Showing papers on "Spin wave published in 1970"

Hidden-Variable Example Based upon Data Rejection

Abstract: A deterministic local hidden-variable model is presented which describes the simultaneous measurement of the spins of two spin-\textonehalf{} particles which emerged from the decay of a spin-zero particle. In this model the measurement of the spin of a particle has one of three possible outcomes: spin parallel to the apparatus axis, spin antiparallel to the apparatus axis, or the particle goes undetected. It is shown that agreement with the predictions of quantum theory is obtained provided the experimenter rejects the "anomalous" data in which only one particle is detected. A reasonably model-independent lower bound to the fraction of undetected particles is also computed: It is found that in 14% of the decays or more, one or both of the particles will go undetected.

Dipole‐Exchange Spin Waves in Ferromagnetic Films

Abstract: The effects of exchange on the magnetostatic surface and bulk spin waves of a ferromagnetic film in the regime where the dipole and exchange fields are both important are determined from the solutions of a sixth‐order differential equation. The eigenstates are admixtures of bulk and surface waves. With an applied magnetic field parallel to the film surface and for small values of the wave vector parallel to the surface, k, the Damon and Eshbach surface state is split into segments which join adjacent bulk branches. No sharp cutoff of the surface branch occurs as the angle between k and the applied field is increased, instead the character of the branch changes continuously from surface‐like to bulk‐like. For values of | k |≳104 a number of branches have significant surface character so that no single branch can be identified with the Damon‐Eshbach surface state. When the applied field is perpendicular to the surface a new type of surface spin wave occurs below the bulk manifold and is characterized by a c...

Spin waves in antiferromagnetic FeF2

Abstract: Spin-wave dispersion in antiferromagnetic FeF2 has been investigated by inelastic neutron scattering using a chopper time-of-flight spectrometer. The single mode observed has a relatively flat dispersion curve rising from 53 cm-1 at the zone centre to 79 cm-1 at the zone boundary. A spin Hamiltonian which includes a single-ion anisotropy constant D (Hsi=-DSiz2), three exchange parameters Jl(Hex12=JlS1.S2) and dipolar interactions has been fitted to the measured energies. The authors find D=6.46 (+0.29, -0.10) cm-1, J1 (coupling neighbouring ions along the c axis)=-0.048 (+or-0.060) cm-1, j2 (coupling neighbouring ions at the corner and body centre of the cell)=+3.64(+or-0.10) cm-1, and J3 (coupling neighbouring ions along the a axis)=+0.194 (+0.060) cm-1.

Inelastic neutron scattering investigation of spin waves and magnetic interactions in α-Fe2O3

Abstract: Spin waves in antiferromagnetic α-Fe2O3 have been studied at temperatures of 240 and 290° K by means of inelastic neutron scattering. We report here the first determination of the acoustical branch throughout the entire Brillouin zone and the first observation of the optical branch. The latter shows little dispersion throughout the zone, and has an energy of 1125° K (97 meV) at the zone centre. At the zone boundaries a gap of 20 to 70° K exists between the two branches. Heisenberg interaction parameters Jm defined through a Hamiltonian were obtained through fitting of the data to theoretical expressions for the dispersion relations. The following values were obtained for the first five nearest neighbours: J1 = 6.0 ± 1.6° K, J2 = 1.6 ± 0.6° K, J3 = −29.7 ± ± 2.0° K, J4 = −23.2 ± 1.0° K and J5 = −1.0 ± 1.0° K. Interactions to farther neighbours were found to be weak. Neutron intensity data were partly invoked in obtaining the interaction parameters, as two sets of parameters could fit the energy data almost equally well, but they predicted different relative intensities for the two branches in certain regions of the reciprocal space. All spin waves of energy larger than 80° K, including the entire optical branch, were found to be unaffected by the Morin spin-flip transition at 261° K temperature. The spin-wave data were used to calculate the sublattice magnetization, the Neel and the Curie-Weiss temperatures, the perpendicular susceptibility at low temperatures and the density of spin-wave state spectrum. [Russian Text Ignored]

New Approach to the Theory of Itinerant Electron Ferromagnets with Local‐Moment Characteristics

