A theoretical model is proposed to describe the quasi-one-dimensional organic ferromagnet polyBIPO. Different from Ovchinnikov’s model, we consider the itineracy of π electrons, the strong electron-phonon interaction, and electron-electron correlation in a one-dimensional system. Within the mean-field theory and Hartree approximation, allowing for full lattice relaxation, a set of self-consistent iterative equations is established to study the effects of π electrons on the ferromagnetic order in the organic materials. The results of our calculations show that the ground state of the system is a stable ferromagnetic state. In the ferromagnetic ground state, the energy levels of the π electrons will split off with respect to different spins. However, owing to the Peierls distortion of the lattice, all of the π electrons will form an antiferromagnetic spin-density-wave chain.

Magnetic properties and interface delocalization of the spin-1/2 Ising model on a cubic lattice with an amorphized defect plane that divides the system into two semi-infinite ones are investigated by the use of the effective-field theory. The global phase diagrams, which represent the connection between amorphized defect-plane order and wetting transitions (from partial to complete wetting), are given in the case of two equivalent semi-infinite systems (the same coupling) and in the case of different semi-infinite systems. The influence of the amorphization in these global phase diagrams is also studied. Moreover, magnetic properties which illustrate some interesting behavior for the amorphized systems are investigated.

In this article, we discuss a system that is appropriately called a domain-wall junction (DWJ). The DWJ is a close analog of the Josephson junction with respect to the usefulness of both systems in studying switching by thermal activation (TA) and by quantum tunneling (QT). We will show through theoretical analysis that it is within our means to fabricate a DWJ that should exhibit switching via quantum tunneling of magnetization. We further lay out experimental procedures for looking for quantum tunneling of magnetization in a DWJ; these procedures are based upon those used in observing macroscopic quantum tunneling in Josephson junctions. Two examples of DWJ’s have been recently made that apparently show a crossover from TA to QT.

Magnetoresistance measurements have been made on a number of single-crystal samples of the metallic charge-transfer salt β’’-(BEDT-TTF${)}_2$${\mathrm{AuBr}}_2$, using magnetic fields up to 50 T. The experiments have been carried out for a wide range of orientations of the sample with respect to the magnetic field and for temperatures ranging between 80 mK and 4.2 K. The magnetoresistance exhibits a complex series of Shubnikov–de Haas oscillations, an anisotropic angle dependence, and, below 1 K, hysteresis. Both the hysteresis in the magnetoresistance and frequency mixing effects observed in the Shubnikov–de Haas spectrum can be explained by the effects of Shoenberg magnetic interaction, and this mechanism has been successfully used to model the observed Fourier spectrum of the magnetoresistance. The complex Shubnikov–de Haas frequency spectrum of β’’-(BEDT-TTF${)}_2$${\mathrm{AuBr}}_2$ is proposed to result from the effects of a spin-density wave on the band structure, which alters the original Fermi surface to produce three two-dimensional carrier pockets. The angle dependence of the Shubnikov–de Haas oscillation amplitudes has been used to deduce the approximate shapes and orientations of these pockets, which are found to be in good qualitative agreement with the proposed model.

In films of partially ordered ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$, with x≊0.28, large electrical-resistance noise from magnetic effects was found. Individual fluctuating domains with large volumes but low magnetic moments were observed. These domains are apparently clusters of constituent ferromagnetic domains which have a net systematic antiferromagnetic alignment. The clusters are larger in the ‘‘reentrant spin-glass’’ regime than in the ferromagnetic regime. Ferromagnetic domains arranged in antiferromagnetic stripes were also imaged by scanning electron microscopy.

The ferromagnet ${\mathrm{CeRh}}_3$${\mathrm{B}}_2$ has a Curie temperature ${\mathit{T}}_{\mathit{C}}$ near 117 K, two orders of magnitude higher than anticipated from simple de Gennes scaling. To shed light on the nature of the anomalous magnetic state, the hydrostatic pressure dependence of ${\mathit{T}}_{\mathit{C}}$ was measured to 7.0 GPa using a diamond-anvil cell with dense He as pressure medium. As a function of pressure, ${\mathit{T}}_{\mathit{C}}$(P) initially increases at the rate of 1.0 K/GPa but then passes through a maximum near 2.5 GPa and falls rapidly at higher pressures. This qualitative behavior can be accounted for by a simple phase diagram proposed some time ago by Doniach for dense Kondo systems. This diagram is also used to compare the evolution of magnetism in related substitutional compounds to that for ${\mathrm{CeRh}}_3$${\mathrm{B}}_2$ under pressure. For selected Ce compounds, a universal relation between the coupling strength J and the ordering temperature ${\mathit{T}}_{\mathit{C}}$ is found.

