Showing papers on "Spin wave published in 1993"
TL;DR: In this paper, a review of magnon properties of yttrium-iron garnet (YIG), a classical object for experimental studies in magnetism, is presented, and a new method of approximate calculation of the magnon spectra in magnets with large unit cell and to obtain by means of this method some basic properties of YIG.
306 citations
TL;DR: In this paper, the spin configurations found in multilayer systems determine macroscopic properties such as the static susceptibility and magnetization, and can lead to anomalous field and temperature behavior.
Abstract: The authors discuss some of the fundamental properties unique to magnetic multilayers. Complex spin configurations are examined for many different systems and are shown to arise from a simple competition between exchange and Zeeman energies. The spin configurations found in multilayer systems determine macroscopic properties such as the static susceptibility and magnetization, and can lead to anomalous field and temperature behaviour. The authors also discuss the dynamic behavior of magnetic multilayers. Emphasis is placed on spin waves in magnetic multilayers with canted spin configurations and the softening of modes at magnetic phase transitions. Furthermore they show that spin wave excitations provide a powerful method for studying exchange interactions and spin configurations. Finally, the phenomenon of giant magnetoresistance in magnetic multilayers where the resistivity of the metallic structure can be changed by over 60% at room temperature, is discussed. Simple theoretical approaches are used to understand and predict the properties of the multilayer systems and comparisons between theory and experiment are stressed.
201 citations
TL;DR: In this article, the low temperature and large volume effects in the d = 2+1 antiferomagnetic quantum Heisenberg model are dominated by magnon excitations, and the leading and next-to-leading corrections are fully controlled by three physical constants, the spin stiffness, spin wave velocity and the staggered magnetization.
Abstract: The low temperature and large volume effects in thed=2+1 antiferomagnetic quantum Heisenberg model are dominated by magnon excitations. The leading and next-to-leading corrections are fully controlled by three physical constants, the spin stiffness, the spin wave velocity and the staggered magnetization. Among others, the free energy, the ground state energy, the low lying excitations, staggered magnetization, staggered and uniform susceptibilities are studied here. The special limits of very low temperature and infinite volume are considered also.
147 citations
TL;DR: Inelastic neutron scattering has been used to study the temperature dependence of the magnetic response in the one-dimensional S = 1/2 Heisenberg antiferromagnet as mentioned in this paper.
Abstract: Inelastic neutron scattering has been used to study the temperature dependence of the magnetic response in the one-dimensional S=1/2 Heisenberg antiferromagnet ${\mathrm{KCuF}}_{3}$. The scattering is consistent with that expected for unbound spinon pair excitations.
142 citations
TL;DR: Using the recently proposed density matrix renormalization group technique, it is shown that the magnons in the S = 1 antiferromagnetic Heisenberg chain effectively behaves as bosons that condense at a critical field hc.
Abstract: Using the recently proposed density matrix renormalization group technique [1] we show that the magnons in the S = 1 antiferromagnetic Heisenberg chain effectively behaves as bosons that condense at a critical field hc. We determine the spin-wave velocity, v = 2.49(1), as well as the gap � = 0.4107(1)J.
87 citations
TL;DR: The ground state wave function for the spin-1/2 quantum antiferromagnet on a 36-site kagome$iaa--- structure is found by numerical diagonalization as discussed by the authors.
Abstract: The ground-state wave function for the spin-1/2 quantum antiferromagnet on a 36-site kagome$iaa--- structure is found by numerical diagonalization. Spin-spin correlations and spin gaps indicate that the ground state of this system does not possess magnetic order. The spin-Peierls order is studied using a four-spin correlation function. The short-range structure in this correlation function is found to be consistent with a simple dimer-liquid model. The spin-Peierls order, if it exists, must be quite small.
87 citations
TL;DR: In this article, a theoretical interpretation of the magnetization process of CsCuCl 3 showing a small jump for the external field applied parallel to the c -axis is given. But the quantum fluctuations are so important in this S = 1/2 triangular antiferromagnet that they can change the ground state spin structure.
