Domain-wall spin dynamics in kagome antiferromagnets

We report magnetization and neutron scattering measurements down to 60 mK on a new family of Fe based kagome antiferromagnets, in which a strong local spin anisotropy combined with a low exchange path network connectivity lead to domain walls intersecting the kagome planes through strings of free spins. These produce unfamiliar slow spin dynamics in the ordered phase, evolving from exchange-released spin flips towards a cooperative behavior on decreasing the temperature, probably due to the onset of long-range dipolar interaction. A domain structure of independent magnetic grains is obtained that could be generic to other frustrated magnets.     

Domain-wall induced coupling between ferromagnetic layers

The remanent magnetization of a hard ferromagnetic CoPtCr layer is progressively decreased by repeated switching of a neighboring soft magnetic layer. We show that this effect depends strongly on the thickness of the CoPtCr layer and the spacing between the hard and soft layers. We propose a model that accounts for these results: An interlayer magnetostatic coupling is induced by large stray fields from domain walls that form within the soft layer during its magnetization reversal.     

Spherical spin waves

The spin waves in a two-sublattice ferrimagnetic sphere are derived from the equation of motion for the sublattice magnetization. The pinning conditions are discussed and the results are compared with those obtained by numerical diagonalization of corresponding Heisenberg hamiltonian. For a sphere with the diameter of about 40 lattice constants the continuum approximation is fully applicable for estimation of some (long wavelength) spin wave energies and angle of the core spins. The transition probabilities seem to be considerably overestimated by this method.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.The author wishes to thank Dr. V. Kamberský for many valuable discussions which enabled him to improve the text.     

Optical characterization of thermal-stress induced by spin waves in thin-film ferrimagnetic structures

It is shown that intense spin-dipole waves (SDWs) excited in thin yttrium iron garnet (YIG) films induce an in-plane thermal stress (σ) of 1-2 MPa in a YIG/GGG structure (where GGG is gadolinium gallium garnet). In YIG/GGG with normal magnetization, σ shifts its ferromagnetic resonance frequency by ≈1 MHz, which is comparable to the linewidth of the absorption curve of YIG/GGG resonators. The effect was characterized by an optical technique that detects σ in the GGG substrate. It was also demonstrated that this effect can be used for the optical-microwave spectroscopy of spin waves in thin ferromagnetic films, by using thermal mapping of SDWs in the substrate. We have shown that this opens up the possibility of determining the contribution of the two-particle magneto-elastic interaction to the microwave heating of the sample.     

Beam splitter for spin waves in quantum spin network

We theoretically design and analytically study a controllable beam splitter for the spin wave propagating in a star-shaped (e.g., a Y-shaped beam) spin network. Such a solid state beam splitter can display quantum interference and quantum entanglement by the well-aimed controls of interaction on nodes. It will enable an elementary interferometric device for scalable quantum information processing based on the solid system.     

Surface spin waves in Heisenberg ferromagnet

The existence conditions and the dispersion laws for surface spin waves in Heisenberg ferromagnet are obtained for any orientation of the surface and any exchange constant or anisotropy field in the surface plane. It is shown that purely decaying modes, both of accoustical or optical types, can occur only for surface having special properties of rotational symmetry.     

Angle-dependent spin waves in antidot bilayers

Ferromagnetic resonance is introduced to examine the microwave frequency response of Ni Fe/Ir Mn bilayers, patterned as antidot arrays. In the experiment, field direction dependence on mode is obtained by rotating the applied magnetic field. We find that at a given resonance frequency, the dependence of the resonance field on the angle has a tendency of sinusoid/cosine variation in the experiment. From micromagnetic simulation it can be seen that spin waves are localized between dots from a given mode profile. This is caused by a demagnetization distribution with a larger value in the center of the two nearest dots than that of the next-nearest dots.     

Focal shifts in diffracted converging spherical waves

It is known from a number of publications that when a converging, monochromatic spherical wave is diffracted at a circular aperture, the point of maximum intensity of the diffracted wave may not be at the geometrical focus of the incident wave, but may be located closer to the aperture. In the present note we show that when the incident wave is uniform and the angular semi-aperture is small, the ratio of such a shift Δ? of the point of maximum intensity to the distance ? between the geometrical focus and the plane of the aperture depends only on the Fresnel number N of the aperture when viewed from the geometrical focus. The effect becomes significant when N ? 5. When N = 1, for example, |Δ?| ≈ 0.4 ? and the maximum intensity is approximately twice as large as the intensity at the geometrical focus.     

