In this work, structural and magnetic properties RE Cu4Al8 (RE = Tb, Dy, Ho and Er) family of compounds are reported. Measurements of X-ray diffraction, temperature (T) and magnetic field (H) dependencies of magnetization are presented for each compound. The analysis of X-ray diffraction patterns shows a small variation in a (8.705824-8.734991 angstrom) and c (5.123802-5.135258 angstrom) lattice parameters as a function of the rare earth. The T-dependent magnetic susceptibility reveals the Curie-Weiss-like behavior at high-T evolving to antiferromagnetic transitions at low-T (6 < T-N < 28 K) for all studied materials. Measurements of magnetization as a function of magnetic field show field-induced transitions for RE = Dy, Ho and Er, which present larger magnetic moments, though no magnetic saturation is attained (4.4 < M-max < 7.3 mu B) for none of the studied compounds. This fact is an indicative of strong crystalline electrical field (CEF) effects. We have conducted a detailed analysis, based on a mean-field spin model, in order to determine the CEF parameters from the macroscopic data of the studied powdered samples. Finally, to show the reliability of our calculations, the obtained CEF energy levels and wave functions are compared with previous CEF studies in single crystalline samples of ErCu4Al8, including results obtained from inelastic neutron scattering data.

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. La1.5Sr0.5CoMnO6 stands out as presenting the largest ZEB reported so far, while for La1.5Ca0.5CoMnO6 the exchange bias field (HEB) is one order of magnitude smaller. Here we show that La1.5Ba0.5CoMnO6 also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate H-EB value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound La2CoMnO6 was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co2+/Co3+ and Mn4+/Mn3+ are already present at the La2CoMnO6 parent compound, and that Ba2+/Ca2+/Sr2+ partial substitution at the La3+ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the L-edge XAS indicate the presence of oxygen vacancies in all samples (0.05 <= delta <= 0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

In this work, the effect of Cu-doping on the structural and magnetic properties of the ErNi3Al9 compound is reported. X-ray diffraction (XRD) and magnetization experiments as a function of temperature and applied magnetic field have been performed on Er(Ni1-xCux)(3+y)Al9-y(x = 0.0, 0.1, 0.8, 0.9, 1.0 and y = 0 or 1) intermetallic compounds. XRD analysis shows that the substitution of Ni by Cu in ErNi3Al9 changes the crystal symmetry from trigonal R32 to tetragonal I4/mmm, even though a clear transition region cannot be established due to the presence of secondary metal phases for intermediate Ni-Cu concentrations. The T-dependence of the magnetic susceptibility shows that the transition temperatures across the series are unaffected by the doping. Moreover, the anisotropic magnetic responses of the MvsT curves measured in the ab-plane and along the c-axis for the series end members may be indicative of strong crystalline electric field (CEF) effects. In fact, we suggest a crystal electric field (CEF)-driven effect due to the different point symmetries for those members. By applying an anisotropic spin model we are able to extract the crystal field parameters, reproduce the macroscopic data and compare these to the previously reported experimental values for ErCu4Al8. Finally, our set of data show clearly that the phase transition is more evident in the X-ray and crystal field effects than in the T-dependent magnetization. (C) 2019 Elsevier B.V. All rights reserved.

While there has been a keen interest in studying computation at the edge of chaos for dynamical systems undergoing a phase transition, this has come under question for cellular automata. We show that for continuously deformed cellular automata, there is an enhancement of computation capabilities as the system moves towards cellular automata with chaotic spatiotemporal behavior. The computation capabilities are followed by looking into the Shannon entropy rate and the excess entropy, which allow identifying the balance between unpredictability and complexity. Enhanced computation power shows an increase of excess entropy, while the system entropy density has a sudden jump to values near one. The analysis is extended to a system of non-linear locally coupled oscillators that have been reported to exhibit spatiotemporal diagrams similar to cellular automata.

It is shown how to reconstruct the stacking sequence from the pairwise correlation functions between layers in close-packed structures. First, of theoretical interest, the analytical formulation and solution of the problem are presented when the exact pairwise correlation counts are known. In the second part, the practical problem is approached. A simulated annealing procedure is developed to solve the problem using as initial guess approximate solutions from previous treatments. The robustness of the procedure is tested with synthetic data, followed by an experimental example. The developed approach performs robustly over different synthetic and experimental data, comparing favorably with the reported methods.

