A rigorous micromagnetic study of nucleation by use of the magnetization curling mode in an elongated particle with heterostructure is presented. We categorize the heterostructure particles into three different types depending on their growth conditions and internal structures. The coercivity of the heterostructure particles critically depends on the relative magnetic parameters of the composite materials. The physical origins of the enhanced coercivity for the three types of heterostructure particles are quite different. The initial slope of the enhanced coercivity of surface-coated particles results from the shell crystalline anisotropy, while the saturation level depends on the interfacial coupling, shell crystalline anisotropy, and shell surface anisotropy. The main contribution to the enhancement of the coercivity of surface-modified particles is the surface anisotropy of the adsorbed layer at the particle surface. The homogeneity of the doped materials within the volume-modified particles affects their coercivity and the saturation level of the coercivity is always higher than that of the other two types. Agreement between theory and experiment is reasonably good.

The isotope ${}_{}{}^{147}{}_{}{}^{}\mathrm{Nd}$ has been oriented by means of low-temperature nuclear orientation on the pseudobinary series ${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{Nd}}_{1\mathrm{-}\mathit{x}}$Ni for x=0.0, 0.1, 0.25, 0.5, and 0.8. The results show that the compounds are magnetically ordered due to Nd-Nd interactions and no magnetic moment exists on cerium. The Kondo effect is present (for x≠0) as revealed by measurements of electical resistivity and thermoelectric power.

${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4$ are both ${\mathrm{K}}_2$${\mathrm{NiF}}_4$-structure oxides. ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$ is a weakly ferromagnetic insulator and ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4$ a paramagnetic metal. The ${\mathrm{Sr}}_2$${\mathrm{Ir}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ru}}_{\mathit{x}}$${\mathrm{O}}_4$ solid solution displays surprising physical properties. On the substitution of Ru to ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$, the Ru first exhibits its expected S=1 local magnetic moment, but at a critical Ru concentration of x≃0.7 the local moment disappears, the apparent density of states at ${\mathit{E}}_{\mathit{F}}$ increases, and an insulator-to-metal transition occurs. These changes in physical properties are accompanied by a change in the crystallographic cell parameters.

We report measurements of the velocity of the sliding spin-density-wave (SDW) condensate in (TMTSF${)}_2$${\mathrm{PF}}_6$, where TMTSF is tetramethyltetraselenafulvalene from their modulation of the proton spin echo as a function of the SDW current. From these measurements and those of the sliding SDW noise we find that the velocity is consistent with the entire condensate participating in the charge transport and that the wavelength of the pinning potential is half that of the SDW.

It is shown in the density-functional theory that a two-dimensional (2D) layer of electrons may have an inhomogeneous spin-density-wave (SDW) ground state, induced by 2 net attraction between nearest-neighbor electrons with antiparallel spins due to the exchange-correlation potentials stemming from the second-order exchange processes. Phase diagrams of the spin-inhomogeneous ground states for such 2D electrons are obtained in terms of the antiferromagnetic order parameters. The results predict that spin-inhomogeneous conductive states exist between the onset of a SDW instability and the eventual antiferromagnetic Mott-insulator states; analyses of the Fermi-surface structures and the densities of states substantiate that these SDW electrons are itinerant. Stability of those SDW states against thermal fluctuations, spatial modulation due to a lattice field, and hole injection is also examined. It is thus suggested that the conductive SDW states may play a key role in elucidating the microscopic mechanisms of high-temperature superconductivity.

We have studied the kinetic phase transition of a three-state-vector Potts system on a triangular lattice in contact with a heat bath and subject to an external energy source. The system evolves via the stochastic dynamics composed of Glauber and Kawasaki dynamics simulating interaction with the heat bath and energy source, respectively. The pair approximation is used. It is found that there is a paramagnetic-ferromagnetic transition at small energy flow, and the transition line is obtained. Also, when the energy is large enough, the system always will be in the paramagnetic state.

