Mechanical control of resonance and relaxation in magnetoelastic composites and MR fluids near microwave frequencies (Conference Presentation)

Work on magnetoelastic particulate composites and magnetorheological (MR) fluids has traditionally focused on frequencies that are small compared to the ferromagnetic resonance (FMR) frequency. Under these conditions the structural or fluid dynamics may be of importance, but the magnetic response is essentially quasi-static. This is in contrast to the response of magnetic materials in microwave devices where the nonlinear spin dynamics present a variety of novel phenomena. Notably, shifting the FMR frequency controls the transmission, absorption, and reflection of electromagnetic waves, with potential non-reciprocal propagation. The present work provides an analysis of a magnetoelastic inclusion in solid and fluid dielectrics, and shows how mechanical loads can control the resonant and relaxation characteristics of these composites at microwave frequencies. Both analytical and numerical analysis of the coupled spin and mechanical dynamics will be provided. In the magnetic inclusion the equations of elastodynamics are coupled to the Landau-Lifshitz-Gilbert (LLG) equation. The considered magnetic anisotropy energies include Zeeman, magnetocrystalline, and magnetoelastic interactions. Altering the magnetocrystalline and magnetoelastic energies allows the magnetic moment to either rotate independent of the lattice or strongly couple to it (i.e., modeling superparamagnetic or permanent magnetics). Non-symmetric stress tensors arise in the strongly coupled case via the Maxwell stress. It will be shown under what loading conditions the effective magnetoelastic field shifts the FMR frequency and characteristic relaxation times. The effects are found to depend strongly on the crystallinity of the magnetic inclusion and nature of the applied load (i.e., hydrostatic pressure or general 3D stress state).