M. G. Cottam

Localization and scaling properties of magnetostatic modes in quasiperiodic magnetic superlattices

The localization and scaling behaviour of quasiperiodic structures are studied for a geometry where the magnetization is perpendicular to the interfaces of the superlattices. Numerical results for the bulk and surface spin waves in the magnetostatic regime are presented for the Fibonacci, Thue-Morse and period-doubling sequences. The results are obtained for both ferromagnetic and antiferromagnetic ordering by using the transfer-matrix method. Interesting features of the localized modes are shown for Fe, EuS and MnF2 .

Ferromagnetic and antiferromagnetic spin-wave dispersions in a dipole-exchange coupled bi-component magnonic crystal

The magnon dispersion relations of the ferromagnetic and antiferromagnetic phases in a dipole-exchange coupled one-dimensional magnonic crystal comprising alternating cobalt and Permalloy nanostripes have been mapped by Brillouin spectroscopy. To elucidate the magnetization dynamics at the interfaces between stripes, the experimental data are analyzed based on a macroscopic theory under Hoffmann-type boundary conditions. Good agreement is obtained between theory and experiment for both the ferromagnetic and antiferromagnetic phases. Results suggest the existence of strong exchange coupling across the cobalt-Permalloy interfaces, comparable with the exchange coupling within each component material.

Microscopic theory of dipole-exchange spin waves in ferromagnetic films: Linear and nonlinear processes

The linear and nonlinear processes in ferromagnetic films at low temperatures T<< Tc are studied in a microscopic theory. Both the long-range magnetic dipole-dipole and the Heisenberg exchange interactions to nearest and next-nearest neighbors are included. The results obtained for the linearized spin-wave spectrum are compared with previous macroscopic theories. For ultrathin films (or for large wave vectors) the microscopic theory provides important corrections. The nonlinear dynamics of the spin waves are studied through a finite-temperature perturbation theory based on Feynman diagrams. We obtain explicit results for the energy shift and damping (or reciprocal lifetime) of the dipole-exchange spin waves due to all possible three-magnon and four-magnon processes involving combinations of the surface and quantized bulk spin waves at low temperatures. To investigate different dipole interaction strengths (relative to the exchange) numerical results are presented using parameters for Fe, EuO, and GdCl3.

One- and two-magnon excitations in NiO

Abstract Spin waves in antiferromagnetic NiO have been investigated by Raman spectroscopy at temperatures up to 300 K. A pair of sharp peaks were observed at low frequencies in this relatively opaque material and tentatively assigned to surface and bulk one-magnon modes from theory. Experiment and theory are also compared for the stronger and broader two-magnon line seen at higher frequency.

Lateral Heterogeneity of Photosystems in Thylakoid Membranes Studied by Brownian Dynamics Simulations

The aggregation and segregation of photosystems in higher plant thylakoid membranes as stromal cation-induced phenomena are studied by the Brownian dynamics method. A theoretical model of photosystems lateral movement within the membrane plane is developed, assuming their pairwise effective potential interaction in aqueous and lipid media and their diffusion. Along with the screened electrostatic repulsive interaction the model accounts for the van der Waals-type, elastic, and lipid-induced attractive forces between photosystems of different sizes and charges. Simulations with a priori estimated parameters demonstrate that all three studied repulsion-attraction alternatives might favor the local segregation of photosystems under physiologically reasonable conditions. However, only the lipid-induced potential combined with the size-corrected screened Coulomb interaction provides the segregated configurations with photosystems II localized in the central part of the grana-size simulation cell and photosystems I occupying its margins, as observed experimentally. Mapping of thermodynamic states reveals that the coexistence curves between isotropic and aggregated phases are the sigmoidlike functions regardless of the effective potential type. It correlates with measurements of the chlorophyll content of thylakoid fragments. Also the universality of the phase curves characterizes the aggregation and segregation of photosystems as order-disorder phase transitions with the Debye radius as a governing parameter.

Magnetostatic modes in quasiperiodic Fibonacci magnetic superlattices

The magnetostatic modes are studied in multilayer structures that exhibit deterministic disorders. Some models that have attracted particular attention are the quasiperiodic magnetic multilayers that obey a substitutional sequence of the Fibonacci type. The spin wave spectra are evaluated in the geometry where the magnetization is perpendicular to the surfaces of the layers of the superlattice by using a transfer-matrix approach. Numerical results are presented for the ferromagnets EuS and Fe and for the antiferromagnet MnF2.

Microscopic dipole–exchange theory for planar nanostriped magnonic crystals

Microscopic (or Hamiltonian-based) calculations are reported for the collective spin-wave bands and gaps in one-dimensional magnonic crystals consisting of a periodic array of ferromagnetic stripes separated by nonmagnetic spacers. This is achieved by generalizing a previous approach for the dipole–exchange spin waves in individual (or non-interacting) stripes and small finite arrays of nanoelements to cases where there is an effectively infinite periodic array of striped elements. This involves introducing a Bloch wavenumber associated with the periodicity property and reformulating the microscopic dipole–dipole and exchange sums for a periodic structure to include the inter-stripe and intra-stripe contributions. The theory gives good agreement when compared with Brillouin light scattering data for Permalloy magnonic structures. Applications of the microscopic theory are also made to cases where the applied magnetic field has a component transverse to the easy axis of the stripes, favouring the formation of edge modes.

Robustness and time-scale hierarchy in biological systems

This study addresses the issue of robustness of biological systems with respect to microscopic parameters, especially the emergence of robustness as a consequence of time-scale hierarchy, applying naive thermodynamic and dynamic assumptions. Theoretical considerations of how the time-scale hierarchy can decouple physiological regulatory mechanisms are illustrated by two model systems involving the photosynthetic apparatus of green plants.

Phononic dispersion of a two-dimensional chessboard-patterned bicomponent array on a substrate

Brillouin measurements of the dispersion relations of surface acoustic- and optical-like waves along Γ-M and Γ-X symmetry directions in a two-dimensional bicomponent nanostructured crystal are reported. The sample, in the form of a periodic chessboard array of alternating Permalloy and cobalt square dots on a SiO2/Si substrate, was fabricated using high-resolution electron-beam lithographic, sputtering, etching, and lift-off techniques. The measured phononic band structures exhibit diverse features, such as a partial hybridization bandgap and unusual surface optical-like phonon branches, where there are out-of-phase vibrational characteristics between nearest-neighbor dots. Numerical simulations, based on the finite element analysis, reproduced the experimental dispersion relations.

Surface magnetic polaritons in ferromagnetic and antiferromagnetic cylindrical tubes

A theory is developed for the localized surface magnetic polaritons in hollow magnetic cylinders or tubes, in the presence of a longitudinal applied field. This involves solving for the dynamical response using the nondiagonal susceptibility tensor for a ferromagnet or antiferromagnet in Maxwell’s equations, with electromagnetic boundary conditions at the inner and outer tube surfaces. Results for wires and antiwires are deduced as limiting cases of this geometry. As the outer radius is increased, the surface polariton branches near the light line are reduced in frequency and the localization properties are modified due to strong retardation effects. Numerical applications are made to yttrium iron garnet and MnF2 materials.