Search

Magnetic skyrmions are nano-sized topologically protected spin textures with particle-like properties. They can form lattices perpendicular to the magnetic field and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both Cu$_2$OSeO$_3$ and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample which can induce metastable lattices. Kinetic measurements show that these metastable skyrmion lattices may or may not relax to their equilibrium positions under macroscopic relaxation times. Furthermore, multidomain lattices may form when two or more equivalent crystallographic directions are favored by spin-orbit coupling and oriented perpendicular to the magnetic field., Comment: Supplementory movie 4 is not uploaded due to size restrictions but is available upon request

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.

The (100) surface of Ni2MnGa ferromagnetic shape memory alloy exhibits intrinsic surface property dissimilar to the bulk as well as influence of compositional variation at the surface. It is shown that by sputtering at room temperature and annealing at high temperature, it is possible to obtain a clean, ordered and stoichiometric surface. However, for even higher annealing temperatures, the surface becomes Mn rich. The (100) surface of Ni2MnGa is found to have Mn–Ga termination. A surface reconstruction to p4gm symmetry is observed in the austenite phase, while the expected bulk truncated symmetry at surface is p4mm. For the stoichiometric surface, the XPS valence band is compared with our calculations based on first principles density functional theory and good agreement is obtained. The ultraviolet photoelectron spectroscopy (UPS) valence band spectra depend sensitively on composition varying from Ni rich to Mn rich surfaces. A satellite feature observed in both Ni 2p core-level and valence band spectra is related to the narrow 3d valence band in Ni2MnGa.

We have employed X-ray photoelectron spectroscopy to characterise the growth and thermal stability of a Bi thin film on the five-fold icosahedral Al-Pd-Mn surface. The growth can be characterised as Stranski–Krastanov, in agreement with previous STM studies. As a function of annealing temperature, the multilayer desorbs first and thereafter coverages of 1 ML and 0.5 ML are stable for significant temperature ranges.

The spontaneous generation of voltage (SGV) in single crystalline Tb 5 Si 2.2 Ge 1.8 and Gd has been studied. Temperature-induced SGVs were observed along the three principal crystallographic axes of Tb 5 Si 2.2 Ge 1.8 , but not in Gd. Field-induced SGVs were observed with magnetic fields less than 40 kOe applied along the a -axis of Tb 5 Si 2.2 Ge 1.8 , and the c -axis of Gd. The absence of the temperature-induced SGV in Gd indicates the key role first-order phase transformations play in the appearance of the effect when temperature varies. The anisotropy of the magnetic field-induced SGV in Tb 5 Si 2.2 Ge 1.8 and the existence of the field-induced SGV in Gd, highlight the importance of the magnetocaloric effect in bringing about the SGV.

The (1 0 0) surface of Ni2MnGa and Mn2NiGa ferromagnetic shape memory alloys have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). It is shown that by sputtering and annealing, it is possible to obtain a clean, ordered and stoichiometric surface that shows a four-fold 1 × 1 LEED pattern at room temperature. For both Ni2MnGa and Mn2NiGa, the surface becomes Ni-rich and Mn deficient after sputtering. However, as the annealing temperature is increased Mn segregates to the surface and at sufficiently high annealing temperature the Mn deficiency caused by sputtering is compensated. The (1 0 0) surface of Ni2MnGa is found to have Mn–Ga termination. The valence band spectra of both Ni2MnGa and Mn2NiGa exhibits modifications with surface composition. For the stoichiometric surface, the origin of the spectral shape of the valence band is explained by calculations based on first principles density functional theory.

This work describes a rapid method to correct the two-fold astigmatism of transmission electron microscope (TEM) objective lens employing caustic curve when no objective aperture is inserted. The method makes use of rounding the caustic curve via the objective lens stigmators after the condenser lens astigmatism has been corrected. It has many advantages over other methods, it is fast, straightforward, and does not need holes or an amorphous material.

There have been many investigations to identify new alloys exhibiting large magnetic field-induced strain and more recently, the giant magnetocaloric effect. For any particular alloy system, it is necessary to adjust composition to alter the magnetic ordering and structural transformation temperatures to achieve the desired field-induced response. For the Ni–Mn–Sn alloy system, this compositional tuning has dramatic effects on microstructural development significantly affecting both transitions. Solidification induced chemical variations, when not fully equilibrated, have been found to mask the intrinsic magnetostructural behavior in Ni50Mn37Sn13. The use of a phase purity criterion was found to be an insufficient descriptor for determining homogeneity. When both structural and chemical homogenization are achieved, the nature and sequence of the transitions were found to be: (1) austenite paramagnetic to ferromagnetic transition, (2) coupled ferromagnetic austenite to paramagnetic martensite magnetostructural transition, and (3) paramagnetic to ferromagnetic martensite transition with Curie–Weiss behavior on heating above its Curie temperature.

