Chemical Synthesis of Magnetically Hard and Strong Rare Earth Metal Based Nanomagnets

We report a general chemical approach to synthesize strongly ferromagnetic rare‐earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra‐large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm 2 O 3 (denoted as SmCo‐O) multipods via decomposition of Sm(acac) 3 and Co(acac) 3 in oleylamine. These multipods were further reduced with Ca at 850 °C to form SmCo 5 NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo 5 NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG‐SmCo 5 NP composite with the room temperature coercivity ( H c ) of 49.2 kOe, the largest H c among all ferromagnetic NPs ever reported, and saturated magnetic moment ( M s ) of 88.7 emu g −1 , the highest value reported for SmCo 5 NPs. The method was extended to synthesize other ferromagnetic NPs of Sm 2 Co 17 , and, for the first time, of Sm 2 Fe 17 N 3 NPs with H c over 15 kOe and M s reaching 127.9 emu g −1 . These REM based NPs are important magnetic building blocks for fabrication of high‐performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications.