Showing papers on "Spin wave published in 1991"

The spin- 1/2 Heisenberg antiferromagnet on a square lattice and its application to the cuprous oxides

Abstract: The spin-\textonehalf{} antiferromagnetic Heisenberg model on a square lattice is used to describe the dynamics of the spin degrees of freedom of undoped copper oxides. Even though the model lacks an exact solution, a solid, accurate, and rather conventional picture emerges from a number of techniques---analytical (spin-wave theory, Schwinger boson mean-field theory, renormalization-group calculations), semianalytical (variational theory, series expansions), and numerical (quantum Monte Carlo, exact diagonalization, etc.). At zero temperature, the effect of the zero-point fluctuations is not strong enough to destroy the antiferromagnetic long-range order, despite the fact that we are dealing with a low-spin low-dimensional system. The corrections to the spin-wave theory, which treats perturbatively the effect of such fluctuations around the classical N\'eel ground state, appear to be small. At any nonzero temperature the order disappears and the correlation length at low temperature $T({k}_{B}\frac{T}{J}\ensuremath{\ll}1$, where $J$ is the antiferromagnetic coupling) follows the singular form $\ensuremath{\xi}(T)=C\mathrm{exp}(\ensuremath{\alpha}\frac{J}{{k}_{B}}T)$. In the long-wavelength limit and at low $T$, the model has the same behavior as the quantum nonlinear $\ensuremath{\sigma}$ model in two spatial dimensions and one Euclidean time dimension, which we also study with available analytical and Monte Carlo techniques. The quasiparticles of the theory are bosons; at low $T$ and for wavelengths shorter than the correlation length they are welldefined spin-wave excitations. The spectrum of such excitations and the temperature-dependent correlation length have been determined by neutron and Raman scattering experiments done on ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$. The good agreement of the experimental data with the predictions of this theory suggests that the magnetic state of the undoped materials is the conventional ordered state. We discuss, within a simple mean-field theory, the effect of weak three-dimensional antiferromagnetic coupling and the role of an antisymmetric term, introduced to explain a hidden ferromagnetic behavior of the uniform susceptibility. We find that understanding the copper-oxide antiferromagnetic insulator is only the first essential step towards the development of a theory of the superconductor created upon doping such materials.

Rare Earth Magnetism: Structures and Excitations

Abstract: Elements of rare earth magnetism Magnetic structures Linear response theory The magnetic scattering of neutrons Spin waves in the ferromagnetic rare earths Spin waves in periodic structures Crystal-field excitations in the light rare earths Perspectives for rare earth research References Index.

Nonlinear surface electromagnetic phenomena

Abstract: Preface to the series. Preface. 1. Second-order nonlinear guided wave interactions (W. Sohler). 2. Third-order nonlinear electromagnetic TE and TM guided waves (A.D. Boardman, P. Egan, F. Lederer, U. Langbein and D. Mihalache). 3. The problem of stability and excitation of nonlinear suface waves (N.N. Akhmediev). 4. Nonlinear waves and switching effects at nonlinear interfaces (A.E. Kaplan, P.W. Smith and W.J. Tomlinson). 5. Second-order nonlinear optical effects at surfaces and interfaces (T.F. Heinz). 6. Nonlinear surface magnetoplasma polaritons in semiconductors (V. Ambrazeviciene, R. Brazis and A. Kungelis). 7. Light scattering from phonons and spin waves on surfaces (F. Nizzoli and D.L. Mills). 8. Surface magnetic and structural phase transitions (M.I.Kaganov, A.P. Levanyuk, S.A. Minyukov and A.V. Chubukov). 9. Laser-induced surface gratings (A.M. Prokhorov et al.). 10. Laser-induced surface polaritons and optical breakdown (A.M. Bonch-Bruevich and M.N. Libenson). Author index. Subject index. Cumulative index.

