Showing papers on "Spin wave published in 1990"

Magnetic properties of layered transition metal compounds

Abstract: to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-Heisenberg models.- 4.1. Series for the 2-d XY model.- 4.2. Series for the 2-d Ising-Heisenberg model.- 5. Applications to magnetic systems.- 5.1. Ising model.- 5.2. Heisenberg model.- 5.2.1. Spin 1/2.- 5.2.2. Spin 1.- 5.2.3. Spin 3/2 and spin 2.- 5.2.4. Spin 5/2.- 5.2.5. Restricted dimensionality.- 5.3. XY and Ising-Heisenberg models.- Acknowledgements.- References.- Spin Waves in Two-Dimensional Magnetic Systems: Theory and Applications.- 1. Introduction.- 2. Magnetic structures and spin Hamiltonians.- 3. Spin wave theory of model systems.- 4. Dispersion relation.- 5. Thermodynamic properties.- 6. Impurities in antiferromagnets.- References.- Neutron Scattering Experiments on Two-Dimensional Heisenberg and Ising Magnets.- 1. Introduction.- 2. 2-d systems with Ising and Heisenberg interactions.- 2.1. K2CoF4: a 2-d Ising system.- 2.2. K2FeF4: a 2-d planar antiferromagnet.- 2.3. K2MnF4 and K2NiF4: weakly anisotropic Heisenberg magnets.- 2.4. Rb2CrCl4: a planar Heisenberg ferromagnet with small anisotropy.- 2.5. K2CuF4: a planar Heisenberg ferromagnet.- 3. 2-d random magnetic systems.- 3.1. Phase transitions and critical phenomena.- 3.2. Excitations.- 3.3. Random field effects.- 3.4. Relaxation front 2-d to 3-d order.- 3.5. Competing anisotropics and interactions.- 4. Triangular lattice antiferromagnet (TALAF).- 4.1. Fluctuations.- 4.2. An additional degree of freedom.- 4.3. Perturbation.- 4.4. Quantum effect RbFeCl3 and CsFeCl3 VX2 (X = Cl, Br, I) AMX2 (A = Li, Na, K M = 3d metal ion X = O, S, Se).- References.- Phase Transitions in Quasi Two-Dimensional Planar Magnets.- 1. Introduction.- 2. Phase transition and excitations in the 2-d XY model.- 3. Crystallographic properties of BaM2(X)4)2 compounds.- 4. Magnetic properties of BaNi2(PO4)2.- 4.1. Static properties.- 4.2. Dynamic properties.- 4.3. Critical properties.- 5. Magnetic properties of BaCo2(AsO4)2.- 5.1. Static properties.- 5.2. Magnetic phase diagrams.- 5.3. Dynamic properties.- 6. Magnetic properties of BaNi2(AsO4)2.- 6.1. Static properties.- 6.2. Dynamic properties.- 7. Magnetic properties of BaCo2(PO4)2.- 8. Other experimental realizations of the 2-d planar model.- 8.1. K2CuF4.- 8.2. NiCl2 and CoCL2 graphite intercalated compounds NiCl2-GIC CoCl2-GIC.- 9. Concluding remarks.- Acknowledgement.- References.- Spin Dynamics in the Paramagnetic Regime: NMR and EPR in Two-Dimensional Magnets.- 1. Introduction.- 1.1. Dynamics of the 2-spin correlation functions.- 1.2. Nuclear magnetic resonance (NMR).- 1.3. Electron paramagnetic resonance (EPR).- 2. General formalism.- 2.1. Diffusion and dimensionality.- 2.2. Cut-off and EPR linewidth.- 3. EPR spectrum.- 3.1. Diffusion of 4-spin correlation functions.- 3.2. Secular contribution D0.- 3.3. Nonsecular contributions.- 3.4. Satellite line.- 4. Experiments on quasi 2-d Heisenberg magnets.- 4.1. NMR experiments.- 4.2. EPR experiments.- 4.2.1. Angular dependence of linewidth.- 4.2.2. Frequency dependence of magic angle linewidth.- 4.2.3. Dynamic shift.- 4.2.4. Lineshape of the main line.- 4.2.5. Satellite lines at half resonance field.- 5. Critical dynamcis.- 5.1. Critical behaviour of the NMR line.- 5.1.1. Isotropic regime.- 5.1.2. Anisotropic regime.- 5.1.3. Experiments.- 5.2. Critical behaviour of the EPR linewidth.- 5.2.1. Ferromagnets.- 5.2.2. Antiferromagnets.- 5.3. AC susceptibility.- 6. Conclusions.- References.- Field-Induced Phenomena in Two-Dimensional Weakly Anisotropic Heisenberg Antiferromagnets.- 1. Introduction.- 2. Effective, field-dependent anisotropies.- 3. The phase diagram.- 4. Random fields and domain walls (solitons).- 5. The spin flop transition.- 6. The bicritical point.- 7. Concluding remarks.- Acknowledgements.- References.- Index of Names.- Index of Chemical Compounds.- Index of Subjects.

