We discuss the fluctuation of domain walls in two-dimensional systems interacting with short-range exchange, anisotropy, and long-range dipolar interactions. The elastic energy of different domain walls are calculated. Some of the domain-wall distortion modes are soft in the long-wavelength limit. The competition of the stabilizing long-range dipolar interaction and low-dimensional fluctuation suggests the possibility of a finite-temperature roughening of an array of one-dimensional walls in the film.

We present evidence for enhanced magnetic moments in epitaxial Fe films in proximity with Ag, Au, Cu, and Pd overlayers prepared by molecular-beam epitaxy on singular Ag(001) substrates in a combined study using polarized-neutron reflection (PNR) and ferromagnetic resonance (FMR). Second versions of equivalent samples were investigated in order to test the repeatability of the results, and all PNR measurements were corrected for background and diffuse scattering. Within experimental error, measurements of the absolute value of the magnetic moment per atom made using PNR were found to agree with measurements of the relative magnetization using FMR, and measurements on repeated samples were also found to agree. An average magnetic moment per Fe atom of 2.58±0.09${\mathrm{\mu }}_{\mathit{B}}$ was determined for a Ag/5.5 ML Fe structure, significantly enhanced from the value of ${\mathrm{\mu }}_{\mathrm{Fe}}$=2.33±0.05${\mathrm{\mu }}_{\mathit{B}}$ obtained for a Ag/10.9 ML Fe reference sample. Corresponding values of 2.48±0.08${\mathrm{\mu }}_{\mathit{B}}$ and 2.50±0.10${\mathrm{\mu }}_{\mathit{B}}$ were determined for Cu/5.8 ML Fe and Cu/5.7 ML Fe structures, respectively. Values of the layer averaged moment of 2.66±0.05${\mathrm{\mu }}_{\mathit{B}}$ and 2.6±0.2${\mathrm{\mu }}_{\mathit{B}}$ were determined for Pd/5.6 ML Fe and Pd/5.7 ML Fe structures, respectively, assuming that no induced polarization occurs in the interface Pd layers.

The degree of enhancement measured in equivalent samples is seen to be in agreement within experimental error. The magnitude of the enhancement (12–20)% we observe in the samples of thickness close to 6 ML is slightly higher than that predicted by recent band-structure calculations, but our results show conclusively that the enhancement can be attributed to the interface atoms. The temperature dependence of the magnetization has also been measured using PNR and the results compared with that predicted from spin-wave theory using FMR measurements of the effective spin-wave gap. Good agreement is found for the Ag/Fe and Cu/Fe systems for which spin-wave gap temperatures of 0.1 and 0.12 K, respectively, are determined by PNR, compared with values of 0.15 and 0.11 K deduced from FMR, where the agreement for Au/Fe is less good. Both FMR and PNR data are consistent with the largest gap occurring for the Pd/Fe interface system but the value of the spin-wave gap determined by FMR is in this case smaller than the value obtained from fitting the PNR data.

We examine the structure of the low-temperature fixed point of a generalized two-level-system (TLS) Hamiltonian using the perturbative renormalization-group transformation. In our model the conduction electrons scattered by the TLS are labeled by an additional flavor quantum number α=1,...,${\mathit{N}}_{\mathit{f}}$, the physically relevant case being ${\mathit{N}}_{\mathit{f}}$=2 corresponding to the two spin directions. Using the second-order scaling equations it is shown that the only stable fixed point is the one where the conduction electrons have an ${\mathit{S}}_{\mathit{e}}$=1/2 orbital pseudospin. Thus, the relevant subspace of the electrons is essentially reduced. Our results, which are exact in the limit ${\mathit{N}}_{\mathit{f}}$→∞, are in a reasonable agreement with the recent experiments made on metallic nanostrictions. We also investigate numerically the effect of orbital electron channels with higher angular momenta on the Kondo temperature and check the results of the analytical calculations.

We recently proposed an isothermal equation of state that was successfully applied to study the high-pressure behavior of molecular liquids. In this work we extend its applicability to liquid metals, polymers, molten salts, and solids. The possibility of considering this equation as an alternative to long-standing equations such as those of Tait and Birch-Murnaghan is emphasized. This suggestion is firmly supported by comparisons with experimental data up to pressures of several GPa. A new physical interpretation of the pressure coefficient of the Tait equation ${\mathit{B}}_{\mathit{T}}$ is given. It can be identified to the divergence pressure along a pseudospinodal curve. We also compare the performance of our equation with the most successful equations of state used to represent isothermal data of solids, with excellent results. Our equation can be applied to systems with phase transitions. An interesting observation is that it seems that the different pseudospinodal curves obtained for the different phases can be put together into the same curve except for characteristic jumps occurring at the phase transitions.

