Showing papers on "Spin wave published in 1974"

Quantum theory of one- and two-dimensional ferro- and antiferromagnets with an easy magnetization plane . I. ideal 1-d or 2-d lattices without in-plane anisotropy

Abstract: A « semi-polar » representation of the spin operators is introduced, which makes possible, in the harmonic approximation, the definition of magnons for any wavelength at low temperature in one-dimensional (= 1-D) or two-dimensional (= 2-D) magnetic systems without long-range order, provided they are of the « planar » type, i. e. they have an easy magnetization plane. The semi-polar representation is used to calculate the spin pair correlation function at low temperature. Its space-time Fourier transform (directly observable by neutron scattering) consists of a relatively broad peak due to spin fluctuations inside the easy plane, plus a narrower peak due to out-of-plane fluctuations. The intensity, width and lineshape of both peaks are calculated in both 1-D and 2-D cases for all momentum transfers, as well as the frequency shift as a function of temperature.

Conductivity and spin-wave stiffness in disordered systems-an exactly soluble model

Abstract: An exact discussion is given of the conductivity of disordered resistor networks and of the related problem of spin-wave stiffness in disordered Heisenberg ferromagnets, by considering the special case of the Bethe lattice. For arbitrary substitutional disorder an exact nonlinear integral equation is obtained for a generating function determining the conductivity and spin-wave stiffness in such branching models. An effective medium (coherent potential) approximation is shown to provide an asymptotically exact solution in the limit of high coordination number z. Its extension, to order (1/z)4, is obtained.

Ferromagnetism and strong correlations in metals

Abstract: A single band description of ferromagnetism in 3d transition metals is given based on a recent formal theory of strong correlations by the author. Correlations are assumed to be important leading to 'Hubbard split bands' and localized moments. Correlations and magnetic effects of the system are calculated selfconsistently. An effective Heisenberg Hamiltonian is obtained. The magnetic excitations of the system are spin waves, their dispersion relation is derived. The spin wave stiffness constant is compared with earlier theories. At the Curie temperature Tc long range magnetic order disappears, localized moments and short range order, however, still exist above Tc. Comparison with experiment is made.

Magnetization of Amorphous Co P Alloy—Spin Waves in Noncrystalline Ferromagnets

Abstract: The Magnetization of several bulk amorphous ferromagnetic CoP alloys is shown to obey a corresponding-states law which differs significantly from comparable crystalline ferromagnets such as Co or Ni. This difference originates in an anomalously large ${T}^{\frac{3}{2}}$ low-temperature demagnetization extending beyond $0.2{T}_{C}$. The temperature and field dependence of this term provide evidence for the presence of spin-wave excitations characterized by a high density of states at low energies.

Excitations in the dilute Heisenberg ferromagnet using the coherent potential approximation

Abstract: The coherent potential approximation (CPA) has been generalized to describe spin waves in a dilute Heisenberg ferromagnet at T=0. The full T matrix of an isolated vacancy in an effective medium is evaluated and the self-energy is obtained selfconsistently. In order to project out the spurious degrees of freedom associated with the fictitious spins on the vacancy sites, a class of pseudopotentials are constructed. The response functions G(k, omega ) and density of states rho ( omega ) for two such pseudopotentials are evaluated in detail. The had core pseudopotential is found to yield the best CPA approximation although it fails near the critical percolation concentration of magnetic sites, as do all the effective medium theories in the face of large fluctuations. The spin-wave stiffness D is calculated and the magnetization and specific heat near T=0. The random phase approximation is used to find Tc, the transition temperature.

The pseudo-spin model and its application to KDP-type materials

Abstract: The applicability of the pseudo-spin model is discussed in relation to the existence of constituents with a restricted number of low-lying energy states. The situation certainly occurs in materials with low-lying electronic levels which undergo co-operative Jahn-Teller phase transitions. The Slater-Takagi model of KDP is analysed to show that under certain approximations it leads to a double well in the collective co-ordinates with two levels in each unit cell and hence to a pseudo-spin model of one S = 1/2 per cell. The spin hamiltonian is the Ising model in a transverse field together with four spin interactions. The thermodynamic properties of this model are discussed and best values of the parameters obtained. The elastic constant anomalies are interpreted in terms of cs', in the adiabatic regime. The spin dynamics is discussed in terms of random phase approximation for spin waves, and more complete treatments are reviewed. Raman scattering data is analysed in terms of a coupled spin-phonon m...

