Magnetic-Induced Deformation in Off-Stoichiometric Ni\u3csub\u3e2\u3c/sub\u3eMnGa: The Competition Between Constraints and Magnetic Energies

Magneto-mechanical properties of off-stoichiometric Ni2MnGa were investigated in three sample configurations: polycrystalline foams, oligocrystalline wires, and a monocrystalline wire. A fundamental understanding of dominating mechanisms in magnetic-inducted deformation is needed to improve, accommodate, or utilize magnetic-field-induced deformation. In this work, we specifically focus on the direct competition between constraints (i.e., surface stresses, grain boundaries, and twin boundaries) and magnetic energies (i.e., magnetostatic energy, Zeeman energy, magneto-crystalline anisotropy energy, and shape anisotropy energy) that ultimately determine the macroscopic deformation of the sample. Macroscopic deformation was studied with a focus on decreasing constraints and minimizing the competing magnetic energies to increase magnetic-induced deformation. Specifically, we wanted to determine how processing and aspect ratio affect magnetic-field-induced strain. Polycrystalline foam samples were selected because they offered an opportunity to test the effectiveness of internal constraint reduction paired with lowered magneto-crystalline energy and Zeeman energy through directional solidification. The oligocrystalline wire samples allowed us to explore the effectiveness of bamboo grains reducing internal constrains and increasing the shape anisotropy. The monocrystalline sample allowed investigation of aspect ratio and applied magnetic field in an attempt to understand the interplay of Zeeman energy, magneto-crystalline energy and shape anisotropy energy. Adding crystallographic texture, to an already porous material, increased magnetic field induced strain by aligning grains that reduced the amount of random internal constrains in a polycrystalline material. Wires with a bamboo structure exhibit magnetic-field-induced bending with a maximum bending strain of 3%. The single crystal wire displayed magnetic-field-induced strain in two clear types axial (~ 2%) and bending (0.5 - 2 %). However, a clear trend revealed that axial strain dominates for short samples and bending strain dominates for long samples. This collection of work has shown that decreasing constraints and minimizing the competing magnetic energies do increase magnetic-induced deformation in Ni2MnGa