Magnetic properties of hard magnetic nanoparticles of Nd2Fe14B synthesized using self-assembled block copolymers

Abstract

Nd2Fe14B nanoparticles are widely used in bonded magnets for actuators in many types of electric equipment, motors in mobile phones and hard disk drives. Magnetic nanoparticles are also used in magnetic storage devices, drug delivery systems, and ferrofluids. Improvement of the magnetic properties of Nd2Fe14B rare-earth magnets would allow for production of small, lightweight, high-torque motors for use in energy-saving household appliances. However, because of the chemical instability of Nd2Fe14B nanoparticles, it is challenging to synthesize them with high purity by chemical synthesis or by conventional metallurgy methods using Nd2Fe14B alloys. In addition, the high reduction potential of Nd3+ makes reduction difficult. In this study, block copolymers were used as templates for chemical synthesis of Nd2Fe14B nanoparticles. In the synthesis, tris(acetylacetonato) iron(III) as an iron precursor, neodymium tris(acetylacetonate) hydrate as a neodymium precursor, and 1,1-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ferrocene as a boron precursor were introduced into a polystyrene-b-poly(2-vinylpyridine) block copolymer template. Then, the block copolymer was removed by oxidation. After reduction with CaH2 and washing with water to remove residual Ca species, highly pure Nd2Fe14B nanoparticles were obtained. Under the optimized experimental conditions, nanoparticles with a coercivity of 3.5 kOe and a saturation magnetization of 158 emu/g were obtained. The saturation magnetization was 93.5% of the theoretical value of Nd2Fe14B (169 emu/g).