The effect of dipole-dipole interactions on coercivity, anisotropy constant, and blocking temperature of MnFe₂O₄ nanoparticles

Superparamagnetic manganese ferrite nanoparticles with mean size of 〈D〉 = 6.5(±1.5) nm were synthesized through a solvothermal method using Tri-ethylene glycol as a solvent. The peak temperature of zero field cooled measurements of magnetization and AC magnetic susceptibility curves shifted toward higher temperatures by applying different pressures from 0 to 1 kbar and increasing the powders compaction. The frequency dependence of AC susceptibility measurements indicated the presence of weak dipole-dipole interactions between nanoparticles. By increasing the powders compaction and interactions strength, the coercive field (H_c) increased and squareness (M_r/M_s) decreased. The obtained effective anisotropy constant (K_eff), by susceptibility measurements, was from 1.72 × 10⁶ to 2.36 × 10⁶ergs/cm³ for pressure of 0 to 1 kbar. These values are larger than those obtained from hysteresis loops at 5 K (0.14 × 10⁶ to 0.34 × 10⁶erg/cm³). Also, the K_eff was two orders of magnitude greater than that of bulk MnFe₂O₄. Size, surface effects, and total energy barrier between equilibrium states were reported as the main causes of large anisotropy. Below 75 K, a signature of weak surface spin glass was observed. However, memory effect experiment indicated that there is no collective superspin glass state in the samples. This study suggests the role of powders compaction on properties of a magnetic nanoparticles system. Furthermore, the coercivity, the anisotropy constant, and the blocking temperature are affected by changing nanoparticles compaction