The anisotropic magnetoresistance (AMR) at low temperatures is theoretically studied for low-dimensional NiFe-based systems in various geometries by solving the Boltzmann transport equation. The AMR is treated by introducing spin-dependent anisotropic mean free paths, making use of anisotropic-scattering parameters that are extracted from experimental spin-resolved resistivity data for bulk dilute NiFe alloys. A first set of calculations comprises the AMR in NiFe thin films and cylindrical wires, as a function of the layer thickness and wire diameter, respectively. For the thin film case we have considered rotation of the magnetization vector within the film plane as well as out of the film plane. For the latter the highest AMR ratio is found, which even slightly exceeds the bulk value. For wires the dependence of the AMR on the dimensions is qualitatively different as compared to the film case due to the relatively enhanced importance of boundary scattering. Finally, the validity of a description of the combined effect of AMR and the giant magnetoresistance in terms of a simple summation of the two effects is studied by performing model calculations for NiFe/Cu/NiFe trilayers.

We have studied the angular dependence of the giant magnetoresistance (GMR) for ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$/Ag/Co/Ag and ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$/Cu/Co/Cu multilayers, in which the Co layers are discontinuous and composed of a collection of clusters. In these structures, the magnetization of the ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ layers can be rotated by a small applied field without affecting the remanent magnetization of the Co clusters. We analyze our results with a method that allows us to separate accurately GMR and anisotropic magnetoresistance and we find that the angular dependence of the GMR does not exhibit the features expected to arise from the existence of interface spin-dependent potential steps.

The magnetostrictive behavior of amorphous ${\mathrm{Tb}}_{\mathit{x}}$${\mathrm{Fe}}_{1\mathrm{-}\mathit{x}}$ (0.10<x<0.45) thin films was studied by measuring the stress derivative of the perpendicular anisotropy. Large values of the magnetostriction were found, of order ${10}^{\mathrm{-}3}$, about the same as the value observed in their polycrystalline counterparts. Within this composition range, ${\mathrm{Tb}}_{45}$${\mathrm{Fe}}_{55}$ shows a large magnetostriction with a relatively low anisotropy which makes it a good candidate for application as a magnetoelastic element. After annealing at low temperature (175 °C), relaxation of both perpendicular anisotropy and magnetostriction was observed. However, the relaxation strength was particularly remarkable in the range of low Tb content (x<0.30). The relaxation behavior seems to indicate that local anelastic strain may be the origin of this bond anisotropy for low Tb content while additional structural anisotropy is likely to be responsible for the anisotropy in higher Tb content films.

A numerical approach for calculating an eigenvalue distribution function is developed and it is applied to the study on a fully frustrated spin system called the Δ-chain model. We examine the magnetic-field dependence of the specific heat to clarify the relationship between the highly frustrated spins and the lower-temperature peak which has been extensively investigated by Kubo. Our numerical data show that with the increase of magnetic field, the peak width becomes broader and the height lower. This sensitive dependence should be caused by lowering the high degeneracy of states contributing to the peak formation. On the other hand, we also find that the higher-temperature peak observed commonly in antiferromagnetic quantum spin chains is almost unchanged.

A mean-field theory for spin-reorientation phase transitions in magnetic thin films is worked out, solving analytically the relevant differential equation. The role of the exchange stiffness A in the competition between the in-plane shape anisotropy ${\mathit{K}}_{\mathit{v}}$ and the perpendicular surface anisotropy ${\mathit{K}}_{\mathit{s}}$ is addressed. The spin-reorientation transition from the perpendicular configuration to the in-plane one with increasing thickness is derived. Two different stable spin configurations are presented at large thickness for ${\mathit{K}}_{\mathit{s}}$< √${\mathit{AK}}_{\mathit{v}}$ and vice versa. The phase transition with temperature is also explored. Asymptotic formulas are given for energy stocked in the film per unit area, which enable one to evaluate the surface anisotropy.

We present a theoretical study of the RKKY coupling in magnetic multilayers based on an effective-mass model with a single quantum well or a superlattice potential. It is found that the oscillation period is determined by the nesting vector of the Fermi surface of the spacer regardless of the depth of the quantum well, while the coupling strength and the phase of oscillation are affected by the presence of the quantum well. The magnetic-layer thickness dependence of the RKKY coupling for the superlattice is investigated. It is found that the RKKY coupling integrated over the magnetic layer is a strong oscillatory function of the magnetic-layer thickness with a period related to the nesting vector of the Fermi surface for the magnetic material. The coupling across a quantum barrier is also studied.