Magnetization of α-Phase Fe-Mn Alloys

Abstract: Magnetization measurements of α-phase Fe-Mn alloys, in which manganese concentration is extended up to 11 at% by cold-working techniques, have been made from liquid He temperature to room temperature. The magnetic moment decreases linearly with increasing manganese concentration and assuming that the magnetic moment of iron atom is constant at 2.217 µ B , one obtains the magnetic moment of manganese atom to be 0.8 µ B . On the basis of the spin wave theory, the exchange intergral J and the exchange stiffness constant D are estimated from precision measurements of magnetization at low temperatures.

Problem of the coexistence of superconductivity and ferromagnetism

Abstract: This review begins with the theory of the paramagnetic effect in a superconductor. The coexistence of superconductivity and ferromagnetism in metals with paramagnetic impurities is considered with the Gorkov--Rusinov theory being developed in another way. The time correlations in the spin system are taken into account, and the coexistence criteria are defined more precisely. The role of spin waves in the coupling interaction in a perfect ferromagnetic superconductor is considered. (47 references) (DLC)

Spin waves in superfluid He 3 : Collisionless regime

Abstract: Spin waves in superfluid $^{3}\mathrm{He}$ are studied in the collisionless regime by introducing a canonical transformation in the kinetic equation for the distribution-function matrix. This transformation is a space-time\char22{}dependent spin rotation, which takes into account the fluctuations of the direction of the order parameter which are coupled to the spin-density fluctuations. After the transformation, the order parameter is invariant. The kinetic equation is then expanded to lowest order in $\ensuremath{\omega}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ and diagonalized. Fermi-liquid effects are taken into account. The result is a set of equations describing the coupled motion of the superfluid parameters and the quasiparticle distribution. In the axial state, only two degenerate spin-wave modes exist. These have their spin polarization perpendicular to the direction of the order parameter in spin space. In the isotropic state, one longitudinal and two transverse modes are obtained. Equations giving the velocity of these modes are written.

Magnetization of the Two-Dimensional Heisenberg Ferromagnet K 2 CuF 4 with Small XY-Like Anisotropy

Abstract: The temperature variation of magnetization in the quadratic layer ferromagnet K 2 CuF 4 has been investigated by measuring the NMR frequency of Cu 63 or out-of-layer F 19 nuclei at zero and applied fields in the temperature region of 0.27≦ T / T c ≦0.67. The results are analyzed by a two-dimensional spin wave theory. Under the external field the dipole interaction is approximated in a form of the effective field. The X Y -like anisotropy of exchange interaction is taken into consideration. Good agreement between the theory and the experimental results is obtained. The X Y -like anisotropy itself is not effective to the magnetization, but its contribution to the magnetization cannot be neglected when it cooperates with the external field. The parameter of the exchange interaction J within the layer is obtained to be 12.37 K. The NMR result at zero applied field is also discussed.

Selfconsistently renormalized spin-wave approximation for some two-dimensional magnetic systems

Abstract: Some temperature-dependent properties of two-dimensional magnetic hamiltonians with uniaxial anisotropy are studied on the basis of the selfconsistently renormalized (SCR) spin-wave approximation. Due to the importance of anisotropy in two- dimensional systems, particular attention is paid to the kinematical consistency of the theory by considering multiple scattering due to anisotropy. In particular the properties of FeCl2, a layer ferromagnet with large single-ion and exchange anisotropy, and K2NiF4, a layer antiferromagnet with small anisotropy have been studied. In both cases good agreement with experiments is found for temperature renormalization of small k spin-wave energies. Improved results with respect to spin-wave theory are obtained for the order parameter. The large k dependence in the temperature renormalization of spin-wave energies is predicted for antiferromagnets with medium and small anisotropy.

