Showing papers on "Magnon published in 1998"

Adiabatic spin dynamics from spin-density-functional theory: Application to Fe, Co, and Ni

Abstract: The adiabatic theory of spin-density waves is developed on the basis of spin-density-functional theory. The wave-number-dependent exchange constant matrix is obtained from spin-density-functional calculations with constrained moment directions. The central assumption considers a fast electronic and a slow magnetic time scale, and postulates negligible correlation of the fast motion between different ionic sites. The parameter-free calculated magnon spectra for Fe, Co, and Ni are in excellent agreement with available experimental data. In the case of Fe, they show strong Kohn anomalies. Using Planck statistics at low temperature, the temperature dependence of the magnetization is well described up to half the Curie temperature. It is conjectured that correlated local-moment clusters survive the Curie transition. On this basis, calculated Curie temperatures are obtained within $10%$ deviation from experiment for Fe and Co, but $30%$ to low for Ni.

Theory of two magnon scattering microwave relaxation and ferromagnetic resonance linewidth in magnetic thin films

Abstract: A detailed analysis of the two magnon scattering contribution to the microwave relaxation and ferromagnetic resonance linewidth in isotropic and anisotropic films and disks has been made. The analysis is based on the Sparks, Loudon, and Kittel (SLK) theory for the scattering of uniform mode magnons into degenerate spin wave states for isotropic spherical samples in the presence of magnetic inhomogeneities in the form of spherical voids or pores. The SLK theory has been extended to include: (i) thin film and thick film samples magnetized in an oblique out-of-plane direction; (ii) uniaxially anisotropic materials with either easy-axis or easy-plane anisotropy and an anisotropy axis perpendicular to the disk plane; (iii) a modified density of degenerate states to account for the nonzero relaxation rate of the scattered spin waves; and (iv) two limiting cases of the scattering interaction: (a) the original SLK case where the inhomogeneities are modeled as spherical voids and the coupling to the degenerate spi...

Spin-flip scattering in magnetic junctions

Abstract: Processes which flip the spin of an electron tunneling in a junction made up of magnetic electrodes are studied. It is found that (i) magnetic impurities give a contribution which increases the resistance and lowers the magnetoresistance, which saturates at low temperatures. The conductance increases at high fields. (ii) Magnon assisted tunneling reduces the magnetoresistance as ${T}^{3/2}$ and leads to a nonohmic contribution to the resistance which goes as ${V}^{3/2}.$ (iii) Surface antiferromagnetic magnons, which may appear if the interface has different magnetic properties from the bulk, gives rise to ${T}^{2}$ and ${V}^{2}$ contributions to the magnetoresistance and resistance, respectively. (iv) Coulomb blockade effects may enhance the magnetoresistance, when transport is dominated by cotunneling processes.

Magnetic anisotropies in dot arrays: Shape anisotropy versus coupling

Abstract: The magnetization and resonance frequencies of submicron Fe magnetic dot arrays are investigated using Brillouin light scattering (BLS) and magneto-optic Kerr Effect (MOKE). Large in-plane anisotropies, evident in both the BLS and MOKE results, are traced to shape anisotropies of the individual dots. The measured magnon frequencies are in good agreement with values calculated on the basis of isolated ellipsoids without interdot coupling.

Ferromagnetic resonance linewidth in thin films coupled to NiO

Abstract: The out-of-plane angular dependence of the ferromagnetic resonance linewidth, ΔH, has been measured for thin magnetic films coupled to NiO and for uncoupled control films. In the control films, ΔH is described by nearly angle-independent damping parameters. In the NiO-coupled films, however, the damping was found to depend strongly on magnetization orientation, with linewidth values comparable to the control samples at normal orientation, but several times larger when the magnetization lies in plane. The additional linewidth in the NiO-coupled films follows the angular dependence of the number of nearly degenerate spin wave modes, in agreement with the predictions of a two-magnon scattering model of damping which incorporates a spin wave dispersion relation suitable for ultrathin films.

