Magnetic and thermal studies of nano-size Co and Fe particles

Abstract The thermal and magnetic properties of nanocrystalline Fe, CoFe 2 , and Co particles as well as their bulk samples were studied between room temperature and 900°C. From the thermogravimetric analyzer measurements, we have observed that near 200°C an endothermal signal occurs abruptly within 10°C for nanocrystalline Fe, and then this signal decreases with increasing temperature and shows a kink near 550°C. However, an exothermal signal starts near 200°C for nanocrystalline Co and CoFe 2 , and it stops to decrease near 310°C and 440°C for Co and CoFe 2 , respectively. Both endo- and exo-thermal effects near 200°C are related to the aggregation and oxidation of the ultra-fine particles. The kink near 550°C for Fe is related to the Curie temperature of Fe 3 O 4 . The magnetization of the ultra-fine particles was slightly smaller than that of the bulk samples, and it decreased very fast with increasing temperature between 100°C and 200°C. The coercivity of nanocrystalline CoFe 2 is roughly 1780 Oe, and is larger than that of Co ( H c ∼1198 Oe) and Fe ( H c ∼883 Oe). After annealing in air, the content of oxygen in nanocrystalline particles increases roughly to 40% for Fe particles, and roughly to 30% for CoFe 2 and Co particles. The oxidation is manifestly reduced after adding the Co element into Fe nanoparticles.