Spin Waves in Manganese Monoxide

Abstract: The spin waves in MnO have been studied at various temperatures from 4.2 K to the Neel temperature T N of 117 K by means of neutron inelastic scattering techniques. The dispersion curves in the [001], [111] and [\bar1\bar11] direction have been determined. Using the spin Hamiltonian with destance dependent n. n. interaction, n. n. n. interaction and dipole-dipole interaction, the spin wave theory involving magnon-magnon interaction has been developed. It has been found that the theory interprets not only the spin wave dispersion curves at 4.2 K but also its temperature dependence up to T / T N =0.9. The magnon density of states has been calculated.

Temperature dependence of the exchange splitting in ferromagnetic metals. I. Information from the de Haas-van Alphen effect in iron

Abstract: High resolution measurements of the de Haas–van Alphen frequency associated with the minority-spin electron 'lens' sheets of the Fermi surface of iron have been carried out between 1 and 4.2 K. The variation of the frequency over this temperature range is found to be less than one part in 105 (the resolution of our experiment) and is virtually identical to that found for the corresponding electron lenses in molybdenum. By contrast, a variation of one part in 104 would be expected for the Fe lenses on the basis of a literal interpretation of the Stoner model, in which the exchange splitting of the energy bands is proportional to the magnetization at all temperatures. The absence of any significant change of the frequency with temperature can be understood if the magnetization decreases almost entirely by spin wave excitations, and if the spin waves have negligible effect on the exchange splitting.

Spin-Wave Resonance in Conducting Films: Parallel Resonance

Abstract: Detailed numerical calculations and simple analytical expressions of the surface impedance for both the pinned and umpinned boundary conditions in parallel resonance are reported. When the small component of the magnetization vanishes on the surfaces of a thin film (the pinned case), the surface impedance, which is proportional to the average power absorbed by the specimen, can be expressed as a sum of two contributions: a general ferromagnetic resonant background (FMRB) controlled by conduction and dipole-dipole effects with superimposed standing spin wave resonant peaks (SWRP). For thick specimens the FMRB dominates the response and possesses a maximum which is shifted to lower fields than that predicted by the Kittel formula. The phenomenon of ferromagnetic antiresonance (FMAR) is treated in detail and shown to be controlled principally by the phenomenological magnetic damping parameter.

Spin Wave Resonance and Exchange Parameters in fcc Fe-Ni Alloys

Abstract: Spin wave resonance for a series of fcc Fe-Ni alloys has been measured in order to study the exchange stiffness constant D . In general the resonance field vs the square of the spin wave mode number ( n ) curve is linear for high values of n , whereas some amount of deviation from linearity occurs for low values of n . This is considered to be due to the inhomogeneous demagnetizing field of the sample. We can determine the value of D from the linear part of the curve, provided we have a sufficient number of observed modes. As a supplementary means, we have also made low temperature magnetization measurements from which the value of D was derived. Consistency between these two kinds of measurements is ascertained. The composition dependence of D is not quite coincident with that derived from the neutron small angle scattering experiments by Hatherly et al. The data are discussed both from the standpoint of localized electron model and collective electron model.

Antiferromagnetic Resonance Frequency in Quadratic Layer Antiferromagnets

Abstract: The temperature variation of the far-infrared AFMR frequency in the quadratic layer antiferromagnets K 2 NiF 4 and Rb 2 NiF 4 has been measured using Fourier transform interferometry techniques. The results are found to be sharply defferent from those in usual three-dimensional antiferro-magnets. Oguchi-Honma's spin wave theory applied to the two-demensional systems is shown to predict properly the observed behaviours.

Magnon Sideband of Linear Chain Antiferromagnets

Abstract: The temperature dependence of the intensity, frequency and line width of a magnon sideband is calculated for a linear chain antiferromagnet by means of classical approximation for spins, and comparison is made with the observed results on N(CH 3 ) 4 MnCl 3 (TMMC) and CsMnCl 3 ·2H 2 O(CMC). At low temperatures, the classical appoximation gives results corresponding to the Neel state for the antiferromagnetic ground state, so that certain discrepancy exists between the present results and that of the simple spin wave theory. At high temperatures, the results should be more reliable. In order ro see the validity of the classical approximation, the internal energy of the system is also calculated by means of a series expansion with respect to 1/ S . The variation of the internal energy as a function of temperature according to this method is compared with the results of various other approximations and also with the exact numerical results.