Preparation for exchange-coupled permanent magnetic composite between α-Fe (soft) and Nd2Fe14B (hard)

Soft magnetic α-Fe nanoparticles were prepared by a coprecipitation route and hard magnetic Nd15Fe77B8 nanoparticles were prepared by ball milling for 20 h by using a shaker mill. A mechanical ball-mill technique was applied to build up exchange-coupled nanoparticles. A mixture of Nd2Fe14B and α-Fe nanoparticles in a stainless steel boat was milled for 2 h and annealed in a vacuum furnace under vacuum (∼10−5 Torr) at 650 °C for 30 min. The crystal structure of the nanoparticles was confirmed by using X-ray powder diffraction (XRD). The surface morphology was identified by FE-SEM. The magnetization curve was measured with a vibrating-sample magnetometer (VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis (TMA) curve showing the downward magnetic force versus temperature.