Softening and Broadening of the Zone Boundary Magnons in Pr0.63Sr0.37MnO3

Abstract: We have studied the spin dynamics in Pr0.63Sr0.37MnO3 above and below the Curie temperature T-C = 301 K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for T T-C, and no evidence for residual spin-wave-like excitations just above T-C. The results are inconsistent with double exchange models that have been successfully applied to higher T-C samples, indicating an evolution of the spin system with decreasing T-C.

Magnon modes and magnon-vortex scattering in two-dimensional easy-plane ferromagnets

Abstract: We calculate the magnon modes in the presence of a vortex on a circular system, combining analytical calculations in the continuum limit with a numerical diagonalization of the discrete system. The magnon modes are expressed by the $S$ matrix for magnon-vortex scattering, as a function of the parameters and the size of the system and for different boundary conditions. Certain quasilocal translational modes are identified with the frequencies which appear in the trajectory $\stackrel{\ensuremath{\rightarrow}}{X}(t)$ of the vortex center in recent molecular dynamics simulations of the full many-spin model. Using these quasilocal modes we calculate the two parameters of a third-order equation of motion for $\stackrel{\ensuremath{\rightarrow}}{X}(t).$ This equation was recently derived by a collective variable theory and describes very well the trajectories observed in the simulations. Both parameters, the vortex mass and the factor in front of $\stackrel{\ensuremath{\rightarrow}}{X},$ depend strongly on the boundary conditions.

Non-Haldane Spin-Liquid Models with Exact Ground States

Abstract: We present a family of spin-ladder models which can be solved exactly for the ground state and exhibit non-Haldane spin-liquid properties as predicted recently by Nersesyan and Tsvelik [Phys. Rev. Lett. 78, 3939 (1997)], and study their excitation spectra using a simple variational Ansatz. The elementary excitation is neither a magnon nor a spinon, but a pair of propagating triplet or singlet solitons connecting two spontaneously dimerized ground states. Second-order phase transitions separate this phase from the Haldane phase and the rung-dimer phase.

SO(5) symmetric ladder

Abstract: We construct an SO(5) symmetric electron model on a two-chain ladder with purely local interactions on a rung. The ground state phase diagram of this model is determined in the strong-coupling limit. The relationship between the spin-gap magnon mode of the spin-gap insulator and the $\ensuremath{\pi}$ resonance mode of the $d$-wave pairing phase is discussed. We also present the exact ground state for an SO(5) superspin model.

Unexpected behavior of the antiferromagnetic mode of NiO

Abstract: Although NiO is often considered the classic example of an antiferromagnetic insulator, recent investigations have revealed unexplained features of the magnon spectrum. The present study of the temperature and polarization behavior of first-order magnetic Raman scattering reveals that the polarization selection rules are not described by the generally accepted antisymmetric scattering tensor. The inclusion of quadratic magneto-optic coupling terms can explain the symmetry of the scattering tensor, but does not lead to results consistent with the accepted [112] spin alignment direction. {copyright} {ital 1998} {ital The American Physical Society}

Evidence for a gap in the excitation spectrum of CrO2

Abstract: Magnetization and resistivity of CrO2 powders and oriented CrO2 films on different TiO2 single-crystal substrates are reported, and the low-temperature specific heat is measured on powders. The M(T) data follow a Bloch T3/2 law, with no sign of a spin-wave gap. The residual resistivity depends strongly on film texture, but a T2 temperature dependence of the resistivity is found in all samples above a cut-off temperature of the order of 70 K. The behavior is attributed to electron-magnon spin-flip scattering which is suppressed at low temperature in a half-metallic ferromagnet.

Magnetic properties of (VO) 2 P 2 O 7 from frustrated interchain coupling

Abstract: Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon dispersion and an unexpected, low-lying second mode. The proximity and intensity of these modes suggest a frustrated coupling between the alternating spin chains. We deduce the minimal model containing such a frustration, and show that it gives an excellent account of the magnon dispersion, static susceptibility and electron spin resonance absorption. We consider two-magnon states which bind due to frustration, and demonstrate that these may provide a consistent explanation for the second mode.