Abstract: A theoretical interpretation is given on the magnetization process of CsCuCl 3 showing a small jump for the external field applied parallel to the c -axis. It is shown that quantum fluctuations are so important in this S =1/2 triangular antiferromagnet that they can change the ground-state spin structure. The observed magnetization jump is successfully explained as a spin flop process caused by the quantum effect.
85 citations
TL;DR: The effective spin Hamiltonian that describes the magnetic properties of La 2 CuO 4 is derived from the nearest-neighbor superexchange interactions, and it is shown that the spin system of the CuO 2 plane of the compound is frustrated.
Abstract: The effective spin Hamiltonian that describes the magnetic properties of La 2 CuO 4 is derived from the nearest-neighbor superexchange interactions. It is shown that the spin system of the CuO 2 plane of the compound is frustrated; the principal axes of the symmetric parts of the one-bond anisotropy tensors are not the same for all the bonds. We also show that, due to a hidden symmetry of the one-bond anisotropic superexchange interactions, the symmetry of the lattice alone leads to the identification of the largest eigenvalue of the mean-field superexchange anisotropy tensor
74 citations
TL;DR: In this article, the spontaneous magnetic order in p-NPNNs was observed using a microscopic magnetic probe, and the behavior of the system is close to that of an isotropic three-dimensional (3D) Heisenberg spin system.
63 citations
TL;DR: In this article, a single crystal La1.65Nd0.35CuO4 was investigated for a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low-temperature orthor Hn-group Pccn atTs=76 K.
Abstract: We report a neutron scattering investigation of a La1.65Nd0.35CuO4 single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) atTs=76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO6 octahedra. Antiferromagnetic long range order is established belowTN=316 K, and a hysteretic reorientation to a noncollinear spin structure occurs belowTs. The low energy spin dynamics of La1.65Nd0.35CuO4 aboveTs are closely similar to those of stoichiometric La2CuO4; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly belowTs.
58 citations
TL;DR: The results for the isotropic point appear to agree with recent series expansion, large-[ital N] expansion, and the original resonating valence bond picture of Anderson, although they cannot be considered as conclusive evidence supporting any of these theories.
Abstract: The spin-1/2 anisotropic Heisenberg antiferromagnet is studied at T=0 on the triangle lattice via numerical diagonalization for system sizes up to N=36 sites. Extrapolation to the thermodynamic limit suggests that the isotropic system possesses no, or very small, \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 magnetic order; no helical or chiral order; and spin-spin correlations consistent with that of a critical phase. For XY-like anisotropy there is long-ranged \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 magnetic order. In contrast to bipartite lattices, the standard first- and second-order spin-wave theories are not quantitatively accurate. Excitation energy gaps suggest that the lowest lying excitations for the isotropic point are not spin-flip excitations in the thermodynamic limit. The results for the isotropic point appear to agree with recent series expansion, large-N expansion, and the original resonating valence bond picture of Anderson, although they cannot be considered as conclusive evidence supporting any of these theories.
TL;DR: Inelastic neutron scattering was used to study the magnetic excitations of the triangular antiferromagnets Mn 3 Sn and Mn 3 Ge and finds the data are remarkably well described by this model with exchange parameters extending to fifth-nearest neighbors and with both axial- and basal-plane anisotropy.
