Showing papers on "Spin wave published in 1972"

A Quantum Theory of Double Exchange. I

Abstract: Metallic double-exchange ferromagnets such as doped LaMnO 3 are investigatd on the basis of an efffective Hamiltonian derived from the s - d model Hamiltonian. Resistance at low temperatures is calculated in the spin wave approximation and it is found to be proportional to T 9/2 . Magnetic behaviors of the system at finite temperatures are examined by the use of a simple molecular field theory. A magnetization-temperature curve which firs well the experimental data of Leung et al. is obtained. Resistance and magneto-resistance are also studied.

Magnetic properties of rare earth metals

Abstract: 1 Introduction.- 2 Phenomenological Theory of Magnetic Ordering: Importance of Interactions with the Crystal Lattice.- 3 Magnetic Structures of Rare Earth Metals and Alloys.- 4 Bulk Magnetic Properties.- 5 Spin Waves.- 6 Energy Band Structure, Indirect Exchange Interactions and Magnetic Ordering.- 7 Transport Properties.- 8 Hyperfine Interactions.

Measurement of Spin-Wave Dispersion in NiO by Inelastic Neutron Scattering and Its Relation to Magnetic Properties

Abstract: Inelastic neutron scattering techniques have been used to measure the spin-wave dispersion relations at 78\ifmmode^\circ\else\textdegree\fi{}K in the fcc antiferromagnet NiO. The energy dispersion has a steep initial slope (\ensuremath{\sim}250 meV \AA{}) and a high maximum energy (\ensuremath{\sim}117 meV) and is further characterized by a relatively low zone boundary energy in certain directions. The exchange parameters defined by ${\mathcal{H}}^{1,2}={J}_{j}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(1)}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(2)}$ were determined by fitting the theoretical expression for the spin-wave energies to the experimental data corrected for instrumental resolution effects. The predominant interaction is a large antiferromagnetic exchange ${J}_{2}=221\ifmmode^\circ\else\textdegree\fi{}$K (19.01 meV) between next-nearest neighbors, which are linked by a 180\ifmmode^\circ\else\textdegree\fi{} superexchange path. The interaction between nearest neighbors, linked by a 90\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{Ni}}^{2+}$---${\mathrm{O}}^{2\ensuremath{-}}$---${\mathrm{Ni}}^{2+}$ configuration, is much smaller and ferromagnetic in sign, ${J}_{1}=\ensuremath{-}15.9\ifmmode^\circ\else\textdegree\fi{}$K (-1.37 meV). A consequence of the relatively small value of ${J}_{1}$ is that the spin waves from the four domains present in the sample can only be resolved in a limited region of reciprocal space. These values of exchange interactions are in accord with simple ideas of covalency and overlap, and the results emphasize the behavior of NiO as a weakly covalent insulator. The density of magnon states, estimates of the transition temperature, and several thermomagnetic properties of NiO have been calculated from the measured exchange parameters using molecular field and random-phase-approximation Green's-function formulas.

Green's-Function Formalism of the One-Dimensional Heisenberg Spin System

Abstract: The one-dimensional Heisenberg model with S= 1/2 is treated with the use of the two­ time Green's functions. The hierarchy of the equations of motion of the Green's functions is decoupled at a stage one-step further than Tyablikov's decoupling. The thermal average of the spin component,

Anomalous Damping of Spin Waves in Magnetic Metals

Abstract: An analysis of long-wavelength magnon damping in magnetic metals is presented in the context of an itinerant-electron picture of ferromagnetism. The decay is into single-particle excitations, and is a result of spin-orbit forces. The predicted dependence of the damping constant on the electron mean free path is in accord with recent experiments on ferromagnetic resonance. In particular, for pure metals it increases as the temperature is lowered, and then saturates. This anomalous rise is closely related to the anomalous electrical conductivity.

Spin-Wave Modes in a Heisenberg Linear Chain of Classical Spins

Abstract: The second, fourth and sixth moments of the time-dependent correlation functions are calculated rigorously for a classical Heisenberg linear chain with the nearest-neighbour inter­ actions. With use of these moments the spectral-line shape of the time-dependent correlation functions is reproduced by a Gaussian assumption at the second step in a continued fraction expansion. At the low temperatures, well-defined spin-wave peaks are obtained. From this result it may be considered that there exist propagating modes in the highly developed local order instead of the long-range order that is required in the simple spin-wave theory. At finite temperatures the calculated spectrum is compared with the inelastic neutron scattering data of (CDs)~MnCls (TMMC) and there can be found a good agreement. The conspicuous discrepancy is found only in the line-width and intensity at the lowest temperature. At the high-temperature limit we can reproduce a spectrum which is very similar to the rigorous solution calculated by Carboni and Richards for a finite chain of quantum spin (S=1/2).

Spin dependence of critical indices in the two dimensional Ising model

Abstract: High temperature series expansions of the susceptibility and second moment of the 1/2, 1, 2 and infinite spin Ising models on a triangular lattice are obtained and the spin dependence of the critical indices gamma and nu investigated.