Spin-wave calculations for multilayered structures.

Abstract: Results are presented on the calculations of spin-wave frequencies in ferromagnetic layers, double layers, and multilayered structures for small, nonzero wave vectors such as can be investigated by, e.g., Brillouin light scattering. The underlying continuum-type magnetostatic theory includes both dipolar and exchange contributions and fully takes into account magnetic surface and interface anisotropies as well as interlayer exchange coupling. For single magnetic layers the detailed influence of surface anisotropies on both film surfaces is studied. For magnetic double layers the interlayer exchange coupling mechanism is investigated. In the case of multilayers consisting of alternating magnetic and nonmagnetic layers, the crossing regime between dipolar and exchange modes shows a strong dependence of the gap width on the amount of interface anisotropy. For small layer thicknesses the interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar modes into the exchange-mode regime. In the case of all-magnetic multilayered structures, a new type of collective spin-wave excitations arising from coupled exchange modes is predicted.

Aharonov-Bohm scattering of particles with spin.

Abstract: The scattering of relativistic spin-one-half particles in an Aharonov-Bohm potential is considered. It is shown that earlier approaches to this problem have neglected a crucial delta function contribution to the potential. By formulating the problem with a source of finite radius which is then allowed to go to zero, it is established that this is the delta function alone that causes solutions that are singular at the origin to become relevant. The changes in the amplitude which arise from the inclusion of spin are seen to modify the cross section for the case of polarized beams. Finally, the calculated Aharonov-Bohm amplitude is shown to describe the scattering of particles with arbitrary spin in the c=\ensuremath{\infty} limit.

On the connection between the one-dimensional S=1/2 Heisenberg chain and Haldane-Shastry model

Abstract: Extra integrals of motion and the Lax representation are found for interacting spin systems with the HamiltonianH = (J/2) ∑ =1, P(j − k) σ j σ k , where one of the periods of the WeierstrassP function is equal toN. The Heisenberg and Haldane-Shastry chains appear as limiting cases of these systems at some values of the second period. The simplest eigenvectors and eigenvalues ofH corresponding to the scattering of two spin waves are presented explicitly for these finite-dimensional systems and for their infinite-dimensional version.

Raman scattering from single crystals of cupric oxide

Abstract: Raman scattering experiments have been carried out on single crystals of cupric oxide (CuO) in the temperature range from 15 K to 300 K. The symmetry of the three first order Raman active phonons has been determined. A broad peak occurs at approximately 1100 cm−1 and is assigned to multiphonon scattering on the basis of the observed temperature dependence. Another peak appears in the Raman spectrum as the temperature of the sample is lowered below the Neel temperature. The frequency, intensity and linewidth of this peak have been studied as a function of temperature. The frequency of this peak cannot be reconciled with the known spin wave spectrum of CuO and thus, on the basis of the measured temperature dependence, it has been assigned to scattering from a magnetic exciton. The Raman spectrum also contains a background continuum which is weakly peaked at about 2000 cm−1 and the possible origin of this scattering is discussed. The observed optical and magnetic properties of CuO are compared, where possible, to results that have been obtained for the high-Tc cuprates.

