Showing papers on "Spin wave published in 1989"

Quantum spin chains and the Haldane gap

Abstract: One-dimensional antiferromagnets have exotic disordered ground states. As was first argued by Haldane (1983), there is an excitation gap for integer, but not half-integer, spin. The authors review the arguments for this behaviour based on field-theory mappings, the Lieb-Schultz-Mattis theorem, exactly solvable models, finite-chain diagonalisation and real experiments.

Modified spin-wave theory of a square-lattice antiferromagnet.

Abstract: A modified spin-wave theory for the Heisenberg antiferromagnet (scrH=J) is formulated under the assumption of zero sublattice magnetization in the same way with the Heisenberg ferromagnet. This theory gives self-consistent equations which are equivalent to those of Auerbach and Arovas, but in our theory the factor of (3/2 in the correlation function does not appear. This theory reproduces the main results of traditional spin-wave theory, as well as those of renormalization-group theory, in a unified picture. For the square lattice at low temperature the susceptibility behaves as a+bT and the correlation length as (c/T)exp(d/T). This correlation length coincides very well with experimental results of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ if we choose J=900 K. Calculation of self-consistent equations is done for the S=(1/2 system and compared with the result of exact diagonalization of a 4\ifmmode\times\else\texttimes\fi{}4 system and high-temperature expansion. The quantitative agreement is surprisingly good, especially at T\ensuremath{\lesssim}0.6J. .AE

Magnetic dynamics of La2CuO4 and La2-xBaxCuO4.

Abstract: High-energy inelastic neutron scattering is used to resolve spin waves in La/sub 2/CuO/sub 4/ The corresponding long-wavelength velocity is 085 +- 003 eV A, well above previous bounds set by thermal neutron scattering Within experimental error, conventional spin-wave theory gives an excellent description of the magnetic dynamics for momentum and energy transfers below 015 A/sup -1/ and 01 eV, respectively Doping with Ba changes the magnetic dynamics; the data indicate a combination of spin-wave softening and damping effects

Spin-wave results for the triangular Heisenberg antiferromagnet

Abstract: We perform the standard spin-wave analysis of the triangluar Heisenberg quantum antiferromagnet. Contrary to the variational calculation of Anderson, we diagonalize exactly the quadratic part of the spin-wave Hamiltonian and obtain results in a 1/S expansion. We compute the ground-state staggered magnetization per spin and the ground-state energy. For spin (1/2, the agreement with the variational values of Huse and Elser suggests that the trangular lattice has long-range magnetic order.

Theory of the spin liquid state of the Heisenberg antiferromagnet

Abstract: We propose that the disordered state of a two-dimensional spin-1/2 Heisenberg antiferromagnet is physically equivalent to the incompressible liquid state of the fractional quantum Hall system. The fractional quantum Hall state for bosons is shown to be an exact spin singlet and to possess a low variational energy for the near-neighbor Heisenberg model on a triangular lattice. Variational wave functions for neutral spin-1/2 excitations are constructed and shown to form an exact spin doublet. Variational energies of these states are calculated, and their spin density profiles are determined. We find that a localized spin-1/2 quaisparticle has a size comparable to a lattice bond length and an excitation energy \ensuremath{\Delta}=1.3J. The energy-momentum dispersion of quasiparticles and spin-1 collective modes, obtained variationally, supports the hypothesis that the spin liquid state has a finite energy gap. The 1/2 fractional statistics exhibited by the quasiparticle excitations is explicitly demonstrated.

Spin-wave theory of the quantum antiferromagnet with unbroken sublattice symmetry

Abstract: We discuss the properties of spin-wave theory of quantum antiferromagnets with an additional constraint that restores the sublattice symmetry. Our treatment is analogous to Takahashi's recent theory of low-dimensional ferromagnets, and closely related to the Schwinger boson theory of Arovas and Auerbach. We obtain exact results for spin-spin correlations of 2, 4, and 8 spins in a singlet state, and exceedingly close agreement with exact results for S=(1/2 in a variety of other cases. The implication for the long-range order of the d=2, S=(1/2 Heisenberg antiferromagnet is discussed.

Model for quasi-one-dimensional antiferromagnets: application to CsNiCl3

Abstract: The approximate mapping of the antiferromagnetic Heisenberg chain onto a (1+1)-dimensional quantum field theory indicates that the ground state is disordered with a finite gap for integer spin. We study a lattice of coupled (1+1)-dimensional field theories as a model for weakly coupled spin chains. This model offers a possible resolution to the mystery of the upper mode in the low-temperature ordered phase of CsNi${\mathrm{Cl}}_{3}$, whose polarization is inconsistent with spin-wave theory.

Ground-state and low-lying excitations of the Heisenberg antiferromagnet.

