Defect-tuning exchange bias of ferromagnet/antiferromagnet core/shell nanoparticles by numerical study

The influence of non-magnetic defects on the exchange bias (EB) of ferromagnet?(FM)/antiferromagnet?(AFM) core/shell nanoparticles is studied by Monte Carlo simulations. It is found that the EB can be tuned by defects in different positions. Defects at both the AFM and FM interfaces reduce the EB field while they enhance the coercive field by decreasing the effective interface coupling. However, the EB field and the coercive field show respectively a non-monotonic and a monotonic dependence on the defect concentration when the defects are located inside the AFM shell, indicating a similar microscopic mechanism to that proposed in the domain state model. These results suggest a way to optimize the EB effect for applications.     

Surface defects characterization with frequency and force modulation atomic force microscopy using molecular dynamics simulations

This paper is devoted to the characterization of the surface defects using a recently developed AFM technique called frequency and force modulation AFM (FFM–AFM). The simulated system includes a recently developed gold coated AFM probe which interacts with a sample including single-atom vacancy and impurities. In order to examine the behavior of the above system on different transition metals, the molecular dynamics (MD) simulation with Sutton–Chen (SC) inter-atomic potential is used. In this study, an online imaging simulation of the probe and sample is performed, and the effects of the horizontal scan speed, the effective frequency set-point, the cantilever stiffness, the tip-sample rest position and the cantilever quality factor on the resulting images are investigated. Using a proposed optimum controlling scheme for the excitation force amplitude, the cantilever horizontal speed can be increased.     

AFM observation of a retgersite crystal surface growing in a water-ethanol solution

In situ investigation of the growth and dissolution of retgersite crystals α-NiSO₄ · 6H₂O in water-ethanol solutions (10–50 wt % of ethanol) was made by atomic force microscopy (AFM). The habit of crystals grown in aqueous and water-ethanol solutions, as well as the Raman scattering spectra, were identical. It is shown that the typical peak of sulphate ions at 981 cm⁻¹ does not change after the addition of ethanol. Absorption spectra of aqueous and water-ethanol solutions also have similar characteristic features. AFM images of hillocks on the (001) face of retgersite crystals were obtained directly in the process of their growth. Using a series of distorted AFM images obtained in situ, a geometrical calculation scheme is given, which allows one to recover the real direction of the growth steps. The kinetic coefficient of growth steps in aqueous and water-ethanol solutions of retgersite is estimated.     

Imaging of surface morphologies of l-arginine trifluoroacetate crystals

The growth mechanism and defect-mediated surface morphologies are investigated by atomic force microscopy (AFM). Both screw-dislocation-controlled growth and 2D nucleation growth operate simultaneously during growth. Hollow core locating at the top of spiral hillock validates that the dislocation has a large Burgers vector. 2D nuclei introduce growth islands and steps with wavy fronts are observed. Impurity-induced step bunching is presumably formed as a result of the imbalance of step speed. The formation of the hollow channels is due to instability of the interface and these may ultimately cause other defects such as liquid inclusions come into being.     

AFM study of the SIMS beam induced roughness in monocrystalline silicon in presence of initial surface or bulk defects of nanometric size

In this paper, the SIMS beam induced roughness (BIR) in monocrystalline Si in presence of initial surface or bulk defects of nanometric size is studied. We follow the development of the BIR by monitoring the increase of Si²⁺ and SiO₂⁺ signals during SIMS sputtering. The topography of the crater bottoms is measured at different steps of the evolution of the roughness using an atomic force microscope (AFM). We show that in presence of nanometric sized defects on the surface or in the bulk, the BIR develops far more rapidly than usual. It appears as soon as the crater reaches the defects and, as reported on Si free from any treatment, the same morphology evidencing waves perpendicular to the sputtering beam develops rapidly. This study of the behaviour of the BIR in presence of voluntarily introduced defects allows us to better understand the basic physical phenomena involved in its apparition.     

