MAGNETIZATION BEHAVIOR OF NANOMETER-SCALE IRON PARTICLES

The magnetic properties (magnetic moment, shape anisotropy, switching field, and distribution) of nanometer-scale ferromagnetic iron particl es are investigated by measuring magnetization curves for different pa rticle orientations. The measured switching fields indicate that magne tization curling accounts for the reversal at ''zero'' temperatures an d an exchange length, lambda(ex)=2.6 nm, is deduced. A phenomenologica l model describing the temperature dependence of the switching field i s used to estimate the activation volume, upsilon(A) = 270 nm(3). This is small compared to the particle volume and may explain the experime ntal fact that the magnetization reversal cannot be described by therm al activation over an average single energy barrier.