Elementary excitations in the coupled spin-orbital model

Abstract: The elementary excitations of a model Hamiltonian that captures the low-energy behavior of a simple twofold-degenerate Hubbard Hamiltonian, with Hund's rule coupling, is studied. The phase diagram in the mean-field limit and in a two-site approach reveals a rich variety of phases where both the orbital and the spin degrees of freedom are ordered. We show that, besides the usual spin waves (magnons), there also exist orbital waves (orbitons) and, most interestingly, in a completely ferromagnetically coupled system, a combined spin-orbital excitation which can be visualized as a bound state of magnons and orbitons. For a completely degenerate system the bound states are found to be the lowest-lying elementary excitations, both in one and two dimensions. Finally we extend our treatment to almost-degenerate systems. This can serve as an example that elementary excitations in orbitally degenerate strongly correlated electron systems in general carry both spin and orbital character.

Direct Two-Magnon Optical Absorption in α'-NaV2O5 : “Charged” Magnons

Abstract: We investigate the temperature-dependent optical conductivity of α'-NaV2O5 in the energy range 4 meV–4 eV. The intensities and the polarization dependence of the detected electronic excitations give a direct indication for a broken-parity electronic ground state and for a noncentrosymmetric crystal structure of the system in the high-temperature phase. A direct two-magnon optical absorption process, proposed here, is in quantitative agreement with the optical data. By analyzing the optically allowed phonons at various temperatures above and below the phase transition, we conclude that a second-order change to a larger unit cell takes place below 34 K.

Dynamical properties of low-dimensional \(\) and \(\) spin-Peierls systems

Abstract: Properties of low-dimensional spin-Peierls systems are described by using a one-dimensional S=1/2 antiferromagnetic Heisenberg chain linearly coupled to a single phonon mode of wave vector \(\) (whose contribution is expected to be dominant). By exact diagonalizations of small rings with up to 24 sites supplemented by a finite size scaling analysis, static and dynamical properties are investigated. Numerical evidences are given for a spontaneous discrete symmetry breaking towards a spin gapped phase with a frozen lattice dimerization. Special emphasis is put on the comparative study of the two inorganic spin-Peierls compounds CuGeO3 and NaV2O5 and the model parameters are determined from a fit of the experimental spin gaps. We predict that the spin-phonon coupling is 2 or 3 times larger in NaV2O5 than in CuGeO3. Inelastic neutron scattering spectra are calculated and similar results are found in the single phonon mode approximation and in the model including a static dimerization. In particular, the magnon S=1 branch is clearly separated from the continuum of triplet excitations by a finite gap.

Dynamical effects of phonons on soliton binding in spin-Peierls systems

Abstract: The role of dynamical magnetoelastic coupling in spin-Peierls chains is investigated by numerical and analytical techniques. We show that a Heisenberg spin chain coupled to dynamical optical phonons exhibits a transition towards a spontaneously dimerized state in a wide range of parameter space. The low-energy excitations are characterized as solitons. No binding between solitons occurs in the isolated spin-phonon chain and the dynamical spin structure factor shows a broad magnon dispersion. However, elastic interchain coupling can lead to the formation of bound states.

Magnonic spectra of ferromagnetic composites versus magnetization contrast

Abstract: It had been shown recently that the calculated mαgnonic spectra of two-dimensional periodic ferromagnetic composites can present frequency ranges forbidden for the propagation of magnon excitations throughout the composite. However, those forbidden energy gaps were found to be highly sensitive to the exchange contrast between the component ferromagnetic materials but were very weakly sensitive to the contrast in spontaneous magnetizations of the two materials. Accordingly, in this paper we introduce a r.ew mathematical definition of the exchange field acting in inhomogeneous medium. With this new definition the present theory gives magnonic spectra reasonably sensitive to magnetization contrast, as they should be from the physical viewpoint; moreover, the magnetization contrast now becomes a gap-creating factor as well. PACS numbers: 75.50.Gg