Green's Function Theory of the Two-Dimensional Heisenberg Model : Spin Wave in Short Range Order

Abstract: The two dimensional Heisenberg model is studied with a Green's function decoupling scheme in the ferromagnetic and the antiferromagnetic cases. We employ Kondo and Yamaji's formulation and improve it quantitatively. The present theory does not violate the rotational symmetry and the sum rule of the correlation function. We calculate excitation spectrum, long range order, correlation functions, correlation length, energy, and spin susceptibility. The results are compared with those of the Monte Carlo simulations and the experiments of La 2 CuO 4 . Qualitative and semi-quantitative agreements are obtained. In particular, the temperature dependence of the susceptibility indicated by Monte Carlo simulations is reproduced over all temperature region.

High-energy spin waves in La2CuO4.

Abstract: Time-of-flight spectrocopy using neutrons produced by a spallation source is used to measure the one-magnon scattering throughout the Brillouin zone for La{sub 2}CuO{sub 4} The zone-boundary magnons have an energy {h bar}{omega}{sub ZB}=0312{plus minus}0005 eV and are good eigenstates of the quantum Heisenberg Hamiltonian in that they possess lifetimes {gt}10/{omega} A multiplicative renormalization of the overall frequency scale of classical spin-wave theory accounts for the quantum effects in the one-magnon spectrum

Spin-wave theory of two-dimensional ferromagnets in the presence of dipolar interactions and magnetocrystalline anisotropy

Abstract: A spin-wave theory of two-dimensional ferromagnets taking into account both the long-range dipolar interactions and a uniaxial magnetocrystalline anisotropy is presented. We show that in the case of the perpendicular easy axis the stabilization of long-range ferromagnetic order at finite temperature is due mainly to the anisotropy-induced gap at the bottom of the spin-wave spectrum, whereas the dipolar interactions play a negligible role; on the other hand, in the case of easy-plane anisotropy, the spin-wave spectrum remains gapless, so that the stabilization of long-range ferromagnetic order is due in turn to the long-range character of the dipolar interactions. The temperature dependence of the magnetization is calculated in both cases. The results are discussed in connection with experimental data reported for fcc (001) cobalt monolayers epitaxially grown onto (001) copper single crystals.

Spin dynamics in the quantum antiferromagnetic chain compound KCuF3.

Abstract: The magnetic excitations of a nearly ideal realization of a one-dimensional S=1/2 Heisenberg antiferromagnet, ${\mathrm{KCuF}}_{3}$, have been studied by inelastic neutron scattering. The experimments were performed using the pulsed neutron source ISIS and show that the magnetic scattering occurs at energies well above the top of the linearized spin-wave band. This result shows that it is essential to take account of quantum effects in discussing the excitations. The results are in excellent accord with theoretical predictions for the dynamical correlation function.

Evidence for Oscillations in the Interlayer Coupling of Fe Films Across Cr Films from Spin Waves and M(H) Curves

Abstract: By means of light scattering from spin waves and magneto-optical Kerr measurements we investigated the exchange interaction of two Fe films across a Cr film as a function of the Cr thickness dCr. Using good-quality epitaxial samples with a special wedge geometry of the interspacer we observed up to four full periods of the long-range oscillations of the exchange, including its ferromagnetic parts. The decay of the oscillations does not follow the dCr-2-dependence predicted by classical two-dimensional RKKY-type theory. A fine structure in the first antiferromagnetic region is also observed.

Rare-earth transition-metal intermetallics

Abstract: The combination of itinerant transition metal (M = Fe) and localized rare-earth (R = Gd-Y) magnetism in RFe 2 compounds has been investigated in self-consistent energy band calculations. The computed and measured total moments are in good agreement for all cases where single crystal data are available. We find, however, that there is a significant contribution to the moment from the R-5d partial moments coupled antiparallel to the Fe-3d moment which results from 3d–5d hybridization. The R-4f moments interact with the conduction band system solely by local exchange interactions, which are calculated ab initio from density functional theory. A sum rule for the total 3d + 5d moment is shown to be obeyed and the effective ferrimagnetic exchange interaction between rare-earth and transition-metal moments is discussed. Finally, the spin wave spectra of these materials are evaluated in terms of a model arising from these calculations.

