Excitation of spin waves in superconducting ferromagnets

This Letter presents a theoretical analysis of propagation of spin waves in a superconducting ferromagnet. The surface impedance was calculated for the case when the magnetization is normal to the sample surface. We found the frequencies at which the impedance and the power absorption have singularities related to the spin-wave propagation, and determined the form of these singularities. With a suitable choice of parameters, there is a frequency interval in which two propagating spin waves of the same circular polarization are generated, one of them having a negative group velocity.     

Quantum effects of nonlinear spin waves in ferromagnets

We investigate a squeezed thermal spin state of nonlinear spin waves in Heisenberg ferromagnets. In this state, the magnon system possesses a new kind of quasiparticle, the dressed magnon, whose mass is a monotonically decreasing function of temperature. The noise of one spin component in the squeezed thermal spin state can be below the noise level in the vacuum state. The magnon system undergoes a first-order phase transition from the normal state to the squeezed thermal spin state. The critical temperature is much lower than the Curie temperature. A possible detection scheme based on a polarized neutron-scattering technique is suggested.     

Suppression of standing spin waves in low-dimensional ferromagnets

We examine the experimental absence of standing spin wave modes in thin magnetic films, by means of atomistic spin dynamics simulations. Using Co on Cu(001) as a model system, we demonstrate that by increasing the number of layers, the optical branches predicted from adiabatic first-principles calculations are strongly suppressed, in agreement with spin-polarized electron energy loss spectroscopy measurements reported in the literature. Our results suggest that a dynamical analysis of the Heisenberg model is sufficient in order to capture the strong damping of the standing modes.     

Dynamical scaling for spin waves in dilute ferromagnets

The ambiguity in the application of the principle of dynamic scaling to spin waves in dilute ferromagnets is resolved by taking account of the fractal nature of the infinite percolation cluster.     

Variational method for spin waves in disordered ferromagnets

A variational method for spin waves in a disordered ferromagnet is described and is shown to give same results as obtained by a Green function method. The results of this calculation are applied to rare earth di-aluminium alloys. The spin wave dispersion relations in these alloys as a function of concentration are predicted.     

Parametric excitation of spin waves in ferromagnets via localized magnon modes

The possibility to excite ferromagnetic magnons with very large k is analyzed theoretically. The proposed excitation mechanism is analogous to the first-order Suhl instability and takes place via non-linear processes involving localized modes associated with imputities in ferromagnetic insulators. A rough estimate of the critical power of a far-infrared laser source necessary to attain the instability threshold shows that the processes are experimentally feasible.     

Cycloidal vortex motion in easy-plane ferromagnets due to interaction with spin waves

The dynamics of a non-planar vortex in a two-dimensional easy-plane ferromagnet of finite size is studied. Spin dynamics simulations show small cycloidal oscillations of the vortex around its mean path. In contrast to an earlier phenomenological theory we give a physical explanation: The oscillations occur due to the interaction of the vortex with coherent spin waves which are excited by this vortex at the moment when it starts to move, in order to conserve the total energy and angular momentum. The calculation of these quantities yields the frequencies and amplitudes of the cycloidal oscillations in good agreement with the simulation data. Received 9 December 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: franz.mertens@uni-bayreuth.de     

Parametric excitation of spin waves in ferromagnets by longitudinal pumping localized in space

The equations of motion for the slowly varying complex amplitudes of spin waves parametrically excited by a localized pumping magnetic field have been derived. A solution of these equations satisfying given boundary and initial conditions has been obtained. The energy dissipated by spin waves decreases with the pumping intensity beyond a certain pumping power, which can be termed the regeneration threshold. The losses vanish and change sign at the instability threshold. Both thresholds depend heavily on the linear dimension L of the pumping zone, increasing with decreasing L. Owing to the regeneration process, the dissipation length of spin waves increases without bound as the pumping power approaches the instability threshold. Consequently, perturbations of a uniform state due to the boundary penetrate throughout the pumping zone, regardless of the dimension L. As a result, the full pattern of parametric instability is strongly affected by the zone boundary: 1) the spatial distribution of wave amplitudes becomes nonuniform everywhere inside the zone; 2) the amplitude growth rate in the unstable regime decreases at all points when perturbations due to the boundary reach these points; 3) the instability threshold is independent of the spin-wave frequency offset from the parametric resonance frequency. The calculated minimum instability threshold as a function of the bias magnetic field (the “butterfly” curve) changes shape with L, in agreement with the available experimental data. Zh. éksp. Teor. Fiz. 111, 199–219 (January 1997)     