Observation of magnetic excitons and spin waves in activation studies of a two-dimensional electron gas

Abstract: Measurements of activated conductivity for well-resolved Landau levels at filling factors v=1, 2, and 3 reveal a large exchange contribution to the energy gap, both at v=1 and 3 (the familiar exchange-enhanced g factor) and for the first time demonstrate a similar contribution at v=2. These results provide a measure of the large-wave-vector limits of the spin-wave and magnetic-exciton dispersion relations. The surprisingly strong magnetic field dependences of the measured energies are caused by the disorder present in the system.

Instability of the Nagaoka ferromagnetic state of the U=∞ Hubbard model

Abstract: We identify a ``${k}_{F}$ instability'' of the Nagaoka ferromagnetic state of the U=\ensuremath{\infty} Hubbard model. We show rigorously that for a large enough hole concentration the ferromagnet possesses an instability with respect to overturning an up-spin electron at the Fermi surface and placing it at the bottom of a down-spin band made very narrow by correlation effects. We find a low-energy scale for spin waves in this strong-coupling limit, in the form of a spin-wave stiffness that is much smaller than its random-phase-approximation value.

The dipole-exchange spin wave spectrum for anisotropic ferromagnetic films with mixed exchange boundary conditions

Abstract: A theory is developed for dispersion characteristics of spin waves in ferromagnetic films taking into account both dipole-dipole and exchange interactions, crystallographic anisotropy and mixed exchange boundary conditions on the film surfaces. An arbitrary orientation of the external bias magnetic field with respect to the orientation of crystallographic axes and the film normal is assumed. The influence of crystallographic anisotropy on the spin wave spectrum of a ferromagnetic film is discussed. The theoretical results obtained are compared with the results of experiments performed in yttrium-iron garnet (YIG) and Mn-ferrospinel films.

A quantum fluids approach to frustrated Heisenberg models

Abstract: Emphasising the close analogies between antiferromagnetism and neutral superfluidity, the authors develop a gauge-invariant quantum fluids description of non-bipartite Heisenberg systems. The antiferromagnet is treated as a spin superfluid with a rotational gauge invariance associated with the continuity of spin flow. They show how an extended Schwinger boson approach naturally incorporates the Onsager reaction fields generated by spin fluctuations, and correctly reproduces the semiclassical behaviour of spin wave theory in the large-S limit. The important short-wavelength physics of fluctuation-stabilised order is also captured by this description. For two-dimensional helimagnets at small S, the method predicts that the twist will survive the loss of sublattice magnetisation, closely analogous to the biaxial-uniaxial transition of nematic liquid crystals.

Random-phase-approximation analysis of NMR and neutron-scattering experiments on layered cuprates.

Abstract: Using a random-phase-approximation expression to approximate the dynamic susceptibility \ensuremath{\chi}(q,\ensuremath{\omega}) of a 2D Hubbard model, we have calculated the spin-relaxation rates and the neutron-scattering intensity Here, using recent static magnetic measurements to set parameters which enter the hyperfine form factors, we show that this simple form for \ensuremath{\chi}(q,\ensuremath{\omega}) exhibits the features of the spin fluctuations that are required to fit the NMR data, and explore its consequences for the temperature and energy dependence of the magnetic neutron scattering

Ground state and electron-magnon interaction in an itinerant ferromagnet: half-metallic ferromagnets

Abstract: Electron and spin Green functions of a Hubbard ferromagnet are calculated, both starting from the Stoner ground state and for a ferromagnet with Hubbard subbands. The temperature dependences of the spin-wave stiffness and damping, the magnetisation, the local moment on a site and the thermodynamic properties are investigated. The role of non-quasiparticle contributions, described by branch cuts of electron Green functions, is discussed. The non-quasiparticle ('ferrospinon') correction to the linear term in the specific heat is obtained. Experimental data on 'half-metallic' ferromagnets (in particular, spin polarisation and longitudinal nuclear relaxation rate) are analysed.

Surface phase transitions and spin-wave modes in semi-infinite magnetic superlattices with antiferromagnetic interfacial coupling.

