Giant nonlinear damping in nanoscale ferromagnets

Magnetic damping is a key metric for emerging technologies based on magnetic nanoparticles, such as spin torque memory and high-resolution biomagnetic imaging. Despite its importance, understanding of magnetic dissipation in nanoscale ferromagnets remains elusive, and the damping is often treated as a phenomenological constant. Here, we report the discovery of a giant frequency-dependent nonlinear damping that strongly alters the response of a nanoscale ferromagnet to spin torque and microwave magnetic field. This damping mechanism originates from three-magnon scattering that is strongly enhanced by geometric confinement of magnons in the nanomagnet. We show that the giant nonlinear damping can invert the effect of spin torque on a nanomagnet, leading to an unexpected current-induced enhancement of damping by an antidamping torque. Our work advances the understanding of magnetic dynamics in nanoscale ferromagnets and spin torque devices.

Fracture-Micromechanics Based Model of Mortars Susceptible to Shrinkage

The addition of stiff aggregates into a relatively compliant matrix results, up to a certain limit, in an increase of the effective mortar stiffness. Conventional micromechanical models are not able to capture this limit and must be coupled with a fracture mechanics model predicting the formation of microcracks between aggregates due to shrinkage of matrix. The proposed model utilizes the Mori-Tanaka (MT) scheme together with an estimation of a crack density, based on requirements for the crack formation determined by a numerical analysis. As a result, the model involves a single phenomenological constant, and provides significant improvement over the basic MT scheme.

A kinetic model for Ascaridia galli populations in chickens treated with mixed salts of copper and zinc

The action of mixed salts of copper and zinc (basic and neutral) on Hisex chickens experimentally infected with Ascaridia galli has been studied. The data show that the lowest host mortality and decrease in body weight gain and the highest reduction in nematode loading occurs in infected chickens treated with basic salts (in comparison with infected chickens, untreated or treated with neutral salts). A mathematical model has been proposed to provide a quantitative interpretation of the observed results. The model solutions of the kinetics of parasite numbers and of the gain in body weight are in a good agreement with the experimental data. One of the kinetic parameters in the model is defined as a phenomenological constant of the host immune response. Its value is determined in the case of infected and untreated chickens.