PROBLEM TO BE SOLVED: To provide a method for preparing a nanoparticle powder having magnetism comparable to neodymium iron borate.SOLUTION: A method of preparing core-shell-shell FeCo/SiO/MnBi nanoparticles comprises: a) a step 1 of reducing an Fe ion and a Co ion from a common solution so as to form FeCo alloy nanoparticles; b) a step 2 of forming a silica coating on the FeCo alloy nanoparticles; and c) a step 3 of forming a MnBi coating on the silica shell, where the step 3 is conducted in a magnetic field of 50 to 800 Gauss applied.
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