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq \delta \leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite BaTi1/2Mn1/2O3, Mn4+ cations, with S = 3/2 spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature (T), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-T magnetic properties of the remaining magnetic degrees of freedom of BaTi1/2Mn1/2O3 are investigated. Heat-capacity data and magnetic susceptibility display no evidence for a phase transition to a magnetically ordered phase but indicate the formation of a correlated spin state. The low-temperature spin dynamics of this state is then explored by mu SR experiments. The zero-field mu(+) relaxation rate data show no static magnetism down to T = 19 mK and longitudinal field experiments support as well that dynamic magnetism persists at low T. Our results are interpreted in terms of a spin-glass state which stems from a disordered lattice of orphans spins and trimers. A spin liquid state in BaTi1/2Mn1/2O3, however, is not excluded and is also discussed.

We report measurements of the temperature dependent specific heat and magnetic susceptibility in single crystals of the series of intermetallic compounds Tb1-xYxRhIn5 (nominal concentrations x = 0.0, 0.15, 0.3, 0.4, 0.5, 0.7). A mean field approximation to simulate the macroscopic properties along the series has been used. Neutron diffraction data in powdered samples of Tb(0.5)Y(0.4)Rhln(5) and Tb0.6La0.4RhIn5 reveal antiferromagnetic (AFM) propagation vector (k) over right arrow =[1/2 0 1/2] with the magnetic moments oriented along the tetragonal c axis or canted from this axis, respectively for Y and La-substitutions. Considering both the simulations of the magnetic exchange and neutron diffraction data, we discuss the role of combined effects of crystalline electric field (CEF) perturbations and dilution in the evolution of magnetic properties with Y and La contents. In particular, we found negligible variations of the B-n(m) parameters along the Y series. The decrease of TN with x is fully dominated by magnetic dilution effects.

In this paper, we report temperature-dependent magnetic susceptibility, electrical resistivity, and heat-capacity experiments in the family of intermetallic compounds RNi3Ga9 (R = Tb, Dy, Ho, and Er). Single-crystalline samples were grown using Ga self-flux method. These materials crystallize in a trigonal ErNi3Al9-type structure with space group R32. They all order antiferromagnetically with T-N < 20 K. The anisotropic magnetic susceptibility presents large values of the ratio chi(easy) / chi(hard) indicating strong crystalline electric-field (CEF) effects. The evolution of the crystal-field scheme for each R was analyzed in detail by using a spin model including anisotropic nearest-neighbor Ruderman-Kittel-Kasuya-Yosida interaction and the trigonal CEF Hamiltonian. Our analysis allows one to understand the distinct direction of the ordered moments along the series-the Tb-, Dy-, and Ho-based compounds have the ordered magnetic moments in the easy ab plane and the Er sample magnetization easy axis is along the <(c)over cap> direction.

We report measurements of the temperature dependence specific heat, magnetic susceptibility in single crystals of the series of intermetallic compounds Tb_1-xY_xRhIn_5 (nominal concentrations x= 0.15, 0.30, 0.40, 0.50 and 0.70). A mean field approximation to simulate the macroscopic properties along the series has been used. Neutron diffraction data in powdered sample of nominal concentration Tb_0.6Y_0.4RhIn_5 reveal AFM propagation vector k=[1/2 0 1/2] with the magnetic moments oriented close to the tetragonal c axis. We discuss the role of combined effects of crystalline electric field (CEF) perturbations and dilution in the magnetic properties evolution with Y content. In particular, we suggest that changes in the Tb-In first neighbors distances, i.e. the TbIn_3 cuboctahedra distortion, are responsible for changes in the Tb crystalline potential and the possible reorientation of Tb magnetic moments for x>0.4. This reflects non negligible variations of the B^m_n crystal field parameters and the energy levels splitting with x.

In this work, experiments of X-ray diffraction, magnetic susceptibility, heat capacitance, and Electron Spin Resonance (ESR) carried out in the Gd1–xYxNi3Ga9 (0 ≤ x ≤ 0.90) compounds grown through a Ga self flux method are reported. The X-ray diffraction data indicate that these compounds crystallize in a trigonal crystal structure with a space group R32. This crystal structure is unaffected by Y-substitution, which produces a monotonic decrease of the lattice parameters. For the x = 0 compound, an antiferromagnetic phase transition is observed at TN = 19.2 K, which is continuously suppressed as a function of the Y-doping and extrapolates to zero at x ≈ 0.85. The ESR data, taken in the temperature range 15 ≤ T ≤ 300 K, show a single Dysonian Gd3+ line with nearly temperature independent g-values. The linewidth follows a Korringa-like behavior as a function of temperature for all samples. The Korringa rates (b = ΔH/ΔT) are Y-concentration-dependent indicating a “bottleneck” regime. For the most diluted sample (x = 0.90), when it is believed that the “bottleneck” effect is minimized, we have calculated the q-dependent effective exchange interactions between Gd3+ local moments and the c-e of 〈Jf−ce2(q)〉1/2 = 18(2) meV and Jf−ce(q=0) = 90(10) meV.