Spin-wave phenomena in liquid3He systems

Abstract: The magnetic effects predicted by Leggett and Rice for Fermi liquids in the collisionless regime have been verified. Spin-echo measurements are reported which demonstrate the existence of spin waves and undamped spin currents in liquid3He and dilute3He-4He solutions. The experimental results are compared with the theoretical expressions of Leggett and Rice, and values of the Fermi liquid interaction parameter $$\lambda = \left[ {1/(1 + F_0^a )} \right] - \left\{ {1/[1 + (F_1^a /3)]} \right\}$$ are derived for three samples: liquid3He atP=0 and 27 atm, and a 6.4%3He-4He solution. In addition, some techniques for direct observation of individual spin-wave modes are explored and the results reported.

Theory of spin waves in disordered antiferromagnets. I. Application to (Mn, Co)F2 and K(Mn, Co)F3

Abstract: The coherent potential approximation has been extended to provide a theory of the magnetic excitations in substitutionally disordered quasi-binary antiferromagnetic alloys. It is shown how the self consistent equations for a two sublattice antiferromagnet may be reduced to a scalar form with the aid of the single site approximation and time reversal symmetry. The theory is applied to the Mn1-cCocF2 and KMn1-cCocF3 systems and numerical results obtained for comparison with recent neutron scattering experiments. The theory correctly predicts the two-peaked spectral function observed experimentally at each wavevector, and gives a good description of the dispersion relations of the two bands, and of the widths, relative intensities and lineshapes of the excitations.

Theoretically predictable effects and new interpretations in spin wave resonance of thin films with asymmetric boundary conditions

Abstract: The eigenvalue problem of the finite linear chain with arbitrary asymmetric boundary conditions is solved strictly. The results are used in quantum theory of spin wave resonance (SWR) in thin asymmetric ferromagnetic films. A theory providing some new interpretations of experimentally known effects in SWR and predicting new as yet unobserved effects is proposed. The results are visualized in an especial surface pinning diagram which should prove highly useful for the comparison of experimental data with the theory.

Nuclear Spin Relaxation of Impurities in Dilute Ferromagnetic Alloys

Abstract: The nuclear spin lattice relaxation time T 1 of the 3 d - and 4 d -series impurities (V, Mn, Co, Cu, Nb, Mo, Ru, Rh, and Pd) dissolved dilutely in Fe metal has been measured as a function of external magnetic field strength by pulsed NMR technique. The relaxation mechanisms for these impurities in high external magnetic field are discussed in terms of the hyperfine interactions with the s - and the d -band electrons. The density of states of the impurity d - electrons in the Fe metal is calculated using the Wolff and Clogston model and used for the calculation of the relaxation time. External field dependence of the relaxation time was discussed by the spin wave mechanism. The relaxation mechanism in the ferromagnetic metal proposed by Weger has been also derived by the susceptibility formalism.

Ferromagnetic Thin Films

Abstract: Publisher Summary From the practical point of view, a thin ferromagnetic film is a ferromagnetic body presenting a microscopic thickness in contrast with the macroscopic extension of its surfaces and satisfying certain conditions of deposition and oxidation. From an ideal point of view, the thin ferromagnetic film occurs as a many-body system of spins, which is finite in one direction, and which obeys some condition of uniformity and cohesion. A rapid progress of the technology together with a remarkable growth of the interest of physicists brought in the past 20 years the subject of films to a position of prominence in the physics of magnetism. The interest of both engineers and physicists in thin films is mainly stimulated by the exciting fact that a typical thin ferromagnetic film would be the only known single ferromagnetic domain of macroscopic extension. This chapter focuses mainly on critical phenomena and phase transitions on one hand, and elementary excitations on the other. It discusses saturation magnetization, Curie temperature, spin wave spectrum, dispersion laws, and resonant excitations. However, a discussion of the magnetic configurations in thin films with special emphasis on the domain structure as well as references to ripple is also given.

Temperature Dependence of Magnon Sideband

Abstract: Temperature dependence of magnon sideband with respect to the intensity, shift and broadening is studied theoretically in the entire temperature range by using the moment expansion method. The assumption of the Neel state for the ground state of the antiferromagnets is reasonable for intensity and shift but is incapable of explaining the magnitude of the width. Numerical calculations are carried out for 6 A 1 g → 4 A 1 g , 4 E g transition of RbMnF 3 by means of the random phase decoupling approximation for spin correlation functions and the results are presented. The results agree with those of the spin wave theory at low temperatures. At the high temperature limit, both the intensity and shift also agree with those given by the pair spectra theory. The effect of exciton transfer is also considered. The numerical result of the shift is compared with experiments.

