The superparamagnetic relaxation time is calculated for spherical particles with cubic magnetocrystalline anistropy, whose easy axis is either [100] or [111] in zero applied field. Results are reported for relatively small particles only, in which no anomaly is found. It is concluded that the experimentally observed anomaly in this size region must be due to incoherent magnetization reversals.

• Received 6 August 1971

DOI:https://doi.org/10.1103/PhysRevB.7.1103