Partial Chemical Ordering Research Articles

Microstructural and multiferroic properties in layered perovskite-related Sm6Ti4Fe2O20

Layered perovskite-related Sm6Ti4Fe2O20 compound was successfully synthesized through the intercalation of bilayer SmFeO3 into the Sm2Ti2O7 with pyrochlore structure by means of floating-zone melting technique. The microstructural properties were characterized using aberration-corrected scanning transmission electron microscopy and X-ray diffraction. Electron energy-loss spectroscopy investigation reveals that the Fe3+ ions prefer to occupy the inner sites within the perovskite-like layers. This compound exhibits clearly the spin glass-like behavior as demonstrated by the magnetic properties measurement. Such complex magnetic behavior could be attributed to the partial chemical order of Ti/Fe over the B sites and the interactions between magnetic ions including Sm3+ and Fe3+. In addition, the multiferroic behavior with the coexistence of the ferroelectricity and ferromagnetism was well established by magnetic and piezoresponse measurements.

Density functional simulations of hexagonal Ge2Sb2Te5at high pressure

We investigated the structural transformations of the hexagonal phase of Ge${}_{2}$Sb${}_{2}$Te${}_{5}$ under pressure by means of ab initio molecular dynamics with a variable simulation cell. To overcome the enthalpy barriers between the different phases we used metadynamics techniques. We reproduced the hexagonal-to-$\mathrm{bcc}$ transformation under pressure found experimentally. The $\mathrm{bcc}$ phase retains a partial chemical order, as opposed to a second $\mathrm{bcc}$ phase we generated by pressuring the amorphous phase. This structural difference is suggested to be responsible for the memory effect uncovered experimentally, the $\mathrm{bcc}$ phase reverting to the amorphous or to the hexagonal phase upon decompression, depending on the type of precursor phase it originates from.

Anisotropic thermal expansion of Lan(Ti,Fe)nO3n+ 2(n= 5 and 6)

Crystal structures are reported for two perovskite-related compounds with nominal compositions La5(Ti0.8Fe0.2)5O17 and La6(Ti0.67Fe0.33)6O20 at seven different temperatures between 90 and 350 K. For both compounds no evidence of a structural phase transition in the investigated range of temperatures was found. The thermal expansions are found to be anisotropic, with the largest thermal expansion along a direction parallel to the slabs of these layered compounds. The origin of this anisotropy is proposed to be a temperature dependence of tilts of the octahedral (Ti,Fe)O6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds AnBnO3n + 2. The crystal structures have revealed partial chemical order of Ti/Fe over the B sites, with iron concentrated towards the centers of the slabs. Local charge compensation is proposed as the driving force for the chemical order, where the highest-valent cation moves to sites near the oxygen-rich borders of the slabs. A linear dependence on the site occupation fraction by Fe of the computed valences leads to extrapolated valence values close to the formal valence of Ti4+ for sites fully occupied by Ti, and of Fe3+ for sites fully occupied by Fe. These results demonstrate the power of the bond-valence method, and they show that refined oxygen positions are the weighted average of oxygen positions in TiO6 and FeO6 octahedral groups.

Anisotropic thermal expansion of Lan(Ti,Fe)nO3n+ 2(n= 5 and 6)

Crystal structures are reported for two perovskite-related compounds with nominal compositions La5(Ti(0.8)Fe(0.2))5O17 and La6(Ti(0.67)Fe(0.33))6O20 at seven different temperatures between 90 and 350 K. For both compounds no evidence of a structural phase transition in the investigated range of temperatures was found. The thermal expansions are found to be anisotropic, with the largest thermal expansion along a direction parallel to the slabs of these layered compounds. The origin of this anisotropy is proposed to be a temperature dependence of tilts of the octahedral (Ti,Fe)O6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds A(n)B(n)O(3n + 2). The crystal structures have revealed partial chemical order of Ti/Fe over the B sites, with iron concentrated towards the centers of the slabs. Local charge compensation is proposed as the driving force for the chemical order, where the highest-valent cation moves to sites near the oxygen-rich borders of the slabs. A linear dependence on the site occupation fraction by Fe of the computed valences leads to extrapolated valence values close to the formal valence of Ti(4+) for sites fully occupied by Ti, and of Fe(3+) for sites fully occupied by Fe. These results demonstrate the power of the bond-valence method, and they show that refined oxygen positions are the weighted average of oxygen positions in TiO6 and FeO6 octahedral groups.

