Electron-paramagnetic-resonance (EPR) spectroscopy has previously been employed in investigating excited levels of the ground-state manifold of the 5${\mathit{f}}^7$-configuration ion ${\mathrm{Cm}}^{3+}$ diluted in single crystals of the tetragonal-symmetry, zircon-structure hosts ${\mathrm{LuPO}}_4$ and ${\mathrm{YPO}}_4$. The three intradoublet resonances that were observed in these earlier studies, however, were significantly broadened due to radiation damage arising from the use of the 18.1-yr half-life isotope ${}_{}{}^{244}{}_{}{}^{}\mathrm{Cm}$. In the present work, the isotope ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$ (${\mathit{T}}_{1/2}$=3.4×${10}^5$ yr) with a significantly lower specific activity has been incorporated in ${\mathrm{LuPO}}_4$-host single crystals in order to reduce the internal radiation-induced damage that can broaden both EPR and optical transitions. By employing these ${}_{}{}^{248}{}_{}{}^{}\mathrm{doped}$ ${\mathrm{LuPO}}_4$ samples, it has been possible to observe an additional interdoublet transition for ${\mathrm{Cm}}^{3+}$, to perform more accurate measurements of the various intradoublet transitions at higher microwave frequencies, and to obtain optical absorption and fluorescence data for this system. These EPR and optical results have led to a more complete and accurate determination of the energy-level structure of ${\mathrm{Cm}}^{3+}$. Additionally, it has been possible to detect magnetic resonance transitions for the ${\mathrm{Cm}}^{3+}$ ion at room temperature.

The optical conductivity is calculated within the t-J model for low hole-dopant concentrations. The calculations are done on the basis of a Green’s-function formalism within a slave-fermion Schwinger-boson representation. The Green’s function for holes is calculated in the self-consistent Born approximation to first order in the dopant concentration δ. The Drude weight D is determined by applying a gauge-invariance condition and Ward’s identity. It is found that D is proportional to δ and the inverse effective mass of quasiparticles in the hole pockets. It increases gradually with increasing ratio J/t. We compare our results with those of exact diagonalizations. The latter are obtained for larger values of δ only and therefore must be extrapolated to low dopant concentrations. The two results differ and we argue that nonlinear δ dependences of D appear already at rather low δ values. The finite-frequency conductivity is also calculated by taking account of the lowest-order diagrams for the Green’s function of the current. Like previous results for exact diagonalizations of small clusters, large weight is found in the vicinity of the frequencies characteristic of spin excitation. It may be associated with the midinfrared band observed in experiments.

The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbM${\mathrm{Br}}_9^{3\mathrm{-}}$ (M=${\mathrm{Cr}}^{3+}$, ${\mathrm{Ho}}^{3+}$) dimers as discrete units, and the magnetic excitations of mixed ${\mathrm{Yb}}^{3+\mathrm{-}}$${\mathrm{Cr}}^{3+}$ and ${\mathrm{Yb}}^{3+\mathrm{-}}$${\mathrm{Ho}}^{3+}$ dimers could thus be observed. The ${\mathrm{Yb}}^{3+\mathrm{-}}$${\mathrm{Cr}}^{3+}$ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of ${\mathrm{Cr}}^{3+}$, has to be included in the exchange Hamiltonian. For the ${\mathrm{Yb}}^{3+\mathrm{-}}$${\mathrm{Ho}}^{3+}$ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated ${\mathrm{Ho}}^{3+}$ ion. Taking into account the full (2J+1) ground-state multiplet of ${\mathrm{Ho}}^{3+}$, as well as anisotropic exchange, gives a satisfactory description of this dimer.