Holes and magnetic textures in the two-dimensional Hubbard model

Abstract: The Hubbard model in a two-dimensional square lattice is studied within the unrestricted Hartree-Fock approximation. It is shown that a large number of self-consistent solutions exist, which correspond to possible metastable spin and charge configurations. A solution, in which the spins are coplanar and form a vortexlike pattern, is described. The solution is stable over most of the parameter range and dominates for large values of {ital U}/{ital t} or at high doping. The influence of this multiplicity of solutions on the phase diagram is discussed.

Second-order spin-wave results for the quantum XXZ and XY models with anisotropy

Abstract: A second-order spin-wave analysis is given for the quantum Heisenberg antiferromagnet with anisotropy on the one-dimensional linear chain, two-dimensional honeycomb lattice, and three-dimensional simple-cubic and body-centered-cubic lattices. A similar analysis is given for the quantum XY model with anisotropy on the one-dimensional linear chain, two-dimensional triangular and honeycomb lattices, and three-dimensional simple-cubic, body-centered-cubic, and face-centered-cubic lattices. The singular behavior at the isotropic point is discussed in detail. We find that the quantum spin reduction of the isotropic Heisenberg antiferromagnet on a two-dimensional honeycomb lattice is much stronger than that on a square lattice.

Spin Dependence of the AHARONOV—BOHM Effect

Abstract: The problem of the proper inclusion of spin in Aharonov—Bohm scattering is considered. It is proposed that this should be accomplished by imposing the requirement that all singularities arising from the presence of spin in the associated wave equations be interpreted as limits of physically realizable flux distributions. This leads to results which confirm the usual cross section in the spinless case but imply nontrivial modifications for the scattering of a polarized spin one-half beam. By applying the technique to a calculation of the virial coefficient for a collection of flux carrying spin one-half particles, some severe obstacles to conventional views of the flux as a parameter which interpolates between bosonic and fermionic statistics are shown to occur. Although similar results for the scattering of arbitrary spin particles obtain in the Galilean limit, it is found that when spin one is considered in the context of a relativistic wave equation the singularity structure is too pathological to yield a consistent interpretation. The exact equivalence of the spin one-half Aharonov-Bohm effect to the Aharonov-Casher effect is also demonstrated and corresponding results for polarized beams are presented. Finally, it is shown that the Aharonov-Bohm effect for arbitrary spin in the Galilean limit is the exact solution in the two-particle sector of a Galilean covariant field theory.

London penetration depth of d-wave superconductors

Abstract: The temperature dependence of the London penetration depth is calculated for a model superconductor stabilized by antiferromagnetic spin fluctuations. The order parameter has {ital d}-wave symmetry. Results are compared with {ital s}- and {ital p}-wave calculations and anisotropic as well as strong-coupling effects are discussed.

Spin-wave, Stoner single-particle and correlated particle-hole pair contributions to thermal demagnetization in amorphous Fe90+xZr10-x

Abstract: Detailed magnetization (M) measurements have been performed on amorphous (a-)Fe90+xZr10-x alloys with x=0 and 1 at temperatures ranging from 4.2 to 300 K in external magnetic fields (H) up to 15 kOe. Arrott plot isotherms of M2(H,T) against H/M(H,T) are nearly linear at high magnetic fields for temperatures well outside the critical region. Spontaneous magnetization varies with temperature as M(0,T)/M(0,0)=1-BT32/-AT2, (M(0,T)/M(0,0))2=a-bT2 and (M(O,T)/M(0,0))2=a'-b'T43/ in the temperature intervals 0

Dipolar interactions and the magnetic behavior of two-dimensional ferromagnetic systems.

