Electromagnetic radiation, such as microwaves, are all the time reflected, transmitted, and/or absorbed by any kind of matter, glasses, conductors, water, ferrites, and so forth. Magnetic materials absorb greatly microwaves. The more magnetic, the more microwaves are absorbed. The aim of this chapter is to present the fundamental physics of the absorption of microwave power (energy per unit time) by ferrimagnetic and ferromagnetic matter in the nano and micro size scale. The magnetic moments and their collective modes are the basic microscopic absorbers under in-resonance and out-of-resonance conditions. Experimental setups and measurement techniques are described. The profiles of microwave absorption are described and connected to the micromagnetic environment that elicits such absorption. Section by section and the overall microwave power absorption profiles are related to the micromagnetic structures. Emphasis is made on nano-and micromagnets. These interactions of microwaves with nano-and micromagnets serve to infer microscopic magnetic information.

The science of ferroelectric materials has long known that transition metal atom and/or rear earth atom substitution in the composition of a ferroelectric material can produce substantial structural and electric dipole changes and ferroelectric behavior. The focus is on first neighbor changes, symmetry, very tiny atomic displacements, hence magnitudes of electric polarization, charge changes, and mechanical-tensile change of parameters. The transition atom used for the substitution can, or, cannot be paramagnetic. When it is paramagnetic as is the case with Cr3+, Mn2+ and so forth, there emerges an advantage for its experimental characterization at atomic level. Electron Paramagnetic Resonance (EPR) allows the identification of its location within the structure and the number and nature of its neighbors. The presence of crystal fields, symmetry and distortions of the first coordination sphere can also be determined. Here, we describe how a set of EPR spectra is analyzed to extract such atomic information.

Nanomagnetic particles respond sensitively and nonlinearly to electromagnetic radiation. Many excitation schemes are now well known. However, nonlinear dynamics determinations have not been examined in detail under FMR conditions. The nonlinear dynamics of a simple nanomagnet is studied and the excitation field, H 1 , is varied. We solve numerically the Landau-Lifshitz nonlinear dynamics of M t .( ) There is a special focus on the spherical degrees of freedom: t , q ( ) t . f ( ) We find that the t q ( ) trajectories converge asymptotically to asym q =constant, while t f ( ) is a linear function of time. The combined dynamics of t q¢( ) and t f¢( ) produce a limit cycle for each value of H 1 . The systematic numerical calculations and analysis show that the limit cycle asym q follows a fourth-degree polynomial on H 1 and an inverse law on frequency ν 1 . It is also found that the limit cycles are established after 12-20 nanoseconds. They cause M t ( ) to sweep a constant precession cone that lasts for more than 200 ns independent of initial conditions. These results bring significant novel knowledge for fast information technology.

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