Magnetic circular x-ray dichroism (MCXD) studies at ${\mathit{L}}_{2,3}$ absorption edges in intermediate-valence (IV) compounds ${\mathrm{CeFe}}_2$, Ce(${\mathrm{Fe}}_{0.8}$${\mathrm{Co}}_{0.2}$${)}_2$, ${\mathrm{CeCo}}_5$, ${\mathrm{Ce}}_2$${\mathrm{Co}}_{17}$, and trivalent 4${\mathit{f}}^1$ systems (${\mathrm{CeRu}}_2$${\mathrm{Ge}}_2$, ${\mathrm{Ce}}_3$${\mathrm{Al}}_{11}$, CePd, ${\mathrm{CeFe}}_2$${\mathrm{H}}_{3.8}$) at the Ce ${\mathit{L}}_{2,3}$ edges are presented. The comparison with reference compounds (La, Lu, Hf, Gd) confirms the existence, in such IV systems, of an ordered 5d magnetic moment on the Ce site antiferromagnetically coupled with the 3d Fe (Co) moment. Moreover, this study suggests, provided the 4f orbital moment is almost quenched, the existence of a simple proportionality between the MCXD intensity and the magnetic moment of the probed atom. For instance the MCXD method leads to a Ce 5d magnetic moment in ${\mathrm{CeFe}}_2$ of about 0.35${\mathrm{\mu }}_{\mathit{B}}$ antiferromagnetically coupled with the 3d Fe moment, in good agreement with band-structure calculations. The MCXD applications in Ce(Fe,Co${)}_2$ and ${\mathrm{CeFe}}_2$${\mathrm{H}}_{\mathit{x}}$ permit us to define precisely the electronic structure of Ce in such compounds. Particularly we will show that MCXD experiments at the ${\mathit{L}}_{2,3}$ absorption edges of Ce are able to yield valuable information on the degree of hybridization of the 4f electrons in the ground state. Indeed, systematic comparison between the MCXD signals of IV and normal trivalent Ce compounds reveals the extreme sensitivity of the ${\mathit{L}}_{2,3}$ MCXD experiments to the degree of localization of the 4f electrons.

Inelastic-neutron-scattering measurements on cerium and plutonium monopnictides, thought to have moderately delocalized f electrons, yield magnetic-excitation spectra with anisotropic dispersion; while reasonably sharp excitations have been observed only for USb and UTe among presumably more-delocalized uranium monopnictides and monochalcogenides. For UTe the broadening as well as the dispersion is quite anisotropic. We have now extended our previous theory for the magnetic behavior of hybridizing partially delocalized f-electron systems to include hybridization-induced relaxation effects in the magnetic response, and this work and results are reported in the present paper. Each partially delocalized f-electron ion is coupled by hybridization to the band sea; and this both leads to a hybridization-mediated anisotropic two-ion interaction giving magnetic ordering and also gives a damping mechanism, via the coupling to the band sea, for the excitations of the magnetically ordered lattice. This coupling also provides a strong renormalization of the magnetic-excitation energies obtained for the ionic lattice coupled by the two-ion interaction. To treat these effects on the magnetic response we have developed a formalism for calculating the dynamic susceptibility based on the projection-operator method developed by Mori and others. We have applied our model and theory to the behavior of CeSb, CeBi, PuSb, UP, UAs, and UTe; and excellent overall agreement with the wide range of unusual experimentally observed anisotropic magnetic-excitation behavior is obtained.

A scaling analysis near the paramagentic-to-ferromagnetic transition temperature has been performed on amorphous ${\mathrm{Fe}}_{90\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Zr}}_{10}$ (x=1.2 and 5.0) alloys. The extracted values for the critical exponents β, γ, and δ are found to be higher than those predicted by the three-dimensional Heisenberg model. This result supports the idea that below ${\mathit{T}}_{\mathit{c}}$ the magnetic state is not a collinear ferromagnetic state.

We develop a bosonic mean-field theory to describe the spiral phases of a Heisenberg antiferromagnet on a one-dimensional chain, in terms of three bosons at each site. The ground state is disordered, and for large values of the spin S, two different and exponentially small energy gaps are found. The spin-spin correlation function is computed and is shown to decay exponentially at large distances. Our mean-field theory is also shown to be exact in a large-N generalization.

A Green-function theory is presented for the Brillouin light scattering from hybridized surface and bulk spin-wave modes propagating in perpendicularly magnetized films with general asymmetric pinning at the surfaces. The calculations include a spin-wave theory with mixed exchange boundary conditions for the excitation spectrum and a Green-function (linear-response) formalism for light scattering from magnetic films. The influence of dipole-dipole hybridization of the spin-wave dispersion branches on the calculated light-scattering intensities is taken into account, and the dependence of the Brillouin spectra on the pinning conditions is evaluated. Applications of the theory to thin films of Fe and EuO are described.

A quantum Hopfield spin-glass model with transverse fields is presented. A set of equations for the order parameters of a large class of spin-glass models with transverse fields is obtained, and the phase transitions are analyzed with a finite number of patterns p. We find that a reentrant phenomenon is possible for a quantum Mattis model with transverse fields, contrary to the quantum transverse Sherrington-Kirkpatrick-like model. The effects of quantum fluctuations (induced by the transverse field) on the ordered phases of systems with competing interactions are also discussed.