Magnetic susceptibility, heat capacity, thermal expansion, and resistivity of a high-quality single crystal of MnSi were carefully studied at ambient pressure. The calculated change in magnetic entropy in the temperature range 0–30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the behavior of the thermal expansion coefficient in the range 30–150 K, which correlates to a large enhancement of the linear electronic term in the heat capacity. A surprising similarity between variation of the heat capacity, the thermal expansion coefficient, and the temperature derivative of resistivity through the phase transition in MnSi is observed. Specific forms of the heat capacity, thermal expansion coefficient, and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as a combination of sharp first-order features and broad peaks or shallow valleys of yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state in MnSi slightly above the transition temperature. Present experimental findings bring the current views on the phase diagram of MnSi into question.

We have studied the lattice dynamics of a Ni 54 Mn 23 Al 23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA 2 phonon branch at the wave number ξ 0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant ( C ′ ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

The self diffusion of Mn and Pd in a single grain icosahedral Al 69.9 Pd 20.5 Mn 96 quasicrystal has been determined by low energy ion scattering (LEIS). The diffusion was determined by depositing different elements (Pd, Mn) on the surface and measuring the rate of change in surface composition as a function of temperature by LEIS. The surface composition was monitored over the temperature range of 355-575 K for Mn and 440-745 K for Pd and compared to model calculations to allow the activation energy for diffusion to be determined. Activation energies of 0.20 ± 0.01 eV for Mn and 0.64 ± 0.03 eV for Pd have then been measured for self diffusion in i-Al-Pd-Mn, respectively. No deviation from Arrhenius behavior was detected in the temperature range covered by the present experiments. From the low values of activation energy we propose that this range of diffusion is phason related, reflecting the specific nature of the icosahedral structure.

The room temperature adsorption behaviour of Fe on the five-fold surface of i-Al–Pd–Mn has been studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). A complex growth scenario for Fe adsorption on this substrate is observed with STM. At coverages up to about 3 MLE (monolayer equivalent), layer-by-layer growth is observed whereby small clusters and islands are formed which eventually coalesce into almost complete monolayers. No LEED pattern is observed, indicating that the layers are disordered. The AES results rule out intermixing. Above this coverage, there is a transition to a multilayer island growth mode. The islands are rotated by 72° and have the bcc(1 1 0) Fe structure. The results are compared with previous work on Fe adsorption on this substrate and on Al and Fe single crystal substrates.

A combination of characterization methods (SEM, X-ray diffraction, DTA, neutron diffraction and phase diagram analyses) has been used to determine the actual composition of a complex intermetallic compound of nominal composition Pr 6 Ni 2 Si 3 where neither X-ray nor neutron diffraction alone are capable of differentiating between vacancies or Si substitution on one of the Ni sites due to the scattering factor of two Si substitutions being equivalent to one Ni atom plus a vacancy. Using a combination of quantitative phase analysis of the diffraction data based on Reitveld refinement, metallography and mass balance calculations, the correct stoichiometry was found to necessarily contain excess Si, and is close to the stoichiometry, Pr 6 Ni 1.6 Si 3.2. Nonetheless this composition still contains around 1% second phase following long term anneal. Due to the close proximity of neighboring phases, this corresponds to an error in stoichiometry of approximately 0.02%. Assuming similar occupancies of Si and Ni on the Ni sites for other members of the homologous series R ( n +2)( n +1) Ni n ( n −1)+2 Si n ( n +1) , compositions of Pr 5 Ni 1.85 Si 3.1 and Pr 15 Ni 6.7 Si 10.2 were predicted for n = 3 and n = 4 alloys, respectively, and verified experimentally. The approach present in this paper is applicable to many other systems.

Nucleation of Ag islands on the five-fold surface of icosahedral Al–Pd–Mn is influenced strongly by trap sites. Submonolayers of Ag prepared by deposition at 365 K and with a flux of 1 × 10−3 monolayers/s exhibit a variation in Ag island densities across different terraces. Comparisons with previous work and with rate equation analysis indicate that trap sites are not saturated under these experimental conditions and that the difference in island densities is not necessarily due to variation in trap densities. While it could have a number of different origins, our results point to a terrace-dependent value of the effective diffusion barrier for Ag adatoms.