We have applied mean-field theory to calculate the spin-density-wave (SDW) order parameter Δ, the chemical potential μ, and the ground-state energy at various band fillings for the Hubbard model. We have found that at zero temperature and in the large-U limit, the SDW order parameter and the ground-state energy can be written as Δ=U(1-x)/2-O(J) and ${\mathit{E}}_{\mathit{g}}$=-U(1-x${)}^2$/4-0(J). The chemical potential obtained here for x=0 and 1 agrees well with the result obtained by the exact-diagonalization calculation on a 4×4 sites cluster by Dagotto et al. [Phys. Rev. B 45, 10 741 (1992)]. In the SDW phase, the mean-field theory gives an upward shift in the chemical potential as one dopes away from half-filling, which is in contrast with the exact-diagonalization calculations. This result indicates that the Mott-Hubbard gap is depressed much more rapidly in the mean-field theory than the exact-diagonalization calculation. The doping dependence of ground-state energy calculated here is in good agreement with the above numerical calculation for doping up to 0.5. Here, U is the intrasite Coulomb repulsion, x is doping concentration, J=4${\mathit{t}}^2$/U is the superexchange interaction parameter, and t is the hopping integral for the nearest neighbors.

We present the general theory of clean, two-dimensional, quantum Heisenberg antiferromagnets which are close to the zero-temperature quantum transition between ground states with and without long-range Néel order. While some of our discussion is more general, the bulk of our theory will be restricted to antiferromagnets in which the Néel order is described by a three-vector order parameter. For Néel-ordered states, ‘‘nearly critical’’ means that the ground-state spin stiffness, ${\mathrm{\rho }}_{\mathit{s}}$, satisfies ${\mathrm{\rho }}_{\mathit{s}}$≪J, where J is the nearest-neighbor exchange constant, while ‘‘nearly critical’’ quantum-disordered ground states have an energy gap, Δ, towards excitations with spin 1, which satisfies Δ≪J. The allowed temperatures, T, are also smaller than J, but no restrictions are placed on the values of ${\mathit{k}}_{\mathit{B}}$T/${\mathrm{\rho }}_{\mathit{s}}$ or ${\mathit{k}}_{\mathit{B}}$T/Δ. Under these circumstances, we show that the wave vector and/or frequency-dependent uniform and staggered spin susceptibilities, and the specific heat, are completely universal functions of just three thermodynamic parameters. On the ordered side, these three parameters are ${\mathrm{\rho }}_{\mathit{s}}$, the T=0 spin-wave velocity c, and the ground-state staggered moment ${\mathit{N}}_0$; previous works have noted the universal dependence of the susceptibilities on these three parameters only in the more restricted regime of ${\mathit{k}}_{\mathit{B}}$T≪${\mathrm{\rho }}_{\mathit{s}}$. On the disordered side the three thermodynamic parameters are Δ, c, and the spin-1 quasiparticle residue scrA. Explicit results for the universal scaling functions are obtained by a 1/N expansion on the O(N) quantum nonlinear σ model, and by Monte Carlo simulations. These calculations lead to a variety of testable predictions for neutron scattering, NMR, and magnetization measurements. Our results are in good agreement with a number of numerical simulations and experiments on undoped and lightly doped ${\mathrm{La}}_{2\mathrm{-}\mathrm{\delta }}$${\mathrm{Sr}}_{\mathrm{\delta }}$${\mathrm{CuO}}_4$.

The properties of the one-dimensional antiferromagnet of spin 1/2 in a strong magnetic field h with nearest- and next-nearest-neighbor interactions are studied in terms of the equivalent Bose-gas problem. We find that below the first critical field ${\mathit{h}}_{\mathit{c}\mathit{u}}$ the phase with a gap in the elementary excitations spectrum and corresponding plateau in the magnetization curve should exist due to the two-particle attraction in the vicinity of the commensurate wave number ${\mathit{p}}_{0\mathit{u}}$=π. In the interval between the first and second critical fields ${\mathit{h}}_{\mathit{c}\mathit{u}}$<h<${\mathit{h}}_{\mathit{c}\mathit{f}}$, the system behaves as the one-dimensional Bose gas with particle repulsion and the magnetization has a typical square-root dependence on field.