Abstract: The understanding of ferromagnets, like iron, which exhibit localized moment behavior above the Curie point yet show itinerancy has long stood as a major theoretical problem. An account will be given of recent progress on this problem which was achieved through functional integral methods. This technique transforms the interacting electron system into an average over a system of noninteracting electrons moving in a Gaussian‐weighted external ``magnetic'' field which acts only on the electronic spins. For a single magnetic impurity in a free electron metal, a single approximation allows one to go from Pauli paramagnetism to localized moment behavior in a smooth manner as the atomic exchange interaction is increased. The two impurity problem leads to an effective exchange coupling as in the Heisenberg model, which is antiferromagnetic for the nondegenerate orbital case studied here. Application of the technique to homogeneous systems leads to damped spin waves in the ferromagnet in lowest approximation.

Spin-Wave Dispersion Relations in Gadolinium

Abstract: Spin-wave dispersion relations have been measured in high-symmetry directions for metallic Gd. Analysis shows that at least five interplanar constants are required for a satisfactory fit to the data. The energy gap at $q=0$ is unmeasurably small. In the $c$ direction the measured dispersion curve gives directly the Fourier-transformed exchange interaction $J(0)\ensuremath{-}J(q)$. This exhibits no other extreme value except that at the origin.

Dynamic Pinning Induced by Nickel Layers on Permalloy Films

Abstract: The dynamical pinning induced by thin layers of nickel on epitaxial permalloy films is investigated by stationary spin wave resonances at 17 and 35 GHz. The effective surface anisotropy is determined by comparing the location of resonance peaks for films without layer and for composite films. The high value and the d.c. field dependence of the effective surface anisotropy are attributed to the presence of exchange coupling between the films.

Thermodynamic properties of a Heisenberg antiferromagnet

Abstract: A finite temperature perturbation expansion is employed to obtain an approximate description of the thermodynamic behaviour of a Heisenberg antiferromagnet for spin half. The calculations are carried out using the drone-fermion representation, and contributions are classified with respect to a high density parameter 1/z, where z is the number of spins interacting with any given spin. This leads to the results of molecular field theory in lowest order (1/z)0. The spin fluctuation effects, which are absent in molecular field theory, are introduced in higher orders. Calculations of the sublattice magnetization, free energy, and internal energy are carried out up to and including (1/z)1. The results are valid provided the fluctuation effects are sufficiently small, which is the case for all temperatures below the Neel temperature TN and also for higher temperatures if there is a suitable applied magnetic field or anisotropy field acting on the system. In a low temperature approximation the results are found to be consistent with linear spin wave theory, including zero point effects.

ESR of TMPD–TCNQ: Spin Excitations of the Heisenberg Regular Linear Chain

Abstract: The detailed electron spin resonance of single crystals of the charge‐transfer complex tetramethylphenylenediamine–tetracyanoquinodimethan (TMPD–TCNQ) at X band and Q band are reported. The ESR spectra are interpreted as resulting from the thermally excited (activation energy, 0.068 eV) spin excitations of the Heisenberg regular antiferromagnetically coupled chain. Although the exact solution to the excited states of the Heisenberg chain with antiferromagnetic coupling is not available, an approximate solution treating the quasiparticles as Wannier spin excitons is very successful at predicting the spin resonance properties of the excitations; fundamental to the Wannier spin exciton model is the absence of spin correlation in the excitations. The spin–spin (T2−1) and spin–lattice (T1−1) relaxation rates are observed as functions of temperature and are determined mainly by spin exchange between excitons. Exchange is proportional to exciton concentration, and the low activation energy in TMPD–TCNQ allows ex...

Effect of Single-Ion Anisotropy on Two-Spin-Wave Bound State in a Heisenberg Ferromagnet

Abstract: We consider the scattering of two spin waves in a uniaxial (easy axis) Heisenberg ferromagnet with single-ion anisotropy. The two-spin-deviation problem is solved exactly at zero temperature. We find (for $Sg\frac{1}{2}$), in addition to the usual two-spin-wave bound states, a new "single-ion bound state," in which at the zone corner the two spin deviations are on the same site. When the magnitude of the anisotropy is comparable to the exchange interaction, the single-ion bound state becomes the dominant feature of the bound-state spectrum. For arbitrary spin there is a critical anisotropy strength above which the single-ion bound state exists throughout the Brillouin zone. We conclude that the presence of single-ion anisotropy enhances the possibility of experimental observation of the bound states.