Universal Behavior of One-Dimensional Gapped Antiferromagnets in a Staggered Magnetic Field

Abstract: We study the properties of one-dimensional gapped Heisenberg antiferromagnets in the presence of an arbitrary strong staggered magnetic field. For these systems we predict a universal form for the staggered magnetization curve. This function, as well as the effect the staggered field has on the energy gaps in longitudinal and transversal excitation spectra, are determined from the universal form of the effective potential in O(3)-symmetric 1+1–dimensional field theory. Our theoretical findings are in excellent agreement with recent neutron scattering data on R2BaNiO5 (R = magnetic rare earth) linear-chain mixed spin antiferromagnets. One-dimensional isotropic Heisenberg antiferromagnets with an exchange gap in the magnetic excitation spectrum have been at the center of theoretical and experimental attention for almost two decades. This class of materials includes integer-spin Heisenberg chains [1] (commonly referred to as Haldane-gap systems) and halfinteger spin ladders with an even number of legs [2]. Due to the presence of strong quantum fluctuations in these systems the staggered magnetization has a finite correlation length. The principal feature of the excitation spectrum is a degenerate triplet of sharp spin-1 excitations commonly referred to as magnons, separated from the ground state by a finite energy gap �. In recent years much work was aimed at understanding the behavior of such gapped 1D antiferromagnets in the presence of an external uniform magnetic field [3–5]. However, the effect of a staggered field, that couples directly to the order parameter of the classical system, has not been investigated in sufficient detail. This is mainly due to the fact that a strong magnetic field modulated on the microscopic scale was thought to be all but impossible to realize experimentally [6]. A breakthrough came with neutron scattering experiments on R2BaNiO5 (R = magnetic rare-earth) linear-chain nickelates and their interpretation in terms of non-interacting Haldane spin chains immersed in a strong effective staggered exchange field [7,8]. In R2BaNiO5 compounds almost perfectly isotropic antiferromagnetic S = 1 chains are formed by the Ni 2+ ions. The effective staggered field is generated by the R 3+ sublattice that becomes ordered magnetically below some Neel temperature TN. The staggered field intensity is proportional to the magnitude of the ordered moment

Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCu2O3 and CaV2O5.

Abstract: We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder system, both by perturbation around the dimer limit and by quantum Monte Carlo simulations. We discuss the influence of the inter-ladder coupling on the spin gap and the dispersion, and present results for the temperature dependence of the uniform susceptibility. The latter was found to be parameterized well by a mean-field type scaling ansatz. Finally we discuss fits of experimental measurements on SrCu 2 O 3 and CaV 2 O 5 to our results.

Magnons and Skyrmions in fractional Hall ferromagnets

Abstract: Recent experiments have established a qualitative difference between the magnetization temperature dependences $M(T)$ of quantum Hall ferromagnets at integer and fractional filling factors. We explain this difference in terms of the relative energies of collective magnon and particle-hole excitations in the two cases. Analytic calculations for hard-core model systems are used to demonstrate that, in the fractional case, interactions suppress the magnetization at finite temperatures and that particle-hole excitations rather than long-wavelength magnons control $M(T)$ at low $T.$

Optical absorption edge in α–Fe2O3: The exciton–magnon structure

Abstract: Transmission spectra of synthetic and natural hematite (α-Fe2O3) crystals are measured at temperatures 10, 25, and 300 K in the wavelength range 500–1100 nm, and the absorption spectra are computed Pure exciton and exciton–magnon d–d transition bands are revealed, the corresponding wavelengths at 10 K being λ0=1020 nm and λ1=965 nm respectively The half-widths and oscillator forces are g0=84 cm−1, f0=4×10−9, g1=60 cm−1, f1=14×10−7 for 10 K, g0=85 cm−1, f0=5×10−9, g1=110 cm−1, f1=21×10−7 for 25 K The mechanisms of band formation for weakly allowed d–d transitions in hematite are analyzed

