Effective Behavior of Heterogeneous Ferromagnetic Materials

摘要

In the project, we have studied a variety of heterogeneous ferromagnetic materials, including ferromagnetic shape memory alloys, multiferroic materials, and ferromagnetic composites. The main achievements have been summarized as following: 1. A constrained theory of magnetoelastic materials has been developed to study the effective behavior of single- crystalline and polycrystalline ferromagnetic shape memory alloys (FSMA) based on nonlinear homogenization theory and variational principle. The actuation strain induced by domain switching under a combined magneto-mechanical loading has been established first for single crystalline FSMA, from which the strain in FSMA polycrystals has been derived using Taylor estimate. Both NiMnGa and FePd have been analyzed, and the necessary fiber textures for large actuation strain in FSMA polycrystals have been identified. It was discovered that both twin boundary motions and magnetization rotations contribute to the magnetic field induced strain, but the twin boundary motions are blocked when the compressive stress exceeds a critical values, leading to substantially smaller magnetic field induced strain and thus relative low blocking stress.