A simple coupled-cluster-type approximation is introduced, which generalizes in momentum (k) space a method recently presented in the literature for real space, and tested on the exactly solvable XXZ anisotropic quantum antiferromagnetic spin-1/2 chain. We implement our approximation in k space on the fermionized version of this model, and obtain improved results, with remarkable quantitative and qualitative features.

The magnetic and metal-insulator transitions in metallic lithium are studied using the norm-conserving pseudopotentials with a partial-core correction and the local-spin-density (LSD) approximation. We examine the ferromagnetic phase of bcc lithium in detail as a function of atomic volume and magnetic moment using the fixed-spin-moment method. We find a ferromagnetic transition at ${\mathit{r}}_{\mathit{s}}$=6.45 a.u. and a metal-insulator transition at ${\mathit{r}}_{\mathit{s}}$=6.85 a.u. The critical volumes for the ferromagnetic instability, full-spin polarization, and metal-insulator transition, however, turn out to be very sensitive to the particular choice of the LSD exchange-correlation energy functional.

We present results for the magnetization of a series of Co/Cu magnetic multilayers that were grown epitaxially in the (111) orientation. The Co layer thickness was fixed at 15 Å and the Cu layer thickness varied from 5 to 20 Å. The magnetization was found to increase linearly with magnetic field for fields up to about 10 kOe, which indicates the presence of antiferromagnetic coupling between the Co layers. The strength of the exchange coupling between the layers was found to oscillate as a function of the Cu thickness, with an oscillation period of about 12 Å, although no corresponding oscillation was observed in the magnitude of the magnetoresistance. However, the observed approach to saturation of the magnetization corresponds closely to that of the magnetoresistance.

We present a detailed study of the critical properties of the two-dimensional (2D) XY model with maximal frustration in a square lattice. We use extensive Monte Carlo simulations to study the thermodynamics of the spin and chiral degrees of freedom, concentrating on their correlation functions. The gauge-invariant spin-spin correlation functions are calculated close to the critical point for lattice sizes up to 240×240; the chiral correlation functions are studied on lattices up to 96×96. We find that the critical exponents of the spin-phase transition are ν=0.3069 and η=0.1915, which are to be compared with the unfrustrated XY model exponents ν=1/2 and η=0.25. We also find that the critical exponents of the chiral transition are ${\nu }_{\mathrm{\chi }}$=0.875, 2β=0.1936, 2γ=1.82, and 2γ’=1.025, which are different from the expected 2D Ising critical exponents. The spin-phase transition occurs at ${\mathit{T}}_{\mathit{U}\mathrm{}\left(1\right)\mathrm{}}$=0.446, which is about 7% above the estimated chiral critical temperature ${\mathit{T}}_{\mathit{Z}2}$=0.4206. However, because of the size of the statistical errors, it is difficult to decide with certainty whether the transitions occur at the same or at slightly different temperatures. Finally, the jump in the helicity modulus in the fully frustrated system is found to be about 23% below the unfrustrated universal value. The most important consequence of these results is that the fully frustrated XY model appears to be in a novel universality class. Recent successful comparisons of some of these results with experimental data are also briefly discussed.

The annealed Ising magnet on percolation clusters is studied by means of a mapping into a Potts-Ising model and with the Migdal-Kadanoff renormalization-group method. The phase diagram is determined in the three-dimensional parameter space of the Ising coupling K, the bond-occupation probability p, and the fugacity q, which controls the number of clusters. Three phases are identified: percolating ferromagnetic, percolating paramagnetic, and nonpercolating paramagnetic. For large q the phase diagram includes a multicritical point at the intersection of the Ising critical line and the percolation critical line. In the case of random bond percolation (q=1) the Ising critical line is: p(1-${\mathit{e}}^{\mathrm{-}2\mathit{K}}$)=1-exp(-2${\mathit{L}}_{\mathit{C}}$), where ${\mathit{L}}_{\mathit{C}}$ is the pure-Ising-model critical coupling.

The orbital contribution to the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR shift is rigorously calculated in the first and second stages of graphite intercalation compounds for a magnetic field perpendicular to the layers. The Kobayashi-Tsukada formalism is applied in the independent-subsystem model, which offers an opportunity to allow a complete analytical derivation of the orbital shifts. The calculated contribution is almost independent of the charge transfer for donors and acceptors, but is different for each group. For the second stage the calculation predicts a splitting of the lines, corresponding to different shifts associated with the different crystallographic sites. Both results are in good agreement with experiment, confirming the role of the orbital contribution of the delocalized carriers to the NMR shifts in these anisotropic conductors.

