The bottom-up synthesis of Sm2Co7 hard magnet with high coercivity

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Wet-chemical synthesis approaches to the preparation of permanent magnet Sm-Co particles have recently attracted attention, due to the possibility of controlling the nanostructure, and in turn, optimizing the coercivity. The conventional arc-melting synthesis produces polycrystalline Sm2Co7 compounds with very poor magnetic properties due to grain coarsening during the arc-melting melting process. Synthesizing Sm2Co7 with fine grain sizes are challenging because high-temperature methods are conventionally employed. Herein, a bottom-up approach was developed to synthesize Sm2Co7 particles with an average particle size of about 840 nm. The synthesized Sm2Co7 particles contain multiple phases, in which the main phase is Sm2Co7 with rhombohedral and hexagonal structures and a small amount of SmCo5
phase has been identified. The best synthesized-sample has an impressive coercivity of 2072 kA m-1 (26.0 kOe). Strong exchange-coupling interaction exits in all samples as demonstrated by Henkel plots and δM(H), leading to high
remanence over saturation magnetization (Mr/Ms) ratios. The Sm-Co powder was directly compacted into a bulk magnet by Spark Plasma Sintering (SPS) technique. The SPS-pellet has isotropic magnetic properties with an extremely high coercivity of 3039 kA m-1 (38.2 kOe). Crushing the SPS-pellet leads to an even higher coercivity of 3168 kA m-1 (39.8 kOe). Transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) reveal that Sm2Co7 particles with the rhombohedral structure decomposed into SmCo5 and Sm2O3 phases during the SPS process. Magnetic force microscopy (MFM) image shows the magnetic domain size in the pellet to be around one micrometer, consistent with the average particle size of ~840 nm. The excellent coercivity can be attributed to, the high anisotropy field of SmCo5 phase, the obtained single-domain particle size, and Sm2Co3 impurities on the boundary acting as pinning stacks, together with the Sm2Co7-H phase.
TidsskriftInorganic Chemistry
StatusAfsendt - 20 feb. 2020

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