Contribution to the theory of a dilute Heisenberg ferromagnet

Abstract: A theory of a dilute Heisenberg ferromagnet is studied in two ways; the molecular field approximation and the Tyablikov approximation. By considering the Heisenberg model in the molecular field approximation, an expression for the averaged moment at any lattice site is obtained by the use of the diagram method. For a simple cubic crystal, numerical calcu­ lations of the averaged moment with a spin t host are presented. The Curie temperature depending on the concentration of magnetic atoms is presented. In the case of the molecular field approximation, it is shown that the critical concentration at which the ferromagnetic state disappears may not exist. By the use of the Green's function another method based on the Tyablikov approximation is formulated. The theory is a straightforward generalization of the previous paper published in Prog. Theor. Phys. 42 (1969), 477, to finite temperature range. The existence of the critical concentration is also indicated. In a previous paper (referred to as K) /) we have developed a spin-wave theory of a dilute Heisenberg ferromagnet by the use of the Green's function method. For simple lattices with nearest-neighbour interactions, a general forma­ lism of the Green's function was expressed in terms of a function of the con­ centration of magnetic elements. The expression for the energy was applied to determine the critical concentration for which the ferromagnetic ground state is unstable with respect to the formation of long-wavelength spin waves. The critical concentration is defined as the minimum concentration of magnetic atoms, arranged at random in a non-magnetic lattice, for which ferromagnetism will occur. Ferromagnetism is caused by exchange interaction for a greater part between neighboring atoms, and it seems there is no need to have a rigorous periodicity in the distribution of magnetic atoms. For this reason, as was firstly suggested by Gubanov, 2 ) amorphous or vitreous ferromagnets can exist at tem­ peratures low enough so that when they contain the right atoms we have reason to believe they will be ferromagnetic. In this paper, we intend to generalize the previous theory to finite tempera­ ture range in order to obtain the Curie point depending on the concentration of magnetic elements. From this approach, it seems to be possible to clarify the existence of ferromagnetism in the amorphous or vitreous states. Experimentally there are some publications opening the possibility of ferromagnetism in amor­ phous or vitreous states. 3 )

Band Structure, Spin Splitting, and Spin-Wave Effective Mass in Nickel

Abstract: We have applied a modified form of the combined interpolation (tight-binding plus pseudopotential) method to the calculation of energy bands in ferromagnetic nickel. A procedure has been developed to enable calculation of the reciprocal spin-wave effective mass in a multiband system in the $t$-matrix approximation. This has been used in conjunction with the calculated band structure. Matrix elements of the electron interaction are treated as parameters. A by-product of the calculation is an estimate, also in the $t$-matrix approximation, of the energy difference between majority and minority spin states in the highest $d$ band. Moderately good results for the spin-wave effective mass are obtained for reasonable values of the electron interaction parameters; however, a discrepancy is found in regard to the spin splitting, which appears to be too large.

Correlation functions of a Heisenberg antiferromagnet

Abstract: The correlation functions of a Heisenberg antiferromagnet are investigated The method employs a drone-fermion representation for spin-half and a classification of terms with respect to a high density parameter 1/z One- and two-particle correlation functions are evaluated up to order (1/z)1 and their connection with the physical properties of an antiferromagnet discussed This enables results with a determined range of validity to be obtained for the spin wave energy and damping, assumed small, as functions of momentum and temperature The damping is found to increase with temperature, but is small for any temperature below TN for sufficiently long wavelengths This is similar to the result obtained for the ferromagnet by Vaks et al The spin wave energy and damping can be deduced from neutron scattering experiments, and the theoretical predictions are found to be in good qualitative agreement with measurements obtained for CoF2 and NiF2

A study of spin-wave excitations in dilute Pd-Fe alloys

Abstract: The temperature dependence of the heat capacity in magnetic fields up to 16 kOe and the magnetization in magnetic fields up to 40 kOe have been determined for a number of palladium-iron alloys containing up to 2 at.% iron. An adequate description of the magnetization behaviour for all of the alloys and the heat capacity of alloys containing more than 1 at.% Fe can be given in terms of spin-wave excitations having an energy given by Dq2, where D is field-dependent