Extensive neutron-scattering experiments have been performed in order to characterize the nature of the two correlation lengths observed in Tb, a phenomenon common to Ho as well as ${\mathrm{SrTiO}}_3$. In the vicinity of the transition temeprature, each of those crystals exhibits an anomalous two-component q profile in the critical scattering; the usual broad peak and an unexpected additional narrow peak. In order to clarify the spatial origin of the narrow component, the (0,0,δ) magnetic statellite peak of Tb has been closely examined using a very narrow neutron beam realized by a high spatial resolution reflectometer at the National Institute of Standards and Technology, which can produce well-defined beam widths of 0.3 mm and less. The small scattering angle (θ≊1.35 °) and the narrow beam width result in an extraordinarily fine q and E resolution. As recently reported, we have confirmed that the intensity of the narrow component is enhanced near the edge of the crystal; it demonstrates that the major part is located within the near surface volume or ‘‘skin’’ of the crystal. A skin thickness of ∼ 0.2 mm full width at half maximum (FWHM) is obtained by fitting the results to a model scatterer distribution which convolutes the beam profile. In contrast to the case of the central peak observed in ${\mathrm{SrTiO}}_3$, this unusual spatial distribution establishes the narrow component as a distinct entity in the critical fluctuations. We have shown that the narrow component possesses a different temperature dependence of δ from that of the broad one. Moreover, high E resolution scans show that the narrow component has a distinct energy width which is smaller than our resolution limit (∼2 μeV FWHM), and that it is essentially temperature independent. We believe that this quasistatic character of the narrow component is the important key to understanding its physical origin.

We propose that strongly paramagnetic systems can show anti-Invar behavior. This is concluded from the model of Invar behavior and from the differences in calculated total energy, in magnetic pressure and equilibrium volume for nonmagnetic and ferromagnetic states. Self-consistent linear-muffin-tin-orbital calculations are performed for nonmagnetic and constrained magnetic configurations for ${\mathrm{ZrV}}_2$, ${\mathrm{ZrZn}}_2$, and ${\mathrm{TiBe}}_2$ in C15 structure at different volumes. It is found that the ground state is nonmagnetic in anti-Invar systems but low moment states are close in energy and have larger volumes. Model calculations based on thermal, magnetic fluctuations give the qualitative trends for the magnetovolume effects. It is discussed how optimal conditions for large anti-Invar anomalies should appear.

A dimensionless discretization parameter Λ controls the accuracy and the computational cost of numerical renormalization-group calculations. For reliable computations of thermodynamical properties of impurities in metals, the upper bound Λ≤3 has long been established. This report discusses a generalization that raises the limit to Λ=10, yielding the same accuracy with significantly smaller computational cost.

We consider general anisotropic nearest-neighbor magnetic exchange in the pyrite structure where the magnetic ions occupy the fcc sites. We show how the Dzyaloshinsky-Moriya interaction can stabilize the anticollinear spin configuration observed in ${\mathrm{MnTe}}_2$ when T is slightly below ${\mathit{T}}_{\mathit{N}}$. As an alternative explanation, we also consider the most general symmetric anisotropic nearest-neighbor coupling allowed in the pyrite structure. For ${\mathrm{MnS}}_2$, we construct a model which is consistent with the observed centering, slightly above ${\mathit{T}}_{\mathit{N}}$, of the diffuse neutron scattering around an incommensurate position. We also discuss the spin arrangement observed in ${\mathrm{MnSe}}_2$ when only bilinear spin-spin exchange is present.

We investigate the nature of the perturbation expansion of the free energy in one of the simplest models of a system with quenched disorder: a randomly diluted magnet in zero dimensions. We show that the fact that the series is non-Borel-summable is a direct consequence of the existence of Griffiths-like singularities. The implications of this result for more realistic systems are discussed.