The surface magneto-optic Kerr effect (SMOKE) was used to investigate the magnetic properties of epitaxial thin films of Co, Ni, and their alloys grown on Cu(100) and Cu(111). The Curie temperature ${\mathit{T}}_{\mathit{C}}$ is higher for the same films of a given thickness on Cu(111) than on Cu(100). All the films show a change in the power-law exponent β of the magnetization density M∼(1-T/${\mathit{T}}_{\mathit{C}}$${)}^{\mathrm{\beta }}$ with reducing film thickness. Ni films on Cu(100) undergo a particularly abrupt crossover at ∼7 monolayers (ML) from three-dimensional Heisenberg (β=0.37) to finite-size two-dimensional XY (β=0.23) behavior as the film thickness is reduced. The characteristic power-law exponent β=0.23 of these films appears to be an experimental realization of Kosterlitz-Thouless behavior over a restricted temperature range. A similar, but more gradual crossover occurs for the Ni films on Cu(111) at 8 to 12 ML. The finite-size scaling behavior in the few-monolayers-thickness range is compared with that reported for Ising thin-film behavior. In all instances ${\mathit{T}}_{\mathit{C}}$ extrapolates with decreasing thickness to zero at one monolayer. The dimensionality crossover and finite-size scaling behavior is discussed in the light of our current understanding of spin-wave quantization, anisotropy, and film microstructure.

The results of the magnetoresistance (Δρ/ρ) measurements of the compounds ${\mathrm{PrCu}}_2$${\mathrm{Si}}_2$, ${\mathrm{PrCu}}_{1.7}$${\mathrm{Ni}}_{0.3}$${\mathrm{Si}}_2$, ${\mathrm{PrCu}}_{0.5}$${\mathrm{Ni}}_{1.5}$${\mathrm{Si}}_2$, ${\mathrm{GdNi}}_2$${\mathrm{Si}}_2$, ${\mathrm{GdCu}}_{0.4}$${\mathrm{Ni}}_{1.6}$${\mathrm{Si}}_2$, and ${\mathrm{PrCu}}_2$${\mathrm{Ge}}_2$ in the vicinity of the Néel temperature (${\mathit{T}}_{\mathit{N}}$) are presented. The magnetoresistance in these samples below ${\mathit{T}}_{\mathit{N}}$ is found to exhibit a variety of behaviors depending upon the magnetic moment of the rare-earth ions and the position of the Fermi level with respect to the antiferromagnetic energy gap. The results also indicate the existence of short-range magnetic-order effects well above ${\mathit{T}}_{\mathit{N}}$ and spin fluctuations (presumably due to Ni) in ${\mathrm{GdNi}}_2$${\mathrm{Si}}_2$. Features characterizing the existence of a metamagnetic transition below ${\mathit{T}}_{\mathit{N}}$ in the ${\mathrm{PrCu}}_2$${\mathrm{Ge}}_2$ around 15 kOe are also observed. The observations are of relevance to the understanding of magnetoresistance behavior in Ce and U materials.

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium α-nitronyl nitroxide (m-${\mathrm{MPYNN}}^{+}$) is a spin-1/2 organic radical. The simple salt, m-${\mathrm{MPYNN}}^{+}$⋅${\mathrm{ClO}}_4^{\mathrm{-}}$⋅1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-${\mathrm{MPYNN}}^{+}$ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ${\mathrm{C}1\mathrm{O}}_4^{\mathrm{-}}$ ions are in the organic layer, joining the m-${\mathrm{MPYNN}}^{+}$ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-${\mathrm{MPYNN}}^{+}$ makes a crystalline solid-solution system, m-${\mathrm{MPYNN}}^{+}$⋅(${\mathrm{ClO}}_4^{\mathrm{-}}$${)}_{\mathit{x}}$⋅${\mathrm{I}}_{1\mathrm{-}\mathit{x}}^{\mathrm{-}}$⋅1/3 (acetone) (0≤x≤1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ${\mathrm{C}1\mathrm{O}}_4^{\mathrm{-}}$ ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction ${\mathit{J}}_1$ forming a triplet state and a weak antiferromagnetic interdimer interaction ${\mathit{J}}_2$ which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that ${\mathit{J}}_2$ quickly weakens with an increase in x, while ${\mathit{J}}_1$ shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagomé antiferromagnet at very low temperatures.

