The Transmission Electron Microscopy Analysis on Grain Boundary Phases of Ca-La-Co M-type Sintered Ferrite Magnets

Abstract

The microstructures of Ca-La-Co M-type sintered ferrite magnets were analyzed by Spherical Aberration Corrected Scanning Transmission Electron Microscopy (Cs-STEM) to devise guidelines for improving performance. It was confirmed that there were Ca-Si-based oxides, which consists of Si, Ca, La and Fe, at multiple-junction phases of the sintered body, after adding only SiO₂ instead of both CaCO₃ and SiO₂, which are sintering aids for ferrite magnets. It was also confirmed that the ratio of Si, Ca, La and Fe at multiple-junction phases was 30:60:2:5. Moreover, we found that a step-terrace structure of Ca-Si-based oxides formed at the boundary of the M-type ferrite grain, and that the maximum width of the intergranular grain boundary was about half (1.15 nm) the edge length of the c-axis of the M phase. This suggested that ferrite grains were magnetically isolated by the presence of Ca-Si-based oxide phases at the intergranular grain boundary, whereby improving coercivity.