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Many experiments have verified the presence of a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. We review the recently proposed interlayer magnetic coupling mediated by a SDW. Unlike previously proposed mechanisms, this magnetic coupling is strongly temperature-dependent. Depending on the temperature and the number N of Cr monolayers (ML), the SDW may be either commensurate (C) or incommensurate (I) with the BCC Cr lattice.

Several recent experiments have detected a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. We use two simple models to predict the behavior of a collinear SDW within an Fe/Cr/Fe trilayer. Both models combine assumed boundary conditions at the Fe-Cr interfaces with the free energy of the Cr spacer. Depending on the temperature and the number N of Cr monolayers, the SDW may be either commensurate (C) or incommensurate (I) with the bcc Cr lattice. Model I assumes that the Fe-Cr interface is perfect and that the Fe-Cr interaction is antiferromagnetic. Consequently, the I SDW antinodes lie near the Fe-Cr interfaces. With increasing temperature, the Cr spacer undergoes a series of transitions between I SDW phases with different numbers n of nodes. If the I SDW has $n=m$ nodes at $T=0$, then n increases by one at each phase transition from m to $m\ensuremath{-}1$ to $m\ensuremath{-}2$ up to the C phase with $n=0$ above ${T}_{\mathrm{IC}}(N).$ For a fixed temperature, the magnetic coupling across the Cr spacer undergoes a phase slip whenever n changes by one. In the limit $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty},$ ${T}_{\mathrm{IC}}(N)$ is independent of the Fe-Cr coupling strength. We find that ${T}_{\mathrm{IC}}(\ensuremath{\infty})$ is always larger than the bulk N\'eel transition temperature and increases with the strain on the Cr spacer. These results explain the very high $\mathrm{IC}$ transition temperature of about 600 K extrapolated from measurements on Fe/Cr/Fe wedges. Model II assumes that the I SDW nodes lie precisely at the Fe-Cr interfaces. This condition may be enforced by the interfacial roughness of sputtered Fe/Cr multilayers. As a result, the C phase is never stable and the transition temperature ${T}_{\mathrm{N}}(N)$ takes on a seesaw pattern as $ng~2$ increases with thickness. In agreement with measurements on both sputtered and epitaxially grown multilayers, model II predicts the I phase to be unstable above the bulk N\'eel temperature. Model II also predicts that the I SDW may undergo a single phase transition from $n=m$ to $m\ensuremath{-}1$ before disappearing above ${T}_{\mathrm{N}}(N).$ This behavior has recently been confirmed by neutron-scattering measurements on CrMn/Cr multilayers. While model I very successfully predicts the behavior of Fe/Cr/Fe wedges, a refined version of model II describes some properties of sputtered Fe/Cr multilayers.

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 average separation of two-dimensional islands in Cu(100) submonolayer homoepitaxy as a function of the deposition flux at 213 K has been studied using spot profile analysis low-energy electron diffraction. As the flux decreases, a large change in the apparent critical island size, from one to a value between seven and 12 atoms, is obtained even though the temperature is held constant. This is shown to be consistent with a recently proposed dimer shearing mechanism which has a significant influence on the stability of islands with eight atoms or less.

A spin-density wave (SDW) is shown to mediate a strongly temperature-dependent magnetic coupling between magnetic proximity layers. For parallel or antiparallel moments in the proximity layers, the order parameters of the SDW oscillate as a function of the spacer thickness with a two monolayer period. The SDW phase transition between incommensurate and commensurate phases can be controlled by flipping the magnetization of one of the proximity layers. {copyright} {ital 1997} {ital The American Physical Society}

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.

The spin polarization of conduction electrons in paramagnetic layers due to adjacent magnetic layers has been recently observed for copper spacer layers, and in a chromium wedge-like film on top of an iron whisker. Also, we explain the relation of this spin polarization to the indirect coupling of the magnetic layers through paramagnetic spacer layers. We show that the effect of spin-dependent potentials in magnetic multilayers is far more pronounced for CPP-MR than for CIP-MR. This can add to the difference between CIP-MR and CPP-MR that is due to the spin-dependent scattering in the multilayers.

Interlayer coupling in metallic multilayers is generally thought to be alun tothe RKKY coupling of magnetic moments through conduction electrons. By analysing recent experimental data on the spin polarisation of chromium on iron substrates we confirm that the RKKY coupling cannot by itself explain the interlayer coupling in Fe/Cr multilayers. An additional coupling is warranted by the data, and we explain its origin

Solitary waves in a spin chain with biquadratic anisotropic exchange interaction are investigated by means of the Holstein-Primakoff bosonic representation of spin operators. It is argued that the modified terms of the nonlinear Schrödinger equation are restricted strongly by the relative ratio of two small parameters η and ε used in long wave approximation and semiclassical approximation respectively. When assuming that η and ε2 have the same order (η−ε2, the so-called “super-long wave approximation”), after retaining terms of the equivalent order O(ε8), we find that the motion of Bose operator in the anisotropic case satisfies Schrödinger equation with high-order nonlinear term α|α|4. Its solitary wave solution and relevant physical results are discussed.

We have studied the localization of multivibrational states in diatomic nonlinear lattice chain. A simple model Hamiltonian presented here describes a system containing two kinds of phonons. The equations of motion for these boson operators are two partial differential equations with nonlinear coupling in long-wave approximation. With the help of the method of multiple scales, these equations are reduced to the nonlinear Schrödinger equation. It is shown that soliton-like phonon localized modes, multi-phonon localized modes can exist. The possibility of observing the gap solitons (phonon localized modes in the frequency gap) in diatomic nonlinear lattice chain is predicted.