The magnetic x-ray scattering from a transuranium material, NpAs, has been measured. We report the intensity of the scattering as a function of photon energy in the range of the ${\mathit{M}}_{\mathrm{IV}}$ and ${\mathit{M}}_{\mathrm{V}}$ absorption edges (3.5 to 4 keV). Large enhancements are found at the M-edge binding energies, as is the case in uranium compounds. We have used the strong ${\mathit{M}}_{\mathrm{IV}}$ resonance (3.852 keV) to examine the magnetic struture of NpAs as a function of temperature. Differences between the phase diagram as studied previously by neutron diffraction and the present experiments may be ascribed to the near-surface sensitivity of x-ray scattering. In particular we observe some unusual ‘‘domain’’ effects that may be attributed to surface anisotropy. The higher resolution available in the x-ray experiments, as compared to the neutron experiments, has led to an understanding of the wave vector dependence as a function of temperature. We suggest that the change of the wave vector from its commensurate value of 1/4 (corresponding to a square wave 4+, 4- arrangement of magnetic moments) may be understood by introducing ‘‘defects’’ into the magnetic structure. We confirm the earlier work that showed the lattice initially distorted to tetragonal (from the NaCl fcc structure in the paramagnetic state) and reverted to cubic in the low-temperature triple-q antiferromagnetic phase.

Using synchrotron x-rays tuned to the Np ${\mathit{M}}_{\mathrm{IV}}$ resonance (E=3.852 keV) we have studied the critical magnetic scattering associated with the phase transition to the antiferromagnetic state at ${\mathit{T}}_{\mathit{N}}$=172 K of single crystals of NpAs. The most important result is that we observe two components to the critical scattering above ${\mathit{T}}_{\mathit{N}}$. One corresponds approximately to the length scale of the magnetic correlations already observed with neutron scattering, whereas the other correlation length is some ten times longer, giving rise to a sharp component in the critical scattering. By analogy with the ‘‘central-peak’’ problem in structural phase transitions, we have analyzed the broad component with a Lorentzian function, and the sharp component with a Lorentzian squared. The long-range correlations may arise either from volumes nucleated by defects or, because the photon beam has a penetration of ∼1500 Å, from surface-related effects. The results for the critical parameters are difficult to reconcile with present theory. NpAs has a single-q magnetic structure near ${\mathit{T}}_{\mathit{N}}$ and thus three separate domains exist. Unusual changes are seen in the domain populations even above ${\mathit{T}}_{\mathit{N}}$. We suggest that this is related to surface dipole energy.

We examine the critical current densities of superconducting/insulating ${\mathrm{Mo}}_{77}$${\mathrm{Ge}}_{23}$/Ge multilayers in magnetic fields applied at arbitrary angles with respect to the superconducting layers. In these measurements, we demonstrate the existence of a threshold field, below which the critical current is governed only by the perpendicular component of the applied field. Also, we observe that there exists a critical angle that divides two different regimes of vortex pinning. Finally, we relate these results to existing theories of vortices in layered superconductors.

Asymptotic states in long Josephson junctions are investigated in an external magnetic field. We show that a choice of the solution of the stationary Ferrell-Prange equation can be carried out with use of an asymptotic solution of the sine-Gordon equation and that an evolution to that stable solution occurs by passing through metastable states, which is determined with a form of quickly damped initial perturbation. The boundary sine-Gordon and Ferrell-Prange problems were carried out with a numerical simulation. An approximated expression for the vortex and antivortex states is obtained in the case of large values of an external magnetic field.

Circular superconducting devices with mesoscopic Josephson junctions are considered in the presence of a dc voltage bias ${\mathit{V}}_0$ and of nonclassical electromagnetic fields. It is shown that when the voltage ${\mathit{V}}_0$ is an integer multiple of a certain value, the current through the junction has a dc component. This is broadly similar to Shapiro steps in macroscopic Josephson junctions, but the details depend on the nature of the nonclassical electromagnetic field. Coherent states, squeezed states, and number eigenstates are considered and the corresponding Shapiro steps are studied in detail. The presence of thermal noise in the nonclassical fields is also considered and its effect on the results is studied.

It is shown that the attractive Hubbard model for the face-centered-cubic lattice possesses a charge-density-wave semimetallic phase with a superconducting ground state. The properties of this phase are consisent with the behavior of alkali fullerides. The overlapping bands result in peaks in the density of states and anomalous mid-infrared absorption. The low density of carriers leads to a high normal-state resistivity, a short coherence length and a long penetration depth, as observed. The difference between this and a spin-density-wave semimetallic state, which is predicted to occur in the repulsive Hubbard model, is briefly discussed.