The anisotropy of interacting, identical spins in a film is investigated with two kinds of anisotropy interaction. Calculated anisotropy critically depends on film thickness, microscopic structures, orientations, and surface roughness. We find that the volume anisotropy results from long-range interaction and perpendicular anisotropy is due to short-range interaction. Surface roughness usually enhances the perpendicular anisotropy. The deduction is compared with published experimental data.

A series of ${\mathrm{Co}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ alloys were made in an induction furnace. To make the the alloys in a disordered state, they were heated up to 950 °C, and then quenched in salt water. Experimental quantities measured include saturation moment, specific heat, residual resistivity, and anisotropic magnetoresistance at low temperatures. By comparing those data with those for the ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$, ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Mo}}_{\mathit{x}}$, ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$${\mathrm{W}}_{\mathit{x}}$, ${\mathrm{Co}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Ni}}_{\mathit{x}}$, and ${\mathrm{Co}}_{100\mathrm{-}\mathit{x}}$${\mathrm{Pd}}_{\mathit{x}}$ alloys, we are able to have a complete discussion from the framework of the split-band model. For Co-Pt alloys, the electronic-specific-heat coefficient has no minimum, and the anisotropic magnetoresistance shows no global maximum either.

The single-site spin-fluctuation theory of metallic magnetism developed by Hubbard and Hasegawa is modified and extended to investigate the effects of spin and charge fluctuations on the magnetic properties at finite temperature. The key point of our scheme is to adopt a continuous decomposition of the interacting part of the Hubbard Hamiltonian in the Stratonovich-Hubbard transformation, resulting in the separation of the charge fluctuations from the spin fluctuations. This separation makes a better description for the contrary nature between the two types of fluctuations. In order to remove the ambiguity, a variational principle is introduced to determine the appropriate form of the decomposition of the Hubbard model. The ground-state properties, the Curie temperature, the magnetization, and the paramagnetic susceptibility, etc., at finite temperature are calculated and compared with the Hasegawa theory. The improvement of the Curie temperature is significant, and better behavior of the temperature dependence of the magnetization is obtained.

The results of an ultrasonic study of the magnetic-field–temperature (H-T) phase diagram of hexagonal ${\mathrm{CsNiCl}}_3$ as a function of the angle between the magnetic field and the c^ axis are presented. Upon rotation of the field, the structure of the phase diagram is modified from four phases joined at a multicritical point for H∥c^ to three phases without a multicritical point at other field directions. A previously unseen transition has been detected at intermediate field angles. The behavior of the spin-flop anomaly on the acoustic velocity is also investigated as a function of field angle and temperature. The amplitude of this anomaly has been found to exhibit a strong dependence on these two parameters. An analysis of these phase diagrams has been made using a nonlocal Landau free-energy approach [Phys. Rev. Lett. 60, 45 (1988)].

Group-theoretical techniques are used to analyze the paramagnetic-antiferromagnetic transition in β-MnS and zinc-blende MnTe, which are both thought to undergo a transition to a canted antiferromagnetic ground state. Although the canting angle itself has remained inaccessible to experiment, the magnetic transition is known to be first order. To date, canting angles of 0, π/2, π, 3π/2 have been analyzed theoretically and found to exhibit first-order transitions. The theoretical analysis is, here, completed for arbitrary canting angles in the hopes of finding and then ruling out any canting angles that give rise to a second-order transition. It is found that the group representation associated with a canted ground state of arbitrary canting angle (other than π/2 or 3π/2) is reducible and an argument is provided for a first-order transition to these states on a stronger basis than the usual reducibility of the order parameter. One then concludes that the first-order nature of the experimental transition provides no clue as to the actual canting angle.

We report neutron-diffraction studies of antiferromagnetism in various forms of epitaxially grown zinc-blende (ZB) MnTe: in semibulk (∼1 μm thick) single-crystal films of pure MnTe, in its magnetically diluted derivative ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with 0.695<x<1, and in strongly strained very thin (30–300 Å) single-crystal MnTe layers in MnTe/ZnTe superlattices. ZB Mn chalcogenides are unique exmples of fcc Heisenberg antiferromagnets (AF) with dominant nearest-neighbor interactions. Such a lattice is one of the basic models of topologically frustrated spin systems. Only ZB MnS can be obtained through natural crystallization (and only in a fine powder form, which seriously limits the scope of possible studies on this system). The single-crystal forms of MnTe obtained using molecular-beam epitaxy have made it possible to study the influence of strain on a frustrated fcc antiferromagnet. We observe that such built-in strain strongly affects the domain structure as well as the phase-transition behavior. Furthermore, high-resolution x-ray diffraction reveals pronounced magnetostriction effects in the MnTe films. Both neutron as well as x-ray data indicate a rather unusual effect of a strong temperature shift in the relative populations of two inequivalent AF domain states, and a magnetostriction mechanism underlying this phenomenon is proposed. Finally, the data obtained on ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te films complement the results of previous magnetic studies on bulk forms of this material with x≤0.68.