We present a perturbation scheme for evaluating the ground-state characteristics of Néel and collinear phases of the spin-1/2 ${\mathrm{J}}_{1\mathrm{-}}$${\mathit{J}}_2$ model. Using the Dyson-Maleev formalism and performing a canonical transformation to exclude strong interaction between bosons, we convert the Hamiltonian of the ${\mathit{J}}_{1\mathrm{-}}$${\mathit{J}}_2$ model to the Hamiltonian ${\mathit{H}}_{\mathrm{DM}}$=${\mathit{H}}_0$+${\mathit{V}}_{\mathrm{DM}}$, where ${\mathit{H}}_0$ represents a noninteracting gas of quasiparticles and ${\mathit{V}}_{\mathrm{DM}}$ is a normal-ordered quartic operator. The description, based on the zero-order Hamiltonian ${\mathit{H}}_0$, turns out to be equivalent to the description of ordered phases, obtained earlier in the framework of modified spin-wave theory (MSWT). We carry out perturbation-type calculations up to second order in ${\mathit{V}}_{\mathrm{DM}}$ for the ground-state energy and magnetization and obtain small corrections in a wide range of the frustration parameter α (=${\mathit{J}}_{\mathit{z}}$/${\mathit{J}}_1$). It is shown that near the phase boundaries the spin-wave interaction causes an essential melting effect. The corrected value of magnetization of the Néel (collinear) phase goes to zero at α≃0.52 (α≃0.57). Thus, within a second-order approximation a window 0.52<α<0.57 instead of the MSWT overlap between Néel and collinear phases is found.

The relaxation of the low-field thermoremanent magnetization has been measured, as a function of temperature and system age, for a reentrant (${\mathrm{Fe}}_{0.65}$${\mathrm{Ni}}_{0.35}$${)}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ ferromagnet with x=0.118. A dynamic crossover is observed in the vicinity of the reentrant transition from a high-temperature (T≥65 K) regime of equilibrium dynamics, where the relaxation isotherms are compatible with a weak power law, and exhibit no measurable dependence on system age, to a low-temperature (T≤60 K) regime, where the isotherms exhibit an age dependence symptomatic of nonequilibrium dynamics, and are describable analytically by a stretched exponential. Moreover, temperature-cycling experiments clearly confirm the instability of this low-temperature reentrant phase to variations in temperature, and thus support the existence of an overlap length scale, which is regarded by droplet scaling theories as a defining feature of the spin-glass state. The data are compared with the functional forms predicted by various phenomenological models of relaxation dynamics in systems with randomness.

A theory is developed describing the drift of a planar domain wall in a collinear antiferromagnet with equivalent sublattices in a weak external magnetic field oscillating with an arbitrary frequency. The character of the dependence of the drift velocity on the frequency and polarization of the oscillating field is determined for different ratios of the parameters of the uniaxial and planar anisotropy. The possiblity of drift of a stripe domain structure is considered.

A general analysis is presented for angle-dependent photoemission from magnetic and oriented atoms using linearly and circularly polarized x-rays. The anisotropy in the angular distribution in a localized material is due to the polarization of the photon, the polarization of the shell from which the electron is emitted, and the Coulomb and exchange interactions of the hole with the polarized valence electrons in the final state of the photoionized atom. The angular dependence of the dipole excited photoemission gives the same information about the multipole orbital and spin magnetic moments of the atom as excitation with quadrupole or octupole radiation. It is shown that the photoemission spectrum at an arbitrary angle is a linear combination of the fundamental spectra, which contain all the physical information of the atom. Redundancy in the measurements makes it possible to obtain these fundamental spectra in completely different ways, allowing to choose the most advantageous geometry and polarization, e.g., the magnetic circular dichroism can be also be measured with linear polarization. Circular dichroism in the angular dependence, magnetic linear dichroism in the angular dependence, and spin polarization are treated using the same analysis. We explain the strong angle dependence observed in the rare-earth 5p photoemission, in the circular dichroism of the 3d transition-metal 2p core-level photoemission and the rare-earth 4f photoemission.