Abstract: Monte Carlo methods are used to determine the exact ground-state energy of the spin-(1/2 Heisenberg antiferromagnet on two-dimensional square periodic lattices up to size 32\ifmmode\times\else\texttimes\fi{}32. The extrapolated ground-state energy for infinite lattice size is -0.334 59\ifmmode\pm\else\textpm\fi{}0.000 05. In addition, splittings between the ground state and the lowest spin-1 and -2 excitations are determined as a function of lattice size. The scaling of both the ground-state energy and the gap are in agreement with that predicted by spin-wave theory over a wide range of lattice sizes. In particular, numerical results demonstrate convincingly the lack of a gap for infinite systems, and that the gap for finite systems scales with the inverse volume of the lattice. Finally, we present results for the ground-state spin-correlation function. Our approximate results for larger lattices indicate that the staggered magnetization is 0.34\ifmmode\pm\else\textpm\fi{}0.01 units where the saturated value is (1/2.

Monte Carlo simulations of the spin-1/2 Heisenberg antiferromagnet in two dimensions

Abstract: The authors describe the results of Monte Carlo simulations of the spin-1/2 Heisenberg antiferromagnet on the honeycomb lattice. They find that the staggered magnetisation is nonzero and given, quite accurately, by spin-wave theory. Similar results have been found earlier for the square lattice.

Evidence of magnetic order of Cu moments in the superconducting and normal region of La2−xSrxCuO4: Observation of Cu-NQR for 0.12≦x≦0.40

Abstract: The observation of Cu-NQR in La2−xSrxCuO4 revealed that the spectra were splitted into two peaks at low temperatures and that its separation decreased with increasing x. This fact indicates that the well-defined hyperfine field due to magnetic ordering of Cu moments exists in the superconducting and normal metallic regions. The nuclear spin-lattice relaxation rate, 1/T1 of the Cu spectra is suppressed apparently by the superconducting energy gap for x≦0.20, while its temperature dependence obeys the Korringa relation for the normal region of x≧0.25 . The nuclear spin-spin relaxation rate, 1/T2 increases proportionally with increasing hyperfine field, indicating the dominant indirect unclear spin coupling via virtual excitation of antiferromagnetic spin waves. The coexistence of superconductivity and the magnetic order of Cu d-electrons with a tiny moment is demonstrated by the NQR study in the La-Sr-Cu-O system

Sublattice-symmetric spin-wave theory for the Heisenberg antiferromagnet

Abstract: We apply the sublattice-symmetric spin-wave theory (SSSW) for Heisenberg antiferromagnets to obtain excited states at zero temperature. We identify a set of spin-wave states that have the correct total spin and correspond to the states with lowest energy in a given sector where ${S}_{z}$, the z component of the total spin, is fixed. This approximation gives results in good agreement with the results of exact diagonalization. We also discuss results of SSSW for finite-temperature and dynamical correlations. We recover the same equations as those obtained in the mean-field Schwinger boson theory of Arovas and Auerbach, except for a factor of 3/2. From comparison with exact results obtained by exact diagonalization, we assess the accuracy of the theory when the temperature is finite.

Effects that can stabilise multiple spin-density waves

Abstract: Antiferromagnetism on the face-centred cubic lattice is frustrated. The different competing spin arrangements are degenerate in the classical limit of the Heisenberg model. The author considers quantum-mechanical spin fluctuations, using the Holstein-Primakoff transformation, as a perturbation to lift this degeneracy and find that the collinear spin arrangement is stabilised. When he considers the inclusion of paramagnetic impurities, however, he finds that non-collinear arrangements are locally preferred. He suggests this scenario as a possible explanation for the fact that manganese, when quenched in a face-centred cubic lattice, can be driven from a collinear to non-collinear spin arrangement by doping with either nickel, iron or iridium.

Mean-field theory for vacancies in a quantum antiferromagnet.

Abstract: The vacancies in a quantum antiferromagnet have dipolar interactions mediated by antiferromagnetic (AF) spin waves. An effective Hamiltonian incorporating this effect is studied within a mean-field approximation. The ground state is found to be an {ital s}- or {ital d}-wave superconductor. In the strong-coupling regime the superconductivity coexists with the incommensurate spiral AF order. The analysis is limited to the regime where the AF correlation length is larger than the distance between vacancies.