Spin waves in quasiequilibrium spin systems

Using the Landau Fermi liquid theory we discovered a new propagating transverse spin wave in a paramagnetic system which is driven slightly out of equilibrium without applying an external magnetic field. We find a gapless mode which describes the uniform precession of the magnetization in the absence of a magnetic field. We also find a gapped mode associated with the precession of the spin current around the internal field. The gapless mode has a quadratic dispersion leading to a T3/2 contribution to the specific heat. These modes significantly contribute to the dynamic structure function.     

Dipolar surface spin waves in antiferromagnets

We present calculations for dipolar surface spin waves in antiferromagnets at low temperatures. We calculate bulk and surface spin wave spectra for long wavelengths in the magnetostatic mode region. The different phases in an applied field along the easy axis are discussed with special emphasis on the spin flop- and paramagnetic phase. Nonreciprocal features of the surface spin waves are discussed. Numerical applications for GdAlO₃ are given.     

Focal shifts in diffracted converging spherical waves

It is known that when a converging, monochromatic, spherical wave is diffracted at a circular aperture, the point of maximum intensity may not be at the geometric focus of the incident wave, but may be located closer to the aperture. In the present note this effect is calculated using somewhat simpler mathematical procedures than have been used in previous publications. The result is interpreted as the combined effect of diffraction and the inverse square law.     

Nonreciprocity engineering in magnetostatic spin waves

Magnetostatic surface spin waves (MSSW) excited from a coplanar waveguide antenna travel in different directions with different amplitudes. This effect, called nonreciprocity of MSSW, has been investigated by micromagnetic simulations. The ratio of amplitude of two counter propagating spin waves, the nonreciprocity parameter κ, is obtained for different ferromagnetic materials, such as NiFe (Py), CoFeAl, yttrium iron garnet (YIG), and GaMnAs. A device schematic has been proposed in which κ can be tuned to a large value by varying simple geometrical parameters of the device.     

Magnetic excitations in a bond-centered stripe phase: spin waves far from the semiclassical limit

Using a spin-only model, we compute spin excitation spectra in a bond-centered stripe state with long-range magnetic order. We employ a bond operator formalism, which naturally captures both dimerization and broken spin symmetry in a unified framework. At low energies, the spin excitations resemble spin waves, but at higher energies they are very similar to spin-one excitations of isolated spin ladders. Our theory does well describe neutron scattering data [Nature (London) 429, 534 (2004)]] on La2-xBaxCuO4, pointing towards bond order in this material.     

Connection between the phase shifts and the coefficients of theM-matrix for the elastic scattering of spin 1 by spin 0 particles

Explicit formulas for theM-matrix in terms of phase shifts and mixing parameters are derived for the case of spin 1 spin 0 elastic scattering. The formulas are applied to the casel, l′≦2.     

Superconductivity and spin density waves

In the electron-electron interaction the r-space structure caused by magnetic fluctuations at the phase transition from a nonmagnetic metal to an antiferromagnetic metal gives rise to a d-wave attractive interaction for Cooper pairing. This is a contribution to some total electron-electron interaction which in total may or may not give rise to Cooper pairing and superconductivity.     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.     

Interaction of solitary spin waves

We present the results of a computer experiment devoted to the problem of the interaction of two magnetic solitary spin waves moving in the direction perpendicular to the axis of easy magnetization in an uniaxial ferromagnet. Such waves being particular solutions of the Landau-Lifshitz equations move like a domain wall under the influence of an external magnetic field. Our computer experiment shows that the two solitary spin waves during their interaction, behave as two solitons and thus the concerned Landau-Lifshitz equations allows N-soliton solutions.     

Intrinsic spin hall effect induced by quantum phase transition in HgCdTe quantum wells

The spin Hall effect can be induced by both extrinsic impurity scattering and intrinsic spin-orbit coupling in the electronic structure. The HgTe/CdTe quantum well has a quantum phase transition where the electronic structure changes from normal to inverted. We show that the intrinsic spin Hall effect of the conduction band vanishes on the normal side, while it is finite on the inverted side. By tuning the Cd content, the well width, or the bias electric field across the quantum well, the intrinsic spin Hall effect can be switched on or off and tuned into resonance under experimentally accessible conditions.