Exploring the facets of “soft crystals” using an Atomic Force Microscope

We obtained monocrystalline droplets in a thermotropic cubic phase, of approximate size 100μm, deposited on a flat surface. The facets of these soft crystals are explored using both an optical microscope and an AFM. The height of individual steps on the principal facets and the lateral distance between steps in vicinal facets are measured using AFM in imaging (tapping) mode. Moreover, the elastic modulus is measured locally, using the AFM tip (in contact mode) as a local rheological probe.     

原子力显微扫描云纹法的相移技术研究

提出一种原子力显微镜扫描云纹法的相移新技术 ,运用原子力显微镜的压电扫描头作为相移元件 ,对所得云纹图像可进行 0 - 2 π范围内的四步相移。对该方法的测量原理和实验技术进行了详细的阐述。作为典型实验和应用实例 ,分别对由全息光栅和含热变形的电子封装试件栅形成的云纹进行相移分析。成功的实验结果表明 ,该方法是可行的 ,为微米云纹方法的条纹处理提供了一种新途径。该方法可望在微观变形的云纹法测量中发挥积极作用     

Atom force microscopy study on HTHP as-grown diamond single crystals

For understanding the mechanism of diamond growth at high temperature–high pressure (HTHP) from a metallic catalyst–graphite system, it is of great interest to perform atomic force microscopy (AFM) experiments, which provide a unique technique different from that of normal optical and electronic microscopy studies, to study the topography of HTHP as-grown diamond single crystals. In the present paper, we report first AFM results on diamond single crystals grown from a Fe-Ni-C system at HTHP to reveal the growth mechanism of diamond single crystals at HTHP. AFM images for as-grown diamond samples show dark etch pits on the (111) surface, indicating dislocations. Some fine particles about 100–300 nm in dimension were directly observed on the (100) diamond surface. These particles are believed to have been formed through transition of graphite to diamond under the effect of the catalyst and to have been transported to the growing diamond surface through a metallic thin film by diffusion. The roughness of the (100) diamond surface is found to be about several tens of nanometers through profile analysis. The diamond growth at HTHP, in a sense, could be considered as a process of unification of these fine diamond particles or of carbon-atom-cluster recombination on the growing diamond crystal surface. Successive growth interlayer steps on the (111) diamond surface were systemically examined. The heights of the growth interlayer steps were measured by sectional analysis. It was shown that the heights of the growth interlayer steps are quite different and range from about 10 to 25 nm. The source of the interlayer steps might be dislocations. The diamond-growth mechanism at HTHP could be indicated by the AFM topography of the fine diamond particles and the train-growth interlayer steps on the as-grown diamond surfaces. Received: 29 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Atomic force microscopy studies of twins in yttrium-doped barium titanate

Barium titanate is the main constituent of PTC materials and their electric properties are sensitive to microstructure and defects, in atomic scale, that are significantly affected by processing parameters. The microstructure of barium titanate doped with yttrium was investigated using topographic images obtained by AFM in contact mode. The AFM images of barium titanate doped with yttrium showed the effect of large grains with double twins at different (1 1 1) planes.     

Novel mesoscale defect structure on NiO(1 0 0) surfaces by atomic force microscopy

Cleaved NiO(1 0 0) surfaces were imaged with atomic force microscopy (AFM) to determine defect concentrations and morphology. Random 〈0 1 0〉 and 〈0 0 1〉 oriented steps, which have been previously characterized, were the most common defect observed on the cleaved surface and formed with step heights in multiples of 2.1 Å, the Ni-O nearest-neighbor distance, and terrace widths in the range of 25-100 nm. In addition, the surface showed novel mesoscale (∼0.5-2 μm) square pyramidal defects with the pyramid base oriented along 〈1 0 0〉 symmetry related directions. Upon etching, the pyramidal defects converted to more stable cubic pits, consistent with (1 0 0) symmetry related walls. The square pyramidal pits tended to cluster or to form along step edges, where the weakened structure is more susceptible to surface deformations. Also, a small concentration of square pyramidal pits, oriented with the base of the pyramid along 〈0 1 1〉, was observed on the cleaved NiO surfaces. For comparison purposes, chemical mechanical polished (CMP) NiO(1 0 0) substrates were imaged with AFM. Defect concentrations were of comparable levels to the cleaved surface, but showed a different distribution of defect types. Long-ranged stepped defects were much less common on CMP substrates, and the predominant defects observed were cubic pits with sidewalls steeper than could be accurately measured by the AFM tip. These defects were similar in size and structure to those observed on cleaved NiO(1 0 0) surfaces that had been acid etched, although pit clustering was more pronounced for the CMP surfaces.     

