E.E. Vogel

Reversal modes in magnetic nanotubes

The magnetic switching of ferromagnetic nanotubes is investigated as a function of their geometry. Two independent methods are used: Numerical simulations and analytical calculations. It is found that for long tubes the reversal of magnetization is achieved by two mechanisms: The propagation of a transverse domain wall or propagation of a vortex domain wall depending on the internal and external radii of the tube.

Relationship between the structure of the ground level and frustration in ±J Ising lattices

Ground states of Ising lattices with ±J exchange interactions (bonds) are highly degenerate. Such degeneracy can be grouped in sets of local ensembles (LEGs) in which states are connected by single spin flips. Here we study LEGs in samples 6×6. Then we go onto the decomposition of the total ground-state degeneracy into partial degeneracies of the LEGs. Each LEG can be generated by flipping spins grouped into clusters, whose sizes are directly related to the degeneracies of the LEGs. We find that all bonds attached to a cluster are frustrated in an itinerant way. Additionally, there is rigid frustration that can be local (associated to some LEGs) or global (associated to all LEGs). The rest of the bonds constitute the diluted (unfrustrated) lattice possessing interesting properties.

The ground state energy of the Edwards–Anderson spin glass model with a parallel tempering Monte Carlo algorithm

We study the efficiency of parallel tempering Monte Carlo technique for calculating true ground states of the Edwards–Anderson spin glass model. Bimodal and Gaussian bond distributions were considered in two- and three-dimensional lattices. By a systematic analysis we find a simple formula to estimate the values of the parameters needed in the algorithm to find the GS with a fixed average probability. We also study the performance of the algorithm for single samples, quantifying the difference between samples where the GS is hard, or easy, to find. The GS energies we obtain are in good agreement with the values found in the literature. Our results show that the performance of the parallel tempering technique is comparable to more powerful heuristics developed to find the ground state of Ising spin glass systems.

Frustration in mixed two-dimensional ±J Ising lattices

Method of the sublattice previously introduced for homogeneous lattices is adapted here to characterize ground state properties of two inhomogeneous lattices: Kagome lattice with coordination 4 and Five-points-star lattice with coordination 5. A representative cell must be chosen in each case in such way that main geometrical and topological properties of the lattice are well represented. Ferromagnetic interactions (in concentration x) and antiferromagnetic interactions (in concentration 1−x) define different possible configurations in the cell. By means of combinatorial and probability analysis weight functions are obtained allowing to calculate properties such as frustration length, energy per bond, and fractional content of unfrustrated interactions for the entire ground manifold. We report analytic expressions as functions of x which are later compared to average results coming from numerically solving 40000 randomly prepared samples in an exact way. The good agreement between numerical and theoretical analysis validates the use of the method for inhomogeneous lattices and helps to interpret results. Values for these topological and physical parameters are also compared to those available for homogeneous two-dimensional lattices, looking for general trends. Roles of coordination number, plaquette shape and topology in the previously mentioned properties are discussed and established.

Ground-state topology of the Edwards-Anderson ±J spin glass model

In the Edwards-Anderson model of spin glasses with a bimodal distribution of bonds, the degeneracy of the ground state allows one to define a structure called backbone, which can be characterized by the rigid lattice (RL), consisting of the bonds that retain their frustration (or lack of it) in all ground states. In this work we have performed a detailed numerical study of the properties of the RL, both in two-dimensional (2D) and three-dimensional (3D) lattices. Whereas in 3D we find strong evidence for percolation in the thermodynamic limit, in 2D our results indicate that the most probable scenario is that the RL does not percolate. On the other hand, both in 2D and 3D we find that frustration is very unevenly distributed. Frustration is much lower in the RL than in its complement. Using equilibrium simulations we observe that this property can be found even above the critical temperature. This leads us to propose that the RL should share many properties of ferromagnetic models, an idea that recently has also been proposed in other contexts. We also suggest a preliminary generalization of the definition of backbone for systems with continuous distributions of bonds, and we argue that the study of this structure could be useful for a better understanding of the low temperature phase of those frustrated models.

