Antiphase Boundaries, Magnetic Domains, and Magnetic Vortices in Ni-Mn-Ga Single Crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni-Mn-Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments and the APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/μm), air quenching in a medium density (≈8/μm), and water quenching in a high density (≈15/μm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in one-to-one correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy became lower than the stray field energy, and magnetic vortices additionally appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.