A diagrammatic technique that treats the kinetic energy as a perturbation on the Coulomb interaction term is used to show the correspondence between the Hubbard Hamiltonian and a disordered alloy of singly and doubly occupied atoms. The method generalizes the alloy-analogy approach. Infinite and finite chains, and finite clusters of the square lattice, are considered. Finite systems are handled by means of a method that includes, in a self-consistent manner, only those configurations with the same number of particles. Total energies and momentum distributions are calculated. The results for chains up to 14 sites and 4×4 clusters of the square lattice compare satisfactorily with exact results.

The electron-spin-resonance spectrum of the quasi-one-dimensional antiferromagnet ${\mathrm{CsNiCl}}_3$ has been observed at various temperatures, using a fixed-frequency, varying field setup. The power absorption, the linewidth, and the g factor were measured as a function of temperature. An attempt is made at explaining the temperature dependence of the power absorption in the disordered phase by use of the nonlinear σ model, in particular taking into account multimagnon contributions.

For a magnetic multilayer system, the physical origin of the difference between the giant magnetoresistances for currents parallel and perpendicular to the layer planes is studied. In order to take into account the two characteristics of a multilayer system, interfacial roughness and layered structure, we adopt the single-cell coherent-potential approximation. The spin-dependent resistivities are calculated for currents parallel and perpendicular to the layer planes by using a single-band tight-binding model and the Kubo formalism. It is shown that the magnetoresistance ratio for perpendicular current is larger than that for parallel current. The difference between magnetoresistances for parallel and perpendicular currents is attributed to the fact that the effects of the anisotropy of the effective mass and the two-dimensional distribution of randomness on the anisotropy of the resistivity depend on the spin-dependent potential. It is also shown that interfacial roughness is favorable to magnetoresistance for parallel currents.

We derive low-temperature properties of the large-U Hubbard model in two and three dimensions starting from exact series-expansion results for high temperatures. Convergence problems and limited available information prevent a direct or Padé-type extrapolation. We propose a method of extrapolation, which is restricted to large U and low hole densities, for which the problem can be mapped on that of a system of weakly interacting holes. In this formulation an extrapolation down to T=0 can be obtained, but it can be trusted for the presently available series data for βt≲20 and for hole densities ${\mathit{n}}_{\mathit{h}}$≲0.2 only. Implications for the magnetic phase diagram are discussed.

We use the recursion method to study the spectral and dynamical properties of the one-dimensional (1D) s=1/2 XXZ method with planar anisotropy at T=0. Distinct methods of continued-fraction analysis have been developed for the weak-coupling and strong-coupling regimes of the corresponding lattice fermion system. The weak-coupling analysis presented here yields detailed information on the spectral-weight distribution in dynamic structure factors and spin autocorrelation functions, notably on the infrared singularities produced by critical fluctuations, and on the bound states for the case of attractive fermion interaction. The same method is then applied to the charge dynamics of the 1D t-J model for strongly correlated electrons. There it yields similar yet distinct results in the regime of weak exchange coupling. The results for renormalized bandwidths of particle-hole excitations are consistent with available results for charge velocities, and the results for the infrared exponent in the charge dynamic structure factor agree with existing results for the exponent of the equal-time charge correlation function.

The effect of pressure on the crystal structure of ${\mathrm{GaAsO}}_4$ has been studied using energy-dispersive powder diffraction up to 19.7 GPa. A structural phase transition was observed between 6.3 and 9.5 GPa from the ambient hexagonal structure to a triclinic structure. A model for this new high-pressure phase is presented. The low- and high-pressure phases were found to have bulk moduli of 17.6 and 177.0 GPa, respectively. The phase transition is reversible and first order with a volume discontinuity of 8%. The implications for the related shape-memory materials are discussed.

It is suggested that an interacting many-electron system in a two-dimensional lattice may condense into a topological magnetic state distinct from any discussed previously. This condensate exhibits local spin-1/2 magnetic moments on the lattice sites but is composed of a Slater determinant of single-electron wave functions which exist in an orthogonal sector of the electronic Hilbert space from the sector describing traditional spin-density-wave or spiral magnetic states. These one-electron spinor wave functions have the distinguishing property that they are antiperiodic along a closed path encircling any elementary plaquette of the lattice. This corresponds to a 2π rotation of the internal coordinate frame of the electron as it encircles the plaquette. The possibility of spinor wave functions with spatial antiperiodicity is a direct consequence of the two-valuedness of the internal electronic wave function defined on the space of Euler angles describing its spin. This internal space is the topologically, doubly-connected, group manifold of SO(3). Formally, these antiperiodic wave functions may be described by passing a flux which couples to spin (rather than charge) through each of the elementary plaquettes of the lattice. When applied to the two-dimensional Hubbard model with one electron per site, this new topological magnetic state exhibits a relativistic spectrum for charged, quasiparticle excitations with a suppressed one-electron density of states at the Fermi level.