Abstract: Inelastic neutron scattering was used to study the magnetic excitations of the triangular antiferromagnets ${\mathrm{Mn}}_{3}$Sn and ${\mathrm{Mn}}_{3}$Ge. These compounds have itinerant d electrons and large magnetic moments localized at the Mn sites and may be regarded as materials that lie in the intermediate regime between local-moment and itinerant-electron systems. The spin-wave spectra exhibit steep dispersion and strong damping, which is characteristic behavior of itinerant-electron systems. Nevertheless, it is useful to analyze the data in terms of a local-moment model with anisotropy. We find the data are remarkably well described by this model with exchange parameters extending to fifth-nearest neighbors and with both axial- and basal-plane anisotropy. The axial-anisotropy parameters were determined from the uniform out-of-plane spin fluctuation, and the signs show that the spins are confined to the basal plane. The second-order basal-plane anisotropy constants were determined by satisfying both the magnitude of the weak basal-plane ferromagnetic moments and the observed splitting of a doubly degenerate acoustic-spin-wave branch. The sixth-order basal-plane anisotropy was determined by adjusting to the observed energy gap associated with spin fluctuations within the basal plane. The exchange parameters have the correct signs to stabilize the triangular antiferromagnetic structure but yield N\'eel temperatures that are higher than those observed by a factor of 3 or 4. This overestimation of the N\'eel temperature is not an uncommon result when a local moment model is applied to an itinerant-electron system.
TL;DR: Inelastic neutron scattering experiments have been carried out on YBa 2 Cu 3 O 6+ξ single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes as discussed by the authors.
Abstract: Inelastic neutron scattering experiments have been carried out on YBa 2 Cu 3 O 6+ξ single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes: the pure and doped AF-states (ξ = 0.15, 0.37), the weakly doped (χ = 0.45, 0.51), the heavily doped (χ = 0.69, 0.92) and the overdoped (χ = 1) metallic states. In the insulating state, the hole doping strongly affects the AF-order and yields a strong renormalization of the spin wave velocity. In the metallic state, dynamical AF-correlations remain, but the in-plane correlation length becomes very short (ξ/a ≌ 0.8) as the doping increases. However, the AF-coupling between the two CuO 2 -layers is not affected, except in the overdoped regime. At low temperatures the spin excitation spectrum exhibits an energy gap in any superconducting samples. Its value, E G / kT c ≌ 3.5, becomes much weaker close to the I-M transition. Moreover, a pseudo-gap persists above T c in the heavily doped regime.
TL;DR: A calculation of spin waves in coupled multilayered structures that is based on exact evaluation of both exchange and dipolar fields is presented and the validity of effectivemedium theory is examined and it is found that it does not completely describe spin-wave excitations in finite antiferromagnetically coupling multilayers.
Abstract: We present a calculation of spin waves in coupled multilayered structures that is based on exact evaluation of both exchange and dipolar fields. We compare our results with earlier continuum treatments. The ground-state spin configuration in antiferromagnetically coupled multilayers can differ significantly from the uniformly canted ground state usually assumed. This nonuniform ground state is found to alter radically the character of the spin-wave modes and sometimes lead to a strong localization of the wave to the outermost magnetic films of the multilayer. In addition, we examine the validity of effective-medium theory---a continuum theory---and find that it does not completely describe spin-wave excitations in finite antiferromagnetically coupled multilayers. Finally, the spin-wave frequencies are found to be nonreciprocal with respect to propagation direction for most directions, i.e., \ensuremath{\omega}(q)\ensuremath{
e}\ensuremath{\omega}(-q) where q is the propagation wave vector. This nonreciprocal behavior is explained from basic symmetry considerations. Again, the nonreciprocity is not properly described within effective-medium theory.
TL;DR: Inelastic neutron magnetic scattering experiments were performed from the well characterized single crystal of La 2 NiO 4 to study the spin-wave excitations in this system, and the large energy gaps are observed at the two dimensional zone center, namely 15.7±0.7 meV for the out-of-plane mode, and 4.1 meV or 7.9 meV below or above 70 K for the inplane mode respectively as discussed by the authors.