We have systematically investigated the magnetotransport properties of five binary metallic systems (Co-Ag, Fe-Ag, Fe-Cu, Fe-Au, and Fe-Pt). Except for Fe-Pt, which forms a solid-solution in sputtering synthesis, all other systems exhibit giant magnetoresistance (GMR) effects and are immiscible or almost immiscible alloys. Using a combination of structural analysis involving transmission electron microscopy, x-ray diffraction, and magnetic characterization, we have revealed the microstructure for some of the granular alloys where the magnetic particles are embedded in a nonmagnetic matrix. Across the whole particle volume fraction range (0≤x≤100 %), a universal peak of GMR at x=0.15∼0.25 is uncovered in every granular system. The suppression of GMR in the high-x region is attributed to the percolation effect, which also causes a concurrent reduction in magnetic coercivity. Magnetic particle size, a variable under thermal annealing, is another important factor affecting GMR. We have observed correlations between transport properties and particle size, supporting the view that magnetic scattering occurs predominantly on the surface of the particles. Quantitatively, both the field and temperature dependence of the GMR effect can be well accounted for by a modified exchange interaction model. Spin-wave excitation and electron-phonon interaction at finite temperatures are detrimental to GMR. Superparamagnetism and large magnetic anisotropy impede the saturation process of GMR, but have no effect on the saturated value of GMR.

We analyze the frustrated Heisenberg antiferromagnet defined on a honeycomb lattice using a Schwinger-boson mean-field theory. The spin-wave velocity and the susceptibility are presented as functions of the strength of the frustrating interaction for spin S=1/2, and the dynamic structure factor is calculated for various temperatures and frustrations. For large S, we find an increased Ne´el stability with respect to the classical case.

Circular magnetic x-ray dichroism is studied at the L edges of the 3d transition metal series (Cr, Mn, Fe, and Co) as impurities in a nickel host, using partial fluorescence yield to monitor the absorption. While the latter 3d metals Mn, Fe, and Co couple ferromagnetically to the Ni host, Cr is found to couple antiferromagnetically. Using a sum rule derived within an atomic description we obtain values of the spin magnetic moment of the impurities, which are in good agreement with calculations.

We compute sample-to-sample fluctuations in the electrical conductance ${\mathit{G}}_{\mathit{N}\mathit{S}}$ of mesoscopic normal-superconducting boundaries. If the superconductor is clean and the normal conductor diffusive, the rms deviation δ${\mathit{G}}_{\mathit{N}\mathit{S}}$ is approximately a factor of 2 larger than the universal value obtained for normal disordered conductors. This is consistent with predictions of random matrix theories and recent numerical simulations. If the normal material is clean, but the superconductor diffusive, the boundary conductance fluctuation is greater than δ${\mathit{G}}_{\mathit{N}\mathit{S}}$, by an amount which increases as the superconducting coherence length decreases. For a diffusive normal material in contact with a dirty superconductor, δ${\mathit{G}}_{\mathit{N}\mathit{S}}$ is insensitive to weak disorder in the superconductor, but decreases in the extreme dirty limit, where quasiparticles within the superconductor become Anderson localized.

The below-gap, nonbolometric photoresponse of granular, thin-film superconductors seen recently by Strom et al. and by other experimental groups is simulated by a numerical study of the dc voltage response to an applied ac current at finite temperature, by ordered and disordered two-dimensional arrays of resistively shunted Josephson junctions. The current-assisted random generation of vortex-antivortex pairs by the temperature fluctuations and the ac current is observed. The photoresponse for perfect arrays shows a large peak at the Kosterlitz-Thouless transition ${\mathit{T}}_{\mathrm{KT}}$ which moves to lower temperature with increasing ${\mathit{I}}_{\mathrm{dc}}$ or ${\mathit{I}}_{\mathrm{ac}}$. For randomly diluted, disordered arrays at p=0.9 (10% of the junctions removed) the peak in the photoresponse is greatly enhanced and moved to lower temperatures. The behavior of the photoresponse versus bias current, ac power, and frequency is reported. A simple model of random localized heating is tried as a numerical approach to explain some features of the above-gap photoresponse.

The critical current and the relaxation rate were measured for temperatures down to 150 mK and fields up to 40 kOe for the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCN${)}_2$Br, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. At low temperatures, the critical current is found to scale with ${\mathit{B}}^{\mathrm{-}1/2}$ while the tunneling rate is found roughly constant above 5 kOe. This disagrees with theories considering bundles of flux lines pinned collectively. Plastic shear of the flux-line lattice might provide an alternative mechanism in this case.