Free Carrier Scattering in Metallic n-GaAs in the Presence of Static Lattice Distortions Due to a Partial Chemical Order of Impurities

Simple electric transport versus T = 20‐400 K in metallic n-GaAs annealed single crystals with Te impurity concentration » (0.4‐1.7) £ 10 19 cm i3 , which is above the equilibrium doping limit, is reported and compared with modern theory of electron mobility in degenerated n-GaAs by Szmyd, Hanna, Majerfeld. An overcome of the equilibrium doping limit in annealed n-GaAs is manifested by a lowered electrical activation of Te donors and by an onset of … 0.1‐1 „m regions of local strain in the crystal lattice known from high resolution X-ray studies. These preliminary results of transport show that the electron mobility „(T) measured for n-GaAs with local strains is not consistent with predictions of Szmyd et al. model for any degree of compensation assumed. This surprising result indicates that electric transport in materials above the equilibrium doping limit is not well understood assuming the scattering by ionized impurities. The nature of defects responsible for an observed strong reduction of free carrier concentration (here … 80%) in annealed heavily doped n-GaAs seems not to be related with electrical compensation. We point here at the possible role of eects of free carrier scattering due to static lattice distortions (local strains) related to a chemical aggregation of impurity atoms. We also notice that transport in metallic n-GaAs with local strains shows features similar to a weak localization aexx / logT.

Magnetic susceptibility contributions and electronic density of states in(Ti,Zr)100−x(Ni,Cu)xmetallic glasses and crystalline compounds

Available experimental data on the magnetic susceptibility of melt-quenched amorphous TE-TL alloys ($\mathrm{TE}=\mathrm{Ti}$ or Zr; $\mathrm{TL}=\mathrm{Ni}$ or Cu) and the corresponding crystalline counterparts are reviewed. In order to analyze the composition dependence of the magnetic susceptibility in these systems, the individual contributions (element-resolved Langevin diamagnetic, Van Vleck and Pauli spin susceptibility, as well as the Landau susceptibility) were determined directly by theoretical calculations for these TE-TL alloys in a face-centered-cubic (fcc) structure. The total susceptibility, both experimental and calculated, was found to decrease with increasing TL content up to about $60--70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$. This variation was mainly ascribed to corresponding changes of the Van Vleck contribution and the spin susceptibility. The composition dependence of the latter term is in line with the previously established trend of variation of the density of states at the Fermi level $n({E}_{F})$ upon alloying. As in previous reports on electronic band-structure calculations, it turned out that to get agreement with experiments on $n({E}_{F})$ in amorphous Zr-Ni alloys, at least some partial chemical ordering should be taken into account. Available experimental data of crystalline stoichiometric Zr-Ni compounds beyond $70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni show a gradual increase of the Pauli susceptibility, indicating an approach toward the onset of ferromagnetic order observed previously experimentally around $90\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni in amorphous Zr-Ni alloys. The susceptibility calculations for fcc Ti-Ni and Zr-Ni alloys indicate a strong increase of the spin susceptibility component above $70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni, and the calculated Stoner enhancement for ${\mathrm{Zr}}_{10}{\mathrm{Ni}}_{90}$ fulfills the condition of ferromagnetism in agreement with the experimental observation.

Synthesis of FePtAu nanoparticles in high-boiling-point solvents

Partially ordered FePtAu nanoparticles with size around 8 nm were prepared by the simultaneous decomposition of iron pentacarbonyl and reduction of gold acetate and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The final temperature was controlled between 300/spl deg/C-360/spl deg/C during synthesis, where disordered FePt particles could be partially transformed into the ordered L1/sub 0/ phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. XRD patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the FePtAu particles. The coercivities of partially transformed as-made particles vary from a few hundred oersteds to a few thousand oersteds, depending on the concentration of gold additive and the temperature of synthesis. Dynamic coercivity measurements yield thermal stability factors and switching volumes that are slightly higher than would be expected for noninteracting particles.

Ion-irradiation induced chemical ordering of FePt and FePtAu nanoparticles

We have studied the effect of ion-beam irradiation on reducing the ordering temperature of FePt and FePtAu nanoparticles. FePt and FePt(Au14%) 4nm particles dispersed on a Si-substrate were irradiated by 300keV Al-ions with a dose of 1×1016 ions/cm2 at 43°C using a water-cooled flange in order to minimize the vacancy migration and voids formation within the collision cascades. Partial chemical ordering has been observed in as-irradiated particles with coercivity of 60–130Oe. Post-irradiation annealing at 220°C enhanced chemical ordering in FePt nanoparticles with coercivity of 3500Oe, magnetic anisotropy of 1.5×107erg/cc, and thermal stability factor of 130. A much higher 375°C post-irradiation annealing was required in FePtAu, presumably because Au atoms were trapped at Fe/Pt lattice sites at lower temperatures. As the annealing temperature increased, anomalous features in the magnetization reversal curves were observed that disappeared at higher annealing temperatures.

