Interparticle interactions and surface contribution to the effective anisotropy in biocompatible iron oxide nanoparticles used for contrast agents

We have investigated the dynamic magnetic properties of dextran-coated magnetite (Fe₃O₄) nanoparticles in the form of (a) particles suspended in a carrier liquid and (b) concentrated powder obtained from lyophilization. The blocking temperature was found to increase from T_B=42(2) to 52(2) K (@μ₀H=10 mT) after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field B_hyp(T) reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy E_a=3.6×10^-21 J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K₁=10 kJ/m³ was obtained by assuming the cubic phase with easy magnetic direction [111] of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at T_V could be observed in our measurements. From the dynamical parameters of ac susceptibility χ(f,T) curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be Ω=4.5×10^-21 J, larger than the single-particle value.