The superconductor-normal-metal phase transitions in V/Cu superlattices have been studied in parallel and perpendicular magnetic fields. Two crossovers from three-dimensional to two-dimensional and from two-dimensional to three-dimensional have been observed in magnetoresistance R(H,T) and in dependencies of the fluctuating conductivity σ’(T) in a parallel magnetic field. The crossover in low magnetic field is caused by the fact that the superconducting coherence length ${\xi }_{\mathit{S}}$(T) becomes of the order of the superstructure period Λ. The crossover in high magnetic field is due to the competition of the normal-metal coherence length ${\xi }_{\mathit{N}}$ and magnetic length ${\mathit{L}}_{\mathit{H}}$. The experimental results are in good agreement with numerical solution of the Ginzburg-Landau equation for superconductor-normal-metal-superconductor (SNS) superlattices.

The theory of macroscopic quantum tunneling is applied to a current-biased dc SQUID which constitutes a system of two interacting quantum degrees of freedom coupled to the environment. The decay probability is obtained in the exponential approximation for the overdamped case. Close to the critical driving force of the system, the decay of the metastable state is determined by a unique instanton solution describing the symmetric decay of the phases in each of the two Josephson juctions. Upon reducing the external driving force a new regime is reached where the instanton splits. The doubling of the decay channels reduces the decreasing of the decay rate in the quantum regime. A current-temperature phase diagram is constructed based on the Landau theory of phase transitions. Depending on the external parameters the system develops either a first- or a second-order transition to the split-instanton regime.

We use a large-N renormalization-group (RG) method to study a model of interacting boson system with a quenched random potential. In the absence of impurities, the pure boson system has a critical point that describes the superfluid–Mott-insulator (SF–MI) transition. The SF–MI transition of d-dimensional bosons belongs to the (d+1)-dimensional XY model universality class. In this paper, we study the dirty-boson critical points in the neighborhood of this pure SF–MI critical point. In general, the on-site random potential in the original lattice model gives two types of randomness in the effective-field-theoretic action. One is the randomness in the effective on-site repulsion w(x) and the other is the randomness of the chemical potential u(x). It turns out that d=2 is the critical dimension for both types of disorder but the roles of these two types of disorder are reversed as d=2 is crossed. Applying ε=d-2 expansion, we found coupled RG equations for both kinds of randomness which reveal several nontrivial critical points. All the weak random fixed points we found have three or more relevant directions. We conclude that the direct SF–MI transition is unlikely to occur near two dimensions.

The transition temperature and the critical fields of electron-beam evaporated Nb/Gd/Nb triple layers and Nb/Gd multilayers have been determined by measurements of the electrical resistivity and the dc susceptibility. For constant thickness ${\mathit{d}}_{\mathrm{Gd}}$ of the Gd layers, we observe a decrease of ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{H}}_{\mathit{c}2\mathrm{\perp }}$ with decreasing thickness ${\mathit{d}}_{\mathrm{Nb}}$ of Nb layers down to a critical thickness ${\mathit{d}}_{\mathit{c}}$, below which superconductivity is completely destroyed. The parallel critical fields mostly show the square-root temperature dependence near ${\mathit{T}}_{\mathit{c}}$, typical for two-dimensional superconductors. As predicted theoretically, competing pair-breaking mechanisms lead to a nonmonotonic dependence of ${\mathit{H}}_{\mathit{c}\mathrm{\parallel }}$ on ${\mathit{d}}_{\mathrm{Nb}}$. We have also studied the dependence ${\mathit{T}}_{\mathit{c}}$(${\mathit{d}}_{\mathrm{Gd}}$) with constant ${\mathit{d}}_{\mathrm{Nb}}$ and find a decrease of the ${\mathit{T}}_{\mathit{c}}$(${\mathit{d}}_{\mathrm{Gd}}$) curve with increasing ${\mathit{d}}_{\mathrm{Gd}}$ and a steplike structure at ${\mathit{d}}_{\mathrm{Gd}}$≊20 Å. To clarify the nature of this step, the ferromagnetic transition of the Gd films is determined with the transverse magneto-optical Kerr effect. Long-range magnetic order is found only above ${\mathit{d}}_{\mathrm{Gd}}$≊20 Å, which is attributed to the formation of a discontinuous film below this thickness. These results indicate a change in the underlying pair-breaking mechanism.

A recently proposed nonperturbative method for evaluation of the thermodynamic scaling functions in the nominal critical region of quasi-two-dimensional type-II superconductors is generalized and extended to arbitrary type-II systems. It is found that, in general, layered superconductors do not exhibit single-parameter scaling except in two opposite limits corresponding to two-dimensional (2D) and (an) isotropic 3D systems. Explicit, closed-form expressions for the scaling functions are constructed in these two limits. The results are found to be in good agreement with experiments and Monte Carlo simulations. The limits on the applicability of the Landau level description of fluctuations are also discussed.