A study is made of the zero-field splitting (ZFS) of ${}_{}{}^6{}_{}{}^{}$S(3${\mathit{d}}^5$) ions in cubic crystals, based on an extended crystal-field (CF) model which assumes two constants ${\mathrm{\zeta }}_{\mathit{t}\mathit{e}}$ and ${\mathrm{\zeta }}_{\mathit{t}\mathit{t}}$ in the description of the spin-orbit (SO) interaction. In addition to the recognized origin for the ZFS, namely, the combined effect of the CF and the SO couplings, a second source is found to arise from the SO interaction alone through a difference between ${\mathrm{\zeta }}_{\mathit{t}\mathit{e}}$ and ${\mathrm{\zeta }}_{\mathit{t}\mathit{t}}$ caused by covalency. To understand this second effect, we have investigated the SO coupling processes which contribute to the ZFS, using the Macfarlane-Zdansky perturbation procedure. Processes in which the couplings are all between states of different configurations ${\mathit{t}}_2^{\mathit{m}}$${\mathit{e}}^{5\mathrm{-}\mathit{m}}$ are found to make a positive contribution proportional to ${\mathrm{\zeta }}_{\mathit{t}\mathit{e}}^4$. Other processes contribute negatively through a term in ${\mathrm{\zeta }}_{\mathit{t}\mathit{e}}^2$${\mathrm{\zeta }}_{\mathit{t}\mathit{t}}^2$. The ZFS is thus determined by the relative magnitudes of these two parts, and a small difference between ${\mathrm{\zeta }}_{\mathit{t}\mathit{e}}$ and ${\mathrm{\zeta }}_{\mathit{t}\mathit{t}}$ will cause a great change in its value. Application of this new theory is successfully made to ${\mathrm{Mn}}^{2+}$ ions in tetrahedral II-VI compounds and in fluoroperovskites.

Ferromagnetic amorphous ribbons (Metglas 2605 CO, Metglas 2605 SM, Metglas 2826 MB, and Metglas 2605-S3A) were exposed isochronally to pulsed-excimer-laser irradiation (λ=308 nm, τ=26 ns) by employing various repetition rates and pulse energies. The effects of laser treatment were studied by transmission Mössbauer spectroscopy and scanning electron microscopy. Complementary information was obtained using conversion electron Mössbauer spectroscopy and energy-dispersive x-ray analysis. At moderate values of the repetition rate and pulse energy, controlled magnetic anisotropy could be induced in the higher magnetostriction samples without the onset of crystallization. Similarly, a random distribution of magnetic-moment directions was obtained in the lower magnetostriction samples. The laser-induced changes in the magnetic anisotropy were correlated to the presence of molten zones subsequently quenched and of correspondingly induced mechanical stresses. Analysis of the magnetic texture of the irradiated surfaces supported the model of closure domain structure. At higher values of the repetition rate and pulse energy, the nature of the observed phase transformation could be elucidated by a combination of complementary methods. The effect of excimer-laser-induced crystallization in metallic glasses was evidenced and explained considering characteristics of high-fluence laser irradiation. Through a suitable selection of irradiation parameters, the effects of induced anisotropy and partial crystallization can be used following the laser treatment proposed for tailoring the magnetic properties of metallic glasses with regard to specific applications.

Using spin-dynamics techniques we have performed large-scale computer simulations of the dynamic behavior of the L×L×L body-centered-cubic classical Heisenberg ferromagnet with L≤40 in the vicinity of the critical point ${\mathit{T}}_{\mathit{c}}$. The temporal evolutions of spin configurations were determined numerically from coupled equations of motion for individual spins by a fourth-order predictor-corrector method with initial spin configurations generated by Monte Carlo methods. The space- and time-displaced spin-spin correlation functions and their space-time Fourier transforms were calculated to determine the neutron-scattering functions. We developed a dynamic finite-size scaling theory for the neutron-scattering function at ${\mathit{T}}_{\mathit{c}}$ and used it to extract the dynamic critical exponent z. Within our resolution limit, the value of z was estimated to be 2.478(28), in excellent agreement with the dynamic scaling prediction, and dynamic scaling was found to be valid for momentum transfer q up to 0.4π in the (1,0,0) direction.