Nuclear Magnetic Resonance and Relaxation of 159Tb in Ferromagnetic Terbium Metal

Abstract: Nuclear magnetic resonance and relaxation of 159 Tb in ferromagnetic Tb metal were investigated by spin echo technique at liquid helium temperatures. The nuclear magnetic resonance spectrum consists of three equally separated lines in the frequency range from 2.4 GHz to 3.9 GHz. The center line frequency which corresponds to the Zeeman frequency is 3120 MHz and the frequency difference of the adjacent lines which represents the electric quadrupole effect is 673.5 MHz. It is found that spin-lattice relaxation is produced mainly by the process in which the nuclear energy relaxes to the kinetic energy of the conduction electrons via spin waves (Weger's mechanism), as in the case of Dy nuclei in ferromagnetic Dy metal, and the spin-spin relaxation time is determined by the spin-lattice relaxation and the I · I coupling.

Spin wave measurements of exchange constant in Ni-Fe alloy films

Abstract: The ferromagnetic exchange constant has been measured using spin wave resonance in a number of iron-nickel alloys covering the composition range 60% to 90% nickel. The exchange constant varies smoothly from a value of 1.3 (10-6) erg . cm-1at 60% Ni to an extrapolated value of 0.77 (10-6) at 100% Ni. The data are in significant disagreement with the values published by Rusov.

Effect of Magnon-Magnon Interaction on Spin Wave Dispersion and Magnon Sideband in MnS

Abstract: The temperature-dependent magnon energies and magnon sidebands in MnS have been studied theoretically by the spin wave approximation including magnon-magnon interaction and neglecting exciton-magnon interaction. Results found are: (1) the temperature renormalization factor for magnon energies depends strongly on magnon wavevector and is different from that for the sublattice magnetization; (2) the line shape is insensitive to temperature, and the peak position, which corresponds to the magnon energy at k =1.2π[1, 1, 0]/ a , shifts slowly with temperature; (3) the integrated intensity of the cold band is almost constant at low temperatures but decreases slightly with increasing temperature in contrast to a rapid increase predicted from the free spin wave theory. For the hot band, the intensity increases more slowly than predicted from the free spin wave theory. The total intensity increases gradually with temperature. The sublattice magnetization, the peak position, and the integrated intensity as function...

Magnons in Thin Films

Abstract: In Heisenberg ferromagnetic thin films with {100} surfaces, the spin waves are analyzed in an exact treatment. The bulk and surface modes are separated and discussed. When the number N of parallel layers is finite, space quantization and phase shifts due to the surface perturbation are precised. Magnon specific heat, ferromagnetic resonance and the contribution to the fluctuation of the surface magnetization from the bulk and surface modes are studied in a microscopical way for very thin films. Anomalous variations of specific heat at very low temperature are found to exist. The ferromagnetic resonance is demonstrated to give a detailed study of the surface perturbation. Bulk and surface contributions occur in catalytic effects such as ortho-para conversion.

Dynamic magnetoelastic coupling in ferromagnets and antiferromagnets

Abstract: Recent advances in the theory and application of dynamic magnetoelastic coupling in single-crystal ferromagnets and antiferromagnets are reviewed. As background, a linear model of a magnetoelastic ferromagnet is constructed by demanding that the small-signal equations lead to small-signal power-energy and stress-momentum conservation. Self-consistent boundary conditions are also discussed. Magnetoelastic excitations are reviewed both as waves and quasiparticles and coupled-mode analysis is used to describe the propagation of spin-elastic waves in a crystal subjected to time-varying and/or space-varying magnetic bias fields. In such cases, the character of a wave packet can change. This is especially true for longitudinal-elastic/spin waves propagating in yttrium iron garnet (YIG). Microwave and optical experiments dealing with this important case are recounted; the latter demonstrates that strong Bragg scattering of coherent 1150-nm wavelength laser radiation can be used to allow one to "see" the microwave (1.5 GHz) spin/elastic conversion in detail.

Linear Spin‐Wave Theory of Librational Motion in the α Phase of N2 and CO

Abstract: A linear spin‐wave theory is used to calculate the frequencies of librational modes in the α phase of nitrogen and of carbon monoxide. Results are presented both for quadrupole‐quadrupole interactions and for the potential of Kohin. From a comparison with the Raman scattering measurements of Anderson, Sun, and Donkersloot, it is concluded that the Kohin potential does not give a satisfactory representation of the anisotropic interactions in either crystal. The present spin‐wave theory is compared with the recent excitonlike theories of Jacobi and Schnepp and of Raich. A numerical error in the classical harmonic calculations of Goodings and Henkelman is rectified.

Contribution to the spin wave theory of amorphous ferromagnets

Abstract: A technique for discussing the behaviour of spin waves in an amorphous ferromagnet is presented. Whether or not the long wavelength spin waves can be clearly defined in amorphous states is discussed. From the viewpoint of the instability of the ferromagnetic ground state with respect to the formation of long wavelength spin waves, a criterion for the existence of amorphous ferromagnets is studied. The possibility of a new amorphous ferromagnet, which has a piezomagnetic effect, or which is not ferromagnetic in its crystalline state, is discussed.