Ising model on mixed two-dimensional lattices

We inform results on physical and topological magnitudes related to the ground level of Ising model on mixed two-dimensional lattices of coordination numbers 4 (Kagome lattices) and 5 (five-point star lattices). We consider little clusters of size N, where N represents the total number of spins, subject to periodic boundary conditions. On these systems we randomly distribute ±J nearest-neighbor interactions (+J: antiferromagnetic, −J: ferromagnetic (F)). Concentration x of F interactions is varied in the interval (0,1). Two different methods are used to obtain results reported here. First, a numerical method related to multiple replicas. Second, an analytical method based on probabilistic analysis of flat and curved plaquettes. Both methods are complementary to each other. Initially, this study is restricted to calculate frustration of plaquettes and bonds, energy and bond order parameter at T=0. The results of magnitudes informed here are compared with the similar ones obtained for honeycomb, square and triangular lattices.

Detailed structure of configuration space and its importance on ergodic separation of ±J Ising lattices

A complete and exact characterization of the configuration space of 2-D ±J Ising lattices is performed. A new algorithm is introduced here representing advantages for reaching all states for small samples and doing a non-biased sampling of ground states for larger samples. We report efficient procedures to find all ground states grouped in local ensembles of ground states (LEGs) and also a convenient way of storing and comparing states. Properties of such LEGs differ from some approximate descriptions reported in the literature. The onset of lattice size dependence of properties is discussed. Four different ways of performing ergodic separation are used to calculate order parameters. The most significant way of doing ergodic separation requires previous classification of states in LEGs.

Infrared luminescence and vibronic coupling in ZnTe:Fe2+

Samples of crystalline ZnTe with different concentrations of iron were prepared by the vertical high-pressure Bridgman method. Absorption and emission spectra were recorded at liquid-helium temperature in the region of the5T2(D)⇌5E(D) infrared transitions of substitutional Fe2+(d6) ions. In the range between 2400 and 2520 cm−1 a rich structure is observed showing more lines than predicted by plain crystal-field theory. The explanation of all these lines is found after introducing a vibronic Jahn-Teller term to the Hamiltonian. A linear coupling between the doubly degenerate vibrational mode ɛ to the electronic orbitals of the ionic multiplet5D is added to the electronic and vibrational terms of the Hamiltonian. A diagonalization follows using just one free parameter: the Jahn-Teller energy representing the strength of the coupling. The corresponding value is 3 cm−1, which is identical to the one already reported for CdTe∶ Fe2+. The calculated spectrum is in good agreement with the one determined experimentally. Measurements of the absorption spectra support conclusions drawn earlier about a Jahn-Teller coupling also for the excited multiplet. Finally a prediction of how the far-infrared would look like is also given.

Simulation of hysteresis for ±J triangular lattices

Abstract We report Monte Carlo simulations of low-temperature hysteresis of ±J Ising triangular lattices. A hysteresis loop similar to the one shown by real spin glasses is found. Such loop is divided in six sectors, according to the coordination number. Average hysteresis curves, obtained over large numbers of randomly prepared samples, are discussed for different values of temperature (T): it is found that the hysteresis loop disappears for T⩾2.2J. Characteristics of the zero-temperature curve are explained by probabilistic analysis.

Site order parameters for ±J Ising lattices

Two different site order parameters are considered to characterize the partial spin-glass behavior of ±J square Ising lattices. On the one hand, q proposed by Edwards and Anderson two decades ago; on the other p, a more drastic parameter introduced recently. Site order parameters depend very strongly on the way ergodic separation is performed. Here, we deal with three different ways of doing so, thus allowing to find the most representative parameter with the appropriate ergodic separation, to have a more sensitive method to characterize the spin-glass phase. Implications of clustering in the configuration space on order parameters are also discussed.