Thermo-mechanical and magneto-mechanical properties were studied for iron substituted with 17.5, 19.1 and 20.5 at.% gallium quenched from 1000 °C and subsequently heated to 400 °C and slow-cooled. A phase change was observed in the 19.1 and 20.5 at.% Ga quenched samples upon heating. X-ray diffraction studies on the single crystal samples showed that the phase change observed was due to the precipitation of D0 3 upon heating from the quenched state. Magneto-mechanical characterization showed that the presence of ordered D0 3 phase in the slow-cooled 19.1 at.% Ga sample reduced the saturation magnetostriction in comparison to the quenched sample of same composition. The magneto-mechanical properties of both the quenched and slow-cooled 17.5 at.% Ga samples were similar owing to the presence of only the disordered A2 phase in both these samples.

The deposition of Si at room temperature on the ten-fold quasicrystalline surface of d -Al–Ni–Co has been investigated by scanning tunnelling microscopy. At a coverage of 0.30 ML, Si pentagons in two orientations related by inversion symmetry are observed on the same substrate terrace. The side-length of the pentagons is 4.2 ± 0.2 A. At this coverage, the Si adlayer displays quasiperiodic order. Depressions related to pentagonal features observed in STM images of the clean d -Al–Ni–Co substrate are proposed as plausible adsorption sites for the Si adatoms. As the Si coverage is increased, the well-defined structures observed are no longer distinguishable. At coverages above the monolayer, the Si overlayer follows a rough three-dimensional growth mode.

The existence of a magnetic moment at the nickel sites in ferromagnetic shape-memory alloys (FSMA) has long been speculated. Using a state-of-the-art X-ray magnetic circular dichroism (XMCD) technique we found that Ni 3d moments indeed exist in Ni–Mn–Ga-based FSMA. Band-structure calculations were used to simulate the XMCD spectra and estimate this moment to be ∼ 0.33 μ B . Such a Ni moment plays a critical role in the dispersions of non-collinear spin structures, which determine the finite-temperature magnetic properties.

The room temperature deposition of 7 ML of Au onto the fivefold symmetric surface of icosahedral Al–Pd–Mn leads to the formation of a several monolayers thick Au–Al alloy film. An AlAu film with 1:1 stoichiometry is formed, which shows no evidence of ordered structure, being either amorphous or polycrystalline. Annealing to 325 °C causes more Al to diffuse into the film, producing Al2Au but still with no indication of structure. Experiments using 0.5 ML of pre-deposited In demonstrated a surfactant effect as the In 'floated' on the surface during growth and produced a reduction in film roughness. However, contrary to previous findings the film was still either amorphous or polycrystalline, with no evidence of quasi-crystalline or aperiodic structure. Experiments were also conducted using smaller doses of Au to look for the formation of an epitaxial layer and, if formed, determine the registry with the substrate. However, no change in the Pd blocking curves for the surface could be seen, suggesting that the Au does not adsorb in well defined sites. This result is not surprising when considering that even for these low doses Al is drawn into the film, changing the composition and probably the structure of the topmost layers of the substrate, so that the potential adsorption sites on the clean surface may no longer exist.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to examine the bulk microstructure of Gd5Si2Ge2 and Gd5Ge4 compounds, and specifically the Gd5(Ge,Si)3 thin plates seen in these systems. The orientation relationship between the matrix and the precipitate thin plates was determined as [ 1 ¯ 0 1 0 ] ( 1 2 ¯ 1 1 ) p / / [ 0 1 0 ] ( 1 0 2 ¯ ) m . High-resolution TEM images of the Gd5Ge4 were used to study the crystallographic relationship between the parent and matrix that exists along the interface boundary. The observed planar alignment and microscopic structure of the interface, which consists of ledges and terraces, indicate the presence of invariant line strain. The results are also consistent with the Δg approach suggested by Zhang and Purdy. A displacive–diffusional mechanism is proposed to explain the rapid formation of the precipitate Gd5(Ge,Si)3 plates.

Fe–Ga alloys have exhibited significant increases in magnetostriction as a function of Ga content, reaching a maximum near the solubility limit of Ga in BCC α-Fe (A2). At this limit, the magnetostriction is dependent on the thermal history of the sample and the formation of long-range chemically ordered D0 3 phase. However, due to nearly identical unit cells of A2 and D0 3 phases and similar atomic scattering factors of Fe and Ga, detection and quantification of phase mixtures in Fe–Ga alloys is problematic. This work demonstrates the use of high resolution powder X-ray diffraction in detecting the presence of two-phase mixtures in Fe–Ga alloys. Peak splitting of the primary reflections is resolved, irrespective of whether or not superlattice reflections from the ordered structure are detectable. This peak splitting is clearly discernible from Kα 1 –Kα 2 splitting at moderate and high 2 θ values and texturing of the samples does not interfere or mask the peak splitting. Quantitative analysis showed Fe-19.5 at.% Ga was 100% A2 when quenched from high temperatures, but was a two-phase mixture, 67% A2 and 33% D0 3 by volume, when slow cooled. Conversely, Fe-22 at.% Ga alloy was a two-phase mix, 40% A2 and 60% D0 3 when quenched and 100% D0 3 when slow cooled.