Direct synthesis and easy axis alignment of L1-FePt nanoparticles

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The reflux temperature of the solution could exceed 360°C, where disordered FePt particles could be partially transformed into the ordered L10 phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. The particles were dispersed in toluene and films of the particles were cast onto silicon wafers from the solution. X-ray diffraction patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the Fe53Pt47 particles. The room-temperature hysteresis loop of the as-made sample reveals a small coercivity (∼600Oe) because of thermal fluctuations; however, the loop is wide open and hard to saturate. The remanence coercivity from the dcd curve is about 2.5kOe, which is four times larger than the hysteresis coercivity. The large remanent to hysteresis coercivity ratio and the shapes of the hysteresis loop and dcd curve suggest a broad distribution of anisotropies in the partially ordered particles. By coating the ordered nanoparticles with a polymer binder, the easy axis of the particles could be aligned under an external field.

Chemical ordering of FePt nanoparticle self-assemblies by rapid thermal annealing

Self-assembled 4 nm FePt nanoparticle arrays were treated with rapid thermal annealing (RTA) process. The phase transformation from the chemically disordered face-centered-cubic structure of the as-synthesized FePt nanoparticles to the chemically ordered face-centered-tetragonal structure was realized by RTA, with both the annealing temperature and time being greatly reduced, as compared to conventional annealing. The onset of chemical ordering occurred at the annealing condition of around 400°C for only 5 s. Temperature-dependent coercivity measurements revealed strong thermal effects of the nanoparticle assemblies. Curie temperatures ( T c) of the annealed assemblies were derived from the temperature-dependent hysteresis measurements. T c of the annealed assemblies increases with increasing annealing temperature, but is about 200–300 K lower than that of bulk FePt (750 K). The drastic reduction of T c may be attributed to size effect and partial chemical ordering of FePt nanoparticles.

Thermal effects in self-assembled FePt nanoparticles with partial chemical ordering

We have studied the thermal stability of self-assembled FePt nanoparticles with partial chemical ordering. The magnetic viscosity was measured at zero field after partial dc demagnetization as a function of the initial remanent magnetization. The viscosity curves are in good agreement with a calculation using a noninteracting Stoner–Wohlfarth, Arrhenius–Néel model with a broad distribution of anisotropy fields and randomly oriented easy axes. The shape of the zero field viscosity curve differs from curves measured for thin film media with in-plane and perpendicular anisotropy, in which interactions are known to be important.

Irradiation-induced magnetic patterning in magnetic multilayers

Irradiation with light ion (He +) can modify in a precisely controlled way the magnetic properties of thin films, with negligible change of surface roughness and optical indices. In Co/Pt multilayers with perpendicular easy axis of magnetization, we have shown that the magnetic anisotropy decreases with the irradiation fluence. In FePt alloys, partial chemical ordering and high perpendicular anisotropy have been induced by irradiation at moderate processing temperatures. Also, we have shown that perpendicular anisotropy in FePd and CoPt 3 alloys can be decreased depending on the temperature of irradiation. These features allow the realization of a new type of magnetic nanostructures. As the structural modifications are localized in the close vicinity of the ion path in matter, planar magnetic patterning at the sub-50-nm scale can be achieved when the irradiation is performed through a mask. By means of 30 keV He + ion irradiation of Co/Pt multilayers, we have produced a nearly optical contrast-free, planar array of magnetically hard lines embedded in a softer matrix. Such irradiation-fabricated nanostructures exhibit specific magnetization reversal processes: low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and non-irradiated areas. The magnetization reversal dynamic is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. Striking images of nucleation and domain wall motion in such arrays are displayed and interpreted within a novel theoretical framework of irradiation through a mask.

Chemical order induced by He+ ion irradiation in FePt (001) films

We show that partial chemical ordering can be obtained at moderate processing temperatures by using postgrowth ion bombardment. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) sputtered FePt(001) films in which the order parameter S was increased up to 0.3 and 0.6, respectively. X-ray diffraction measurements indicate that the high crystalline quality of the films is not modified as irradiation proceeds. The changes on the magnetic hysteresis loops are compatible with the expected perpendicular magnetic anisotropy increase. This novel method could have a great impact on the current race toward high anisotropy materials to increase magnetic recording density.

