Showing papers on "Spin wave published in 1980"

Solvable Model with a Roughening Transition for a Planar Ising Ferromagnet

Abstract: An exactly solvable modification of the planar Ising ferromagnet is proposed which has a roughening transition below the Curie temperature. The computation confirms the de Gennes-Fisher scaling theory of correlations with homogeneous surface fields, giving an exponent value ${\ensuremath{\Delta}}_{1}=\frac{1}{2}$.

Solitons in Spin-Peierls Systems and Applications to Polyacetylene

Abstract: Analytical estimate of the formation energy, E s , of a soliton is performed for a system of strongly correlated one-dimensional π-electrons coupled to lattice distortions. The problem is first transformed into that of a spin-Peierls systems and then the spin degrees of freedom are represented by the phase Hamiltonian with the help of the boson representation of the spinless fermions. Following Cross and Fisher, parameters in this phase Hamiltonian are adjusted so that the spin wave velocity and the critical exponent of the correlation function agree with the exact results. The soliton solution of the resulting quantum sine-Gordon systems is constructed by the method of Dashen et al. The result is applied to polyacetylene, where we obtain E s ≈0.3 eV, which is close to the value estimated by Su et al. for the non-interacting systems.

Analytic theory of the ground state properties of a spin glass. I. Ising spin glass

Abstract: For pt.I see ibid., vol.10, no.12, p.2769 (1980). The general theory, developed in the preceding paper to study the ground state properties of spin glasses, is applied to two-component spin systems. As an extension of the Edwards-Anderson model (1975) a system of the two-component spins, each spin of which can take only n directions in a plane ('discrete planar model'), is introduced to count (gn)-the average degeneracy in the ground state. This procedure allows the authors to see how the fundamental properties change by the spin symmetry. For n=2, the theory reproduces the previous results for the Ising model. They can study the XY model as a limit of n to infinity . Precise calculations are performed for the infinite-range exchange interactions. They find (ginfinity )=e0.51691N.

Calculations of the dynamic susceptibility of nickel and iron

Abstract: The spin dynamics of the transition-metal ferromagnets nickel and iron are investigated within the framework of the itinerant theory of magnetism. The theory is developed in terms of a generalized random-phase approximation which incorporates the band and wave-vector dependence of relevant interaction matrix elements. In contrast to constant-matrix-element approximations, this formalism generates band- and wave-vector-dependent splitting of the energy bands, possible ''optical'' spin-wave modes, and a different interpretation of the spin-wave disappearance phenomena. First-principles numerical calculations of the neutron scattering intensity based on this model have been found to be in excellent agreement with experiments. Recent neutron scattering experiments have also verified the existence of an ''optical'' spin-wave branch predicted by this theory.

Excitation of propagating spin waves in ferromagnetic films

Abstract: The linear excitation of spin waves by an inhomogeneous microwave magnetic field (the field of a microwave antenna) is investigated. The investigation is based on the spin-wave normal-mode theory which takes into account the specific features of spin waves (dipole and exchange interactions and surface-spin-pinning conditions) travelling in perpendicularly and in tangentially magnetised thin ferromagnetic films. Explicit dispersion equations and expressions for the spin-wave magnetisation generated are derived. These expressions enable one to calculate the characteristics of spin waves excited by the microwave field of an antenna.

Spin-Wave Evolution Crossing from the Ferromagnetic to Spin-Glass Regime of Fe x Cr 1 − x

Abstract: Inelastic-neutron-scattering studies on ${\mathrm{Fe}}_{x}{\mathrm{Cr}}_{1\ensuremath{-}x}$ for $x=0.34 \mathrm{and} 0.26$ reveal well-defined spin-wave excitations in the ferromagnetic regime and show a significant decrease of the spin-wave stiffness as the temperature is lowered towards the mictomagnetic or spin-glass regime. Within the spin-glass phase, no well-defined excitations are seen, but an intense quasielastic peak is observed.

