Magnetic nanoparticles with combined anisotropy

We study the influence of the distribution of the particles' aspect ratio on the magnetostatic properties of an assembly of nanoparticles with cubic magnetocrystalline anisotropy, such as iron, nickel, iron oxides, and ferrites. Because of the large values of the dimensionless ratio M-s(2)/vertical bar K-c vertical bar for these particles, the shape anisotropy energy makes considerable contribution to the total nanoparticle energy even for relatively small shape distortions, with equivalent ellipsoids having aspect ratios xi >= 1.1. As a result, the magnetostatic properties of a randomly oriented assembly of particles with combined anisotropy at xi >= 1.1 are similar to those for an assembly of particles with purely uniaxial magnetic anisotropy. This conclusion is shown to be valid not only for the assembly hysteresis loops but also for the magnetic relaxation characteristics, at least in the high damping limit. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749799]