Abstract: A simple model is presented for spin-wave excitations and the low-temperature magnetization of ferromagnetic ultrathin films. The films are assumed to be quasi-two-dimensional arrays of fixed magnetic moments and both exchange and dipolar interactions are taken into account. Discrete and continuous models are used to examine the wave-vector dependence of the spin-wave frequency in the long-wavelength limit. This spin-wave theory can be used to calculate the temperature dependence of the saturation magnetization in the limit of small fluctuations. The role of dipolar interactions is examined for a single monolayer and for thin films consisting of two or three atomic layers.

Dynamics of magnetic fluctuations in high-temperature superconductors

Abstract: A Neutron Scattering- Antiferromagnetic Spin Fluctuations in Cuprate Superconductors- Neutron Scattering Measurements of the Magnetic Excitations of High-Temperature Superconducting Materials- Neutron Scattering Study of the Spin Dynamics in YBa2Cu3O6+x- Disordered Low Energy Component of the Magnetic Response in Both Antiferromagnetic and Superconducting Y-Ba-Cu-O Samples- B NMR- Copper and Oxygen NMR Studies On The Magnetic Properties of YBa2CuO7-y- 17O and 63Cu NMR Investigation of Spin Fluctuations in High Tc Superconducting Oxides- Local Hyperfine Interactions of Delocalized Electron Spins: 205TI Investigations in Tl Containing High Tc Superconductors- Weak Coupling Analysis of Spin Fluctuations In Layered Cuprates- Influence of the Antiferromagnetic Fluctuations On The Nuclear Magnetic Resonance In The Cu-O High Temperature Superconductors- Microscopic Models for Spin Dynamics in the CuO2 Planes with Application to NMR- C SR- Recent Topics OF SR Studies Of High-Tc Systems- Recent Results In The Application Of SR To The Study Of Magnetic Properties Of High-Tc Oxides- On the phase diagram of bismuth based Superconductors- D Raman Scattering- Raman scattering from spin fluctuations in cuprates- E Photoemission- Electronic structure of Bi2Sr2CaCu2O8 single crystals at the fermi level- Calculation of the photoemission spectra for the t-J model and the extended hubbard model- F Macroscopic Fluctuations- Microwave absorbtion of superconductors in low magnetic fields- Magnetic properties of a granular superconductor- G Transport Properties- Thermodynamic fluctuations and their dimensionality in ceramic superconductors out of transport properties measurements- H General Theory- Models Of High Temperature Superconductors- BSC Theory Extended To Anisotropic And Layered High-Temperature Superconductors- Correlated Electron Motion, Flux States And Superconductivity- Orbital Dynamics And Spin Fluctuations In Cuprates- Magnetic Frustration Model And Superconductivity On Doped Lamellar CuO2 Systems- I One Band Hubbard Models- Strong Coupling Regime In The Hubbard Model At Low Densities- How Good Is The Strong Coupling Expansion Of The Two Dimensional Hubbard Model- The Hubbard Model For n ? 10: New Preliminary Results- J Heisenberg Model Dynamics- Exact Microscopic Calculation Of Spin Wave Frequencies And Linewidths In The Two Dimensional Heisenberg Antiferromagnet At Low Temperature- Magnetic Excitations In The Disordered Phase Of The 2-D Heisenberg Antiferromagnet- K Quasiparticles and Magnetic Fluctuations- Exact Diagonalization Studies Of Quasiparticles In Doped Quantum Antiferromagnets- Copper Spin Correlations Induced By Oxygen Hole Motion- Spin Polarons In The t-J Model- Analytic Evaluation Of The 1-Hole Spectral Function For The 1-D t-J Model In The Limit J ?- Doping Effects On The Spin-Density-Wave Background- L Magnetic phases- Phase Separation In A t-J Model- Spiral Magnetic Phases As A Result Of Doping In High Tc Compounds

Inelastic polarized neutron scattering from S=1 antiferromagnetic CsNiCl3 in an applied field

Abstract: The authors report inelastic polarized neutron scattering measurements of the excitations in the S=1, quasi-one-dimensional antiferromagnet CsNiCl3 with applied magnetic field. A field up to 5.9 T was applied perpendicular to the chain direction and the field dependence of spin excitation modes at wave vectors corresponding to the one-dimensional zone centre (i.e. ( xi , xi ,1)) was studied. Near the antiferromagnetic ordering point Q=(1/3,1/3,1) two types of field dependence are observed. The behaviour of the higher energy mode suggests that it is the novel longitudinal mode recently proposed by Affleck (1989). The application of the field also reveals a near-degeneracy of the excitations observed at Q=(0,0,1) in zero field. This degeneracy is incompatible with the result of the classical spin wave theory based on the Heisenberg Hamiltonian with a single-site anisotropy.