Spin and phonon excitations in actinide systems

Abstract: Recent Chalk River neutron scattering experiments on antiferromagnetic UN, on ferromagnetic UTe and US, and on non-magnetic UPd 3 , are reviewed. The spin excitations may be sharp or diffuse and the phonons exhibit anomalies reminiscent of mixed-valent compounds. A very detailed study of the dispersion of the sharp collective excitations seen in dhcp UPd 3 has shown that the uranium is in a stable f 2 , J = 4, ground state subject to a conventional crystal field and exchange. Thus, UPd 3 is the canonical 5f system, analogous to dhcp Pr in the 4f series. Although UN is cubic, large anisotropies are found in its critical scattering, and large anisotropy gaps are found in the spin wave spectra of UN and UTe.

Excitation spectrum and low-temperature thermodynamics of the Ising-Heisenberg linear ferromagnet

Abstract: Important new analytic results have been obtained for the linear, ferromagnetic, spin-\textonehalf{}, Ising-Heisenberg model in a small magnetic field. Specifically, for zero field an expression, which changes its form at an intermediate value of the variable anisotropy parameter, has been obtained for the thermal excitation energy gap. This special anisotropy value does not correspond to a symmetry change in the Hamiltonian, but is associated with an important difference in the physical significance of the results. For anisotropy greater than the special value, the dominant excitations correspond to bound spin complexes. For anisotropy less than the special value, the dominant excitations are spin waves. These results govern the low-temperature behavior of the specific heat. The effective magnetic excitation gap, which determines the low-temperature susceptibility, is dominated at zero field by the bound states for all anisotropy. More complex crossover effects occur in both specific heat and susceptibility when the analysis is extended to nonzero field. These results may have an important bearing on the quantum soliton problem in the linear ferromagnet.

Long-Wavelength Surface Spin Waves on Antiferromagnets

Abstract: The first discussion of a long-wavelength, magnetostatic, surface spin-wave mode on uniaxial antiferromagnets is presented. In the absence of an applied field, the frequency of the surface wave depends on the angle between the direction of propagation and the easy axis. With a magnetic field applied parallel to the surface, and along the easy axis, the surface mode does not propagate in all directions, but is limited to a restricted set of angles.

The Double Sine-Gordon Equations: A Physically Applicable System of Equations

Abstract: We treat two physical problems--the propagation of ultra-short optical pulses in a resonant 5-fold degenerate medium, and the creation--and propagation of spin waves in the anisotropic magnetic liquids 3HeA and 3HeB at temperatures below the transition to the A phase at 2.6 mK. The optical pulse problem is governed by a ‘double’ sine-Gordon equation, the 3HeA problem by the sine-Gordon and the 3HeB by a double sine-Gordon with changed sign. The double sine-Gordons are not integrable: we study them analytically and numerically and predict a “wobbling” 4π pulse in the optical case, a result supported by experiments. We solve certain initial value problems and show how they could be used for the creation of spin excitations in 3HeA and 3HeB. Finally we develop singular perturbation theory in terms of sine-Gordon scattering data in order to describe the 4π wobbler and a similar 0π pulse.

Stability of the collinear spin structure of γ-manganese

Abstract: The bandstructure of antiferromagnetic gamma -manganese was recently calculated using the local spin functional approach and assuming, as is usual, as simple collinear spin configuration. This paper investigates the stability of this spin configuration with respect to the formation of a multiple spin density wave corresponding to a tetrahedral spin arrangement which also agrees with present experiments. The calculations which use the linear response theory within the local spin density functional approach show that the collinear spin arrangement is indeed unstable, thus favouring the multiple spin density wave as the ground-state configuration of antiferromagnetic gamma -Mn.

Chapter 4 Microwave ferrites

Abstract: Publisher Summary This chapter describes a microwave ferrite. The physical phenomenon exploited is the gyromagnetic resonance that appears when the material is subjected to both a static magnetic field and to a microwave field. This effect is inherently non-reciprocal, which accounts for the tensorial nature of the magnetic permeability. This non-reciprocity is widely used in various devices. The saturation magnetization is a fundamental quantity, not only as a factor of efficiency, but also because of the existence of a natural resonance in the internal fields. Consequently, magnetization is the first quantity to be considered in the choice of material for a given application. As in any resonance effect, the linewidth is of prime importance. It is fundamentally related to the damping effects of spin movements, which seem possible to approach by considering either the linewidth off-resonance or the non-linear effects related to the spin waves. Conversely, the linewidth of the main mode observed at resonance, on the polycrystals, is broadened by the magnetocrystalline anisotropy effects or by spurious effects related to faults and porosity. Ferrites now used at microwave frequencies have spinel, G, or hexagonal structures.