The Ni-Pr phase diagram on the Pr-rich side was revised using differential thermal analysis, differential scanning calorimetry, and scanning electron microscopy. The existence of four stoichiometric compounds, Ni2Pr, NiPr, Ni3Pr7, and NiPr3, was confirmed; however, the melting temperatures of 766 °C (NiPr), 566 °C (Ni3Pr7), and 562 °C (NiPr3) are considerably different from the previously reported values. The same was true for the four eutectic reactions, which were determined to be as follows: liquid (L) (45±1 at.% Pr) ↔ Ni2Pr+NiPr at a mean temperature of 732±5 °C; L (65±1 at.% Pr) ↔ NiPr+Ni3Pr7 at a mean temperature of 550±2 °C; L (72±2 at.% Pr) ↔ Ni3Pr7+NiPr3 at a mean temperature of 557±2 °C; and L (77±1 at.% Pr) ↔ NiPr3+Pr.

R5(SixGe1−x)4 alloys, where R is Tb, Dy, and Er, have been examined. Their microstructures consisted of large grains with thin plates of a second phase. Energy dispersive spectrometry of the plates revealed a rare earth rich composition. It is proposed that the plates are R5(SixGe1−x)3 compounds, similar to results seen in Gd5(SixGe1−x)4.

The five-fold surface of an Al70Pd21Mn9 quasicrystal has been studied under a variety of surface preparation conditions using medium energy ion scattering. Argon ion sputtering of the surface was seen to give rise to Mn depletion and the formation of a thick five-domain AlPd(1 1 0) layer with 〈1 0 0〉 azimuths oriented close to, but not exactly on the five-fold symmetric directions of the substrate. Annealing to just 450 °C was sufficient to restore the Mn content and the icosahedral structure in the surface and near-surface region. Annealing to increasingly higher temperatures was seen to cause Mn enrichment in the surface region above the ideal composition for icosahedral structure and this was accompanied by increased amounts of disorder in the sample. For the highest temperature anneals the surface showed low apparent roughness and exhibited blocking patterns consistent with a contraction of the outermost layer of atoms in line with previous studies of this material.

Single crystals of Gd 5 (Si x Ge 1− x ) 4 have been prepared by the Bridgman method using tungsten crucibles. The tungsten crucible was found to be chemically inert with respect to Gd, Si, Ge but did have limited solubility in liquid Gd 5 Si 2 Ge 2 , resulting in the precipitation of pure tungsten throughout the bulk crystal. Overall, the bulk crystal solidified in the monoclinic phase and a slight increase in Si content and decrease in Ge content at the growth proceeded was found. This increase in Si:Ge ratio resulted in a slight increase in lattice parameter of the monoclinic phase and an increase in the magnetostructural transformation temperature of Δ T = 10 K. AC susceptibility measurements indicated a small fraction of orthorhombic phase was present throughout the ingot which accounts for the decrease in MCE values along the length of the ingot.

We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface.

The growth and properties of a Pd thin film deposited on a structurally complex surface are studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission spectroscopy. The substrate is the pseudo-10-fold surface of the ξ ′ -Al 77.5 Pd 19 Mn 3.5 crystal, an approximant of the AlPdMn quasicrystalline phase. Spectroscopic data are consistent with a low density of electronic states around the Fermi level for the clean surface, in agreement with the poorly conducting nature of this intermetallic alloy. Deposited Pd atoms readily form small metal particles with relatively homogeneous size on the substrate, for coverages ranging from the submonolayer regime up to 7 ML. The Pd particles do not coalesce with time and are relatively stable upon annealing up to 920 K. Their average size increases slightly with film thickness. The local electronic structure of the Pd thin film has been studied by STS and appears to be particle-size dependent, with a transition from non-metal to metal, deduced from I ( V ) characteristics with increasing particle size. A shift of the Pd-3d core level is also measured by photoemission, and it can be correlated with a shift of the Pd-d band toward the Fermi level. The growth mode of the Pd film on the ξ ′ approximant surface and its electronic structure are discussed in relation to Pd thin films grown on metal-oxides that are used as model catalysts.

