We present results of absorption, near-steady-state and long-time photoinduced absorption, and light-induced electron-spin resonance (LESR) studies for the pernigraniline-base polymer (PNB), which has a Peierls ground state with multiple order parameters. Two kinds of defects have been observed and differentiated by photoinduced-absorption spectroscopies in different time domains, namely polarons and solitons. Direct absorption of PNB shows the Peierls energy gap at 2.3 eV and π-${\mathrm{\pi }}^{\mathrm{*}}$ transition energies at 3.8 and 4.3 eV. Pumping at 2.41 eV (into the Peierls gap), the near-steady-state photoinduced absorption spectrum at room temperature shows three induced absorption peaks at 1.0 eV (LE), 1.5 eV (ME), and 3.0 eV (HE); at a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). With pump photon energy of 3.8 eV (into the π-${\mathrm{\pi }}^{\mathrm{*}}$ gap) and at 10 K, features similar to those observed by pumping into the Peierls gap were found, except the intensity of each of the features increased by a factor of 4. The LE features are short lived with a lifetime on the millisecond time scale and have associated short-lived infrared vibrations. The ME peak is very long lived; in addition, there are very long-lived, relatively weak infrared vibrations. The lifetime of the ME peak is measured to be greater than 24 h for temperatures below 200 K. The defect masses for long-lived and short-lived defects are estimated to be ∼300${\mathit{m}}_{\mathit{e}}$ and ∼(5–10)${\mathit{m}}_{\mathit{e}}$, respectively, using the amplitude-mode formalism. Light-induced ESR studies show the long-lived 1.5 eV peak has spin while the short-lived defect has no spin. We suggest that the LE peaks originate from charged soliton-antisoliton pairs (dominated primarily by the bond-length order parameter u), while the ME peak originates from massive ring-torsional polarons (with dominant ring-torsion-angle order parameters δ and ${\mathrm{\Psi }}_0$).

A two-dimensional anharmonic lattice model to describe the behavior of coupled nonlinear displacement modes is constructed. The equations of motion and the underlying Hamiltonian of the anharmonic lattice are found. The equations of motion are analyzed using the fourth-order Runge-Kutta method. The integrability of the system is found to depend on its energy as well as the regularity of the system potential. A continuous transition between regular and chaotic behavior is found and is illustrated using Poincare´ sections. As an example, the effects of ordering on a (100) tungsten surface are discussed in this context.

Previous theories of dynamic response for many magnetic layered structures appear to be in conflict with macroscopic electromagnetic theory. We present a method for constructing dynamic magnetic susceptibilities that resolves these difficulties. Our results are valid for a large class of easy-plane magnetic systems and are particularly interesting for systems with antiferromagnetic coupling. The macroscopic approach is justified by microscopic calculations which use exact dipolar and exchange fields and thus avoid the assumptions of a macroscopic continuum theory.

We report an NMR study of solid ${\mathrm{H}}_2$ formed by the recombination of doubly polarized atomic hydrogen at a temperature of ∼0.3 K. The molecules are formed in a highly excited ortho state with ${\mathit{M}}_{\mathit{I}}$=-1, thermalize through collisions with the ${}_{}{}^4{}_{}{}^{}\mathrm{coated}$ walls of the containment chamber, and form a solid. The solid is essentially pure ortho ${\mathrm{H}}_2$ in the fcc phase, as expected. However, the nuclear polarization is close to zero, in contrast to the value of -1 if there was no relaxation, and +0.034 if the spin temperature was in equilibrium with the surface of the cell. Possible mechanisms for depolarization are discussed, though no plausible explanation for the loss has yet been found.

A epitaxial orientation of Fe/Cr superlattices—Fe/Cr(211) on MgO(110)—is grown by magnetron sputtering. Its structural and magnetic characterizations are presented and compared to those for Fe/Cr(100) superlattices grown simultaneously onto MgO(100) substrates. The epitaxial orientation of the Fe/Cr(211) superlattices is Fe/Cr[01¯1]∥MgO[001] and Fe/Cr[1¯11]∥MgO[11¯0], while that for Fe/Cr(100) is Fe/Cr[001]∥ MgO[011]. A uniaxial, in-plane surface anisotropy for the Fe/Cr(211) superlattices along the Fe[01¯1] of 0.06 erg/${\mathrm{cm}}^2$ is obtained from analysis of the magnetization hysteresis loops. Four oscillations in the antiferromagnetic interlayer coupling and giant magnetoresistance (GMR) are observed with a period of 18 Å for both orientations. The strength, oscillation period, and phase of the magnetic coupling are identical for the two orientations. The GMR values increase by a factor of ∼4 to 5 on cooling from room temperature to 4.2 K. At 4.2 K the maximum GMR value of the [Fe(14 Å)/Cr(8 Å)${]}_{50}$ superlattice is 70% for the (211) orientation and 150% for the (100) orientation.

