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Based on a simple model with antiferromagnetic interactions at the Cr - Fe interfaces, we construct the magnetic phase diagram for the Cr spacer within Fe/Cr multilayers and wedges. The spin-density wave is found to undergo transitions between incommensurate (I) phases with a decreasing number of nodes n as the temperature increases for a fixed number N of Cr monolayers. In the limit , the incommensurate to commensurate (C) transition temperature from n = 1 to n = 0 nodes is independent of the Cr - Fe coupling strength but sensitively depends on the strain and doping. These results agree with recent neutron-scattering studies that the SDW inside Fe/Cr multilayers may be either C or I depending on the temperature and Cr thickness.

The full set of high-energy spectroscopy measurements including X-ray photoelectron valence band spectra and soft X-ray emission valence band spectra of both components of FeSi (Fe K_beta_5, Fe L_alpha, Si K_beta_1,3 and Si L_2,3) are performed and compared with the results of ab-initio band structure calculations using the linearized muffin-tin orbital method and linearized augmented plane wave method., 11 pages + 3 PostScript figures, RevTex3.0, to be published in J.Phys.:Cond.Matter

We demonstrate the consistency of the method that introduces two parameters, the inverse of the square root of the spin length and the characteristic soliton length, in the study of solitons in spin systems.

We study the interlayer coupling between metallic ferromagnetic materials, separated by a thin insulator spacer, modelled as a Kondo insulator, in an attempt to understand the behavior of Fe/FeSi multilayered structures. We find that interlayer magnetic coupling is antiferromagnetic at large band filling and ferromagnetic at small band filling with only one change in sign as a function of electron density. This dependence of the coupling on the electron density is contrasted with the pure RKKY coupling that would arise in a simple metal. It gives a way of understanding the photoinduced antiferromagnetic interlayer coupling in Fe/FeSi superlattices seen in recent experiments.

The interlayer coupling for thin spacer layers is predominantly antiferromagnetic in some metallic multilayered structures. We interpret this tendency towards antiferromagnetism to arise from a competition between RKKY-like and superexchange contributions to the coupling. For a free-electron gas with no structure in the density of states above the Fermi level there is a complete cancellation of these contributions, so that coupling derived from the s-d model is indistinguishable from a point contact spin-dependent scattering potential. However, in real solids the non-cancellation of the non-oscillatory parts of these two contributions, i.e. antiferromagnetic bias, can arise from the structure of the density of states of the spacer layer close to the Fermi surface.

An inhomogeneous Heisenberg spin Hamiltonian with single ion anisotropy is used to investigate the nonlinear excitations in ferromagnetic chain. By means of the Holstein-Primakoff transformation and Glauber's coherent–state representation, the equation of motion for anni-hilation operator a(j) is reduced to a nonlinear Schrodinger-like equation in the semiclassical approximation and the long wave approximation. For a homogeneous system, the exact and explicitly single soliton (localized magnon state) and two–magnon bound state solutions are Given by the inverse scattering transform.

The behavior of the antiferromagnetic spin 1/2 Heisenberg Toda chain is investigated in a space of interactions which include exchange anisotropy, bond alternation and Toda-like spinlattice coupling by means of coherent-state for spin. We analyse magnetically driven lattice instabilities and find that multiphase structure only occurs under the condition of bond alternation. Three phase diagrams and tricritical points are obtained and some relevant physical properties are discussed.

An inhomogeneous Heisenberg spin Hamiltonian with single-ion anisotropy is used to investigate the non-linear excitations in a ferromagnetic chain. By means of the Holstein-Primakoff transformation and Glauber's coherent-state representation, the equation of motion for the annihilation operator a(j) is reduced to a non-linear Schrodinger-like equation with variable coefficients. Its non-linear modified terms are strongly restricted by the relation between the continuum approximation ( eta =a/ lambda 0, where eta is the 'degree' of the long wavelength, a is the lattice constant and lambda 0 the characteristic wavelength of the excitation) and the semiclassical approximation ( in =1/ square root S, where in is the degree of truncation of the operator expansion and S is the spin length). When assuming that eta =O( in ) and after retaining the terms to O( in 4), the motion of the coherent amplitude for the homogeneous case satisfies the non-linear Schrodinger equation. The single-soliton and two-soliton bound-state solutions are given. The results show that the magnon localization and two-magnon bound state are possible in the chain. Other relations between eta and in ( eta =O( in 1/2), eta =O( in 3/2 and eta =O( in 2)) are also discussed.

The effect of annealed antiferromagnetic bond randomness on the phase transitions of the Quantum Ising Model (QIM) is studied by using mean-field renormalization group method. It is argued that bond randomness drastically alters multicritical phase diagram via transverse field. Multicritical points and coexistence region of ferromagnetic and antiferromagnetic case exist only at weak transverse field, and are entirely eliminated at strong transverse field. The coexistence region diminishes in reducing the fluctuation interaction. This physical picture demonstrates that the competition between transverse field and exchange interaction and fluctuation interaction via bond randomness play an important role in generating multiphase structure. Another consequence of competition is that tricritical points of first-second order phase transitions are not entirely eliminated by bond randomness in two-dimensional QIM.

The formation of magnetic domains in thin films with large perpendicular anisotropy is investigated. By using a simple model of flux closure strip domain patterns, the author finds the domain size in very thin films depends linearly on film thickness. This interesting result agrees well with the experimental observation of magnetic domains in thin epitaxial Co/Au(111) films (by Allenspach and co-workers).

A double-chain mean-field renormalization group approach for the Ising model is formulated. With the approach, transition temperatures and critical exponents for several numbers of nearest-neighbor chains are obtained.

The behaviour of the antiferromagnetic nearest-neighbour spin-1/2 Heisenberg chain is studied in a space of interactions that include exchange anisotropy ( alpha ), bond alternation ( lambda ), and spin-lattice interaction ( gamma ). Using the coherent state method for spin, the authors analyse magnetically driven lattice instabilities and find that phase transitions occur under the condition of large bond alteration or strong spin-lattice coupling, which means that more staggered interactions make it easy for phase transitions to take place. The three-phase diagram and the tri-critical point are obtained.