The nucleation and growth of Ag thin films deposited on structurally complex alloy substrates is studied by scanning tunneling microscopy and low-energy electron diffraction. The substrates are high-symmetry surfaces of either icosahedral or decagonal quasicrystals or of related periodic crystals called approximants. At low coverage, completely different film morphologies are observed depending on the surface free energy of the substrate (relative to Ag). At high coverage, the formation of hexagonal nanocrystals is observed in all cases. These have fcc symmetry and pyramid-like multilayer stacking along the 〈1 1 1〉 direction. The constituent hexagonal islands have one, two or five different orientations depending on the rotational symmetry of the substrate on which the growth occurred. The films can be artificially smoothened by O 2 induced coarsening, leading to a patchwork of Ag nanocrystals with a perfect texture, which are separated by grain boundaries.

We report on investigations of the potential for using quasicrystal surfaces as templates for molecular adsorption. The quasicrystalline surfaces of the i -Al–Pd–Mn and the d -Al–Ni–Co quasicrystals have been dosed with various molecules and the surface reactivity characterised by Fourier transform infrared spectroscopy (FTIR). The reflected signal intensities obtained are much weaker than those obtained from other crystals such as metals. Although both quasicrystals are Al-based their surface reactivities are seen to differ. For the Al–Pd–Mn surface, NO and HCOOH both dissociate at the surface and CO does not adsorb. CD 4 O adsorption leads to multilayers as in other crystals. C 6 H 6 molecules stick at low coverages and at low temperatures whilst not affecting the LEED pattern. For the Al–Ni–Co surface, CO molecules stick via activated adsorption to atop sites with vibrational frequencies very similar to CO adsorption on Ni. HCOOH forms multilayers at low temperatures which is reduced to a monolayer upon heating to 173 K; changes in the symmetry of the bonding are observed, which is also seen for HCOOH on Ni surfaces. NO and CD 4 O showed no characteristic spectra, which may be due to dissociation or a very low sticking probability. We summarise these observations and draw some conclusions as to the most favourable route for molecular overlayer formation.

The adsorption of sulphur on the fivefold flat-terraced surface of the icosahedral Al–Pd–Mn quasicrystal ( i -Al–Pd–Mn) has been studied using extended X-ray absorption fine structure (EXAFS), Auger electron spectroscopy and low energy electron diffraction (LEED). Surface EXAFS data collected by Auger electron yield from the S saturated surface give average bond lengths for S–Al and S–Pd of 2.10±0.06 and 2.40±0.06 A respectively. The data indicate that single-site adsorption on the unreconstructed surface may be ruled out; this is consistent with near-edge EXAFS data and the disappearance of the LEED pattern upon adsorption of sub-monolayer amounts of S. The implication of this finding is that adsorbate-induced reconstruction and/or multiple-site adsorption occurs in this system.

We investigate the atomic structure of the fivefold surface of an icosahedral Al-Cu-Fe alloy, using scanning tunneling microscopy (STM) imaging and a special dynamical low energy-electron diffraction (LEED) method. STM indicates that the step heights adopt (primarily) two values in the ratio of tau, but the spatial distribution of these two values does not follow a Fibonacci sequence, thus breaking the ideal bulk-like quasicrystalline layer stacking order perpendicular to the surface. The appearance of screw dislocations in the STM images is another indication of imperfect quasicrystallinity. On the other hand, the LEED analysis, which was successfully applied to Al-Pd-Mn in a previous study, is equally successful for Al-Cu-Fe. Similar structural features are found for both materials, in particular for interlayer relaxations and surface terminations. Although there is no structural periodicity, there are clear atomic planes in the bulk of the quasicrystal, some of which can be grouped in recurring patterns. The surface tends to form between these grouped layers in both alloys. For Al-Cu-Fe, the step heights measured by STM are consistent with the thicknesses of the grouped layers favored in LEED. These results suggest that the fivefold Al-Cu-Fe surface exhibits a quasicrystalline layering structure, but with stacking defects.

We discuss and compare approaches to using Auger data for semi-quantitative determination of surface compositions of Al-based quasicrystals. We have examined two quasicrystalline phases: icosahedral Al–Cu–Fe and Al–Pd–Mn. We find that sensitivity factors obtained from pure elemental standards lead to underestimation of concentrations by as much as a factor of 2.5 for the minor constituents, Fe and Mn, and to overestimation by as much as a factor of 1.2 for the major constituent, Al. Surprisingly, calibration with a crystalline phase close in composition to the quasicrystal does not necessarily lead to more accurate results. Auger analyses of fractured surfaces, which are then compared to bulk analyses, is the best option for sensitivity factor calibration. Even without absolute calibration of sensitivity factors, however, surface analysis is extremely useful for detection of secondary phases, and is superior to common bulk analysis methods.