The electrical resistivity in the system β-ErH(D${)}_{2+\mathit{x}}$ was measured in the range 1.3≤T≤330 K through the whole region of the pure β phase, 0≤x≤0.09. The excess H(D) atoms on octahedral sites, x, adjust for x≳0.05 first in a short-range order (SRO) and then in a long-range order (LRO) of the ${\mathrm{H}}_0$ sublattice in the region 150 to 250 K. The activation energies for H(D${)}_0$ migration, determined after a quench across the ordering interval, are: ${\mathit{E}}_{\mathit{m}}$(SRO)=0.17(1) eV and ${\mathit{E}}_{\mathit{m}}$(LRO)=0.30(1) eV, without a notable isotope effect. ${\mathit{E}}_{\mathit{m}}$(LRO) decreases slightly with increasing x. A magnetic transition is observed at ${\mathit{T}}_{\mathit{N}}$=2.32(2) K for the pure dihydride (x=0); with increasing x, ${\mathit{T}}_{\mathit{N}}$ decreases first due to decreasing carrier density, then grows again after the formation of structural LRO, probably because of perturbed crystal-field symmetry. A magnetic phase diagram is proposed using the present data and earlier susceptibility and neutron-diffraction results. The specimens with the highest x concentrations exhibit metal-semiconductor transitions, both at high temperatures (230–290 K) and at low temperatures (40–120 K). The former are related with the order-disorder transformation in the ${\mathit{H}}_0$ sublattice, the latter attributed to carrier localization via a Mott mechanism.

Using a microscopic and quantum approach, we study the ground-state configuration and the spin-wave excitations of a model of two ferromagnetic monolayers, interacting via both bilinear and biquadratic exchange, and subject to an easy-plane anisotropy, a quartic in-plane anisotropy and an external field. In the framework of a Green’s-function method, we obtain analytical results for the spin-wave dispersion curves. Peculiar features are found in the field dependence of the acoustic and optical frequency gaps, in correspondence to the critical fields for which the canting angle between the magnetizations of the two coupled layers presents discontinuities. In particular, a minimum for the optical gap at the saturation field is predicted, which could be experimentally observed by Brillouin light scattering in real systems.

The capping effect in magnetic double-film systems has been studied theoretically by means of a variational method for magnetic energies in a continuum model. The coercive force of the double-film system is given explicitly in terms of the magnetic constants of the two films. A field ${\mathit{H}}_{\mathrm{cap}}$ is defined regarding the exchange coupling between the films as an effective Zeeman energy, and it is found that ${\mathit{H}}_{\mathrm{cap}}$ enhances the external field in recording processes. Since ${\mathit{H}}_{\mathrm{cap}}$ felt by the magnetization in the recording film is localized near the interface, a domino phenomenon of magnetization in the recording film is expected. It is triggered by accidental nucleations at the interface, as part of the dynamic process of recording. A phase transition between different alignments of magnetization is found when the thickness of the capping film is varied and the mechanism of this phase transition is clarified. The critical point is derived analytically which gives also the minimal thickness of the capping film that shows the capping effect. It is found that the critical behavior of ${\mathit{H}}_{\mathrm{cap}}$ in the vicinity of the minimal thickness is characterized by an exponent β=1. The capping effect at large thickness limit is also discussed.

We present a variational wave function for the ground state of the two-dimensional spin-1/2 Heisenberg antiferromagnet. It is a modification of a Monte Carlo variational wave function proposed by Huse and Elser which gives excellent results for the ground-state energy but breaks the rotational symmetry of the squared staggered magnetization. In their case the x component of the squared staggered magnetization (${\mathit{m}}_{\mathit{x}}^{\mathrm{°}}$${)}^2$ is much larger than (${\mathit{m}}_{\mathit{y}}^{\mathrm{°}}$${)}^2$ and (${\mathit{m}}_{\mathit{z}}^{\mathrm{°}}$${)}^2$, which are the y and z components, respectively. In this paper we show how to restore the symmetry between ${\mathit{m}}_{\mathit{x}}^{\mathrm{°}}$ and ${\mathit{m}}_{\mathit{y}}^{\mathrm{°}}$. However, ${\mathit{m}}_{\mathit{z}}^{\mathrm{°}}$ is still much smaller than ${\mathit{m}}_{\mathit{x}}^{\mathrm{°}}$. We also study the enforcement of the full rotational symmetry in our trial wave function using only two spin interaction terms. Finally, we discuss how errors in Monte Carlo data affect the efficiency of algorithms which determine more systematically the best variational parameters for the ground-state wave function.