Abstract: We analyze the bulk and surface spin-wave modes of a semi-infinite Fe/Gd superlattice. If an externally applied magnetic field is increased from zero, one surface mode is driven soft, indicating a surface phase transition which occurs at a field value about 5 times lower than that necessary to cause a bulk phase transition. The phase transition is shown to nucleate at the surface and has a large penetration depth (on the order of a few hundred angstroms) even for fields just slightly above the value needed to induce the transition

Spin fluctuations in superconducting YBa2Cu3O6.5.

Abstract: Antiferromagnetic spin fluctuations in superconducting YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} have been studied using inelastic neutron scattering In a crystal with {ital x}=05 and {ital T}{sub {ital c}}=50 K, we have observed spin fluctuations at 12 K having a cross section which increases with increasing excitation energy The data are consistent with highly overdamped spin waves and a very short spin-spin correlation length

Phase diagram of the frustrated square Heisenberg lattice based upon a modified spin-wave theory.

Abstract: La theorie classique de l'onde de spin ne peut fournir un diagramme de phases correct pour l'antiferromagnetique frustre a reseau carre. Presentation d'une theorie de l'onde de spin modifiee analogue a la theorie recente de Takahashi du modele de Heisenberg bidimensionnel sans frustre. Obtention d'un nouveau diagramme de phases dans lequel il n'existe aucun etat de liquide de spin pour S=1/2. Les resultats sont en bon accord avec ceux des simulations numeriques recentes pour de petits reseaux

Motion of a Bloch domain wall.

Abstract: We study the velocity versus applied-field relation of a moving Bloch domain wall, using the ``collective-coordinate'' method employed in the theory of soliton motion. For a sufficiently large field, the wall emits spin waves, thereby self-limiting its velocity. That velocity is equal to the common value of the phase and group velocities of a particular spin wave, and may be above or below the Walker limit ${\mathit{V}}_{\mathit{W}}$, depending on the intrinsic damping.

Spin waves in the flux-phase description of the S=1/2 Heisenberg antiferromagnet.

Abstract: A so-called ``flux-phase'' projected-fermion mean-field theory has been shown to give a good account of the ground state and excitations of the S=1/2 Heisenberg antiferromagnet on a square lattice. In this paper it is shown that random-phase-approximation spin waves (or paramagnons) of the flux phase have an unusual spectrum, especially near the zone boundary. There is no singularity in the spin-wave density of states at maximum energy, and the small-momentum spin-wave-velocity renormalization is obtained. Noninteracting spin waves are shown to give reasonable description of Raman scattering. It is argued that the conventional picture of strongly interacting, damped Holstein-Primakoff spin waves may in fact be described in terms of less incoherent excitations in this new basis. The flux-phase basis arises through considering the antiferromagnet as the large-U limit of the half-filled Hubbard model. In that model it is suggested that the flux should turn on for Ug${\mathit{U}}_{\mathit{c}}$\ensuremath{\sim}3.1.

Experimental observation of magnetostatic wave envelope solitons in yttrium iron garnet films

Abstract: Ces solitons se forment dans les regions spectrales de dispersion relativement faible a partir des pulsations d'entree rectangulaires courtes (T = 4-50 ns). On interprete les phenomenes non lineaires observes en termes de modele d'equation de Schrodinger non lineaire avec la dissipation

Effective spin Hamiltonian for the CuO planes in La2CuO4 and metamagnetism.

Abstract: We show that the effective spin Hamiltonian used previously to describe the CuO planes of La{sub 2}CuO{sub 4} does not lead to a net ferromagnetic moment for CuO planes and hence does not describe the metamagnetic behavior seen experimentally. We construct for the first time a Hamiltonian from the symmetries of the crystal structure which does lead to metamagnetism. The linear spin-wave spectrum is also calculated. This work points to the necessity of constructing effective spin Hamiltonians for metamagnetic systems which have the same symmetries as the system they are to describe.