Room temperature adsorption of C 60 on the flat quasicrystalline surface of Al 70 Pd 21 Mn 9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C 60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (∼0.065 ML) provides evidence of adsorbate local order, with Fibonacci ( τ -scaling) relationships between the C 60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

An X-ray standing waves experiment was performed at the ID32 beam line of the ESRF on an Al–Pd–Mn quasicrystal with the X-ray beam at normal incidence. The X-ray photoemission core levels for each element were recorded to probe the surface. The drain current, i.e. the electron current from ground needed to neutralize the irradiated sample and to compensate the photoemitted electrons was also recorded as a bulk probe that integrates over all atoms of the sample. Within the two-beam approximation applied to quasicrystals, a simple analysis of this effect is proposed. This provides a direct-space illustration of the diffraction process in a quasiperiodic structure.

Flux-growth techniques have been successfully applied to the preparation of several families of quasicrystals and related approximant phases. Large (up to 1 cm 3 ) single-grain samples have been obtained for icosahedral R–Mg–Zn (R=Y, Gd, Tb, Dy, Ho and Er), decagonal Al–Ni–Co, icosahedral Al–Pd–Mn, icosahedral Al–Ga–Pd–Mn, and the ξ′ Al–Pd–Mn approximant phase. The samples are obtained by slowly cooling ternary (or quaternary) melts of a composition that intersects the primary solidification surface of the desired phase in the equilibrium ternary alloy phase diagram (i.e. this is a self-flux technique). The technique clearly reveals the growth habit, and results in remarkably strain-free quasicrystals.

Directional solidification and quenching experiments have been conducted to examine the formation and origin of the faceted porosity commonly observed in icosahedral Al–Pd–Mn alloys. Faceted porosity has been previously observed in both Czochralski grown and Bridgman grown single crystals during single-phase solidification of the icosahedral phase. Detailed observations in Bridgman grown single crystals indicate that the distribution and quantity of pores are non-uniformly distributed along the length of the as-grown ingot. This non-uniform distribution suggests that their formation is influenced by both chemical segregation during solidification as well as convection in the liquid. Controlled solidification of this alloy followed by quenching of the solid–liquid interface provides for a time–temperature history of the porosity formation and morphological development as a function of processing conditions.

Fivefold surfaces of Al–Pd–Mn quasicrystal have been prepared under ultra-high vacuum (UHV) by sputtering and annealing above 700 K and characterized by electron diffraction, Auger spectroscopy and X-ray photoemission spectroscopy (LEED, AES, XPS). X-ray reflectivity at grazing incidence reveals that an in situ temperature annealing at 900 K is necessary to irreversibly produce the signature of a flat and bulk terminated surface. Air-AFM investigations of surfaces prepared below or in the vicinity of this transformation temperature show a large scale structure in the micrometer range with high steps and parallel terraces. Terrace morphology is dominated by clusters with a typical size of about 10 nm. Nanometer steps can coexist with this cluster structure. With UHV-STM, terrace roughness based on cluster assembly is found to extend down to the nanometer scale.

It is shown that low-energy electron diffraction (LEED) patterns of the three high-symmetry surfaces (fivefold, threefold and twofold) of icosahedral Al–Pd–Mn are all compatible with quasicrystallinity, under specific conditions of preparation. This conclusion results from comparing symmetries of experimental surface LEED patterns with bulk X-ray diffraction data which are converted to the conditions of the LEED experiment. This conclusion is also based upon an analysis of relative diffraction spot spacings in LEED. Hence, none of the three surfaces exhibits a massive lateral reconstruction, i.e. massive deviation from quasicrystallinity. The LEED pattern of the fivefold surface is distinct from the LEED pattern of the pseudo-tenfold surface of an orthorhombic approximant. We believe that this rules out the possibility that the fivefold surface of the icosahedral quasicrystal reconstructs to an approximant with tenfold or pseudo-10-fold symmetry. The twofold and threefold surfaces facet more readily, indicating qualitatively that they are less stable than the fivefold surface.

We have observed dynamical diffraction in the $[[\left(0240\overline{2}4\right)$ and $[[\left(0460\overline{4}6\right)$ reflections of the icosahedral quasicrystal AlPdMn in the back-reflection geometry ( ${\ensuremath{\theta}}_{B}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}90\ifmmode^\circ\else\textdegree\fi{}$). The x-ray fluorescence from the Al and Pd atoms exhibits strong standing wave behavior, similar to that observed in crystalline materials. The data indicate a long-range order of each species of atoms, with the coherent positions attributable to distributions of the Al and Pd, which we compare to a centrosymmetric model. We observe deviations from the model which imply small departures from inversion symmetry along the twofold symmetry axis and from the expected coherent fractions for Al.