Microscopic theory of spin arrangements and spin waves in very thin ferromagnetic films

Abstract: We present theoretical studies of the classical ground-state spin configuration and spin waves, in very thin ferromagnetic films, with thickness that ranges from a monolayer to a few tens of layers. The analyses are based on a microscopic model that includes dipolar and exchange interactions between the spins, surface (or interface) anisotropy of single-site character quadratic and quartic in the spin components, along with an external magnetic field applied at an arbitrary direction with respect to the film normal. Issues explored include the nature of spin canting induced by surface anisotropy in ultrathin (few-atomic-layer) films, and in films with thicknesses of up to 100 layers. Also, we explore the nature of spin waves in ultrathin films, in the presence of spin canting, and in thicker films with attention to the interplay between dipolar and exchange contributions to the excitation energy. Most particularly, in the thicker films, we examine the transition from the dipole-dominated Damon-Eshbach waves to the exchange-dominated surface spin waves that emerge from the Heisenberg model.

Spin gap in a generalized one-dimensional t-J model.

Abstract: The existence of a spin gap is studied in a generalization of the one-dimensional {ital t}-{ital J} model. A frustrated next-nearest- neighbor exchange interaction, {ital J}{prime}, gives rise to a spin gap at half-filling that survives for small doping and arbitrary values of {ital J}. Regions of phase separation and divergent superconducting fluctuations are distorted but still present in the extended model.

Wave functions and other properties of spin-reversed quasiparticles at ν=1/m Landau-level occupation

Abstract: A family of wave functions appropriate for arbitrary-spin quasiparticles of the fully polarized, \ensuremath{ u}=1/m incompressible fluid is constructed. The spectrum with spin degrees of freedom included, becomes more complex, and in particular, is shown to be massively degenerate (no Zeeman energy). As a result, for finite systems in the absence of Zeeman energy the absolute ground states for two sectors differing by one flux quantum are spin singlet and maximally polarized. I study the spectrum with two reversed spins near \ensuremath{ u}=1 for up to 160 electrons and find the signature of this trend to persist in the thermodynamic limit. I also find that for all experimentally accessible magnetic field values, the single spin-reversed quasiparticle at \ensuremath{ u}=1 is irrelevant and is preempted by quasiparticles having additional reversed spins.

Enhanced temperature-dependent magnetoresistivity of Fe/Cr superlattices

Abstract: The temperature dependence of the magnetoresistivity is found to be much larger in Fe/Cr superlattices with rough interfaces (high magnetoresistance) than those with sharp ones (small magnetoresistance). We discuss the mechanisms which produce this temperature dependence and point out that symmetry conditions restrict the putative mechanisms in superlattices. We show that the enhanced temperature dependence of the magnetoresistance can be explained by the existence of local spin excitations at the roughened Fe/Cr interfaces.

Envelope solitons of highly dispersive and low dispersive spin waves in magnetic films (invited)

Abstract: Theoretical description of spin wave (SW) envelope solitons in nonlinear dispersive media with dissipation is given in the framework of a nonlinear Schrodinger equation model with relaxation term. The conditions necessary for envelope soliton observation in magnetic films (MF) and film structures are formulated. Different types of SW dispersion laws are discussed in the context of envelope soliton formation. Formation of envelope solitons at microwave frequencies in periodic structures and in multilayered MF structures are predicted. Theoretically calculated parameters of envelope solitons of both highly dispersive and low dispersive SW in MF are compared to the results of experiments performed in thin yttrium‐iron garnet (YIG) films. New experimental results showing the process of formation and propagation of envelope solitons of low dispersive dipolar SW in YIG films are presented.

