.0 kG, Hc=2.9 kOe, and (BH)_max=3.0 MGOe were obtained in Co₈₂Zr₁₄Ti₄ ribbons produced at a wheel speed of 30 m/s. In this work, we found that Ti was one of the few large atomic radius elements, which could improve hard magnetic properties of Co-Zr alloy.     

Structures of Co and Pt nanoclusters on a thin film of Al2O3/NiAl(1 0 0) from reflection high-energy electron diffraction and scanning-tunnelling microscopy

With reflection high-energy electron diffraction (RHEED) and scanning-tunnelling microscopy (STM), we made measurements on Co and Pt nanoclusters grown by vapour deposition on a thin film of Al₂O₃/NiAl(1 0 0). The results show that the annealed Co nanoclusters (with mean diameters 2.5, 3.4, 5.8 nm and heights 0.7, 1.5, 1.5 nm, respectively) and Pt nanoclusters (with mean diameter 2.25 nm and height 0.4 nm) are highly crystalline and that their structures are significantly affected by the oxide substrate. Structural analysis based on the RHEED patterns indicates that both Co and Pt clusters have a fcc phase and grow with their (0 0 1) facets parallel to the θ-Al₂O₃(1 0 0) surfaces, and with their [1 1 0] and [−1 1 0] axes along the [0 1 0] and [0 0 1] directions of the oxide surface, respectively, so (Co(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0] and Pt(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0]). This growth is optimal as the Co and Pt fcc (0 0 1) facets match well with the oxygen mesh. To minimize the lattice mismatch, the lattice parameter of the Co clusters expands 4-5% relative to fcc Co bulk, whereas the lattice parameter of the Pt clusters remains near the bulk value, as the Pt fcc (0 0 1) plane has a close lattice match with the oxide surface.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Effects of compositional phase transitions on luminescence of Sr1−xCaxTiO3:Pr

The mixed-compound of Sr_1−xCa_xTiO₃ has shown several compositional phase transformations. Photoluminescence and excitation spectra of the samples with different x and doped with 0.2% Pr³⁺ were investigated. Changes in the emission spectra were observed in different phases. The blue emission at 491 nm from ³P₀ state was found quite strong in the tetragonal phase, and was thermally quenched in the orthorhombic phases. The intensity of the red luminescence from ¹D₂ increases with increasing content of calcium. The strongest red emission is obtained from CaTiO₃:Pr³⁺. The results are discussed based on the configuration coordinate model and interaction of Pr with the charge transfer exciton state of the Ti complex.     

Optical characterization of InxGa1−xN alloys

InGaN layers were grown by molecular beam epitaxy (MBE) either directly on (0 0 0 1) sapphire substrates or on GaN-template layers deposited by metal-organic vapor-phase epitaxy (MOVPE). We combined spectroscopic ellipsometry (SE), Raman spectroscopy (RS), photoluminescence (PL) and atomic force microscopy (AFM) measurements to investigate optical properties, microstructure, vibrational and mechanical properties of the InGaN/GaN/sapphire layers.The analysis of SE data was done using a parametric dielectric function model, established by in situ and ex situ measurements. A dielectric function database, optical band gap, the microstructure and the alloy composition of the layers were derived. The variation of the InGaN band gap with the In content (x) in the 0 < x ≤ 0.14 range was found to follow the linear law E_g = 3.44-4.5x.The purity and the stability of the GaN and InGaN crystalline phase were investigated by RS.     

