Abstract
Multi-scale electron microscopy technique that combines transmission electron microscopy (TEM) and focused ion beam - scanning electron microscopy (FIB-SEM) was utilized to investigate the influences of the change in microstructure of Sm2(Fe0.95Mn0.05)17Nx by overnitridation on the magnetic properties. The recent high-contrast backscatter electron imaging technique in SEM machines enabled us to reveal the formation events of cell-like microstructure from a quite large field of view. In addition, detailed TEM observations revealed the crystallographic orientations of the cells as well as the local chemical fluctuation. The combination of these techniques allows us to understand the microstructural hierarchy in this material, verifying that the original orientations of the individual particles are inherited to the nanocrystalline cells formed by overnitridation.
Highlights
Several researchers demonstrated that overnitridation of relatively coarse powder (∼ 60 μm) of Sm2(FeMn)17Nx magnets (x > 3) introduces a unique microstructure that is composed of nanocrystalline cells (= below 100 nm) surrounded by amorphous cell walls into formerly monocrystalline powder, making it possible to achieve quite high coercivity (∼12 kOe) without reducing the particle size.[3,11,13]
Takagi high as the record values reported in literature (e.g. 8∼12 kOe) but clearly higher than the Mn-free Sm-Fe-N coarse powder (∼2 kOe)
Overnitridation treatments were performed for Sm-Fe-Mn alloy coarse powder
Summary
Sm2Fe17N3 is an excellent permanent magnet that exhibits extremely high anisotropy field and the high Curie temperature.[1,2,3] It has been argued that bulk Sm-Fe-N magnets cannot be produced because of the thermal decomposition, but this drawback is being overcome according to recent research activity.[4,5,6,7] On the other hand, many researchers have been striving to improve the coercivity of the ingredient powder for producing bulk magnets by refining particle size.[3,8,9,10,11,12] Several researchers demonstrated that overnitridation of relatively coarse powder (∼ 60 μm) of Sm2(FeMn)17Nx magnets (x > 3) introduces a unique microstructure that is composed of nanocrystalline cells (= below 100 nm) surrounded by amorphous cell walls into formerly monocrystalline powder, making it possible to achieve quite high coercivity (∼12 kOe) without reducing the particle size.[3,11,13] It is known that similar coercivity enhancement phenomena can be observed when Cr, Ti, V are added to Sm-Fe-N powder.[14,15,16] there was a drawback that remanence and squareness are significantly degraded when coercivity is reasonably enhanced by increasing x. The primary reason of this degradation has been attributed to the randomized orientations of the individual nanocrystalline cells for overnitrided powder (x > 5.5).[3,13]. Regions of the sample.[18] The combination of this low TOA BSE imaging and TEM observations enabled us to study the whole image of the overnitrided powder at various length scales,[17] clarifying that the original orientation of the individual particles are inherited to the nanocrystalline cells even if the nitrogen content x approaches 6. It is quite curious to see if the similar microstructure can be observed for Sm-Fe-Mn-N magnet powder by the low TOA BSE image at lower magnifications. This is something that has never been reported to date. Multi-scale electron microscopy technique will be used to study the microstructure of the overnitrided Mn-added Sm-Fe-N powder at various length scales to check if randomization by overnitridation of the crystallographic orientations truly occurs or not
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