Spin-wave/Carrier-wave interactions

Abstract: The possibility of spin-wave/carrier-wave interactions has been considered with a more rigorous stability analysis than that used in previous studies. It is concluded that unstable bulk interactions in magnetic semiconductors will not be possible until resistivities are reduced to several orders of magnitude below those presently available. Composite YIG-InSb structures are potentially unstable at room temperature. The composite system can provide high-gain, slow-wave amplification at X-band with delay line capability (2300 dB/cm at 7.5 GHz). In a uniform magnetic field, the instability exhibits an exceedingly narrow bandwidth, 2.5 MHz, which would be a great disadvantage in any practical application as an amplifier. This could be offset somewhat, with a resulting loss of gain, by applying an inhomogeneous magnetic field.

Macroscopic and Microscopic Theories of Dipole-Exchange Spin Waves in Thin Films: Case of the Missing Surface States

Abstract: Calculations of dipole-exchange spin waves in thin films using a microscopic model show no evidence of surface modes above the lowest bulk frequency, in contrast to recent results based on a macroscopic model. The apparent absence of the surface branch in the former model is explained in terms of the admixture of bulk and surface waves. Nearly exact quantitative agreement between the two models is obtained for wave vectors as large as ${10}^{7}$ ${\mathrm{cm}}^{\ensuremath{-}1}$.

Cyclotron resonance devices controllable by electric fields

Abstract: Electric field controllable devices which operate on the principle of velocity change of a wave passing therethrough. These waves can be magneto-elastic or spin waves, including surface waves. The materials used in these devices include Ga2-xFexO3, Cr2O3, and YIG. When the electric field across the device is changed, the cyclotron resonance frequency of the device is greatly shifted, resulting in wave velocity changes up to about 50 percent. A bias magnetic field is generally applied across the devices to establish a resonance frequency. Devices include variable delays, modulators, frequency translators, wave guides, tunable filters, etc.

Spin Waves in a Spin-Pair System and in a System of Magnetic Ions with Large Anisotropy Energy

Abstract: It has recently been shown, by using a molecular field theory, that in these systems a new type of spin ordering occurs under external magnetic fields. In the present paper, the physical properties of these systems at low temperatures are investigated on the basis of spin wave theory. In the ordered state, the spin wave frequency is proportional to the wave number when it is small, and the proportionality coefficient is strongly dependent on the intensity of external field. The field dependence of spin temperature in adiabatic magnetization process is modified from that obtained by the molecular field theory.

Determination of Spin-Wave Boundary Conditions by dc Effects in Spin-Wave Resonance

Abstract: A galvanomagnetic spin-wave experiment is described which, in conjunction with resonance absorption measurements in both uniform and nonuniform microwave fields, yields more information about the distribution of magnetization in spin-wave modes in ferromagnetic films than hitherto available, particularly for spin waves of low mode number. The method is applied to determine the spin-wave boundary conditions in 1200-\AA{} Permalloy films, with the static field either parallel or perpendicular to the film plane. Homogeneous films show unpinned surfaces for the parallel orientation. In the perpendicular case, these films have a partly pinned film-glass interface, with anisotropies of a few tenths of 1 erg/${\mathrm{cm}}^{2}$, and an unpinned film-air surface. Parallel-orientation results for a film with a thin Ni over-coat indicate the excitation of a surface mode, which is also predicted by theory.

High density corrections for a Heisenberg antiferromagnet

Abstract: The high density expansions derived for the thermodynamic quantities and correlation functions of the spin half Heisenberg antiferromagnet are extended to higher order, for the low temperature case. The (1/z)2 terms in the sublattice magnetization, free energy and internal energy are evaluated and contact made with the interacting spin wave theory of Oguchi. The selfconsistency of the method in this order is demonstrated, and the criteria for the validity of the results determined. The spin wave damping in order (1/z)2 is also investigated. This includes spin wave scattering contributions and gives the dominant contribution at low temperatures.