The phase diagram of chromium alloys sensitively depends on the presence of an electron reservoir. With the formation of a spin-density wave (SDW), electrons are depleted from the nested electron-hole surfaces and for a finite reservoir the chemical potential will decrease. If the power ρ of the electron reservoir were infinite, then the chemical potential would remain constant and lightly doped CrMn alloys would experience a first-order transition from an incommensurate (I) to a commensurate (C) SDW state with decreasing temperature. With a power less than about 10, the reservoir will flip the phase boundary from one side of the triple point to the other, allowing a commensurate to incommensurate transition with decreasing temperature as observed experimentally. A finite reservoir also suppresses the first-order jumps in the SDW order parameter and wave vector. When ρ≤2, the CI transition is second order for all temperatures. When ρ>2, the transition is second order near the triple point but first order at lower temperatures.

We carry out Monte Carlo simulations of the two-dimensional (2D) classical lattice Coulomb gas of half-integer charges on a square lattice, which is believed to be in the same universality class as the fully frustrated 2D XY model, and find evidence for two transitions. At the lower temperature, we find a Kosterlitz-Thouless-type transition with a larger than universal jump in the dielectric constant ${\mathrm{\epsilon }}^{\mathrm{-}1}$. At the higher temperature, we find a second-order charge lattice melting transition with a scaling behavior different from the pure Ising transition, in contrast to expectations from the symmetry analysis.

The effect of an external magnetic field on a pair of interacting methyl groups in a solid lattice is considered. Because of the consequent appearance of a gauge potential in the total Hamiltonian of the system, its time-reversal invariance is broken, leading to the splitting of otherwise degenerate E levels. Since transitions between these states influence the time evolution of the Zeeman polarization in the rotating frame, calculations are presented from which the spectra for a particular energy-level scheme are obtained in the case of a fictitious crystal and a polycrystalline sample. It is shown that the appearance of spectral features at frequencies other than integer multiples of the rf frequency ${\mathrm{\omega }}_1$ can be taken as evidence of transitions between magnetic-flux-split E states. The polycrystalline spectrum is compared to the experimental spectrum of two solids with qualitative agreement if the effective charge on the protons is 0.15 and 0.10, respectively, of the elementary charge.

We study the effect of a random magnetic field on the behavior of the density of states in the Harper problem. By complete diagonalization of lattices of 100×100 sites we show the manner in which the Landau subbands disappear with increasing disorder. At maximum disorder zero-counting techniques for lattices with a large number of sites, up to several million, show the density of states becoming flat and with a narrow bandwidth compared to the pure model in zero field. This allows detailed comparison with previous analytical calculations of the Lifshitz tails. The self-retracing-path approximation gives a good account of the results, except for the tail.

We propose an approach to the numerical study of quantum spin systems. Our method is based on the fact that one can use any set of states for the path integral as long as it is a complete set. We apply our method to one-dimensional quantum spin systems with next-to-nearest-neighbor interactions. We find significant improvement in the negative-sign problem.

The electronic structure and band magnetism of disordered ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$ and ${\mathrm{Ni}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_{\mathit{x}}$ bulk alloys and of corresponding disordered overlayers on an fcc Cu(001) substrate were studied by means of the first-principles tight-binding linear muffin-tin orbital coherent potential approximation method. For the Fe-Co system, we found that the composition dependence of the averaged and the local Fe magnetic moments is strongly affected by the dimensionality of the system while the local Co moments remain nearly constant. In the bulk bcc alloys, a nonmonotonic behavior of the averaged magnetization occurs due to the transition from weak to strong ferromagnetism. The averaged magnetization of the random overlayers depends linearly on the composition while the local Fe and Co moments are constant since the strong ferromagnetism is stabilized by the substrate throughout the whole composition range. For the Ni-Cu system, a transition from ferromagnetism to paramagnetism is found both in the bulk fcc alloys and in the overylayers. The ferromagnetism of the overlayers is reduced in comparison to the bulk systems of the same composition. The concentration trends of local densities of states, local numbers of electrons, and work functions are discussed.

Conductivity measurements are reported in the organic linear-chain compound (TMTSF${)}_2$${\mathrm{PF}}_6$, in both the metallic and spin-density-wave states. The components of the complex conductivity were established by measurements in the radio-frequency, micro- and millimeter wave, and infrared spectral ranges. At temperatures above the spin-density-wave transition, a Drude-like metallic behavior was found together with a temperature-independent feature at higher frequencies. An observed Drude scattering rate of 3 ${\mathrm{cm}}^{\mathrm{-}1}$ was found, placing the material well into the clean limit. In the spin-density-wave state, the low-field dc resistivity shows an activated behavior similar to a standard semiconductor with a gap value 2Δ/${\mathit{k}}_{\mathit{B}}$≊45 K. The ac response shows a strong frequency dependence, and most importantly, we observe two subgap modes: a very broad one in the radio frequency range, due to internal deformations of the spin density wave, and a narrow mode near 0.1 ${\mathrm{cm}}^{\mathrm{-}1}$, which we interpret as the response of the q=0 phason. Furthermore, as expected for a material in the clean limit, we do not see evidence for a single particle gap in the infrared spectral range. In this paper, we will compare our experimental results with the various models of spin-density-wave dynamics and comment on the current status of the understanding of the dynamical response of spin density waves.