Abstract: Inelastic neutron magnetic scattering experiments were performed from the well characterized single crystal of La 2 NiO 4.00 ( T N =328 K) to study the spin-wave excitations in this system. The large energy gaps are observed at the two dimensional zone center, namely 15.7 meV for the out-of-plane mode, and 4.1 meV or 7.9 meV below or above 70 K for the in-plane mode respectively. Combined with the higher energy scans carried out by the spallation neutron source, the whole dispersion relation was determined, which follows a linear spin-wave model. It provides the microscopic parameters of nearest neighbor exchange interactions in plane, two anisotropy energies corresponding to the out-of-plane and in-plane modes, respectively, 28.7±0.7 meV, 1.26±0.12 meV and 0.10±0.02 meV at 10 K. The scattering intensities also nicely follow the linear spin-wave model at the lowest temperature. The results are discussed with the previous experiments on the spin dynamics of La 2 CuO 4 .
TL;DR: In this article, the magnetic structure of Mn3Sn in the modulated triangular phase was examined in an attempt to elucidate the energy gap observed in the spin-wave spectrum, and two distinct moment modulations were found, both of which are commensurate with the lattice at low temperatures but which become incommensurate as increasing temperature.
TL;DR: A dis ordering transition with exponents different from the usual Heisenberg transition is found and tentatively identified as a pure spin wave disordering transition.
Abstract: A three-dimensional lattice of Heisenberg spins with nearest-neighbor interactions is studied by numerical simulation under the constraint that no free topological singularities (hedgehogs) are allowed. Only nearest-neighbor pairs of oppositely charged hedgehogs are permitted in the sum over configurations. A disordering transition with exponents different from the usual Heisenberg transition is found and tentatively identified as a pure spin wave disordering transition.
TL;DR: The good agreement between the two formalisms reinforces the validity of spin-wave theory and the results with recent series-expansion estimates are compared.
Abstract: We have calculated the dynamical transverse and longitudinal spin-correlation functions for the two-dimensional Heisenberg antiferromagnet at zero temperature, by the Dyson-Maleev spin-wave theory to second order in perturbation theory. The transverse correlation function is characterized by a dominant one-magnon peak and a broad three-magnon continuum. For spin 1/2 the contribution of the three-magnon excitations is small but not negligible, and might be detected in highly sensitive neutron-scattering experiments in the undoped layered cuprates. We have also computed the transverse equal-time correlations and compared the results with recent series-expansion estimates. The good agreement between the two formalisms reinforces the validity of spin-wave theory. The longitudinal structure factor, to leading order, displays a two-peak structure similar to that obtained by the Schwinger-boson mean-field formalism. The magnon interaction reduces the second peak, in some cases substantially. We discuss how umklapp processes affect the multiple-magnon excitations. We have finally computed the staggered magnetization and transverse susceptibility corrected to second order. For spin 1/2 we find m=0.3069\ifmmode\pm\else\textpm\fi{}0.00020 for the staggered magnetization, and ${\mathit{Z}}_{\mathrm{\ensuremath{\chi}}}$=0.4844\ifmmode\pm\else\textpm\fi{}0.00010 for the susceptibility renormalization constant, in agreement with the results obtained by other techniques.
TL;DR: The renormalization of the staggered-order parameter as a function of hole doping is studied for a two-dimensional doped antiferromagnet in the framework of the t-J model and it is shown that magnon softening is mainly due to the incoherent motion of the holes.
Abstract: The renormalization of the staggered-order parameter as a function of hole doping is studied for a two-dimensional doped antiferromagnet in the framework of the t-J model. The self-consistent Born approximation is used to calculate the Green's functions of holes and spin waves. It is shown that magnon softening is mainly due to the incoherent motion of the holes. The staggered-order parameter vanishes at a small critical density of holes δ c . The calculated value for δ c as well as the concentration dependence of both the staggered moment and spin-wave velocity are consistent with experimental data for La 2-δ Ba δ CuO 4 and YBa 52 Cu 3 O 6+x
TL;DR: In this article, the authors derived an expression for spin wave absorption curves with Bloch type damping for the reentrant Ni 76 Mn 24 amorphous thin films coated with Ni and Ag layers.
