Micromagnetic phase transitions and spin wave excitations in a ferromagnetic stripe

Magnetic excitations of micrometer-wide ferromagnetic stripes subjected to a transverse applied field have been measured between 1 and 20 GHz. The complexity of the observed response is attributed to the spatially nonuniform equilibrium spin distribution. This one is modeled analytically and numerically, which allows one to distinguish two micromagnetic phases governing the ground state. The nucleation-related phase transitions are evidenced by soft modes, while the different observed resonances are attributed to spin wave modes localized in the two phases and at their interface.     

Longitudinal spin waves as secondary excitations of the spin system of a ferromagnet

The method of two-time Green's functions is used to calculate the natural oscillations of the longitudinal component of the spontaneous magnetization of a ferromagnet. The spectrum of these oscillations or spin waves has no gap, even when magnetic interactions are taken into account. For smallk the spectrum of longitudinal spin waves has the same properties as secondary excitations of the spin system of a ferromagnet In which the primary excitations are transverse (ordinary) spin waves.Translated from Izvestiya VUZ. Fizika, No. 4, pp. 82–85, April, 1970.     

Magnetic spin excitations in Mn doped GaAs: a model study

We provide a quantitative theoretical model study of the dynamical magnetic properties of optimally annealed Ga_{1− x} Mn _x As. This model has already been shown to reproduce accurately the Curie temperatures for Ga_{1− x} Mn _x As. Here we show that the calculated spin stiffness are in excellent agreement with those which were obtained from ab-initio based studies. In addition, an overall good agreement is also found with available experimental data. We have also evaluated the magnon density of states and the typical density of states from which the“mobility edge”, separating the extended from localized magnon states, was determined. The power of the model lies in its ability to be generalized for a broad class of diluted magnetic semiconductor materials, thus it bridges the gap between first principle calculations and model based studies.     

Magnetic excitations in sinusoidally modulated spin structures

We show by using a simple model that in incommensurate sinusoidally modulated spin structures, such as those found in Er and Tm just below their magnetic ordering temperatures and in the ordered phase of Cr, the magnon modes are broadened due to the spatial fluctuations of the exchange and anisotropy energies.     

Domain-wall induced phase shifts in spin waves

We study the interaction between two important features of ferromagnetic nanoparticles: magnetic domain walls and spin waves. Micromagnetic simulations reveal that magnetostatic spin waves change their phase as they pass through domain walls. Similar to an Aharonov-Bohm experiment, we suggest to probe this effect by splitting the waves on different branches of a ring. The interference of merging waves depends on the domain walls in the branches. A controlled manipulation of spin-wave phases could be the first step towards nanoscaled ferromagnetic devices performing logical operations based on spin-wave propagation.     

Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory

Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmittance and the optical conductivity σ(ω) of undoped La_xCa_14−xCu₂₄O₄₁ for x=4, 5, and 5.2. Using two different theoretical approaches (Jordan-Wigner fermions and perturbation theory), we calculate the dispersion of the elementary triplets, the optical conductivity and the momentum-resolved spectral density of two-triplet excitations for 0.2≤J_∥/J_⊥≤1.2. We discuss phonon-assisted two-triplet absorption, the existence of two-triplet bound states, the two-triplet continuum, and the size of the exchange parameters.     

The Mourre estimate in the semiclassical limit

The Mourre estimate is proved in the semiclassical limit for Schrödinger operators satisfying a nontrapping condition. Moreover, an abstract form of a theorem concerning the absence of resonances is given.     

Dephasing in the semiclassical limit is system-dependent

Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/λ_F 〉 1. The weak localization correction g ^wl to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition to the universal algebraic suppression g ^wl ∝ (1 + τ_D/τ_ϕ)⁻¹ with the dwell time τ_D through the cavity and the dephasing rate τ _ϕ ⁻¹ , we find an exponential suppression of weak localization by a factor of ∝ exp[− /τ_ϕ], where is the system-dependent parameter. In the dephasing probe model, coincides with the Ehrenfest time, ∝ ln[L/λ_F], for both perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, ∝ ln[L/ξ] depends on the correlation length ξ of the coupling potential instead of λ_F. The text was submitted by the authors in English.     