A Monte Carlo algorithm has been used to study the cubic ferromagnet with a random uniaxial single-site anisotropy on simple cubic lattices, in the strong anisotropy limit. For both two-component and three-component spins the phase diagram contains a ‘‘domain-type’’ ferromagnetic phase for strong random anisotropy, in addition to the simple ferromagnetic phase which is seen for weak random anisotropy. For two-component spins both paramagnet-ferromagnet transitions seem to be second order. For three-component spins the magnetization probably disappears discontinuously, and there appears to be a thin layer of infinite susceptibility phase between the paramagnetic and ferromagnetic phases.

A Monte Carlo study of a quite realistic model of ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Mg}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cl}}_2$ (a diluted anisotropic antiferromagnet in a field) has been performed in the strong concentration limit (x=0.9, 0.8, and 0.7). Simulations of field cooling (FC) and heating (FH) processes as well as isothermal field cycles are presented. A modification to the standard Metropolis algorithm to deal with layered systems has been introduced in the spirit of cluster-flip algorithms. Our results show that dilution does not destroy the tricritical point at least until x=0.7. We also show that the equilibrium temperature loops are reversible and the antiferromagnetic long-range order is established in a FC process for values of the applied field lower than the threshold field for the metamagnetic transition at zero temperature. On the other hand, irreversibilities found when performing rapid FH+FC loops in the same region of the phase diagram are studied.

We report self-consistent, all-electron, local-spin-density calculations of energy bands of iron and cobalt pyrites (${\mathrm{FeS}}_2$ and ${\mathrm{CoS}}_2$). We find ${\mathrm{FeS}}_2$ to be a moderately small-band-gap semiconductor and ${\mathrm{CoS}}_2$ to be an almost half-metallic ferromagnet. The predicted Fermi surface of ${\mathrm{CoS}}_2$ is described.

A Chern-Simons-gauged nonlinear Schro¨dinger equation is derived from the continuous Heisenberg model in 2+1 dimensions. The corresponding planar magnets can be analyzed within the anyon theory. Thus, we show that static magnetic vortices correspond to the self-dual Chern-Simons solitons and are described by the Liouville equation. The related magnetic topological charge is associated with the electric charge of anyons. Furthermore, vortex-antivortex configurations are described by the sinh-Gordon equation and its conformally invariant extension. Physical consequences of these results are discussed.

The spectrum of spin-wave and single-particle excitations is studied in a half-filled-band Hubbard antiferromagnet for interaction strength values in the intermediate and weak-coupling regimes. The spin-wave-energy scale, which goes essentially as ${\mathit{t}}^2$/U in the strong- and intermediate-coupling regimes, exhibits a crossover and goes down with decreasing interaction strength in the weak-coupling regime. This crossover is found to occur at U/t=3.26, and it demarcates the Heisenberg behavior in the strong- and intermediate-coupling regimes from the spin-density-wave behavior in the weak-coupling regime. The spin-wave-energy scale gets compressed by the charge gap in the weak-coupling regime, and consequently the spin-wave branch, which approaches the single-particle excitation band quite closely, never actually merges into it.

Neutron-diffraction experiments have been carried out on amorphous iron powder prepared from the ultrasonic irradiation of Fe(CO${)}_5$. The structural information from the shape of the radial distribution function is consistent with the random-packing-model (RPD) calculations previously made for films of amorphous iron. The atomic magnetic moment of iron in the amorphous bulk phase has been obtained and its value is close to that of liquid iron. A comparison is made between the present ${\mathrm{\mu }}_{\mathrm{Fe}}$ value and those previously extracted by extrapolation from the concentration dependence of the binary ${\mathrm{Fe}}_{\left(1\mathrm{}\mathrm{-}\mathit{x}\right)}$${\mathrm{B}}_{\mathit{x}}$ metallic glasses.