Chemical functionalization of single walled carbon nanotubes

Chemical modification has been performed on purified single walled carbon nanotubes. XPS spectrum shows that the peak corresponding to C (1s) centered at 284.38 eV in pure nanotubes (graphitic C) is 0.4 eV downshifted in chlorinated sample. Subsequent coupling reactions were carried out with diamine molecules to form intertube connections. Tripropylentetramine and phenylendiamine have been chosen as a molecular linker. End-to-side and end-to-end nanotube interconnections are formed and then observed by atomic force microscopy (AFM). Statistical analysis made from AFM images shows around 30% junctions in functionalized and less than 2% in pristine material. Remarkable features can be observed in the Raman spectra at different functionalization steps. Simple conductance measurements on bucky papers prepared from prestine nanotubes and from nanotubes modified at various steps have been made and are discussed.     

Surface morphology of InGaAs on GaAs(100) by chemical beam epitaxy using unprecracked monoethylarsine, triethylgallium and trimethylindium

Temperature-dependent evolution of surface corrugation and the interface dislocation in In_0.15Ga_0.85As epilayer on GaAs(100) substrate grown by chemical beam epitaxy using unprecracked monoethylarsine have been investigated by atomic force microscope (AFM) and transmission electron microscopy (TEM). AFM images showed that the line direction of surface ridge changes from [011] to [0 1] with increasing temperature. However, TEM micrographs showed that dislocation networks are formed along both [011] and [0 1] directions at the interface. These results indicate that growth kinetics on the terrace and at surface steps generated by the dislocations play an important role in determining the direction of surface corrugation. We suggest that the temperature-dependent change of surface corrugation is caused by an anisotropic surface diffusion on the terrace and different sticking probability of adsorbates on the surface steps which were produced by interface misfit dislocation along the two orthogonal surface directions.     

Magnetic defects generated by crystal defects

Linear magnetic defects (LMDs), i.e. linear singularities of the fields of magnetic order parameters in ferromagnets (FMs) and antiferromagnets (AFMs) are investigated. It is shown that disclinations in collinear AFM and in cubic FM generate LMDs. These and similar crystallomagnetic singularities are considered from the point of view of the topological theory of defects in ordered media.     

In situ gas-phase catalytic properties of TiC-supported size-selected gold nanoparticles synthesized by diblock copolymer encapsulation

TiC-supported size- and shape-selected Au nanoparticles with well defined interparticle distances were synthesized by diblock copolymer encapsulation. Atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) have been used to investigate the correlation between the nanocatalyst morphological/electronic structure and its chemical reactivity. Using the low-temperature oxidation of CO as a model reaction, our TPD results showed an enhancement of the catalytic activity with decreasing particle size. Two desorption features were observed and assigned to kinks/steps in the gold surface and the Au-TiC interface. The role of the interparticle distance on the activity is discussed. AFM measurements showed drastic morphological changes (Ostwald ripening) on the nanoparticles after CO oxidation when the initial interparticle distance was small (∼30 nm). However, no sintering was observed for Au nanoparticles more widely spaced (∼80 nm).     

Observation of antiphase domains in Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te films on silicon by the phase contrast method in atomic force microscopy

It has been shown that phase contrast in atomic force microscopy (AFM) can be used to obtain adequate information on the density and distribution of antiphase domains on the surface of CdHgTe films grown by molecular beam epitaxy on a Si(301) substrate. By comparing the AFM phase images of the film surface with TEM images of structural defects in the near-surface region, the relation between microstructure and micromorphology of the films is revealed.     

