In Situ observations of magnetization process in Alnico magnets by electron holography and Lorentz microscopy

The magnetic microstructure of Alnico 5 and Alnico 8 and their magnetization process were investigated systematically at a nanometer scale by means of electron holography and Lorentz microscopy. In particular, the magnetization process in Alnico alloys was visualized for the first time by utilizing a sharp magnetic needle made of sintered Nd₂Fe ₁₄B in a transmission electron microscope. It was found that the direction of lines of magnetic flux changed at the boundaries between the α₁ and α₂ phases which were aligned in the direction of the magnetic field applied in the thermomagnetic treatment. In the Lorentz microscope image, these boundaries were observed as white lines and black bands, whose shapes reflected the difference in the shape anisotropy between Alnico 5 and Alnico 8. With an increase in the magnetic field induced by the magnetic needle, the magnetization direction of the domains magnetized in the direction opposite to that of the approaching needle was reversed, and finally a large reversed domain was formed. In both Alnico 5 and Alnico 8, it was shown that the magnetization process was accomplished through an entire magnetization reversal in each grain of the α₁ phase.