Effective spin spin interactions in the damped ferromagnets

Simanek's theory is extended by taking into account the nuclear spin excitations due to the motion of the electronic system. Damping of the magnons is found to be an extra criteria for having a finite interaction range for the nuclear spin system. The resonance excitation of Simanek's theory is due to the neglect of the second order in the hyperfine interaction.     

On the heat capacity discontinuity for a gas of nuclear spin waves in ferromagnets

It is demonstrated that the heat capacity discontinuity for a gas of nuclear spin waves occurs in ferromagnetic materials. The heat capacity discontinuity manifests itself at ultralow temperatures and stems from the specific spectrum of these quasiparticles. The effect is observed at the temperature T _K?2.37?ω_n. The heat capacity discontinuity is found at this temperature.     

The damping of spin waves in dirty conducting ferromagnets due to the electron-magnon interaction

The Green's function of a magnon interacting with conduction electrons which move in an arbitrary non-periodic potential field is calculated within thes-d(f) exchange model. The contribution of the lowest order damping to the spin-wave resonance linewidth is discussed. The temperature and wavevector dependences of the second order damping due to the electron-magnon scattering are investigated. The local and optical magnon modes are considered.     

Magnetostatic interface waves in ferromagnets

We derive the dispersion relation for magnetostatic surface waves guided by the interface between two ferromagnets. The applied field and saturation magnetization are parallel to the interface. The propagation of surface waves parallel to the interface is nonreciprocal and is limited to a restricted set of directions. An analytical expression is found for the critical angle, beyond which propagation is not allowed.     

Spin waves in quantum ferromagnets

Low-temperature properties of Heisenberg quantum ferromagnets (spin waves) are derived within aconfiguration space formalism. Most of the work is done without explicitly assuming translational invariance. We provide a general criterion, classical domination, to decide about the nature and uniqueness of ground states for a large class of quantum ferromagnets. We also analyze and clarify the Dyson formalism and indicate why an energy gap between the physical ground state and the improper (unphysical) states does not exist. This is of particular relevance to the kinematical interaction. Using reflection positivity we provide upper and lower bounds to the contribution of the dynamical interaction to the free energy. In a certain approximation, these bounds imply that the dynamical interaction may be dropped if the inverse temperature and the spin quantum numberS are large enough.Suported in part by FAPESP.Supported in part by CAPES.Partial support by FAPESP and CNPq.     

μ+ spin relaxation in ferromagnets

We review the recent theoretical studies of the muon spin relaxation mechanisms in metallic ferromagnets.     

Spin waves in the easy-plane ferromagnets

The modified version of the spin operator diagram technique is presented with regard to the dynamical properties of the easy-plane ferromagnets. The non-interacting spin wave (SW) spectrum and scattering amplitudes are obtained with the help of the spin Hamiltonian diagonalization procedure⁸). The SW relaxation frequencies due to the processes of the SW scattering on each other and on the longitudinal spin components thermal fluctuations are calculated.     

Thermal collapse of spin polarization in half-metallic ferromagnets

We propose two novel approaches to study the temperature dependence of the magnetization and the spin polarization at the Fermi level in magnetic compounds, and apply them to half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab initio calculations. The thermal properties are studied by mapping ab initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.     

Effects of spin fluctuations in itinerant electron ferromagnets

Effects of spin fluctuations on the paramagnetic spin susceptibility, magnetization and specific heat are calculated in gaussian statistics as a power series of k_BT. The results are applied to nickel and iron. It is shown that in effect spin fluctuations reduce the molecular field coefficient above the Curie temperature.