Magnetic excitations in cobalt oxide

Raman measurements of the temperature dependence of several low-lying magnetic excitations in CoO are reported. The large q = 0 value of the lowest excitation (143 cm^?1) and its q-independence are attributed to the large single-ion anisotropy of Co²⁺.     

Magnetic excitations in solids

This article presents a review of low to moderate frequency magnetic excitations, termed magnons or spin waves, in magnetically ordered materials. The emphasis is on intuitive behavior rather than analytical theory. Topics include spin waves, magnetostatic modes, dipole-exchange modes, surface anisotropy, dispersion properties, nonlinear effects, and relaxation. These phenomena are illustrated with experimental examples based on magnon light scattering results as well as conventional microwave techniques.     

Magnetic excitations in TmCu

The magnetic excitations have been studied in the rare-earth intermetallic compound TmCu by means of inelastic neutron scattering. Dispersion curves have been followed through the cubic Brillouin zone, in particular an intense and dispersive transverse branch: this latter presents a lifting of the degeneracy along particular directions. Within a generalized susceptibility formalism, this is related to the coupling of the excitations at q and q + Q, where Q = (π/a, π/a, 0) is the antiferromagn etic propagation vector in TmCu. The inter-ionic isotropic bilinear exchange parameters are deduced and compared with those of isomorphous compounds.     

Magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in silicon oxide

The magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in the amorphous silicon oxide matrix are investigated as a function of the implantation dose. The analysis of the field dependences of the magnetization and the magneto-optical Faraday and Kerr effects demonstrates that, as the ion implantation dose increases, the superparamagnetic behavior of an ensemble of cobalt nanoparticles at room temperature gives way to a ferromagnetic response with the anisotropy characteristic of a thin magnetic film. The magnetization curves for the superparamagnetic and ferromagnetic ensembles of cobalt nanoparticles are simulated to determine their average sizes and the filling density in the irradiated layer of the silicon dioxide matrix. It is revealed that the spectral dependences of the Faraday and Kerr effects for ion-synthesized cobalt nanoparticles differ substantially from those for continuous cobalt films due to the localized excitations of free electrons in the nanoparticles.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

Magnetic excitations in amorphous ferromagnets

Energy spectra of long wavelength spin wave excitations and additional low-lying spin wave excitations near the first peak in the static structure factor of an amorphous ferromagnet Co₄P are calculated in the Heisenberg model.     

Current-induced excitations in single cobalt ferromagnetic layer nanopillars

Current-induced excitations in Cu/Co/Cu single ferromagnetic layer nanopillars ( approximately 50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current-induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin-wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.     

Localized electronic excitations in nickel oxide

Valence-electron ionization and optical excitation energies of NiO have been calculated by the SCF?Xα cluster method and transition-state procedure. Ni 3d-like spin orbitals are localized above the O 2p band, leading to well defined peaks in the photoemission spectra. Ligand-to-metal charge-transfer excitations are responsible for strong optical absorption between 4 and 7eV.     

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.     

Magnetic excitations of undoped iron oxypnictides

We study the magnetic excitations of undoped iron oxypnictides using a three-dimensional Heisenberg model with single-ion anisotropy. Analytic forms of the spin-wave dispersion, velocities, and structure factor are given. Aside from quantitative comparisons that can be made to inelastic neutron scattering experiments, we also give qualitative criteria that can distinguish various regimens of coupling strength. The magnetization reduction due to quantum zero point fluctuations shows clear dependence on the c-axis coupling.     

Magnetic excitations of a ferromagnetic superlattice

By modifying the mixed-Bethe-lattice model, the properties of spin wave spectra of a ferromagnetic supperlattice are studied. The local density of magnon states (LDOMS) are obtained.     

Magnetic anisotropy in icosahedral cobalt clusters

Magnetic anisotropy has been measured in multiply twinned, icosahedral cobalt clusters. It is found that the low-temperature magnetization of deposited cluster layers is well defined with the Stoner–Wohlfarth model by averaging over clusters with a range of anisotropy energy. Anisotropy energy calculation based on Néel's pair model shows that the icosahedral structure and the layer-by-layer growth of the clusters induce oscillations of the magnetic anisotropy as a function of the filling of the outer surface of the particle. The magnetization measurement at room temperature indicates a weakly correlated cluster glass, as deduced from the approach to saturation that is well described with 2D random anisotropy model.     

Magnetic excitations in Pr - a second look

The temperature-dependent excitation spectra of the paramagnetic excitons in the d.h.c.p. praseodymium single crystal are derived in the RPA of the double-time Green function method. Results are compared with the experimental data using Rainford's level scheme.     

Magnetic excitations in the ferromagnetic superconductor UGe2

We report that the uniform magnetization is not conserved in the magnetic excitation spectrum of UGe2. The measured spectrum is therefore different from that in d-electron ferromagnetic metals in a way that would facilitate magnetically mediated superconductivity.     

Magnetic excitations in crystal-field split 4f systems

A survey is given of magnetic excitations in metallic 4f systems when the crystal-field splitting is comparable to or larger than the magnetic interaction between different sites. Particular emphasis is put on the interaction of crystal-field split rare-earth ions with conduction electrons. A number of physical effects is discussed which result from that interaction. The modifications of the magnetic excitations are described which result when the coupling of phonons to the crystal-field levels is so strong that a bound state between the two types of excitations is formed. For a study of these phenomena the method of neutron scattering is an indispensable tool. For that reason it plays a central part in this review.