Magnetic behavior of zero-field-frozen ferrofluid

In contrast to the spin glass-like state described in the literature, an alternative approach to explain the temperature dependence of the magnetization in a zero-field-frozen ferrofluid (ZFFF) is proposed in this work. It is claimed that the presence of a well-defined peak in the magnetization (M) versus temperature (T) curve results from the following combined effects: the temperature dependence of the reorientation of the magnetic moment associated to the nanomagnetic particle, saturation magnetization and magnetic anisotropy. The sample used in this work is a Nickel ferrite-based ferrofluid, which was investigated using magnetometry and transmission electron microscopy, the latter indicating the presence of nanomagnetics with a mean particle diameter of 11.1 nm and standard deviation of 0.37. Excellent agreement between theory and experiment is found for the M vs T curve using the ferrofluid sample containing 3×1016 particle/cm3 and submitted to magnetic fields of 1, 3 and 5 kG.