This paper reports the kinetics of desorption of interstitial H of peculiarly stabilized (washed in water) ${\mathrm{Nd}}_2$${\mathrm{Fe}}_{14}$${\mathrm{BH}}_{\mathit{x}}$, x≤5, hydrides with passivated (amorphous) grain surfaces. These hydrides are stable at room temperature but release H on heating over high temperatures, usually between 450 and 750 K. The rate of desorption of H peaks at a certain temperature ${\mathit{T}}_{\mathit{P}}$, which is found to be a function of heating rate β. The value of ${\mathit{T}}_{\mathit{P}}$ thus regularly increases from 533 to 657 K with raising the β from 3 to 20 K/min, following a linear ln(${\mathit{T}}_{\mathit{P}}$/β) vs ${\mathit{T}}_{\mathit{P}}^{\mathrm{-}1}$ relation. The slope (${\mathit{E}}_{\mathit{a}}$/R, with R the gas constant) of the plot gives the average activation energy ${\mathit{E}}_{\mathit{a}}$=39.2 kJ/mol of the associated desorption process. More curiously, an isothermal heating, at a given temperature ${\mathit{T}}_{\mathrm{iso}}$, exhibits four distinct maxima (${\mathrm{H}}_1$, ${\mathrm{H}}_2$, ${\mathrm{H}}_3$, and ${\mathrm{H}}_4$) of the H desorption rate sequenced over the time intervals ${\mathit{t}}_{\mathit{P}}$. These are attributable to desorption of H from four different interstitial sites accompanied with four separate activation energies. The desorption maximum ${\mathrm{H}}_2$ is the most prominent one and it was possible to study in detail by recording the isothermograms at several ${\mathit{T}}_{\mathrm{iso}}$, in the range 483–543 K. An activation energy of ${\mathit{E}}_{\mathit{a}}$=38.2 kJ/mol is estimated from the slope of the ln${\mathit{t}}_{\mathit{P}}$ vs ${\mathit{T}}_{\mathrm{iso}}^{\mathrm{-}1}$ plot, which is fairly consistent with ${\mathit{E}}_{\mathit{a}}$=39.2 kJ/mol achieved by nonisothermal heating. The close similarity between the two ${\mathit{E}}_{\mathit{a}}$ values suggests that the same desorption process, i.e., attributed to ${\mathrm{H}}_2$, dominates in either measurement.

The quasi-two-dimensional antiferromagnetic Heisenberg model is used to describe the magnetic properties of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. In this model, the in-plane interaction is J and interplane coupling is J’. Studied with nonlinear spin-wave theory the Heisenberg model gives a connection between the ground-state energy and the parameter λ=J’/J, the Néel temperatures, and the softening temperatures for different λ. At λ=0 and λ=1, these results are in satisfactory agreement with the results of other theories of two- and three-dimensional antiferromagnetic Heisenberg models, respectively.

We use the single-cluster Monte Carlo update algorithm to simulate the Ising model on two-dimensional Poissonian random lattices with up to 80 000 sites which are linked together according to the Voronoi-Delaunay prescription. In one set of simulations we use reweighting techniques and finite-size scaling analysis to investigate the critical properties of the model in the very vicinity of the phase transition. In the other set of simulations we study the approach to criticality in the disordered phase, making use of improved estimators for measurements. From both sets of simulations we obtain clear evidence that the critical exponents agree with the exactly known exponents for regular lattices, i.e., that (lattice) universality holds for the two-dimensional Ising model.

Single crystals of the linear chain phase ${\mathrm{Nd}}_2$${\mathrm{BaNiO}}_{5+\mathrm{\delta }}$ [δ=0.005(1)] have been prepared and the magnetism studied by neutron scattering. The Ni-O chain direction is along the short axis a. Long-range antiferromagnetic order sets in below ${\mathit{T}}_{\mathit{N}}$=48 K with Nd and Ni moments parallel to [001]. The ${\mathrm{Nd}}_2$${\mathrm{BaNiO}}_5$ magnetic cell is doubled along a and c relative to the chemical unit cell. The magnitude of the moments at 4.9 K is 2.7(2) and 1.2(1)${\mathrm{\mu }}_{\mathit{B}}$ for Nd and Ni, respectively.

The effects due to the coupling interaction between planes of a layered classical XY model are investigated here. We analyze the renormalization caused by spin waves using an analytical self-consistent theory that includes effects of anharmonic terms and, also, the role played by vortex excitations. In order to study the effects of topological excitations, we calculate the in-plane dynamical correlation function using a phenomenological model of an ideal gas of particles. Our theory is in agreement with some Monte Carlo and Monte Carlo–molecular-dynamics calculations.

