Getting the most out of neutron powder diffraction – revealing the microstructure evolution during sintering of magnetic nano-particles using parametric refinement

Abstract

Nano-structuring is a crucial step in optimizing permanent magnet materials, where both the size and morphology of the individual particles heavily affect the properties of the compacted bulk magnet. The size is tuned to ensure magnetic single-domain particles, which is crucial for optimizing the coercivity of the magnet (the field strength required to flip the magnetic orientation), as this is lowered by the mobility of walls between domains. Generally, the minimum required coercivity of a permanent magnet is half of the saturation magnetization, at which point the remanence (the magnetization at zero field) is the limiting factor. The remanence, in turn, is primarily tuned by the alignment of the particles, i.e. the texture of the bulk magnet. One way to control the alignment is through the morphology of the nano-particles, as the right particle shape leads the powder to selfalignment during compaction and sintering into dense pellets, thus optimizing the alignment of the magnetic domains in the resulting bulk magnet [1] . The overall figure-of-merit for permanent magnets is evaluated from the volume-weighted energy product (a.k.a. the BHmax), reported in kJ/m 3 (MGOe in cgs-units). It follows that the density of the final bulk magnet is an important parameter, which again is correlated with the particle microstructure. The combination of crystallite size, texture, and density emphasizes the compaction process when going from powder to bulk, which also often includes a sintering step. Powder diffraction is a powerful technique for studying the compaction and sintering processes, as proper refinement of the data can provide valuable information about phases, crystallite size, texture, and, in the case of neutron diffraction, the magnetic moments, as they develop during the compact. Neutron powder diffraction also comes with the benefit of a large probing volume, which is useful for studying bulk behavior.Using parametric refinements allows us to get the most out of our neutron powder diffraction data! [1] Stingaciu, Marian, et al. "Optimization of magnetic properties in fast consolidated SrFe12O19 nanocrystallites."