In the ferromagnetic state the alloy ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ shows strong Invar effects governed by magnetoelastic interactions, including a negative hydrostatic pressure dependence of the bulk modulus: it becomes easier to squeeze under pressure. This remarkable behavior has prompted study of its nonlinear acoustic properties by determining its third-order elastic stiffness tensor components (TOEC) from measurements of the influence of uniaxial stress on the velocities of ultrasonic waves between 230 and 370 K, covering the range in which ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ has a negative thermal expansion, a property central to the Invar behavior. To obtain complete sets of the TOEC from 230 K up to the Curie point (367 K), the results have been combined with data for the hydrostatic pressure dependences of ultrasonic wave velocities. The TOEC quantify the first-order anharmonic terms in the interatomic potential and hence the long-wavelength acoustic phonon anharmonicities which are central to the Invar properties of this alloy. The tensor components ${\mathit{C}}_{111}$, ${\mathit{C}}_{112}$, and ${\mathit{C}}_{123}$ are anomalously positive, in accord with the negative values previously determined for the hydrostatic pressure derivatives (∂${\mathit{C}}_{11}$/∂P${)}_{\mathit{T},}$${\mathit{P}}_{=0\mathrm{}}$ and (∂${\mathit{B}}^{\mathit{S}}$/∂P${)}_{\mathit{T},}$${\mathit{P}}_{=0\mathrm{}}$. Thus, the large stress-induced longitudinal-acoustic-mode softening in the ferromagnetic phase is confirmed, reinforcing the suggestion that the vibrational anharmonicities of the longitudinal acoustic modes, which have large negative mode Grüneisen parameters, play an important part in causing the thermal expansion of ${\mathrm{Fe}}_{72}$${\mathrm{Pt}}_{28}$ to be negative.

We report Monte Carlo simulations of Villain’s periodic Gaussian XY model on ${\mathit{L}}^2$×N lattices of film geometry (L≫N) with up to N=16 layers, employing the single-cluster update algorithm combined with improved estimators for measurements. The boundary conditions are periodic within each layer and free at the bottom and top layer. Based on data for the specific heat, the spin-spin correlation function, and the susceptibility in the high-temperature phase we study the crossover from three- to two-dimensional behavior as criticality is approached. For the transition temperatures, determined from Kosterlitz-Thouless fits to the correlation length and susceptibility, we observe a pronounced scaling behavior with N. The associated critical exponent, however, deviates from theoretical expectations. More qualitatively, we further discuss the distribution and shapes of vortex loops in the crossover region.

Exchange hardening of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using an approach which yields analytic nucleation fields from the micromagnetic vector equation, and accounts for interactions between the soft regions. In suitable structures the nucleation field is proportional to the volume-averaged anisotropy constant. For example, a multilayer composed of alternating 2.4 nm hard-magnetic ${\mathrm{Sm}}_2$${\mathrm{Fe}}_{17}$${\mathrm{N}}_3$ layers and 9 nm ${\mathrm{Fe}}_{65}$${\mathrm{Co}}_{35}$ layers can have an energy product as high as 1 MJ/${\mathrm{m}}^3$ (120 MG Oe), with a rare-earth content of only 5 wt %. Giant energy products may also be achieved in suitable cellular and disordered structures.

An additional perpendicular surface anisotropy term resulting from lattice misfit is found for both coherent and incoherent states. From magnetic strain theory it can be concluded that the magnetic surface anisotropy depends not only on the thickness of the magnetic layer but also on the thickness of the nonmagnetic layer. The influence of surface roughness on magnetostrictive anisotropy is also calculated. We find surface roughness usually reduces the magnitude of surface anisotropy irrespective of its sign. Furthermore, a strong 1/${\mathit{t}}^2$ dependence of magnetic anisotropy for multilayers with identical elastic constants is found.

Within a model of the coherent rotation, we assume underdamped dynamics of magnetization in single-domain ferromagnetic particles and calculate the prefactor ${\mathit{f}}_0$(h) of the thermal decay rate as a function of the applied field h. Four different models of dissipative coupling are considered. As opposed to the previously reported temperature dependence of the prefactor, the field dependence is remarkably independent of the chosen coupling mode. The prefactor ${\mathit{f}}_0$(h) drops off sharply as h approaches the nucleation field and significantly enhances the coercivity whose model calculation, together with that of the switching-field distribution, is carried out.