Na1−xLixNbO3 ceramics studied by X-ray diffraction, dielectric, pyroelectric, piezoelectric and Raman spectroscopy

Na_1−xLi_xNbO₃ ceramics with composition 0.05≤x≤0.30 were prepared by solid-state reaction method and sintered in the temperature range 1100-1150 °C. These ceramics were characterised by X-ray diffraction as well as dielectric permittivity measurements and Raman spectroscopy. Dielectric properties of ceramics belonging to the whole composition domain were investigated in a broad range of temperatures from 300 to 750 K and frequencies from 0.1 to 200 kHz. The Rietveld refinement powder X-ray diffraction analysis showed that these ceramics have a single phase of perovskite structure with orthorhombic symmetry for x≤0.15 and two phases coexistence of rhombohedral and orthorhombic above x=0.20. The evolution of the permittivity as a function of temperature and frequency showed that these ceramics Na_1−xLi_xNbO₃ with composition 0.05≤x≤0.15 present the classical ferroelectric character and the phase transition temperature T_C increases as x content increases. The polarisation state was checked by pyroelectric and piezoelectric measurements. For x=0.05, the piezoelectric coefficient d₃₁ is of 2pC/N. The evolution of the Raman spectra was studied as a function of temperatures and compositions. The results of the Raman spectroscopy study confirm our dielectric measurements, and they indicate clearly the transition from the polar ferroelectric phase to the non-polar paraelectric one.     

Formation of GaAs1−xNx nanofilm on GaAs by low energy N2 implantation

Nitridation of GaAs (1 0 0) by N₂⁺ ions with energy E_i = 2500 eV has been studied by Auger- and Electron Energy Loss Spectroscopy under experimental conditions, when electrons ejected only by nitrated layer, without contribution of GaAs substrate, were collected. Diagnostics for quantitative chemical analysis of the nitrated layers has been developed using the values of NKVV Auger energies in GaN and GaAsN chemical phases measured in one experiment, with the accuracy being sufficient for separating their contributions into the experimental spectrum. The conducted analysis has shown that nanofilm with the thickness of about 4 nm was fabricated, consisting mainly of dilute alloy GaAs_1−xN_x with high concentration of nitrogen x ∼ 0.09, although the major part of the implanted nitrogen atoms are contained in GaN inclusions. It was assumed that secondary ion cascades generated by implanted ions play an important role in forming nitrogen-rich alloy.     

Desorption of surfactant and sintering of surface-modified PdxNi1 − x nanoparticles

A series of Pd_xNi_1 − x nanoparticles in a diameter of 6-7 nm were prepared by wet chemical reduction. They were then modified with two surfactants, stearic acid (SA) and polyethylene glycol (PEG). Desorption of the surfactant was studied using a temperature programmed desorption technique, and the sintering behavior of surface-modified Pd_xNi_1 − x nanoparticles was examined. Since surface energy of the nanoparticles depends on the alloy composition, it can be correlated with the desorption temperature of surfactant from the nanoparticle surface. Because Ni has a higher surface energy, the surfactant desorption temperature increases as the Ni content increases. With the same stoichiometry, the desorption temperature of SA is always higher than that of PEG. The SA-modified nanoparticles have higher thermal stability and are less sintered than PEG-modified nanoparticles. The sintering and growth behavior of the nanoparticles can be correlated with variation of surface energy due to different surface modification.     

Effect of a LaSrCoO3 buffer layer on Pb1 − xLaxTi1 − x/4O3 films studied by polarized Raman spectroscopy

A confocal Raman investigation of Pb_1 − xLa_xTi_1 − x/4O₃ (PLT) thin films grown by RF magnetron sputtering on PbO_x/Pt/Ti/SiO₂/Si substrates with an intermediate LaSrCoO₃ (LSCO) layer was performed. The influence of the LaSrCoO₃ buffer layer was analyzed taking advantage of the observed Raman spectral band variation, which varied according to different manufacturing procedures. In the presence of a LSCO layer, the A1(1TO) Raman mode, which was indicative of tetragonal distortion, was pronouncedly enhanced, and a slight deviation from the (0 0 1) plane of the film was observed from the angular dependence of the polarized Raman spectral intensity. Furthermore, the spectral band variation as well as the residual stress along the in-depth direction was measured in the film from cross-sectional spectral line scans. This latter measurement showed a relaxation of the lattice mismatch in the presence of LSCO and PbO layers.