Raman scattering study of LuFeO3

Abstract: The zone‐center magnon modes of LuFeO3 have been studied by first‐order Raman scattering At room temperature, the two magnon modes (M1 and M2) associated with the canted, antiferromagnetically ordered Fe3+ moments are seen at 185 and 22 cm−1, respectively Their corresponding symmetries, determined through polarization measurements on single crystals, are consistent with the magnetic point group (m’m’m) of the crystal The distinct scattering observed from phonons is also discussed in the context of the dependence of the phonon spectra on the mass of the rare‐earth atom

Spin waves in superlattices. III. Magnetic polaritons in the Voigt configuration

Abstract: For pt.II see J. Phys. C, vol.21, p.4097 (1988). Dispersion relations for bulk and surface magnetic polaritons in semi-infinite layered structures composed of two different magnetic materials are analysed theoretically. The considerations are restricted to the case of in-plane magnetisation and to wave propagation in the Voigt geometry. Some exemplary wave spectra are presented for structures composed of magnetic and non-magnetic materials as well as for structures where both constituents are magnetic.

Modified Spin Wave Theory of the Two-Dimensional Frustrated Heisenberg Model

Abstract: The modified spin wave theory of Takahashi is applied to the antiferromagnetic Heisenberg model with next nearest neighbor interactions on the square lattice in the ground state. The result indicates existence of ordered states for any value of the ratio of the nearest and next nearest neighbor interactions in contrast to some of the other theories.

High-power ferromagnetic resonance without a degenerate spin-wave manifold.

Abstract: The nonlinear dynamics of the main magnetoexchange branch of a circular thin (0.45 \ensuremath{\mu}m) film of yttrium iron garnet are investigated at perpendicular resonance. Distinct ``fingers of auto-oscillation'' arise from each magnetoexchange mode in the low-power spectrum. A set of equations of motion derived from a microscopic Hamiltonian are integrated to compare with experiment. In the first finger, the model predicts the power for onset of auto-oscillation to within 1 dB of the experiment and the frequency to within 15%.

Damping of spin waves in a two-dimensional Heisenberg antiferromagnet at low temperatures.

Abstract: The Dyson-Maleev formalism is used to calculate the damping of spin waves in the two-dimensional Heisenberg antiferromagnet at asymptotically low temperatures and long wavelengths, both in the quantum and in the classical case. The calculations are done self-consistently. Various regimes are found for the decay rate depending on the relative size of the reduced temperature \ensuremath{\tau} and the dimensionless wave vector ka. In all cases, the decay rate is found to be much smaller than the frequency of the excitations, leading to well-defined spin waves, provided that k\ensuremath{\xi}\ensuremath{\gg}1, where the correlation length \ensuremath{\xi} is of order exp(const/\ensuremath{\tau}). At low but finite temperatures, we take into account fluctuation renormalizations which tend to increase the damping. The result of simulations on the classical lattice rotor model are presented and compared with the calculations. The agreement is qualitatively good. The simulations are also used to test the scaling form for the decay rate in the regime k\ensuremath{\xi}\ensuremath{\sim}1, which is outside the limit of validity of our direct spin-wave calculations.

Neutron scattering study of YBa2Cu3O6+x single crystals

Abstract: Inelastic neutron scattering studies of YBa 2 Cu 3 O 6+ x single crystals are reported. In the pure AF phase ( x = 0.15, T N = 410 K) the 2D-character of the spin wave spectrum and the exchange parameters have been established. A small amount of p holes in CuO 2 planes ( x = 0.37, T N = 180 K,m n h = 1.8%) strongly modifies the spin dynamics at low q : strong damping of in-plane spin excitations and renormalization of the spin wave velocity. In the superconducting state ( x = 0.45, T c = 34.8 K) we have found dynamical magnetic correlations and an anomalous decrease of the intensity at low energy in the vicinity of T c .

Magnetic dynamics in copper-oxide-based antiferromagnets: The role of interlayer coupling.