A broadband microwave signal to noise enhancer

Abstract: A signal to noise enhancer exhibits a high attenuation to small noise level signals and a lower attenuation to larger above threshold signals. The devices described rely on magnetostatic waves as saturable absorbers of power from a microwave transmission line, typically microstrip. Experiments performed with epitaxial YIG show that instantaneous bandwidths of 900 MHz with threshold levels of less than 0 dBm can be obtained below 4.2 GHz. Enhancement of greater than 20 dB can be achieved for an increase in power level of 20 dB above threshold. Characteristics obtained with lithium ferrite in the 4-8 GHz band are comparable to those obtained with YIG in the 2-4 GHz range. Owing to the increased fmr and spin wave linewidth of the lithium ferrite, the threshold level is higher, + 6 dBm, and the frequency selectivity poorer than observed with YIG.

Neutron scattering investigation of the magnetic excitations in CoBr 2

Abstract: In order to investigate the two-dimensionality and the easy plane anisotropy, the spin waves propagating along the [h, 0, 0], [h, h, 0], [h, 0.5h, 0] and [0, 0, l ] directions in CoBr 2 were observed using inelastic neutron scattering technique. The spin dynamics of CoBr 2 can be described by the anisotropic exchange Hamiltonian for S =1/2 with intraplanar exchange constants J 1 =2.01 meV, J 2 =0.019 meV and J 3 =-0.338 meV, interplanar exchange J '=-0.97 meV, and planar anisotropies D 1 =1.14 meV and D '=-0.06 meV. As shown by these numerical values, CoBr 2 has fairly strong XY characters, but the two-dimensionality is definitely poor. We point out, however, that in a planar antiferromagnet, the easy plane type anisotropy should remove the degeneracy of the excitation spectrum for the `in-plane' and the `out-of-plane' correlations, and in fact, these excitations are observed separately in this experiment.

The spin correlation functions of a finite-thickness ferromagnetic slab

Abstract: Explicit expressions are derived for the Green functions between a pair of spin operators at any two sites within a Heisenberg ferromagnetic slab of finite thickness. These Green functions are found to contain a description of the bulk spin waves and surface spin waves, together with their spectral weighting factors. The results are employed to evaluate the magnetisation in the ferromagnet as a function of temperature and distance from the surfaces.

Optical detection of ferromagnetic resonance in K2CuF4

Abstract: Ferromagnetic resonance (FMR) was investigated in K2CuF4 by observing scattering of light from the crystal under microwave excitation at frequencies around 35 GHz. The spectrum of the scattered light contains satellites whose frequency differs from that of the incident light by the amount of the FMR frequency. The intensity of the satellites is used to estimate the number of the uniform precession magnons and to study their relaxation in a wide range of pumping powers. The power dependencies of the different magnetostatic modes are separately measured and show marked non-linearities. Investigations with light propagating along the c and a axes of the crystal are used to obtain information on the spin wave energy gap.

Neutron scattering study of spin dynamics in CsCoCl 3

Abstract: Neutron profiles of spin waves in the 1d Ising-like antiferromagnet CsCoCl/sub 3/ were reexamined. Instead of well-defined magnon peaks, the spin-wave spectrum consists of a band of magnon states arising from the motion of domain-wall pairs. At the magnetic zone center, the energy of this band extends from 2J(1-2epsilon) to 2J(1+2epsilon), whereas at the zone boundary a sharp resolution limited peak at ..omega.. approx. 2J is observed.

Magnetic Excitations in an Ordered Ferromagnetic Alloy Fe 3 Pt

Abstract: Magnetic excitations in an ordered ferromagnetic alloy of Fe 78 Pt 22 have been studied by neutron scattering. Magnetic dispersion relations have been measured in the [001], [110] and [111] directions. The dispersion curve attained to the zone boundary in [001], while in other two directions, well defined magnon groups could be detected up to 80 meV. The dispersions still have steep slopes, sugggesting that magnon energies at zone boundaries would be quite high. The results have been analyzed in terms of the spin wave theory of the four sublattice Heisenberg system, which indicates that any solutions for fitting the observed dispersions could not explain consistently the whole experimental data. An anomalous damping of the magnons in the [001] direction was detected at high temperatures.