TL;DR: Spin-wave perturbation theory for the Heisenberg antiferromagnet at zero temperature is used to compute the finite-lattice corrections to the ground-state energy, the staggered magnetisation, and the energy gap.
Abstract: Spin-wave perturbation theory for the Heisenberg antiferromagnet at zero temperature is used to compute the finite-lattice corrections to the ground-state energy, the staggered magnetisation, and the energy gap. The dispersion relation, the spin-wave velocity, and the bulk ground-state energy to order O(1/S 2 ) are also computed for the square lattice. The results agree very well with the predictions of Neuberger and Ziman and the predictions of Fisher
TL;DR: In this paper, two-magnon excitation processes in the spin-wave expansion of CsNiCl 3 at T = 0 were investigated and the coupling between longitudinal and transverse fluctuations was found to be important.
Abstract: Recent neutron experiments have revealed that the excitation spectrum in quasi-one-dimensional ABX 3 -type antiferromagnets cannot be explained within the conventional linear-spin-wave theory even in the ordered phase. To explain this result, we take into account two-magnon excitation processes in the spin-wave expansion and apply it to CsNiCl 3 at T =0. We find that in addition to modifying the energy and intensity of the one-magnon spectrum, these processes can result in large peaks due to two-magnon continuum. The coupling between longitudinal and transverse fluctuations is found to be important as previously suggested by phenomenological Landau-Ginzburg models. Our results agree reasonably well with available experiments. Similar effects can be expected in other quasi-one-dimensional systems having non-collinear spin structures.
TL;DR: In this article, a new method for a systematic spin-wave expansion for the quantum fluctuations of a generic spin Hamiltonian in a finite lattice is presented, where the inverse spin magnitude 1/S is a well-defined expansion parameter.
Abstract: We present a new method for a systematic spin-wave expansion for the quantum fluctuations of a generic spin Hamiltonian in a finite lattice, where the inverse spin magnitude 1/S is a well-defined expansion parameter. The first two leading contributions of the spin-spin correlation function are evaluated for the J1-J2 Heisenberg model. Very good agreement between our finite-size predictions and the exact diagonalization and Monte Carlo results is found for J2/J1 0.3 the expansion is poorly converging, suggesting a possible breakdown of the spin-wave approximation. Here our calculation seems consistent with a possible spin liquid ground state.
TL;DR: The square-lattice extended Hubbard model at half filling is studied by a perturbation scheme and the ground-state energies are compared to investigate the SDW-CDW transition.
Abstract: The square-lattice extended Hubbard model at half filling is studied by a perturbation scheme By taking into account the ground state of the mean-field theory as a reference state, the scheme treats the collective fluctuations beyond the mean field as the predominant perturbation The system at ground state can be a spin-density wave (SDW) or a charge-density wave (CDW) depending on the relative strengths of the on-site electron interaction and the nearest-neighbor interaction The excitation modes above the ground state are analyzed through diagonalizing an effective Hamiltonian The energy spectra and the wave functions are self-consistently given by the formalism By comparing the ground-state energies, we investigate the SDW-CDW transition
TL;DR: It is found that in very thin films the lifetime of long-wavelength spin waves is relatively insensitive to scattering from even large numbers of imperfections, and therefore cannot explain large observed linewidths observed in Brillouin light-scattering experiments, but it is possible to have large bands with bandwidths on the order of 10 GHz in rough ultrathin films due to the sensitive dependence of effective anisotropy fields on thickness.
Abstract: The effects of localized imperfections on spin-wave propagation in very thin ferromagnetic films are examined. The imperfections are assumed to be localized to a few lattice sites and cause local changes in anisotropy and exchange fields. These imperfections may be due to thickness variations or other geometrical imperfections. We find that in very thin films the lifetime of long-wavelength spin waves is relatively insensitive to scattering from even large numbers of imperfections, and therefore cannot explain large observed linewidths observed in Brillouin light-scattering experiments. On the other hand, we find that a band of long-wavelength spin-wave modes can exist in an inhomogeneous film with a distribution of effective anisotropy fields. It is possible to have large bands with bandwidths on the order of 10 GHz in rough ultrathin films due to the sensitive dependence of effective anisotropy fields on thickness.
