Non-monotonic Behavior of the Blocking Temperature of Interacting Magnetic Nanoparticles

The superparamagnetic behavior of magnetic nanoparticles plays a key role in the scientific and/or technological use of these particles. Blocking temperature is a remarkable quantity that allows one to determine the regime, superparamagnetic or blocked, in which the nanoparticles can be found. Even though it is a well-known fact that the blocking temperature is strongly dependent on the magnetostatic interaction, the overall effect of the dipolar interaction remains unclear. In a system of single-domain magnetic nanoparticles, placed along a linear chain, we address the effect of the dipole-dipole interaction on the blocking temperature of the system. We add disorder into the system by allowing random sizes of the particles and random directions of their uniaxial anisotropy axis. We perform numerical simulations of the stochastic Landau-Lifshitz-Gilbert equation, taking into account an explicit dependence of the saturation magnetization and uniaxial anisotropy energy density on temperature, relevant for systems where the blocking temperature is high. From ZFC magnetization measurements for different strengths of the dipolar coupling, we show that the blocking temperature is a non-monotonic function of the mean distance between particles. The blocking temperature reaches its maximum value when particles touch each other, then decreases as the mean distance between particles increases, and attains a minimum value, starting to increase again up to a constant value where the dipolar interaction becomes negligible.