Magnetic properties of Co/Pd multilayers determined by Brillouin light scattering and SQUID magnetometry

Abstract: Co/Pd multilayers with modulation wavelengths between 4 and 220 A have been prepared by magnetically enhanced dc‐triode sputtering on single‐crystal sapphire substrates. Their saturation magnetization and volume and interface anisotropies have been investigated using Brillouin light scattering from collective spin waves and by SQUID magnetometry. The saturation magnetization of Co is found to be independent of the Co layer thickness and reduced by about 20% from the Co bulk value. From the comparison of the results of the two experimental methods, clear evidence for a Pd polarization is found and the polarization depth is estimated. Samples with Co thicknesses of 2 atomic layers and Pd thicknesses ≥5 atomic layers exhibit a perpendicular magnetization due to a large negative out‐of‐plane interface anisotropy. The properties of spin waves in Co/Pd multilayers with the direction of magnetization pointing out‐of‐plane are discussed with respect to an appropriate theoretical model.

Path-integral approach to the Hubbard model.

Abstract: A path-integral approach to the Hubbard model is developed for the whole range of the coupling strength U. At half filling, the strong-coupling results are readily reproduced within the simple Gaussian fluctuations. The low-lying spin wave is shown to be described by the nonlinear \ensuremath{\sigma} model. The effective coupling of the doped hole with the background fluctuations also agrees with that obtained from the t-J model in the small-doping limit. At finite doping, such a formalism may provide a starting point for investigating the short-range spin-liquid state.

Spin-wave modes in layered magnetic sandwich structures.

Abstract: A calculation of spin-wave propagation through a nearly nonmagnetic film sandwiched between two magnetic films is given as a function of the thickness of the nonmagnetic film and of a parameter that describes the decay of the exchange through this film and can be related to structural features such as interface roughness or spin-polarization penetration depth. The comparison with experiment enables us to determine this exchange decay parameter for a series of experiments in Permalloy-copper-Permalloy sandwiches prepared in an UHV system.

Collective excitations in a spin-polarized quasi-two-dimensional electron gas

Abstract: The dispersion relations of the collective modes of an optically created two-dimensional spin-polarized electron plasma, treated as a confined spin-polarized electron gas, are calculated. These exhibit many significant qualitative differences from their spin-unpolarized counterparts, such as emergence of the intrasubband spin wave and splitting of the intersubband spin waves and the longitudinal spin-density oscillations.

Dipole-exchange modes in thin ferromagnetic films with strong out-of-plane anisotropies

Abstract: Dipole-exchange modes are calculated for ultrathin ferromagnetic films with strong out-of-plane surface anisotropies large enough to force the magnetization out of plane. The direction of the magnetization is controlled by an in-plane static applied field. It is shown that dipolar-dominated spin-wave modes go soft for applied field strengths near where the magnetization begins to turn out of plane. For ultrathin films, however, the surface mode may contain a significant exchange energy that results in a nonzero minimum frequency at the applied field where the magnetization begins to turn out of plane. A potentially useful finding is a sensitive dependence of this minimum frequency on the exchange constant of the material.

Ordering Due to Quantum Fluctuations in the Frustrated Heisenberg Model

Abstract: The Heisenberg antiferromagnet with both the nearest neighbor and the next nearest neighbor interactions is studied in two and three dimensions. The spin wave excitation spectrum is obtained based ...

Zero-temperature properties of the quantum XY model with anisotropy

Abstract: The spin-1/2 XY model on a square lattice is studied via series expansions around the Ising limit, and spin-wave theory. Series are calculated for the ground-state energy, mass gap, magnetization, and magnetic susceptibilities. Extrapolating these series to the isotropic limit, we find extremely good agreement with the predictions of second-order spin-wave theory.