An infinite-range quantum Ising spin glass with cubic anisotropy (K) is studied using the imaginary-time representation with the n-replica approach and the thermofield dynamic method. Mean-field-theory phase diagrams in the temperature-anisotropy plane (T,K) for quantum spins S ranging from 2 to 9/2 are presented. For integer-spin values and large cubic anisotropy (positive or negative, depending on S) a condensation into a nonmagnetic spin state occurs, accompanied by the destruction of the spin-glass order as indicated by the finite critical value ${\mathit{K}}_{\mathit{c}}$(T=0). For half-integer S and sufficiently low temperature the spin-glass phase persists for arbitrary K.

We present the results of a numerical study of the zero-temperature remanence and coercivity for the random anisotropy model (RAM), showing that, contrary to early calculations for this model, the coercive field increases monotonically with increases in the strength D of the random anisotropy relative to the strength J at the exchange field. Local-field adjustments with and without spin flips are considered. Convergence is difficult to obtain for small values of the anisotropy, suggesting that this is the likely source of the nonmonotonic behavior found in earlier studies. For both large and small anisotropy, each spin undergoes about one flip per hysteresis cycle, and about half of the spin flips occur in the vicinity of the coercive field. When only non-spin-flip adjustments are considered, at large anisotropy the coercivity is proportional to the anisotropy. At small anisotropy, the rate of convergence is comparable to that when spin flips are included.

The anisotropic quantum XY spin chain in a modulating magnetic field incommensurate with the periodicity of the chain is studied. Using Jordan-Wigner transformation, the model is mapped to a tight-binding model in fermions. In the absence of symmetry-breaking terms, the fermion model is identical with the well-known Harper equation exhibiting extended and localized phases with the onset of transition being a critical point with fractal spectrum and wave functions. The effect of O(2) symmetry breaking is to fatten the critical point resulting in a critical phase with power-law localization existing in a finite window of size determined by the anisotropy. In this three-phase spectral diagram, the transition from critical to localized phase is accompanied by the magnetic transition to long-range order. Furthermore, the scaling properties of the fractal spectrum vary in the critical phase. This leads to interesting consequences on the low-temperature thermodynamical properties of the system.

Boundary conditions are derived for spin dynamics of spin-polarized quantum gases near nonmagnetic walls. We are interested mostly in boundary-induced line shifts and attenuation of spin waves, and in the possibility of having a macroscopic boundary condition for systems close to a Knudsen ballistic regime. We consider the effects caused by roughness of the wall and by surface adsorption. By a proper coordinate transformation, we reduce the problem of particle collisions with an inhomogeneous nonmagnetic wall to an equivalent problem with a specular homogeneous wall but with stochastic bulk imperfections. As a result, the boundary effects are described by some additional bulklike transverse spin-diffusion coefficient inversely proportional to the angular harmonics of the correlation function of surface inhomogeneities. This leads to an effective macroscopiclike boundary condition for transverse spin dynamics responsible for the boundary effects in spin-wave resonances. The situation changes drastically at low temperatures because of an appearance of an adsorbed boundary layer which renormalizes the molecular field near the wall, and leads to additional effective spin-exchange processes. The experimental implications for helium and hydrogen systems are discussed.

Tunneling measurements have been performed on both sides of the insulator-superconductor transition in quench-condensed granular Pb films. The results indicate that the Pb film consists of fully superconducting grains with essentially bulk values for the superconducting energy gap Δ and transition temperature ${\mathit{T}}_{\mathit{c}}$, even when the film exhibits insulating behavior in transport measurements. The density of states inferred from measurements in perpendicular magnetic field are consistent with magnetic spin-flip broadening.

We examine geometric and finite-size effects in supeconductors, using a purely microscopic method. The case where the coherence length ${\xi }_0$ is short and phenomenological methods cannot be used is emphasized. We focus on the single-particle density of states, the energy gap, and the order parameter, and obtain results as a function of temperature, coherence length, system size, distance to the boundary, and pair-breaking impurity mean free path. We discuss the roles of all of the relevant lengths and the experimental implications of our results, in particular, how data from surface probes can yield information on bulk properties.