For a topological antiferromagnet on a square lattice, with the standard Hartree-Fock, spin-density-wave decoupling of the on-site Hubbard interaction, there is an exact mapping of the low-energy one-electron excitation spectrum to a relativistic Dirac continuum field theory. In this field theory, the Dirac mass gap is precisely the Mott-Hubbard charge gap and the continuum field variable is an eight-component Dirac spinor describing the components of physical electron-spin amplitude on each of the four sites of the elementary plaquette in the original Hubbard model. Within this continuum model we derive explicitly the existence of hedgehog Skyrmion textures as local minima of the classical magnetic energy. These magnetic solitons carry a topological winding number μ associated with the vortex rotation of the background magnetic moment field by a phase angle 2πμ along a path encircling the soliton. Such solitons also carry a spin flux of μπ through the plaquette on which they are centered. The μ=1 hedgehog Skyrmion describes a local transition from the topological (antiperiodic) sector of the one-electron Hilbert space to the nontopological sector. We derive from first principles the existence of deep level localized electronic states within the Mott-Hubbard charge gap for the μ=1 and 2 solitons. The spectrum of localized states is symmetric about E=0 and each subgap electronic level can be occupied by a pair of electrons in which one electron resides primarily on one sublattice and the second electron on the other sublattice. It is suggested that flux-carrying solitons and the subgap electronic structure which they induce are important in understanding the physical behavior of doped Mott insulators.

CeRhSb is a mixed valent Ce-based compound which shows a rapid rise in resistivity at low temperatures. This resistivity rise has been attributed to the opening of a gap in the electronic density of states. On the other hand, isostructural LaRhSb is a superconductor with ${\mathit{T}}_{\mathit{c}}$ of about 2.5 K. We have investigated the system ${\mathrm{Ce}}_{1\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$RhSb to study the effect of La substitution for Ce on the gap formation in CeRhSb, and the effect of Ce substitution for La on the superconductivity in LaRhSb. It is observed that the rapid rise in resistivity, and hence the gap formation, is suppressed even when a small amount of La is substituted for Ce in CeRhSb, though susceptibility continues to show mixed valent behavior. This suggests that a coherent Kondo lattice state is essential for the gap formation in CeRhSb. On the La rich side, superconductivity is suppressed even by 0.5% replacement of La by Ce. In the intermediate region of substitution, a gradual transformation from a mixed valent to a Curie-Weiss behavior in the susceptibility is observed.

Specific heat, magnetic susceptibility, and resistivity were measured on polycrystalline samples of U(${\mathrm{Be}}_{1\mathrm{-}\mathit{x}}$${\mathrm{B}}_{\mathit{x}}$${)}_{13}$ with 0≤x≤5.2×${10}^{\mathrm{-}3}$. Small amounts of B greatly affect the specific-heat discontinuity at the superconducting phase transition in ${\mathrm{UBe}}_{13}$, while only weakly decreasing the superconducting transition temperature. The presence of B impurities also influences the resistivity. These observations are discussed in terms of possible impurity-induced changes in the symmetry and/or pairing interaction of the superconducting state in ${\mathrm{UBe}}_{13}$.

Utilizing monochromatized synchrotron radiation strong photoemission enhancements in the valence-band region and the Cu 3p core level have been measured in the vicinity of the Cu ${\mathit{L}}_3$ threshold of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_6$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ superconductors. For both the valence-band and the Cu 3p level in both crystals the satellite structures show the strongest enhancements. The line shapes of these enhanced satellites are shown to be in good agreement with calculated atomic Auger spectra for the 3${\mathit{d}}^8$ and 3${\mathit{p}}^5$3${\mathit{d}}^9$ final states, respectively. Measurements of the valence-band emission in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ using a number of different photon energies in the vicinity of the Cu ${\mathit{L}}_3$ threshold show that the enhancement occurs at a constant binding energy for photon energies tuned to the low-energy flank of the Cu ${\mathit{L}}_3$ x-ray-absorption peak. This behavior supports the interpretation that the enhancement is due to a photoemission resonance. The implications of the copper nature of the valence-band satellite on the description of the electronic structure of these compounds are briefly discussed.