E. Estevez-Rams

Direct determination of microstructural parameters from the X-ray diffraction profile of a crystal with stacking faults

The practical aspects of a recently deduced formalism for the direct solution of the powder diffraction pattern of a layer crystal are addressed. It is shown that the obtained solution is particularly well suited to combination with a deconvolution procedure based on a series development of the peak profile. The influence of noise and step size on the obtained solution is discussed. Computer simulation is used to assess the robustness of the proposed procedure. The developed procedure is applied to a powder sample of Y2Co17 that is affected by planar disorder.

Stacking and twin faults in close-packed crystal structures: exact description of random faulting statistics for the full range of faulting probabilities

The classical model of independent random single deformation faults and twin faulting in face-centered-cubic and hexagonal close packing is revisited. The model is extended to account for the whole range of faulting probabilities. The faulting process resulting in the final stacking sequences is described by several equivalent computational models. The probability sequence tree is established. Random faulting is described as a finite-state automaton machine. An expression giving the percent of hexagonality from the faulting probabilities is derived. The average sizes of the cubic and hexagonal domains are given as a function of single deformation and twinning fault probabilities. An expression for the probability of finding a given sequence within the complete stacking arrangement is also derived. The probability P(0)(Delta) of finding two layers of the same type Delta layers apart is derived. It is shown that previous generalizations did not account for all terms in the final probability expressions. The different behaviors of the P(0)(Delta) functions are discussed.

Giant magnetic viscosity in SmCo5−xCux alloys

Hysteresis loops in SmCo5−xCux as-cast and annealed magnets (1<x⩽3) were measured at room temperature using a pulsed-field magnetometer and a static vibrating sample magnetometer. At field sweeps between 2 and 17 (GA/m)/s a giant—in this field range nearly constant—magnetic viscosity Sv was obtained. The viscosity is approximately proportional to the Cu concentration and to the coercive field. A heat treatment at 1273 K for three weeks changes the microstructure and enhances strongly the intensity and the time dependence of the coercivity. The decay of the inverse of the coercive field Hc with time follows a simple ln(t) relation between 10−5 and 10 s. Data analysis allows a density of stacking faults N to be extracted for each sample. Sv (and Hc) increases with N which increases with x.

Transmission electron microscopy study of structural disorder in Sm(Co0.5Cu0.5)5 alloy

Abstract Transmission electron microscopy studies have been carried out in heat-treated Sm(Co 0.5 Cu 0.5 ) 5 samples. The occurrence of heavily planar faulted regions have been observed. X-ray microanalysis shows a relative increase of Sm and Cu content in the faulted regions compared to the non-faulted ones. The origin and type of disorder is discussed from the crystallographic point of view and related to the pinning magnetisation mechanism reported for this type of alloy.

Magnetic and structural properties of YCo/sub 5-x/Cu/sub x/

The influence of the addition of Cu in the YCo/sub 5/ matrix, YCo/sub 5-x/Cu/sub x/ with 0/spl les/x/spl les/3, on magnetic and structural properties was investigated. With increasing Cu concentration a decreasing saturation magnetization, anisotropy energy and Curie temperature were obtained. Contrary to the results obtained in Sm(Co,Cu)/sub 5/ alloys neither an enhancement of the coercive force nor a dH/dt dependence of the coercive force was found. The temperature dependencies of both the saturation magnetization and magnetic anisotropy constant K/sub 1/ for YCo/sub 3/Cu/sub 2/ were analyzed using the spin-wave and Yang model, respectively.

The symmetry of HK codes representing close-packed structures and the efficient generation of non-equivalent polytypes of a given length.

The HK representation of close-packed polytypes is studied as a binary code. It is shown that the HK code can be seen as operators forming a group. The neutrality condition is then translated to HK sequences that result in the identity operator. The symmetry of an HK word can be related to the space-group symmetry of the corresponding polytype. All HK code types corresponding to all possible close-packed space groups are reported. From a coding perspective, equivalent HK codes correspond to bracelet equivalent classes. An efficient algorithm with execution time constant per generated object is modified to generate all non-equivalent polytypes of a given length.

On the algebra of binary codes representing close-packed stacking sequences.

A systematic use of binary codes derived from the Hagg symbol are used to study close-packed polytypes. Seitz operators acting over the corresponding binary codes are defined and used. The number of non-equivalent polytypes of a given length are calculated through the use of the Seitz operators. The same procedure is applied to the problem of counting the number of polytypes complying with a given symmetry group. All counting problems are reduced to an eigenvector problem in the binary code space. The symmetry of the binary codes leads to the different space groups to which polytypes can belong.

Planar faulting in rare earth–Co 2:17 alloys

Abstract Planar faulting in R2Co17 intermetallic alloys were studied by high-resolution X-ray diffraction. For the light rare-earth elements, the Th2Zn17 rhombohedral structure is found, while for the heavy rare-earth atoms, the Th2Ni17 structure prevails. For intermediate rare-earth atom, the R2Co17 structure shows planar disorder. The occurrence of planar faulting is associated with the interlayer distance in the structure. A correlation was found between interlayer distance and stacking order. Simple qualitative explanation is given for the energy preference for one particular type of stacking order. Planar disorder does not introduce new nearest-neighbors environments for the rare-earth atoms apart from those already found in the hexagonal and rhombohedral variants, but merely changes the probability of occurrence of each neighborhood. Simple stacking fault models should prove insufficient to explain the observed planar disorder.

Lempel-Ziv complexity analysis of one dimensional cellular automata

Lempel-Ziv complexity measure has been used to estimate the entropy density of a string. It is defined as the number of factors in a production factorization of a string. In this contribution, we show that its use can be extended, by using the normalized information distance, to study the spatiotemporal evolution of random initial configurations under cellular automata rules. In particular, the transfer information from time consecutive configurations is studied, as well as the sensitivity to perturbed initial conditions. The behavior of the cellular automata rules can be grouped in different classes, but no single grouping captures the whole nature of the involved rules. The analysis carried out is particularly appropriate for studying the computational processing capabilities of cellular automata rules.

Phenomenology of coupled nonlinear oscillators

A recently introduced model of coupled nonlinear oscillators in a ring is revisited in terms of its information processing capabilities. The use of Lempel-Ziv based entropic measures allows to study thoroughly the complex patterns appearing in the system for different values of the control parameters. Such behaviors, resembling cellular automata, have been characterized both spatially and temporally. Information distance is used to study the stability of the system to perturbations in the initial conditions and in the control parameters. The latter is not an issue in cellular automata theory, where the rules form a numerable set, contrary to the continuous nature of the parameter space in the system studied in this contribution. The variation in the density of the digits, as a function of time is also studied. Local transitions in the control parameter space are also discussed.