The isochronal spectra of magnetic aftereffect in the amorphous alloys ${\mathrm{Fe}}_{90\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{Zr}}_{10}$ were measured within a temperature range of 77–450 K and over a time interval of 1 to 180 s. The spectra are characterized by a temperature-independent relaxation process at lower temperatures (less than 190 K) and a well-defined relaxation peak around 350 K which, for the samples with x<50, is accompanied by a small but distinct peak near the Curie temperature of the ${\mathrm{Fe}}_{90}$${\mathrm{Zr}}_{10}$ phase (${\mathit{T}}_{\mathit{C}}$=230 K). The relaxations in the Fe-rich samples with x<30 can be reexcited by demagnetization even after several cyclical temperature measurements are performed within the 77–450 K temperature range, whereas the relaxations in the samples with x>30 are completely irreversible. The current results can be described by a two-level system constructed of mobile atom pairs, and a quantitative analysis performed on the typical Fe-rich sample leads to an activation enthalpy spectrum ranging from 0.6 to 1.6 eV and a small preexponential factor ${\mathrm{\tau }}_0$=2.224×${10}^{\mathrm{-}16}$. In addition to the atom-pair mechanism, the rearrangement of the spin direction is also assumed to explain the concomitant relaxation peaks in the Fe-rich samples.

The results of a detailed histogram Monte-Carlo study of critical-fluctuation effects on the magnetic-field temperature phase diagram associated with the hexagonal Heisenberg antiferromagnet with weak axial anisotropy are reported. The multiphase point where three lines of continuous transitions merge at the spin-flop boundary exhibits a structure consistent with scaling theory but without the usual umbilicus as found in the case of a bicritical point.

We study the effect of a transverse hopping ${\mathit{t}}_{\mathrm{\perp }}$ in two chains of both spinless and spinning repulsively interacting fermions, by means of renormalization group and bosonization techniques. We show that, independent of the presence of spin, ${\mathit{t}}_{\mathrm{\perp }}$ strongly modifies the asymptotic long-wavelength behavior of the two chains, opening gaps in the excitation spectra. The origin of the instability of the gapless Luttinger-liquid behavior is identified in the flavor (==chain index) anisotropy induced by ${\mathit{t}}_{\mathrm{\perp }}$. In the case of spinning fermions, it leads to dominant pair fluctuations, in spite of the repulsive interaction. The role of spin is further analyzed in a model of two coupled chains showing, in the absence of ${\mathit{t}}_{\mathrm{\perp }}$, spin-charge separation without anomalous exponents. We solve this model exactly by the bosonization technique, and we find that the interesting analytical properties induced by spin-charge separation persist in the presence of transverse hopping, although ${\mathit{t}}_{\mathrm{\perp }}$ does modify the shape of the Fermi surface. The asymptotic expression of the single-particle Green function is also obtained.

The nearest-neighbor Ising antiferromagnet on a stacked triangular lattice is a frustrated cooperative system in which it is known that at least two long-range ordered states exist at low temperature. This model has also been of considerable interest as it is known to be a reasonable description of two antiferromagnetic insulators, ${\mathrm{CsCoBr}}_3$ and ${\mathrm{CsCoCl}}_3$. It has also been the subject of previous theoretical and simulation studies which have yielded conflicting results for the critical phenomena displayed near the transition from the paramagnetic to the high-temperature ordered phase. We have carried out a detailed Monte Carlo study of this system using the recently developed multiple-histogram technique and finite-size scaling analysis, with the purpose of extracting estimates for the critical exponents relevant to this continuous transition. Our results give β=0.311(4), γ=1.43(3), α=-0.05(3), and ν=0.685(3) which are not in agreement with previous Monte Carlo work. In addition, although they are close to the expectations from previous symmetry arguments, there are systematic differences between our results and these theoretical predictions. A possible interpretation of these Monte Carlo exponent estimates is that they do not correspond to those calculated for any known universality class, and add to the growing number of simple models of interacting spins, in which geometrical frustration is relevant, which appear to exhibit novel critical behavior. Finally, we have examined the evolution of real-space spin configurations and have seen that a buildup of correlations between anti-phase-domain walls, or solitons, along the stacking direction precedes the transition, an observation which is consistent with recent neutron-scattering measurements on ${\mathrm{CsCoBr}}_3$.