The mechanical properties and the dislocation microstructure of single crystals with a range of compositions within the (Fe,Ni)–Al pseudobinary system have been investigated, with the purpose of bridging the behavior from FeAl to NiAl. Initial experiments are focused on compression testing of (001) oriented single crystals with a family of compositions of the form Fe x -Ni 60 _ x –Al 40 (where x = 60, 50 and 10 at.%). Measurements of yield strength and work hardening rates have been correlated with overall dislocation morphology, dislocation dissociation width and the presence of APB tubes.

Here, we present an electrical transport study in single crystals of LaFe$_{0.92}$Co$_{0.08}$AsO ($T_c \simeq 9.1$ K) under high magnetic fields. In contrast to most of the previously reported Fe based superconductors, and despite its relatively low $T_c$, LaFe$_{1-x}$Co$_x$AsO shows a superconducting anisotropy which is comparable to those seen for instance in the cuprates or $\gamma_H = H_{c2}^{ab}/H_{c2}^{c} = m_c/m_{ab} \simeq 9$, where $m_c/m_{ab}$ is the effective mass anisotropy. Although, in the present case and as in all Fe based superconductors, $\gamma \rightarrow 1$ as $T \rightarrow 0$. Under the application of an external field, we also observe a remarkable broadening of the superconducting transition particularly for fields applied along the inter-planar direction. Both observations indicate that the low dimensionality of LaFe$_{1-x}$Co$_x$AsO is likely to lead to a more complex vortex phase-diagram when compared to the other Fe arsenides and consequently, to a pronounced dissipation associated with the movement of vortices in a possible vortex liquid phase. When compared to, for instance, F-doped compounds pertaining to same family, we obtain rather small activation energies for the motion of vortices. This suggests that the disorder introduced by doping LaFeAsO with F is more effective in pinning the vortices than alloying it with Co., Comment: 7 figures, 7 pages, Phys. Rev. B (in press)

Pseudomorphic growth of thin elemental metal films is often observed on a variety of crystalline solids. On quasicrystalline surfaces with their complex structure and the absence of translational periodicity, the situation is different since elemental metals do not exhibit quasicrystalline order, and hence the specific interaction between overlayer and substrate is decisive. Here we study the growth of manganese films on an icosahedral i-Al-Pd-Mn alloy with a view to establishing the growth mode and electronic structure. Although we observe an exponential intensity variation of the adlayer and substrate related x-ray photoemission spectroscopy (XPS) peaks, low energy electron diffraction (LEED) shows that Mn adlayers do not exhibit quasicrystallinity. The detailed structure of the Mn 2p core level line reveals considerable electronic structure differences between the quasicrystalline and elemental metal environment. Evidence of a substantial local magnetic moment on the Mn atoms in the overlayer (about 2.8 µ(B)) is obtained from the Mn 3s exchange splitting.

The growth of Bi on a pseudomorphic Bi monolayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal has been investigated using low energy electron diffraction and scanning tunnelling microscopy. Initially randomly oriented pseudocubic islands are formed with a preference for an even number of layers. Subsequently a morphological transformation takes place to hexagonal Bi islands, which align along high symmetry directions of the substrate. The Bi flux is found to have a strong effect on which island structure is preferred. When C(60) is adsorbed on the three different allotropes of Bi present in this system, hexagonal C(60) islands are formed in each case. On the pseudocubic and hexagonal islands, the C(60) islands are aligned with the substrate. We discuss the energetic, kinetic and geometrical factors which influence the morphological transformation referred to above.

Electrical resistivity and magnetic susceptibility measurements of the itinerant ferromagnet ${\mathrm{CoS}}_{2}$ at high pressures reveal that its magnetic ordering temperature is tuned to zero at a critical pressure 4.8 GPa, indicating the existence of a quantum-phase transition. The ambient pressure continuous magnetic-phase transition in ${\mathrm{CoS}}_{2}$ becomes first order on a very slight pressure increase, with a tricritical point probably located almost at ambient pressure. No deviations from Fermi-liquid behavior were found in the vicinity of the quantum-phase transition in ${\mathrm{CoS}}_{2}$, implying its strong first-order nature.

