Direct observation of nano-dimensional internal structure of ferromagnetic domains in the ferromagnetic shape memory alloy Co-Ni-Ga

Highlights: • DAM of TEM for visualization of internal structure of magnetic domains in FSMA. • Nano-dimensional observation of magnetic domains. • Quasiperiodic twinning of crystal lattice. • Nanoscale twin structures, which form the secondary mesoscale tween. • Description of magnetic and crystallographic microstructure of FSMAs. The Displaced Aperture Method (DAM) of Transmission Electron Microscopy (TEM) is used to visualize internal structure of magnetic domains in the Co₄₉-Ni₂₁-Ga₃₀ Ferromagnetic Shape Memory Alloy (FSMA). The preparation of the FSMA specimens and the details of DAM enabling visualization of these domains and their internal details are described. The magnetic domains of ca. 100-nm width with the domain walls of ca. 20 nm width are observed. The theoretical estimation supporting the possibility of appearance of such domains in the thin foil prepared for TEM observations is presented. The superfine crystallographic microstructure possessing the features, which are inherent to the quasiperiodic twinning of crystal lattice with period of ca. 6 nm, is observed inside the magnetic domains. The obtained TEM data may be useful for examination of the influence of external magnetic field on the magnetic and crystallographic microstructure of FSMAs.