Structure and magnetic anisotropies in sputtered FePd(1 1 0) thin films

The growth temperature dependence of structure and magnetic anisotropies is studied for epitaxial FePd(1 1 0) alloys grown by sputtering. Partial chemical order is obtained for growth temperatures of 600°C and above. Strong in-plane uniaxial magnetic anisotropy is found for chemically disordered samples grown at 500°C, with a reduction in the anisotropy constants for samples grown at higher temperatures. The obtained anisotropy values can be interpreted as the result of the competition between magnetocrystalline and magnetoelastic anisotropy.

Total-energy and entropy considerations as a probe of chemical order in amorphous silicon carbide

Total energy calculations of various atomic configurations, carried out within the pseudopotential-density-functional (PDF) formalism, show that partial chemical ordering is by far the most favored phase in amorphous silicon–carbon alloys. The random phase, on the other hand, is the least favored configuration. Configurational entropy contributions to the free energy are not able to reverse this picture, yielding an exceedingly high transition temperature.

Ion-beam synthesis of amorphous SiC films: Structural analysis and recrystallization

The analysis of SiC films obtained by carbon ion implantation into amorphous Si (preamorphized by Ge ion implantation) has been performed by infrared and Raman scattering spectroscopies, transmission electron microscopy, Rutherford backscattering, and x-ray photoelectron spectroscopy (XPS). The data obtained show the formation of an amorphous Si1−xCx layer on top of the amorphous Si one by successive Ge and C implantations. The fitting of the XPS spectra indicates the presence of about 70% of Si–C bonds in addition to the Si–Si and C–C ones in the implanted region, with a composition in the range 0.35<x<0.6. This points out the existence of a partial chemical order in the layer, in between the cases of perfect mixing and complete chemical order. Recrystallization of the layers has been achieved by ion-beam induced epitaxial crystallization (IBIEC), which gives rise to a nanocrystalline SiC layer. However, recrystallization is not complete, observing still the presence of Si–Si and C–C bonds in an amorphous phase. Moreover, the distribution of the different bonds in the IBIEC processed samples is similar to that from the as-implanted ones. This suggests that during IBIEC homopolar bonds are not broken, and only regions with dominant Si–C heteropolar bonds recrystallize.

Atomic bonding in amorphous hydrogenated silicon carbide alloys: A statistical thermodynamic approach

The free-energy model (FEM) previously developed for predicting the bonding in amorphous covalent alloys has been extended to include tetrahedra, the fundamental structural units in the a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{\mathit{y}}$${\mathrm{H}}_{\mathit{z}}$ alloys under consideration. It is proven that the tetrahedron probabilities P(i) can be obtained by randomly distributing, according to statistics, the bonds predicted by the FEM among the possible Si- and C-centered tetrahedra. The short-range order present in these alloys therefore corresponds, in general, to partial chemical ordering (CO) with a homogeneous dispersion of the bonds among the available tetrahedra. The nature of the CO predicted for these a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{\mathit{y}}$${\mathrm{H}}_{\mathit{z}}$ alloys is shown to vary with alloy composition. For example, Si-C bonds are favored over Si-Si and C-C bonds in stoichiometric alloys, Si-Si and C-H bonds are favored over Si-C and Si-H bonds in Si-rich alloys, while Si-C and C-H bonds are favored over C-C and Si-H bonds in C-rich alloys. Detailed predictions are presented for the bond fractions, tetrahedron probabilities, and tetrahedral and polymeric volume fractions in a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ alloys and also in a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{\mathit{y}}$${\mathrm{H}}_{\mathit{z}}$ alloys with both high H content and lower H content. In the high-H-content alloys, C is predicted to be present primarily in ${\mathrm{CH}}_{2}$ and ${\mathrm{CH}}_{3}$ units, in good agreement with experiment, and a significant polymeric component is predicted to be present. In the lower-H-content alloys, on the other hand, more Si-C bonds and a smaller polymeric component are predicted. It is therefore demonstrated that the H content plays a dominant role in controlling the optical and electronic properties of these technologically important alloys. The simplest way of improving the usefulness of these alloys is to lower the H content, thereby promoting the random bonding of C and H atoms in the amorphous Si network.

Structure of Unhydrogenated Amorphous Silicon Carbide

AbstractSimulations of unhydrogenated amorphous silicon carbide, formed by condensing a vapor of silicon and carbon atoms, indicate partial chemical ordering, with heteronuclear bonds predominating. We find no evidence of phase separation. Carbon atoms are predominantly three-fold coordinated while silicon atoms have four-fold coordination. By combining these results with results of experimental studies, we speculate on the role of hydrogenation in determining the structure of amorphous silicon carbide.