We consider a spin-glass system with two interacting replicas. In the spin-glass phase we derive the long-time behavior within the Sompolinsky scheme. There are infinitely many spin-glass solutions, which reflect the infinite degeneracy and ultrametric structure of equilibrium states. The two replicas are found to remain correlated even as the replica coupling field goes to zero. The replica coupling field plays the role of symmetry-breaking field for the existence of many solutions. The susceptibility conjugate to the replica coupling field is found to diverge in the whole spin-glass phase.

We report on an acoustic study of insulating glasses doped with dysprosium, gadolinium, and europium ions, which have Kramers degeneracy. The magnetic-ion content is varied between 1 and 10 at. %. The measurements are performed down to 10 mK, using acoustic frequencies up to 900 MHz and in magnetic fields up to 50 kG. There is an effect of the magnetic-ion concentration and the magnetic field on the sound velocity at low temperature. This effect is explained by assuming the existence of two kinds of low-energy excitations: the usual elastic tunneling states and some magnetic tunneling states, which are magnetic moments whose local anisotropy axis is almost perpendicular to the local magnetic field. These magnetic tunneling states have relaxation times shorter than the elastic ones by several orders of magnitude at low temperature. They couple strongly to the latter whose dynamics is then drastically changed. This one is directly observed in saturation-recovery and phonon-echo experiments. The relaxation time ${\mathit{T}}_1$ of the elastic tunneling states is strongly shortened by the magnetic ions, whereas their dephasing time ${\mathit{T}}_2$ remains almost unchanged. This effect implies a relaxation process much more efficient than the usual one-phonon process at low temperature.

We have studied the electrical transport properties of superconducting clusters, periodic arrays of Josephson-coupled superconducting islands containing only a few cells. In such small samples, variations in the coupling between adjacent islands are not averaged out and produce asymmetries in the magnetoresistance as a function of bias current and applied magnetic field. By comparing experimental results with computer simulations of the Josephson phase dynamics in the clusters, we are able to understand the origin of the observed asymmetries and obtain a quantitative estimate of the coupling disorder in our arrays.

Nonlinear dynamics of vortices in current-carrying long Josephson contacts is considered for the cases of weak (${\lambda }_{\mathit{J}}$≫λ) and strong (${\lambda }_{\mathit{J}}$≪λ) couplings, where ${\lambda }_{\mathit{J}}$ and λ are the Josephson and London penetration depths, respectively. The first case concerns the Josephson vortices described by the sine-Gordon equation, whereas the case ${\lambda }_{\mathit{J}}$≪λ corresponds to Abrikosov-like vortices with highly anisotropic Josephson cores which are described by an integral equation for the phase difference cphi within the framework of a nonlocal Josephson electrodynamics. At ${\lambda }_{\mathit{J}}$≪λ, an exact solution for the moving vortex in the overdamped regime is obtained, the fluxon velocity v(j) and the voltage-current characteristic V(j) are calculated. It is shown that the lack of the Lorentz invariance of the integral equation for cphi in the nonlocal regime leads to specific features of the vortex dynamics as compared to the Josephson vortices. The results obtained are employed for the description of nonlinear viscous motion of magnetic flux along planar crystalline defects in superconductors. It is shown that any percolating network of planar defects can considerably reduce the critical current, change the field dependence of the flux-flow resistivity, and result in a nonlinear V(j) in the flux-flow regime.

The impact of increased disorder on superconducting ${\mathit{T}}_{\mathit{c}}$ and ${\mathit{H}}_{\mathit{c}2}$ of amorphous Zr-Cu and Hf-Co has been studied. In these highly disordered materials, disorder was incrementally increased by irradiation with fast neutrons of low intensity. Increased resistivity, increased magnitude of dρ/dT, and smearing of the x-ray-diffraction patterns, confirm the increased disorder. The irradiation effects on ρ and ${\mathit{T}}_{\mathit{c}}$ depend on the amorphous system, but when the ${\mathit{T}}_{\mathit{c}}$ depression is plotted vs resistivity increase, all data fall on the same curve. ${\mathit{T}}_{\mathit{c}}$ data are well described by the Fukuyama-Ebisawa-Maekawa theory. The normalized critical field ${\mathit{h}}^{\mathrm{*}}$(t) increases at low temperatures with increasing disorder. This cannot be explained by present theories.