Mössbauer effect and magnetism of single domain Fe₃O ₄ particles in ferrofluids

Three ferrofluids, consisting of suspensions of Fe₃O₄ particles of ≅100-200 Å diameter in diester, hydrocarbon, and water have been investigated at temperatures 4.2-300 K by means of Mössbauer effect and magnetic measurements. The magnetic data displays almost all the features observed in the spin glasses. The Néel model is used to explain the behavior of the fluids. From the temperature variation of the initial susceptibility and the isothermal remanence, K=(3.5±0.7)×10 ⁵ erg/cc and D₀=(80±5) Å are obtained for the anisotropy constant and the median diameter, respectively, of the diester based ferrofluid. The ac susceptibility gives K=3×10⁵ erg/cc and D₀=92 Å. For the water based ferrofluid measurements give K=1.5×10⁵ erg/cc. Values obtained from Mössbauer measurements for K and D₀ agree with those obtained from magnetization-susceptibility data.