Flow equations and extended Bogoliubov transformation for the Heisenberg antiferromagnet near the classical limit

Abstract: The Heisenberg spin-S quantum antiferromagnet is studied near the large-spin limit, applying a new continuous unitary transformation which extends the usual Bogoliubov transformation to higher order in the 1/S-expansion of the Hamiltonian. This allows to diagonalize the bosonic Hamiltonian resulting from the Holstein-Primakoff representation beyond the conventional spin-wave approximation. The zero-temperature flow equations derived from the extension of the Bogoliubov transformation to order \(\) for the ground-state energy, the spin-wave velocity, and the staggered magnetization are solved exactly and yield results which are in agreement with those obtained by a perturbative treatment of the magnon interactions.

Low-lying magnetic excitations in amorphous and alloys

Abstract: Results of high-resolution magnetization (M) measurements performed on amorphous (a-) (x = 0, 1, 2, 4, 6, 8, 10) and (y = 0, 1) alloys over wide ranges of temperature (T) and external magnetic field are presented and discussed in the light of existing theoretical models. The magnetization at 5 K does not saturate even for fields as high as 70 kOe particularly for the alloys with and y =0, 1. The high-field differential susceptibility at , is extremely large for the alloys with x = 0, 1 and y = 0, 1, and decreases rapidly with x for such that it possesses values typical of crystalline ferromagnets such as Fe, Co, Ni for x > 6. The dominant contribution to the thermal demagnetization of the spontaneous as well as `in-field' magnetization comes from spin-wave (SW) excitations at low temperatures and from enhanced local spin-density fluctuations over a wide range of intermediate temperatures and for temperatures close to the Curie point, , for all of the alloys studied. The spin-wave stiffness, D, is independent of for all of the compositions and the ratio possesses a value characteristic of amorphous ferromagnets with competing interactions for the alloys with and y = 0, 1. For these alloys, thermomagnetic and thermoremanent effects generally associated with the cluster spin-glass behaviour have been observed in the re-entrant state which sets in at a temperature . In accordance with the predictions of the spin-fluctuation model, D renormalizes with temperature as and the spin fluctuations get strongly suppressed by Co substitution and . While the spin-fluctuation (SF) model provides a consistent theoretical basis for the observed temperature dependence of the spontaneous and `in-field' magnetization over the entire temperature range , the infinite three-dimensional (FM) matrix plus finite FM spin-clusters model extends the scope of the SF model in that it offers a straightforward explanation for the absence of SW peaks in the inelastic neutron scattering spectra taken over a certain wave-vector-transfer range, the softening of spin-wave modes for , the existence of a significant contribution due to diffusons, in addition to magnons, to the -decrease of the magnetization, and the composition dependence of D(0), M(0,0) and .

Giant-magnetoresistance calculation for 111 co/cu/co spin valves

Abstract: We calculate the canting-angle and thickness dependence of the current-in-plane giant magnetoresistance (GMR) of a system consisting of a copper slab between two cobalt slabs, where the canting angle is the angle between the magnetization vectors of the two cobalt slabs. We utilize the layer-Kohn-Korringa-Rostoker method to self-consistently calculate the electronic structure. Electron scattering by impurities, phonons, magnons, etc., is modeled with a layer- and spin-dependent complex self-energy. Scattering rates are chosen to match Cu and Co resistivities. The scattering rate for minority Co is assumed to be seven times larger than for majority Co, the same ratio as for the Fermi energy density of states in the two channels. The nonlocal layer-dependent conductivity is calculated using the Kubo-Greenwood formula. We find that the GMR decreases with copper thickness in a nonuniform manner due to changes in the behavior of waveguidelike modes in the copper slab. For fixed copper thickness the GMR dependence on costhinsp{theta}, where {theta} is the relative angle between the cobalt slabs magnetization directions, deviates from a linear dependence on costhinsp{theta}. {copyright} {ital 1998} {ital The American Physical Society}

