Structure of the new metastable grain boundary phase in melt-spun Nd₁₅Fe₇₇B₈ ribbons and its roles on high coercivity

Microstructures and magnetic properties were studied of melt-spun Nd₁₅Fe₇₇B₈ ribbons with various thicknesses. High coercive force up to 1824 kA·m^-1 (22.8 kOe) was found for ribbons with 45-100 μm in thickness. Interspaces among Nd₂Fe₁₄B grains were filled by a non-magnetic phase. Electron diffraction patterns of the grain boundary phase were analyzed to reveal that this phase is of tetragonal structure with lattice constants of a=0.512 nm an c=0.801 nm. An orientation relationship was found such that [001] of the boundary phase lies parallel to the [001] direction of the host Nd₂Fe₁₄B phase. The boundary phase is metastable, since it was decomposed into fcc Nd and a small amount of NdFe₄B₄ phase on heating above 690 K. Coercive force is discussed in terms of a nucleation controlled coercivity mechanism. The smoothness of the interface between Nd₂Fe₁₄B and the boundary phase controls the nucleation behavior of reverse domains. Thus, as-melt spun ribbons exhibited smooth boundaries and high coercive force, heat treated specimens exhibited rough boundaries and low coercive force.