  The specific absorption rate (SAR) of a dilute assembly of spherical iron
nanoparticles with cubic anisotropy distributed in a viscous liquid is
calculated using the solution of stochastic Landau - Lifshitz equation for unit
magnetization vector and stochastic equations for multiple particle directors
that specify the spatial orientation of the nanoparticle in a liquid. The
viscous and magnetic magnetization reversal modes of particles are revealed at
low and sufficiently high amplitudes of alternating magnetic field,
respectively. The SAR of iron nanoparticle assembly is shown to exceed
significantly that of iron oxide nanoparticles with uniaxial anisotropy at the
same amplitudes and frequencies of applied magnetic field. The linear response
theory is shown to be valid only at small magnetic field amplitudes, H0 < 50 -
70 Oe.
