We have grown ultrathin films of Mn on fcc Co(001) substrates and have investigated their magnetic properties using the element-specific techniques of soft-x-ray absorption (SXA) and x-ray magnetic circular dichroism (XMCD). We find that a one monolayer film of Mn is ferromagnetically ordered at room temperature and magnetically aligned with the fcc Co (001) substrate. The absorption spectrum of monolayer Mn is in excellent agreement with calculations for an isolated high-spin atom. By comparing our XMCD and SXA results, we conclude that the surface layer of Mn is in a high-spin state. The ferromagnetic ordering of monolayer Mn results from this high-spin layer being in direct contact with the ferromagnetic Co substrate.

The validity of finite-size scaling in the presence of an inhomogeneous external field vanishing in the thermodynamic-limit is studied using a fully finite three-dimensional mean spherical model. The external field is chosen to change sign stepwise in one space dimension and to be translationally invariant in the other two dimensions, in which the lattice is assumed periodic. The boundary conditions in the direction of broken translational invariance are (i) periodic, and (ii) free, and (iii) fixed. Exact expressions for the magnetization profile are derived and studied. An extended, coordinate-dependent finite-size scaling is found to hold near the shifted critical temperature. Different scaling forms hold near the bulk critical temperature: in case (ii) the distance from the boundary scales with the finite-size correlation length, and in case (iii) with the linear size of the system.

This paper reports the results of a study of the distribution and localization of the magnon modes in one-dimensional Heisenberg spin glasses with nearest-neighbor interactions. The analysis is limited to high fields and frequencies near the precession frequency. Both symmetric and asymmetric distributions of exchange interactions of the form P(J)∝‖J${\mathrm{\Vert }}^{\mathrm{-}\mathrm{\alpha }}$ (α<1) are treated in detail. The results of approximate calculations based on the coherent-exchange approximation are shown to be in good agreement with numerical data obtained by applying mode-counting techniques to arrays of ${10}^7$ spins. Particular emphasis is placed on the qualitative differences in the behavior that arise depending on whether the average values of ${\mathit{J}}^{\mathrm{-}1}$ and ${\mathit{J}}^{\mathrm{-}2}$ are zero, nonzero, or infinite.

Magnetic circular x-ray dichroism experiments at the ${\mathit{M}}_{4,5}$ absorption edges of cerium in the intermetallic compounds CeCuSi, ${\mathrm{CeRh}}_3$${\mathrm{B}}_2$, and ${\mathrm{CeFe}}_2$ are reported. By applying general sum rules, it is shown that these experiments are able to yield both the magnitude and the direction of the 4f magnetic moment on Ce. An estimation of the orbital contribution to those 4f moments is given. Our experiments demonstrate the existence of a 4f magnetic moment on Ce in ${\mathrm{CeFe}}_2$ and confirm the extreme sensitivity of the 4f orbital contribution to the degree of localization of the 4f electrons. This 4f orbital contribution is significantly higher than the one predicted from spin-resolved band-structure calculations.

We report the results of resonant x-ray magnetic scattering experiments on bulk and thin-film single crystals of holmium. The scattering at the principal magnetic reflection has been characterized as a function of the temperature in the spiral phases near and below their respective Néel temperatures. The integrated intensity of the principal magnetic peak in both samples shows power-law behavior versus reduced temperature with nearly equal exponents. The exponents for the scattering at the resonant second and third harmonics in the bulk sample are not simple integer multiples of the first, and motivate the consideration of simple scaling corrections to mean-field theory. We also present and compare the results of high-resolution measurements of the temperature dependence of the magnetic wave vectors, c-axis lattice constants, and correlation lengths of the magnetic scattering of the two samples in their spiral phases. Although the qualitative behavior is similar, systematic differences are found, including uniformly larger magnetic wave vectors and the suppression of the 1/6 phase in the film. The spiral magnetic structure of the film forms a domain state at all temperatures in the ordered phase. The magnetic correlation lengths of both samples are greatest near the Néel temperature, where that of the film appears to exceed the translational correlation lengths of the lattice. As the temperature decreases, the magnetic correlation lengths also decrease. These results are discussed in terms of the strain present in the samples.

A detailed investigation of the structure and magnetic properties of ${\mathrm{Er}}_2$${\mathrm{Fe}}_{17}$${\mathrm{C}}_{\mathit{x}}$ compounds is presented in this paper. The interstitial carbides ${\mathrm{Er}}_2$${\mathrm{Fe}}_{17}$${\mathrm{C}}_{\mathit{x}}$ with x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 have been synthesized by arc melting and by the gas-phase-interaction method. The unit-cell volume of the compounds is found to increase with increasing carbon concentration. The magnetic properties were studied in a Faraday balance and by measurements of the ac susceptibility and the high-field magnetization. The dependencies of Curie temperatures, spin-reorientation temperatures, and the Er-Fe magnetic-coupling strength on the carbon content were determined. The results have been discussed, including the difference of the effect of carbon and nitrogen atoms on the Er-Fe coupling.