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.

An Fe/Au monatomic multilayer, consisting of alternating single Fe and Au layers, has been studied by means of the self-consistent full-potential linearized augmented plane wave method. We show by total energy minimization that this artificial thin film is in the tetragonal $L1_{0}$ ordered structure with the ratio of the interlayer spacing to the intralayer lattice constant at 0.865. In this configuration, the magnetic moment in each monolayer, the spin-polarized electronic density of states, and the corresponding band structure are calculated. The results are discussed in connection with recent experiments., Comment: 7 page Revtex, 5ps Figures, uuencode, to appear in PRB

The spin polarization of conduction electrons in a paramagnetic layer due to an adjacent magnetic layer has been derived based on the mixing interaction between the local and conduction electrons. Specifically, based on the band structure of paramagnetic chromium, we have calculated the spin polarization in a Cr layer on Fe(100) substrates. The results are in reasonably good agreement with recent experimental data.

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).

Many experiments have verified the presence of a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. The authors review the recently-proposed interlayer magnetic coupling mediated by a SDW. Unlike previously proposed mechanisms, this magnetic coupling is strongly temperature-dependent. Depending on the temperature and the number N of Cr monolayers (ML), the SDW may be either commensurate (C) or incommensurate (I) with the bcc Cr lattice.

We develop series expansions for the ground state properties of the Hubbard model, by introducing an Ising anisotropy into the Hamiltonian. For the two-dimensional (2D) square lattice half-filled Hubbard model, the ground state energy, local moment, sublattice magnetization, uniform magnetic susceptibility and spin stiffness are calculated as a function of $U/t$, where $U$ is the Coulomb constant and $t$ is the hopping parameter. Magnetic susceptibility data indicate a crossover around $U\approx 4$ between spin density wave antiferromagnetism and Heisenberg antiferromagnetism. Comparisons with Monte Carlo simulations, RPA result and mean field solutions are also made., 22 pages, 6 Postscript figures, Revtex

The $T=0$ spin stiffness $\rho _{s}$ and the transverse susceptibility $\chi _{\perp }$ of the square lattice half-filled Hubbard model are calculated as a function of the Hubbard parameter ratio $U/t$ by series expansions around the Ising limit. We find that the calculated spin-stiffness, transverse susceptibility, and sublattice magnetization for the Hubbard model smoothly approach the Heisenberg values for large $U/t$. The results are compared for different $U/t$ with RPA and other numerical studies., Comment: 9 Revtex pages, 3 Postscript figures, Europhys. Lett. in press

We present an application of high-order series expansion in the coupling constants for the ground state properties of correlated lattice fermion systems. Expansions have been generated up to order $(t/J)^{14}$ for $d=1$ and $(t/J)^8$ for $d=2,\ 3$ for certain properties of the symmetric Kondo lattice model. Analyzing the susceptibility series, we find evidence for a continuous phase transition from the ``spin liquid'' phase characteristic of a ``Kondo Insulator'' to an antiferromagnetically ordered phase in dimensions $d\ge2$ as the antiferromagnetic Kondo coupling is decreased. The critical point is estimated to be at $(t/J)_c\approx0.7$ for square lattice and $(t/J)_c\approx0.5$ for simple-cubic lattice., Comment: 12 pages, Revtex, replace previous corrupted file

The Anderson model of local-state conduction electron mixing is applied to the p roblem of interlayer magnetic coupling in metallic multilayered structures with palladium (Pd) spacer layers. An oscillation period of $5$ spacer monolayers and the tendency towards ferromag netic bias of the interlayer magnetic coupling that we obtain are consistent with the experimental data., Comment: 6 Latex pages, 5 figures upon requires

The intrinsic localized modes in a homogeneous nonlinear one-dimensional lattice are investigated by use of the method of multiple scales combined with a quasidiscreteness approximation. It is shown that, in the presence of cubic anharmonicity, some asymmetric intrinsic localized modes with zero-group velocity can exist in the lattice and that the eigenfrequencies of these localized modes are above the top of the harmonic frequency band.

A partir du point de vue du liquide reticulaire quantique, ou meme une tentative pour comprendre les excitations non lineaires de type soliton dans la chaine quantique de Heisenberg anisotrope; a l'aide de la methode de l'etat coherent. On montre que les termes modifies de l'equation de Schrodinger non lineaire sont fortement restreints par la relation entre l'approximation du continuum et l'approximation semi-classique representee par deux parametres de la petitesse η et e, respectivement. Lorsque l'on considere que η et e sont du meme ordre, apres retention des termes d'ordre O(e 6 ), l'equation du mouvement des operateurs de Bose satisfait a l'equation de Schrodinger non lineaire. Obtention des excitations des solitons d'enveloppe

Magnetic coupling in multilayer materials is a fascinating phenomenon that manifests itself in a wide range of effects, including novel magnetic arrangements and anomalous transport properties. We employ the locally self-consistent multiple scattering method,1 which has recently been extended to treat general noncollinear magnetic systems,2 to study interlayer magnetic coupling in Fe/Cr(100) multilayer structures. The direction and magnitude of magnetization of each monolayer in a magnetic multilayer with 10 monolayer (ML) Fe layers separated by a 5 ML Cr spacer are obtained. We found that the Fe layers are nearly 90° coupled, and the Cr spacer is in spiral magnetic configuration. We attribute this biquadratic coupling in Fe/Cr multilayers to the nature of itinerant antiferromagnetism of the Cr spacer. These results will be discussed in connection with recent experiments.

Antiferromagnetic (AF) coupling has been observed in sputtered Fe/Si multilayers at room temperature, with thin spacers (

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.