Recent work on the growth of thin metal films on quasicrystalline substrates has indicated the formation of so-called “magic height” islands with multiples of 4 atomic layers (AL) arising as a result of quantum size effects, which lead to enhanced stability. Here the results of a study are reported of Bi deposition on i-Al-Pd-Mn using medium-energy ion scattering to characterize the island thickness and the structural arrangement of Bi atoms within the islands. In addition, data were taken from annealed surfaces after Bi cluster desorption to leave a single aperiodic monolayer of Bi at the surface. Scattered-ion energy spectra from the Bi islands are consistent with a single Bi monolayer covered with mainly 4 AL islands for both 1.8 and 3.2 monolayer equivalent coverages but with some occupation of 2 and 8 Al islands as well. The angular dependence of the scattered-ion intensity (“blocking curve”) from Bi has been compared with simulations for various models of both rhombohedral Bi and a distorted “black-phosphorus”-like structure. The data demonstrate bilayer formation within the Bi islands. In the case of the aperiodic Bi monolayer, the blocking curves from substrate scattering are found to be inconsistent with two high-symmetry sites on the quasicrystalline surface that theory indicates are energetically favorable but do not exclude the formation of pentagonal arrangements of Bi atoms as seen in other recent experimental work.

A survey of a number of different techniques including electron beam float zone, inductively coupled plasma pedestal growth, and induction float zone in atmospheric and pressurized chambers was conducted for the synthesis of MoSi 2 single crystals. These techniques were evaluated as to their suitability for growth of MoSi 2 . Of concern is the high vapor pressure of silicon at the melting point which can result in silicon loss and the formation of Mo 5 Si 3 . Of the techniques surveyed, pressurized induction float zone offers the best potential for compositional control during crystal growth. Some success is demonstrated using induction float zone techniques but, unfortunately, in order to maintain a stable liquid zone, crystal growth rates 3–5 times faster than typical float zone growth rates are necessary. As a result, the size of crystals prepared thus far is limited to approximately 3 mm in diameter.

Several surface phases have previously been identified on the five-fold surface of the Al70Pd21Mn9 quasicrystal. Two of these, the clustered and terraced phases, have been discovered by scanning tunnelling microscopy (STM) studies. In this paper, we report STM images showing the existence of a new phase on this surface. This new topography was obtained by annealing the sample close to its melting point for several minutes. A model is presented which exhibits the main features of the structure. The unit-cell dimensions match those of the Al3Mn orthorhombic phase. Mechanical polishing followed by sputtering and annealing restored a pentagonal quasicrystalline surface indicating that the decomposition is a near-surface phenomenon.

Using electron diffraction, we show that free-electron metals, such as sodium and potassium, form a highly regular quasiperiodic monolayer on the fivefold surface of icosahedral Al-Pd-Mn and that the quasiperiodicity propagates up to the second layer in sodium. Our photoelectron spectroscopy results show that the quasicrystalline alkali-metal adlayer does not exhibit a pseudogap near the Fermi level thought to be characteristic for the electronic structure of quasicrystalline materials. Calculations based on density functional theory provide a model structure for the quasicrystalline alkali-metal monolayer and confirm the absence of a pseudogap.

International audience; The nucleation of Pb clusters on the five-fold Al-Pd-Mn quasicrystal surface has been investigated using scanning tunneling microscopy STM and ab initio calculations based on density-functional theory DFT. In the submonolayer regime, Pb atoms are highly mobile and adsorb preferentially within equatorially truncated pseudo-Mackay clusters present at the surface. The decoration of these unique adsorption sites leads to the formation of five-fold islands dubbed " starfish " and eventually to a quasiperiodic Pb monolayer. From a comparison of measured and calculated STM images it is concluded that most starfish clusters on all terraces are composed of ten Pb adatoms. A model of the structure of the starfish cluster has been proposed. Our total-energy calculations confirm its stability. The experimentally measured height profile of the starfish cluster is also reproduced by the DFT calculations.

International audience; Quasicrystals are long range ordered materials which lack translational invariance so that the study of their physical properties remains a challenging problem. In order to study the respective influence of the local order and of the long range order (periodic or quasiperiodic) on lattice dynamics, we have carried out inelastic x-ray and neutron scattering experiments on single grain samples of the Zn-Mg-Sc icosahedral quasicrystal and of the Zn-Sc periodic cubic 1/1 approximant. Besides the overall similarities and the existence of a pseudo gap in the transverse dispersion relation, marked differences are observed, the pseudo gap being larger and better defined in the approximant than in the quasicrystal. This can be qualitatively explained using the concept of pseudo Brillouin zone in the quasicrystal. These results are compared to simulations on atomic models and using oscillating pair potentials which have been fitted against ab-initio data. The simulated response function reproduces both the dispersion relation but also the observed intensity distribution in the measured spectra. The partial vibrational density of states, projected on the cluster shells, is computed from this model.