The concentration dependence of the Curie temperature in binary, ferromagnetic Ising systems on the simple-cubic lattice is studied by high-accuracy Monte Carlo simulations using the Swendsen and Wang algorithm. Our results are in good agreement with the known mean-field-like approaches. Based on former theories, a formula for estimating the Curie temperature of these systems is proposed.

The dopant-agent–π system magnetic interactions in ${\mathrm{FeCl}}_4^{\mathrm{-}}$-doped poly(3-alkylthiophenes) (PAT’s) have been studied. In particular, PAT’s were subjected to magnetic susceptibility and magnetization measurements. The main goal of the experiments was to obtain temperature and magnetic-field dependencies of the above-mentioned magnetic quantities. The PAT samples were doped with ferric trichloride to different doping levels. The susceptibility and magnetization studies have shown that in the doped conjugated system the transition from a paramagnetic phase to an ordered phase occurs as the temperature is lowered. The magnetic interactions observed are associated with the dopant-agent–charge-carrying-species antiferromagnetic coupling. The coupling is stabilized by a retro or forward virtual charge transfer between the impurity molecule and the polymer chain.

${\mathrm{LaCoO}}_3$ exhibits two magnetic-electronic transitions, one near 90 K and a second near 500 K. A previous study of the paramagnetic scattering using polarized neutrons demonstrated that the low-temperature transition is associated with the thermal excitation of ${\mathrm{Co}}^{3+}$ ions from the low-spin to the high-spin state. In the present work, we extend the paramagnetic-scattering measurements up to a temperature of 700 K. We find that the magnetic-scattering intensity decreases monotonically for temperatures above 300 K, indicating that the high-temperature transition is not dominantly magnetic in origin. Furthermore, the anomalous thermal expansion associated with the low-temperature transition is measured and shown to be consistent with a simple theoretical model for the spin-state transition. For comparison, paramagnetic-scattering measurements for ${\mathrm{La}}_{0.92}$${\mathrm{Sr}}_{0.08}$${\mathrm{CoO}}_3$ are also presented. In this material the ferromagnetic correlations are substantially stronger than in the undoped compound, and no transition to the low-spin state is observed. Instead, the paramagnetic scattering increases steadily with decreasing temperature until saturating below 24 K, the same temperature at which the magnetization of the zero-field-cooled specimen shows a sharp cusp. These results suggest that the magnetic moments in the doped compound freeze into a spin-glass state at low temperature.

We show that zero-field muon-spin-relaxation studies of the critical paramagnetic fluctuations in ferromagnets are mostly sensitive to fluctuation modes at small wave vectors. The transverse-fluctuation modes can be probed very close to the zone center when the temperature approaches the Curie temperature. A quantitative determination of the zone probed in Ni and Fe is performed.

We present results of exact diagonalization of the isotropic antiferromagnetic S=1 Heisenberg chain by the Lanczos method, for finite rings of up to N=22 sites. The Haldane gap G(N) and the ground-state energy per site e(N) converge, with increasing N, faster than a power law. By VBS and Shanks transformations, the extrapolated values are G(∞)=0.41049(2) and e(∞)=-1.401485(2). The spin-spin correlation function is well fit by exp(-r/ξ)/ √r with ξ=6.2.

The dynamical structure factor S(Q,ω) of the S=1 antiferromagnetic Heisenberg chain with length 20 at zero temperature is calculated. The lowest-energy states have the δ-function peak at the region π≥‖Q‖>0.3π. At ‖Q‖<0.3π the lowest-energy states are the lower edge of the continuum of the scattering state, the strength of which decreases for large systems. This gives a reasonable explanation for the experimental fact that no clear peak is observed at the region Q<0.3π. This situation is more apparent for the valence-bond solid state. On the contrary for S=1/2 antiferromagnetic Heisenberg chain the lowest-energy states are always the edge of the continuum.

High-temperature series expansions of the spin-spin correlation function for the XY (or plane rotator) model on the triangular lattice are extended by two terms up to order ${\mathrm{\beta }}^{14}$. Tables of the expansion coefficients are reported for the correlaton function spherical moments of order l=0 and 2. Our analysis of the series supports the Kosterlitz-Thouless predictions on the structure of the critical singularities and leads to fairly accurate estimates of the critical parameters.