The magnetoresistive effect induced by stress in spin-valve structure

Using a method of free energy minimization, this paper investigates the magnetization properties of a ferromagnetic (FM) monolayer and an FM/antiferromagnetic (AFM) bilayer under a stress field, respectively. It then investigates the magnetoresistance (MR) of the spin-valve structure, which is built by an FM monolayer and an FM/AFM bilayer, and its dependence upon the applied stress field. The results show that under the stress field, the magnetization properties of the FM monolayer is obviously different from that of the FM/AFM bilayer, since the coupled AFM layer can obviously block the magnetization of the FM layer. This phenomenon makes the MR of the spin-valve structure become obvious. In detail, there are two behaviors for the MR of the spin-valve structure dependence upon the stress field distinguished by the coupling (FM coupling or AFM coupling) between the FM layer and the FM/AFM bilayer. Either behavior of the MR of the spin-valve structure depends on the stress field including its value and orientation. Based on these investigations, a perfect mechanical sensor at the nano-scale is suggested to be devised experimentally.     

Dynamic magnetic behavior of the mixed spin （2, 5/2） Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.     

Processes of DNA condensation induced by multivalent cations： Approximate annealing experiments and molecular dynamics simulations

The condensation of DNA induced by spermine is studied by atomic force microscopy （AFM） and molecular dynamics （MD） simulation in this paper. In our experiments, an equivalent amount of multivalent cations is added to the DNA solutions in different numbers of steps, and we find that the process of DNA condensation strongly depends on the speed of adding cations. That is, the slower the spermine cations are added, the slower the DNA aggregates. The MD and steered molecular dynamics （SMD） simulation results agree well with the experimental results, and the simulation data also show that the more steps of adding multivalent cations there are, the more compact the condensed DNA structure will be. This investigation can help us to control DNA condensation and understand the complicated structures of DNA--cation complexes.     

Regular step formation on concave-shaped surfaces on 6H–SiC(0 0 0 1)

A concave-shaped surface has been prepared in a 6H–SiC(0 0 0 1) substrate by mechanical grinding. As a consequence, the different crystallographic planes building up the 6H–SiC polytype are cut under continuously changing polar angles in all azimuthal directions. Through hydrogen etching, this curved surface breaks up into a whole set of surfaces vicinal to the initial 6H(0 0 0 1) orientation. The local structural reorganisation after hydrogen etching has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Two types of local bond environments are present at the step edges leading to a strong anisotropy in the surface etching with hydrogen. As a result, the distribution of the terrace width and the step heights varies with the azimuthal angle and reflects the sixfold symmetry of the bulk crystal. For most azimuthal directions, an alternation of large and small terraces, separated by steps of 0.75 nm heights (height of half the 6H polytype, three bilayers) is observed and only for well defined azimuthal directions, equally spaced terraces separated by steps of 1.5 nm height (one unit cell of 6H–SiC, six bilayers) are found. In addition, the polar variations have been studied by taking various line-scans along the concave-shaped surface with AFM. It seems that for polar angles above 3°, step bunching of several SiC steps occurs whereas below 3° the bimodal terrace width distribution is observed.     

Intercalation and deintercalation of transition metal dichalcogenides: Nanostructuring of intercalated phases by scanning probe microscopy

Cu was locally deintercalated from intercalated 1T TaSe₂ and 1T TiSe₂ single crystals by applying a voltage pulse between the sample surface and an atomic force microscope (AFM) tip under ambient conditions. Using this method, clusters with diameters of approximately 60 nm were grown. This would allow the use of intercalated transition metal dichalcogenides as mass-storage devices. The intercalated samples were prepared in ultrahigh vacuum (UHV) and characterised by X-ray as well as ultraviolet photoelectron spectroscopy (XPS, UPS). It is shown that the preparation of metal-free surfaces requires prolonged heating steps. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.