We study the phase diagram of the one-dimensional U-V model at quarter filling in the most general case where the on-site and first-neighbor interactions U and V can be both attractive and repulsive. The results have been obtained using exact diagonalization of small clusters and variational techniques, as well as exact results in various limits. We have analyzed four properties of the ground state: (i) whether it is insulating or metallic; (ii) whether it is homogeneous or phase separated; (iii) whether it has a spin gap; (iv) whether it has dominant superconducting fluctuations. With eight phases, the resulting phase diagram is unexpectedly rich. The four phases not found in the weak coupling limit are: (i) an insulating phase when U and V are large enough; (ii) a region of phase separation when V is attractive; (iii) another region of phase separation when V is large enough and U small; (iv) a region with dominant superconducting fluctuations when V is intermediate and U small. The actual nature of this last plase, which has pairs but no spin gap, is not fully clear yet.

The hexagonal Heisenberg model, in which the spins along the c axis interact via a ferromagnetic exchange coupling and those in the c plane via an antiferromagnetic one, undergoes nontrivial changes when an external magnetic field is applied in the c plane. In the classical approximation this model has the same phenomenology as the two-dimensional planar triangular antiferromagnet: it is characterized by infinitely many minimum-energy configurations, thermal fluctuations select one configuration out of the manifold, and an intermediate collinear phase intervenes between the low-field 120° three-sublattice configuration and the high-field asymmetric fan phase. We find that the quantum nature of the model leads to first-order phase transitions between the helix and fan phase and between the fan and saturated phase. This model is suitable to describe ${\mathrm{CsCuCl}}_3$ and agreement between theoretical expectations and experiment is found.

The cluster algorithm in the fully frustrated Ising model on the square lattice is essentially different from the ones used in other systems. Thus its better understanding is particularly important for finding new lines of development. Therefore we investigate it in detail. In our simulations of high statistics more appropriate choices of the probability for freezing bonds and of that for flipping spins are seen to lead to better results. In an analysis of the topological properties we derive a set of rules for possible cluster configurations and give a classification by pairs of winding numbers.

The electronic and magnetic properties of ${\mathrm{LaCo}}_{13}$ are investigated by performing linearized muffin-tin-orbital band-structure calculations for both paramagnetic and ferromagnetic phases. It is found that, for two types of Co atoms in the paramagnetic phase of ${\mathrm{LaCo}}_{13}$, the local density of states of Co(1) atoms at ${\mathit{E}}_{\mathit{F}}$ is smaller than, whereas that of Co(2) atoms is comparable to that of hcp Co metal. Calculated magnetic moments for Co(1) and Co(2) atoms are 1.34${\mathrm{\mu }}_{\mathit{B}}$ and 1.65${\mathrm{\mu }}_{\mathit{B}}$, respectively. The estimated average magnetic moment of Co atoms in the ferromagnetic phase of ${\mathrm{LaCo}}_{13}$ is 1.60${\mathrm{\mu }}_{\mathit{B}}$, in reasonably good agreement with experimental values of 1.56–1.68${\mathrm{\mu }}_{\mathit{B}}$. The electronic structure of unstable ${\mathrm{LaFe}}_{13}$ is also calculated and compared with that of ${\mathrm{LaCo}}_{13}$ in order to discuss the correlation between the structural stability and the electronic structure.

The recursion method is applied to the T=0 dynamics of the S=1/2 XXZ model on a linear chain and a square lattice. By means of new calculational techniques for the analysis of the continued-fraction coefficients pertaining to specific dynamical quantities, we obtain reliable information on the type of ordering in the ground state, on the size of gaps in the dynamically relevant excitation spectrum, on the bandwidths of dominant structures in spectral densities, on the exponents of infrared singularities, and on the detailed shape of spectral-weight distributions. We investigate some characteristic properties of the dynamic structure factors ${\mathit{S}}_{\mathrm{\mu }\mathrm{\mu }}$(q,ω) and the spin autocorrelation functions ${\mathit{S}}^{\mathrm{\mu }\mathrm{\mu }}$(ω)=${\mathit{N}}^{\mathrm{-}1}$ t${sum}_{\mathbf{q}}$${\mathit{S}}_{\mathrm{\mu }\mathrm{\mu }}$(q,ω), specifically their dependence on the uniaxial anisotropy, i.e., on the parameter which controls the type of ordering and the amount of quantum fluctuations in the ground state. We find, for example, that the different degrees of ordering in the planar regime of the one-dimensional and two-dimensional systems (criticality versus antiferromagnetic long-range order) have characteristic signatures in the dynamical properties which are conspicuously displayed in our results.