Abstract: It is shown that thermal excitation of spin waves in a highly anisotropic antiferromagnet results in a characteristic temperature dependence of sublattice magnetization with a crossover from a 3D to a quasi-2D behavior. The magnetic dynamics in several copper-oxide-based antiferromagnets is analyzed in this context in terms of subtle details of their structural characteristics, and the temperature depen- dence of the Cu moment is used to determine the planar and interplanar exchange energies. PACS numbers: 75.10.Lp, 75.30.Ds, 75.40.Gb neutron scattering, and the general trend expected from structural characteristics. For La2Cu04, we obtain Z,J = 1600 K, where Z, -1.16 is the renormalization of the spin-wave velocity. This is in agreement with the reported values for J in other works: 0. 16/Z, eV (neutron-scattering studies), 0. 14 eV (Raman scatter- ing), 0. 13 eV (by fitting the spin-correlation length within the nonlinear sigma model), 1450 K (by fitting the spin-correlation length within a Monte Carlo simula- tion of the spin- —, ' The remarkable manifestation of the almost 2D anti- ferromagnetism in high-T, cuprate superconductors has provided a great impetus in efforts to understand low- dimensional antiferromagnetism. Specifically, the dis- coveries of long-range antiferromagnetic (AF) order, ' spin-wave excitations, and long-range, 2D AF spin correlations above the Neel temperature3 have contribut- ed much to clarifying important theoretical issues. Thus, the 2D aspects of antiferromagnetism, manifested as T) Ttv, are beginning to be understood. However, in the temperature regime T( Ttv, where 3D AF ordering sets in, the weak interlayer magnetic coupling becomes a most relevant piece in the physics. The very weak interlayer coupling affords us with a high- ly anisotropic antiferromagnet, and therefore an investi- gation of how it controls the magnetic dynamics is of much interest. Furthermore, the magnetic interlayer coupling in the copper-oxide systems depends, in a very subtle manner, on details of their structural characteris- tics. For example, if it were not for the orthorhombic distortion in the La2Cu04 there would be no net ex- change coupling between two neighboring layers. Thus, as a supplement to the conductivity anisotropy, magnetic dynamics can be used as a probe to investigate magnetic aspects of the interlayer coupling, which is of importance in some theories of high-T, superconductivity. In this Letter we report a microscopic study aimed at understanding the magnetic dynamics of several parent compounds (of the high-T, superconductors) in terms of subtle details of their structural characteristics. We first examine, within an itinerant-electron model, the magnet- ic dynamics of thermally excited spin waves in a highly anisotropic antiferromagnet, as revealed in the tempera- ture dependence of sublattice magnetization M(T). We show that there are really two energy scales in the dy- namics, namely, J (=4t /U), the exchange energy, and Jr, where r is the ratio of an effective interplanar- to planar-hopping strength. For kgT & 2Jr, the magnetiza- tion falls off' as T, characteristic of a 3D system. How- ever, for kg T & 2Jr, we show that there is a crossover to a TlnT behavior, which is a quasi-2D behavior. We also fit the M(T) vs T behavior to experimental data for several systems and find the fits to be excellent. Moreover, the value obtained from the best fits for J and r are, respectively, in agreement with results known from

Time-dependent behavior of classical spin chains at infinite temperature

Abstract: An ultrafast, vectorized spin-dynamics method is used to study the time-dependent properties of classical XY and Heisenberg spin chains at infinite temperature. The decay of the energy-energy and spin-spin correlation functions is oscillatory for short times and at long times is consistent with classical diffusion, although the approach to the asymptotic behavior is extremely slow. We have also calculated S(q,\ensuremath{\omega}) and find clear indication of spin-wave peaks in both models.

Phase Hamiltonian and depinning electric field in the charge-density wave and the spin-density wave.

Abstract: Making use of the phase Hamiltonian derived from microscopic models, we study theoretically the threshold electric field when the charge-density wave (CDW) or the spin-density wave starts sliding. Unlike earlier phenomenological models, we can describe the temperature dependence of the threshold field. Most of experimental results on the threshold field in quasi-one-dimensional CDW's are interpreted in terms of the three-dimensional CDW. Curious exceptions are the CDW's in ${\mathrm{NbSe}}_{3}$, which appear to be described better by the two-dimensional CDW.

Theory of surface and bulk excitations in ferromagnetic semiconductors.

Abstract: Ces excitations sont etudiees a l'aide du modele d'interaction s−d (ou s−f). On etablit un formalisme pour obtenir les fonctions de Green qui fournissent la reponse dynamique du systeme et a partir desquelles les excitations de surface et de volume, et leurs relations de dispersion, sont calculees. Analyse des possibilites d'application aux semi-conducteurs de type spinelle