Dephasing in the semiclassical limit is system-dependent

Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/λ_F 〉 1. The weak localization correction g ^wl to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition to the universal algebraic suppression g ^wl ∝ (1 + τ_D/τ_?)^?1 with the dwell time τ_D through the cavity and the dephasing rate τ _? ^?1 , we find an exponential suppression of weak localization by a factor of ∝ exp[? $\tilde \tau $ /τ_?], where $\tilde \tau $ is the system-dependent parameter. In the dephasing probe model, $\tilde \tau $ coincides with the Ehrenfest time, $\tilde \tau $ ∝ ln[L/λ_F], for both perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, $\tilde \tau $ ∝ ln[L/ξ] depends on the correlation length ξ of the coupling potential instead of λ_F.     

Magnetic resonance of spin clusters and triplet excitations in a spin-Peierls magnet with impurities

The magnetic resonance spectrum of spin clusters formed in spin-Peierls magnets in the vicinity of impurity ions is investigated. The observed temperature dependences of the effective g-factor and the linewidth of the electron paramagnetic resonance (EPR) in crystals of Cu_1?x Ni_xGeO₃ are described in the model of the exchange narrowing of the two-component spectrum with one component ascribed to spin clusters and exhibiting an anomalous value of the g-factor and the other related to triplet excitations. An estimation of the size of the suppressed dimerization region around the impurity ion is obtained (this region includes about 30 copper ions). The dependence of the effective g-factor and the EPR linewidth on the impurity concentration at low temperatures indicates the interaction of clusters.     

Finite-amplitude longitudinal excitations in nuclear matter: A semiclassical treatment

Finite-amplitude longitudinal collective excitations in spin and isospin saturated zero-temperature nuclear matter are investigated in the semiclassical limit, using a self-consistent density-dependent interaction of the Skyrme type. There are no solitons and no shocks, only zero-sound modes. Due to trapping of particles in the troughs of the self-consistent potential, there is a relatively small maximum amplitude that a zero-sound mode of a given wavelength can have. The theory of slowly modulated wave-trains of finite-amplitude zero sound is also developed. This theory shows that zero sound is stable, and that a modulation of finite extent ultimately breaks into two separated disturbances. In order to test the adequacy of nuclear hydrodynamics in the regime of large amplitudes, all calculations are performed twice, once exactly using the Vlasov equation, and once using a hydrodynamic approach. In the small-amplitude limit, hydrodynamics is semi-quantitatively correct, while in the large amplitude region it is qualitatively incorrect. It incorrectly predicts the existence of zero-sound solitons, and fails to take into account some of the most interesting new features which can arise, such as particle trapping, discontinuity of Fermi surface, and splitting of the Fermi sea into two disconnected parts.     

Magnetic properties and spin waves of bilayer magnets in a uniform field

The two-layer square lattice quantum antiferromagnet with spins 12 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations throughout the whole phase diagram. Received 23 April 2001     

Laser-induced orbital and spin excitations in ferromagnets: insights from a two-level system

A recent time-resolved measurement showed that laser-induced orbital and spin excitations proceed in unison and the spin-orbit ratio is held constant during demagnetization. Here a two-level model shows that these orbital and spin excitations originate from state population and state interference effects. For an addressed state, spin and orbital dynamics are solely from the state interference, where the spin and orbital momenta oscillate with the laser frequency and match the dipole moment exactly, an unambiguous test case for the time-resolved magneto-optical Kerr effect. For an undressed state, the interference effect introduces a rapid beating in orbital momentum, which is observed in the first-principles calculation in fcc Ni. The state population change leads to a constant spin-orbit ratio, which explains the linear dependence between spin and orbital momentum changes within 2 ps upon the arrival of a pump pulse in Fe.     

Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements

Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.     

Dynamical entanglement as a signature of chaos in the semiclassical limit

The relationship between classically chaotic dynamics and the entanglement properties of the corresponding quantum system is examined in the semiclassical limit. Numerical results are computed for a modified kicked top, keeping the classical dynamics constant while investigating the entanglement for several versions of the corresponding quantum system characterized by a different value of the effective Planck constant ℏ _eff. Our findings indicate that as ℏ _eff → 0, the apparent signatures of classical chaos in the entanglement properties, such as characteristic oscillations in the time-dependence of the linear entropy, can also be obtained in the regular regime. These results suggest that entanglement is not a universal marker of chaotic dynamics of the corresponding classical system.     

Magnetic ordering and spin waves in Na0.82CoO2

NaxCoO2, the parent compound of the recently synthesized superconductor Na(x)CoO(2):yH(2)O, exhibits bulk antiferromagnetic order below approximately 20 K for 0.75相似文献