Spectral and magnetic properties of the two-dimensional t- J model in the quantum disordered regime

Abstract: We apply the modified spin-wave theory with the constraint of zero staggered magnetization to investigate normal-state spectral and magnetic properties of the two-dimensional (2D) t – J model in the paramagnetic state. A set of self-energy equations for hole and magnon Green's functions is solved numerically in the self-consistent Born approximation. The constraint can be fulfilled in the ranges of hole concentrations 0.02×0.17 and temperatures T =100 K. In this region, the hole spectrum is nonmetallic which is manifested in the variation with x of the quasiparticle weights of states and in the violation of Luttinger's theorem. With decreasing x from x ≈0.17 hidden parts appear in the hole Fermi surface which can be interpreted as the opening of a pseudogap near (± π ,0), (0,± π ). Obtained size, symmetry and concentration dependence of the pseudogap are in agreement with photoemission data in Bi2212. Calculated temperature dependencies of the spin correlation length, spin-lattice relaxation times at the Cu and O sites, and static susceptibility are typical for the quantum disordered regime with a pseudogap in the spectrum of magnetic excitations. These quantities are in qualitative and in some cases in quantitative agreement with experiment in underdoped YBa 2 Cu 3 O 6+ δ . At x =0.12, the considered nonmetallic phase borders the phase of conventional metal.

Transverse Ising model at zero temperature

Abstract: The method of correlated basis functions is applied at the variational level to the Ising model consisting of Pauli spins arranged on a simple cubic lattice, experiencing nearest-neighbor interactions through their $x$ components and subject to a transverse field in the $z$ direction of strength $\ensuremath{\lambda}$. Full optimization of a Hartree-Jastrow trial wave function is performed by solving two Euler-Lagrange equations: a renormalized Hartree equation for the order parameter characterizing the ferromagnetic phase and a paired-magnon equation for the optimal two-spin spatial distribution function. The optimized trial wave function yields a second-order transition with a numerically determined critical coupling of ${\ensuremath{\lambda}}_{c}=5.10$. Explicit results are presented for (i) the magnetization order parameter, (ii) the energy per spin and its potential component, (iii) the static structure function at zero wave number, (iv) the spin-exchange strength, and (v) the magnon energy gap corresponding to a Feynman description of the elementary excitations.

Low Energy Spin-Wave Excitation in Highly Conductive Thin Films and Surfaces

Abstract: In this lecture we shall study the low-energy elementary magnetic excitations, spin-waves (magnons), using the methods of microwave spectroscopy, in ferromagnetic materials with high electrical conductivity (metals). Special attention will be given to the material parameter describing the magnetic energy at the surface of a sample, the magnetic surface anisotropy.

Thermal transport in Sr1−xCaxRuO3

Abstract: We have studied the thermal conductivity K(T,B) (6

Incommensurability in the magnetic excitations of the bilinear-biquadratic spin-1 chain

Abstract: We study the magnetic excitation spectrum of the S=1 quantum Heisenberg spin chain with Hamiltonian : H = sum_i cos(theta) S_i S_i+1 + sin(theta) (S_i S_i+1)^2. We focus on the range -pi/4 < theta < +pi/4 where the spin chain is in the gapped Haldane phase. The excitation spectrum and static structure factor is studied using direct Lanczos diagonalization of small systems and density-matrix renormalization group techniques combined with the single-mode approximation. The magnon dispersion has a minimum at q=pi until a critical value theta_c = 0.38 is reached at which the curvature (velocity) vanishes. Beyond this point, which is distinct from the VBS point and the Lifshitz point, the minimum lies at an incommensurate value that goes smoothly to 2pi/3 when theta approaches pi/4, the Lai-Sutherland point. The mode remains isolated from the other states: there is no evidence of spinon deconfinement before the point theta =+pi/4. These findings explain recent observation of the magnetization curve M approx (H -H_c)^1/4 for theta =theta_c.