Magnetism in very thin films of permalloy measured by spin polarized cascade electrons

Abstract: The spin polarizationP of the low energy cascade electrons excited with a primary unpolarized electron beam is measured with ultrathin films of permalloy (Ni80Fe20) as a function of film thickness, external magnetic field, and temperatureT. Surface adsorbates of small concentrations of less than 10% of a monolayer can change the Curie point and the saturation value ofP 0(T→0) by as much as 30%. The Ta-substrate induces a magnetically dead region in permalloy. Conventional spin wave theory cannot account for the observed smallT-dependence of the magnetizationM. Films on a nonmagnetic substrate are compared to similar films coupled to bulk permalloy over an interface of Ta. TheT-dependence ofM with the coupled films can be explained by spin wave theory. At lowT, the films coupled to the bulk exhibit a faster decrease ofM than the uncoupled films. We propose that this thermal stabilization of the magnetization in very thin ferromagnetic films is due to quenching of the long wavelength spin modes.

Spin Wave Theory of the Two-Dimensional Heisenberg Antiferromagnet Coupled with Localized Holes

Abstract: The spin wave theory of the two-dimensional Heisenberg antiferromagnet on a square lattice coupled with localized holes, ie, extra spins, is investigated This model is a limiting case of the coupled spin-fermion model presented recently to investigate the magnetic mechanism of high- T c superconductivity Two schemes of the spin wave theory are proposed according to the relative magnitude of J K (coupling between the extra and the substrate spins) to J S (coupling between the substrate spins) The following conclusions are obtained (1) Small antiferromagnetic (AF) J K as well as ferromagnetic (F) J K enhances the AF long-range order suppressing the quantum fluctuations Large AF J K , however, reduces the AF long-range order and enhances the quantum fluctuations We obtain the large attractive interaction only in the latter case (2) The susceptibility χ ⊥ perpendicular to the staggered magnetization is logarithmically divergent with respect to the external frequency ω when one extra spin is introduced

Systems and markers using magnetic or spin resonance phenomena

Abstract: Both an antipilferage marker and a system therefor is provided that uses magnetic spin resonance phenomena to generate unique and identifiable signals. The marker generally comprises a substrate which supports a material that generates and identifiable signal as a result of either nuclear magnetic resonance, nuclear quadrupole, electron spin resonance, electron paramagnetic resonance, ferromagnetic resonance, ferrimagnetic resonance, antiferrimagnetic resonance, domain wall resonance or spin wave resonance or spin-echoes. A magnetic material is further provided on the substrate for applying a magnetic field to the resonant material. In the system of the invention, a microwave or radio frequency source emits electromagnetic radiation which is absorbed by the resonant material which in turn reemits electromagnetic radiation having a specific and readily detectable frequency signature.

Brillouin light scattering from spin waves in magnetic layers and multilayers

Abstract: We give an overview on our experimental and theoretical investigations of Brillouin light scattering in magnetic thin films, layered magnetic structures and superlattices. For epitaxial Fe(1 10) layers on W(1 10) the in-plane and out-of-plane magnetic surface anisotropy constants are determined, and the influence of Pd overlayers on the surface anisotropies is studied. For Fe/Pd superlattices a magnetic polarization of the Pd at the interfaces is established and the interface anisotropy constant is determined. For “second order” Fe/Pd superlattices, formed by alternating two Fe/Pd bilayers with different repeat periods, the Brillouin spectrum is obtained and compared to calculations. In the case of magnetic/nonmagnetic multilayered structures we investigate theoretically the crossing regime between dipolar and exchange-dominated modes. For small spacer-layer thicknesses, interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar mode into the exchange-mode regime. In case of all-magnetic multilayered structures, such as Fe/Ni multilayers, a new type of propagating collective excitations arising from coupled exchange modes is predicted.

Influence of surface anisotropy on the magnetization of the Heisenberg ferromagnet.

Abstract: We examine theoretically the influence of surface anisotropy on the magnetization of the Heisenberg ferromagnet in its outermost layers. Emphasis is on the case where the surface spins sense an easy axis normal to the surface and at temperatures where spin-wave theory may be applied. For parameters in the range of those that emerge from recent analyses of Fe surfaces and interfaces, we find the influence of surface anisotropy to be very modest, except at rather low temperatures (10-K range). A key element in the analysis is a cancellation between the surface spin-wave contribution to the magnetization and that from a ``hole'' produced in the density of the bulk waves, when the surface wave is removed from the bulk spin-wave bands. Our theoretical analysis is based on a Green's-function method.

Light scattering from spin waves in thin films and layered magnetic structures

Abstract: The spinwave modes occurring in layered magnetic structures are discussed. Experimental examples obtained by means of light scattering are presented. Emphasis is put on coupled modes occurring in magnetic double layers, i.e. two ferromagnetic films, separated by a nonferromagnetic interlayer. The coupling can be via oscillating dipolar fields caused by the preccessing spins, or via the exchange interaction. It is shown, how the coupled modes can be used, to explore such interactions. These investigations have led to the discovery of the interlayer exchange interaction.