“…When the nanoparticle volume, V P , is reduced to a certain extent that magnetic anisotropy energy, KV P , is comparable to the thermal energy, k B T , there exists a finite probability that the magnetization vector will reverse its direction by thermal fluctuation at certain temperature T . Both theoretical and experimental data indicated that the frequency dependence of the susceptibility for a single domain particle depended on the ratio of the magnetic anisotropy energy to the thermal energy ( α = KV P / k B T )1516. In the case of
, the real part of the complex susceptibility x ′( f ) decreases monotonically with increasing f , whilst the imaginary component, χ "( f ), has a maximum at a frequency f b = 1/ τ .…”