Cascade of magnetic field induced spin transitions in LaCoO3

We present magnetization and magnetostriction studies of LaCoO3 in magnetic fields approaching 100 T. In contrast with expectations from single-ion models, the data reveal two distinct first-order transitions and well-defined magnetization plateaus. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by interactions between different electronic configurations of Co3+ ions. We propose a model that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.

Transfer-matrix approach based on modal analysis for modeling corrugated long-period fiber gratings

A transfer-matrix method is developed for modeling a corrugated long-period fiber grating. Cladding-mode resonance in such a corrugated structure can be controlled by the applied tensile stress based on the photoelastic effect. A first-order vectorial perturbation expansion is used to derive the mode fields of the two basic regions under the strain-induced index perturbation. Because the etched cladding radius is much smaller than the unetched radius, the effect of the corrugated structure on cladding modes cannot be treated as a small perturbation. Thus the conventional coupled-mode theory is inadequate for the modeling of such a structure. Based on a self-consistent mode-matching technique, mode coupling within the corrugated structure can be described by a set of transfer matrices. We apply the formulation to the calculation of the transmission spectra of a corrugated long-period grating and compare the calculated with the experimental results. The transfer-matrix approach is found to account well for the features of the transmission spectra of the corrugated long-period gratings.

Analysis and design of almost-periodic vertical-grating-assisted codirectional coupler filters with nonuniform duty ratios

A systematic approach to the analysis of almost-periodic vertical-grating-assisted codirectional couplers with nonuniform duty ratios is presented. The Poisson sum formula is used to expand rigorously a nonuniform rectangular grating into a quasi-Fourier series, and local grating parameters such as period, width, and duty ratio can be defined unambiguously in such a procedure. On the basis of this expansion the coupled-local-mode formulation is the most natural extension for the analysis of such a nonuniform grating-assisted codirectional coupler filter. By transformation of the coupled-local-mode equations into the Zakharov-Shabat system the Gel'fand-Levintan-Marchenko inverse-scattering method is then used to synthesize special grating-assisted codirectional coupler filters. The design is illustrated by two typical examples: One is a third-order Butterworth filter, and the other is a linear filter.