TL;DR: The existence of envelope solitons of spin waves is predicted and conditions for their creation are discussed.
Abstract: The propagation of nonlinear dipole spin waves in a film consisting of a two-sublattice, uniaxial, antiferromagnetic material has been investigated. The external magnetic field is assumed to be parallel to the anisotropy axis of the antiferromagnetic film and is directed parallel, or perpendicular, to the film surface. The existence of envelope solitons of spin waves is predicted and conditions for their creation are discussed
TL;DR: In this article, the ground state properties of the S = 1/2 Heisenberg antiferromagnet (HAF) on the triangular lattice with nearest-neighbour (J) and next-nearest neighbor (αJ) couplings were studied.
Abstract: We study the ground state properties of theS=1/2 Heisenberg antiferromagnet (HAF) on the triangular lattice with nearest-neighbour (J) and next-nearest neighbour (αJ) couplings. Classically, this system is known to be ordered in a 120° Neel type state for values-∞<α≦1/8 of the ratio α of these couplings and in a collinear state for 1/8<α<1. The order parameter ℳ and the helicity /gC of the 120° structure are obtained by numerical diagonalisation of finite periodic systems of up toN=30 sites and by applying the spin-wave (SW) approximation to the same finite systems. We find a surprisingly good agreement between the exact and the SW results in the entire region-∞<α<1/8. It appears that the SW theory is still valid for the simple triangular HAF (α=0) although the sublattice magnetisation ℳ is substantially reduced from its classical value by quantum fluctuations. Our numerical results for the order parameterM of the collinear order support the previous conjecture of a first order transition between the 120° and the collinear order at α≅1/8.
TL;DR: It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter that is calculated for a square lattice by means of linearspin-wave theory.
Abstract: The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite Neel temperature is calculated for a square lattice by means of linear spin-wave theory
TL;DR: In this article, the spin-wave dispersion and the sublattice magnetization up to second order in the 1/S expansion for the antiferromagnetic Heisenberg model with nearest-neighbor (J 1 ) and second-nighbor couplings on a square lattice were calculated.
Abstract: We calculate the spin-wave dispersion and the sublattice magnetization up to second order in the 1/ S expansion for the antiferromagnetic Heisenberg model with nearest-neighbor ( J 1 ) and second-neighbor ( J 2 ) couplings on a square lattice. The corrections to the linear spin-wave theory for the spin-wave dispersion and the sublattice magnetization grow large when the frustration increases. The expansion seems to converge asymptotically well for J 2 / J 1 <0.35, leading to quantitative estimates for both quantities. The corrections are positive for J 2 / J 1 <0.4, making the Neel ordered state more stable than what the linear spin-wave theory predicts. When J 2 / J 1 exceeds 0.4, the second-order corrections grow very large with negative sign.
TL;DR: Experimental susceptibilities for ions with different effective spin are presented which are consistent with predictions of antiferromagnetic order, and which confirm the existence of unusually large quantum spin fluctuations in these materials.
Abstract: We report the first observation of dipolar magnetic order in a diamond lattice, in the RPO 4 -(MoO 3 ) 12 -30H 2 O compounds (R=Gd, Dy, Er). Theory predicts antiferromagnetic order for a diamond lattice of dipoles, with a susceptibility at T=0 depending only on lattice symmetry and spin. We present experimental susceptibilities for ions with different effective spin which are consistent with these predictions, and which confirm the existence of unusually large quantum spin fluctuations in these materials. The fluctuations substantially exceed those of the Heisenberg antiferromagnet