In-plane and out-of-plane transverse susceptibility in close-packed arrays of monodisperse Fe nanoparticles

The transverse susceptibility (TS) of arrays of self-assembled Fe nanoparticles has been studied using a sensitive radio-frequency resonant technique. Symmetrically located broad peaks in the TS data are observed below the blocking temperature as the applied field is swept from positive to negative saturation. These peaks occur at the effective anisotropy fields $(\pm H_K)$ with the peak width determined by the distribution in $H_K$ in the nanoparticle array system. These features are observed to be strongly affected by dipolar interactions as well as thermal relaxation. Systematically tracking the evolution of the TS curves across the superparamagnetic transition reveals distinct temperature ranges over which thermal activation and dipolar energy overcome the effective magnetic anisotropy energy. Hysteresis loops measured using a superconducting quantum interference device magnetometer indicate a smaller coercive field for in-plane field orientation compared to that for out-of-plane orientation. This is also reflected in the TS measurements. A comparison of the TS over a wide range in temperature and magnetic fields, applied in plane and out of plane, reveals the distinct influence of variation in dipolar interaction strengths for the two geometries.