The stacking problem is approached by computational mechanics, using an Ising next-nearest-neighbour model. Computational mechanics allows one to treat the stacking arrangement as an information processing system in the light of a symbol-generating process. A general method for solving the stochastic matrix of the random Gibbs field is presented and then applied to the problem at hand. The corresponding phase diagram is then discussed in terms of the underlying ℇ-machine, or optimal finite-state machine. The occurrence of higher-order polytypes at the borders of the phase diagram is also analysed. The applicability of the model to real systems such as ZnS and cobalt is discussed. The method derived is directly generalizable to any one-dimensional model with finite-range interaction.

This is the second contribution in a series of papers dealing with dynamical models in equilibrium theories of polytypism. A Hamiltonian introduced by Ahmad & Khan [ Phys. Status Solidi B (2000), 218 , 425–430] avoids the unphysical assignment of interaction terms to fictitious entities given by spins in the Hägg coding of the stacking arrangement. In this paper an analysis of polytype generation and disorder in close-packed structures is made for such a Hamiltonian. Results are compared with a previous analysis using the Ising model. Computational mechanics is the framework under which the analysis is performed. The competing effects of disorder and structure, as given by entropy density and excess entropy, respectively, are discussed. It is argued that the Ahmad & Khan model is simpler and predicts a larger set of polytypes than previous treatments.

The correlated biased random walk with latency in one and two dimensions is discussed with regard to the portion of irreducible random movement and structured movement. It is shown how a quantitative analysis can be carried out by using computational mechanics. The stochastic matrix for both dynamics are reported. Latency introduces new states in the finite state machine description of the system in both dimensions, allowing for a full nearest neighbor coordination in the two dimensional case. Complexity analysis is used to characterize the movement, independently of the set of control parameters, making it suitable for the discussion of other random walk models. The complexity map of the system dynamics is reported for the two dimensional case.

Planar faulting in in rare earth - cobalt alloys with type structures Th2Ni17 and Th2Zn17 are studied. The structure changes from hexagonal for heavy rare earth to rhombohedral for light rare earth, all exhibiting different degrees of planar disorder. Faulting is quantitatively studied beyond the model of independent faulting events showing the strength of direct solutions to extract defects information from the diffraction patterns. It is showed that the analysis of the decaying term of the probability correlation function, does not only allow to quantify the loss of memory in the system through the correlation length but also, the strength of interaction between faults. The behavior of the decaying terms turns not to be of universal character when scaled against the correlation length and instead depends on the interaction of faulting. The reconstructive phase transition in this system seems to follow a path where the new phase appears within the disordered phase and grows without homogeneous reordering of the disordered arrangement.

The complete framework for the $\epsilon$-machine construction of the one dimensional Ising model is presented correcting previous mistakes on the subject. The approach follows the known treatment of the Ising model as a Markov random field, where usually the local characteristic are obtained from the stochastic matrix, the problem at hand needs the inverse relation, or how to obtain the stochastic matrix from the local characteristics, which are given via the transfer matrix treatment. The obtained expressions allow to perform complexity-entropy analysis of particular instance of the Ising model. Three examples are discussed: the 1/2-spin nearest neighbor and next nearest neighbor Ising model, and the persistent biased random walk.

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.

The use of Hilbert curves to visualize massive vector of data is revisited following previous authors. The Hilbert curve mapping preserves locality and makes meaningful representation of the data. We call such visualization as Hilbert plots. The combination of a Hilbert plot with its Fourier transform allows to identify patterns in the underlying data sequence. The use of different granularity representation also allows to identify periodic intervals within the data. Data from different sources are presented: periodic, aperiodic, logistic map and 1/2-Ising model. A real data example from the study of heartbeat data is also discussed.

We report a structural-magnetic investigation by x-ray absorption spectroscopy (XAS), neutron diffraction, dc susceptibility (X-dc), and electron spin resonance (ESR) of the 12R-type perovskite BaTi1/2Mn1/2O3. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn4+ and Ti4+ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and X-band (9.4 GHz) electron spin resonance, both in the temperature interval of 2 <= T <= 1000 K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants J(a)/k(B) = 200 (2) K and J(b)/k(B) = 130 (10) K, and orphan spins. Thus, BaTi1/2Mn1/2O3 is proposed as a rare case of an intrinsically disordered S = 3/2 spin gap system with a frustrated ground state.