MOSSBAUER STUDY AND MAGNETIC CHARACTERIZATION OF THE OXIDIZED SURFACE-LAYER OF FINE IRON PARTICLES

In this paper the superparamagnetism of the oxidized surface layer of fine iron particles is reported. Fine iron particles in the range of s ize from 15 to 50 nm with oxidized surface layer were prepared by evap oration in nitrogen atmosphere. The sizes were measured by the BET met hod and transmission electron microscope (TEM). X-ray and electron dif fraction analysis show that the particle is composed of a alpha-Fe Cor e and a surface layer of Fe3O4. The thickness of the outer oxidized la yer is about 2 nm and is independent of the average diameter of the pa rticles. The results indicate that the transition of the domain struct ure plays an important role in the superparamagnetic behavior of the o xide surface layer of iron particles. The emergence of the superparama gnetism at low temperature may be a diagnostic of the transition from the single domain to multi-domain structure for inner core. The critic al particle size of the single domain is estimated at about 40 nm.