Spin waves in holmium

Abstract: The propagation of spin-wave excitations in the conical and spiral magnetic phases of holmium has been studied by neutron inelastic scattering techniques. The results are analysed in terms of an anisotropy parameter and the wave vector dependent exchange. It is proposed that the cone to spiral transition in holmium results from the instability of the spin-wave mode of wave vector equal to the characteristic spiral wave vector. Experimental evidence for this suggestion is presented

Observation of Libron-Libron Interactions in Solid Hydrogen

Abstract: The anharmonic interactions between librational waves in solid hydrogen are found to lead to significant perturbations in the single-libron spectrum. This large anharmonicity is also responsible for two-libron processes whose frequencies and Raman intensities are calculated. Our results for the oneand two-libron spectra are in excellent agreement with, and hence explain, the optical data. Disciplines Physics | Quantum Physics This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/315 VOLUME 25, NUMBER 1$ PHYSICAL REVIEW LETTERS 28 SEPTEMBER 1970 however, still remain to be delineated in detaill before the phenomena reported here can be properly analyzed and understood. I. B. Sanders and P. S. Dobson, Phil. Mag. 20, 881 (1969). C. H. Lane, IEEE Trans. Electron. Devices 15, 998 (1968). B. N. Ghoshtagore, to be published. I. M. Mackintosh, J. Electrochem. Soc. 109, 392 (1962). S. Maekawa, J. Phys. Soc. Jap. 17, 1592 (1962). OBSERVATION OF LIBRON-LIBRON INTERACTIONS IN SOLID HYDROGEN* C. F. Coll, III, A. B. Harris, and A. J. Berlinsky Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19204 (Received 22 July 1970) The anharmonic interactions between librational waves in solid hydrogen are found to lead to significant perturbations in the single-libron spectrum. This large anharmonicity is also responsible for two-libron processes whose frequencies and Raman intensities are calculated. Our results for the oneand two-libron spectra are in excellent agreement with, and hence explain, the optical data. Ever since Dyson's famous paper in 1956 on spin-wave interactions in a ferromagnet, ' great efforts have been made to observe these interactions. However, since long-wavelength spin waves interact only very weakly, this has been a difficult experimental problem, and only in the last ferv years has the optical observation' of two-magnon states in antiferromagnets' shown unequivocally the existence of these interactions. The existence of anharmonicity in phonon systems is also documented, ~ but here the fundamental interactions are less well known and hence many calculations' use anharmonic force constants which are not evaluated from a microscopic point of view. In contrast, in so1id hydrogen the relevant orientational interactions between molecules are determined from first principles, ' and hence the associated anharmonic force constants are well known. Furthermore, since these anharmonic interactions are large, ' the orientational excitations in solid hydrogen constitute a unique many-body system. The elementary excitations of this system are the small librational motions of the molecules about their equilibrium orientations. In the orientationally ordered phase [which occurs for the pure (J = 1) solid below about 3'K for H, and 4'K for D,] the fcc crystal consists of four interpenetrating simple-cubic sublattices, each of which consists of molecules oriented along one of the various [111]directions. " Since there are four molecules per unit cell, each of which can librate in two perpendicular directions, the librational excitation (libron) spectrum has eight branches. ' ' In this approximation the effects of zero-point phonon motion and phonon-libron interactions are taken into account only insofar as they renormalize the orientational interactions, ""of which the quadrupole-quadrupole interactions scaled by the parameter' ' 1" are the most important. From the symmetry of the foursublattice structure, space group T„', one expects at k =0 one twofoM degenerate and two threefold degenerate libron energies. These elementary excitations have been observed directly via Raman scattering of light. " However, the interpretation of this spectrum has been unclear for two reasons. First, there were observed five lines in the Raman spectrum instead of three as predicted from theory. Second, the calculated libron energies did not agree very well with any reasonable assignment of the observed lines. Accordingly, a distortion to a lower symmetry structure was suggested. '7 From x-ray work" a similar distortion has been suggested for solid N, . However, the distortion in solid N, has not been confirmed by subsequent optical data, "and hence its existence is uncertain. For solid hydrogen, a distortion is not a plausible explanation of the spectrum, because it must be supposed to produce rather large splittings in the Raman spectrum. Recently Nakamura' has suggested that the extra lines may be due to two-libron processes. However, the mechanism he proposed relied on the zeropoint disorder in the orientational system, and consequently the intensity associated with this process was very small. Here we propose a mechanism for a two-libron absorption which can account for these two extra