Standard Monte Carlo cluster algorithms have proven to be very effective for many different spin models. However, they fail for frustrated spin systems. Recently, a generalized cluster algorithm was introduced that works extremely well for the fully frustrated Ising model on a square lattice by placing bonds between sites based on information from plaquettes rather than links of the lattice. Here we study some properties of this algorithm and some variants of it. We introduce a practical methodology for constructing a generalized cluster algorithm for a given spin model, and apply this method to some other frustrated Ising models. We find that such algorithms work well for simple fully frustrated Ising models in two dimensions, but appear to work poorly or not at all for more complex models such as spin glasses.

The single-bond superexchange anisotropy resulting from the 2${\mathit{p}}_{\mathrm{\sigma }}$ and the 2${\mathit{p}}_{\mathit{z}}$ orbitals on the ligand ion of the cuprates is analyzed for a general case in which the oxygen octahedra surrounding each copper rotate around a general axis in the ${\mathrm{CuO}}_2$ plane. An extension of Moriya’s calculation shows that when the oxygen orbitals are degenerate, the single-bond spin interaction is rotationally invariant, contrary to previous claims. When the orbitals are nondegenerate the single-bond Hamiltonian is anisotropic, but the anisotropy is small. The ground state of the entire ${\mathrm{CuO}}_2$ plane is found to be completely antiferromagnetic in the low-temperature tetragonal phase. The weak ferromagnetism of the low-temperature orthorhombic phase is mainly due to the anisotropy resulting from frustration of the single-bond interactions. These frustration effects also generate an anisotropy which dominates that due to the nondegeneracy of the oxygen orbitals. We present explicit results for the weak ferromagnetic moment and for the spin-wave spectrum, as a function of the direction of the octahedra rotation axis.

We solve numerically the dissipative Landau-Lifshitz-Gilbert equations to consider hysteresis in fine magnetic particles. Finite-size effects are studied for two models with uniaxial anisotropy—bulk random axis and surface anisotropy only. It is demonstrated that the latter model introduces considerable effects for small enough particles when the coupling to the anisotropy is equal or greater than the coupling to the isotropic Heisenberg exchange. We show that some features of magnetization reversal are associated with spins at the surface of fine particles.

The magnetic structure of Er with a magnetic field applied in the hexagonal basal plane has been studied using a combination of experimental techniques and mean-field modeling. From neutron-scattering and magnetization measurements, phase diagrams are constructed. At temperatures above approximately 20 K, the application of a field is found to favor cycloidal structures with modulation wave vectors of ${\mathbf{q}}_{\mathit{c}}$=(6/23)${\mathbf{c}}^{\mathrm{*}}$,(4/15)${\mathbf{c}}^{\mathrm{*}}$, and (2/7)${\mathbf{c}}^{\mathrm{*}}$. For fields above ≊40 kOe, the (2/7) structure dominates the phase diagram. From a detailed study of this most stable cycloid, we determine how it distorts as the field is increased. In low fields, there is a spin reorientation, so that the plane of the cycloid becomes perpendicular to the applied field, while in larger fields, the cycloid collapses through a series of fanlike structures. At lower temperatures, as the field is increased there is a first-order transition at ≊20 kOe from a cone to a fan in the a-c plane.

The effect of a magnetic field on Mott-Hubbard systems is investigated by studying the half-filled Hubbard model in the limit of infinite dimensions. A first-order metamagnetic transition between the strongly correlated metal and the Mott insulator is found for a critical value of the applied field. The field and temperature dependence of the magnetization, one-particle properties, and susceptibility are studied and compared to the Gutzwiller approximation. The experimental relevance for transition-metal oxides and liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is discussed.

In this work, we continue the study of the supersymmetric t-J model with 1/${\mathit{r}}^2$ hopping and exchange without translational invariance. A set of Jastrow eigenfunctions are obtained for the system, with eigenenergies explicitly calculated. The ground state of the t-J model is included in this set of wave functions. The spectrum of this t-J model consists of equidistant energy levels which are highly degenerate.

We investigate a class of resonating-valence-bond wave functions on the triangular lattice which interpolate between the √3 × √3 Néel state and a dimer state according to the range of the bonds and the similar two classes of resonating-valence-bond wave functions on the kagome$aa— lattice constructed from the √3 × √3 and q=0 Neél states. Numerical calculations show that a √3 × √3 wave function gives for the triangular lattice a variational energy and spin-spin correlations in very good agreement with diagonalization results on 12- and 36-site systems. Rather low variational energies are also obtained with trial functions of the √3 × √3 and q=0 type for the kagome$aa— lattice but spin-spin correlations beyond first neighbors are not in good agreement with diagonalization results on 12- and 36-site systems. For the 12-site system, the spin-spin correlations of the best q=0 wave function most resemble those of the first excited state. The q=0 and perhaps the √3 × √3 wave functions may describe excited states close to the ground state in the case of larger systems.