Magnetization reversal of two-dimensional superlattices of Mn ₃O₄ nanocubes and their collective dipolar interaction effects

Magnetic properties of two-dimensional (2D) paddy-field like superlattices of Mn₃O₄ cubic nanoparticles have been investigated by magnetization measurements. The 2D ordered structure extends over several microns in size. Each nanocube is of single-crystal about 6 nm in size. The magnetic properties are investigated with the powders dispersed in nonmagnetic n -eicosane to "dilute" the dipolar interaction. By accounting for the temperature variation effect of the magnetocrystalline anisotropy, Kmag (T), the temperature dependent coercivity, H_C (T), can be well described by the equation, H_C (T) = H₀ k_mag (T) / m _S (T) { 1- [kB_T ln (t/ t₀) / E₀ kmag (T)] 3/4 }, in which k_mag (T) = K_mag (T) / K _mag (0) is the reduced temperature dependent magnetocrystalline anisotropy and m_S (T) = M_S (T) / M_S (0) is the reduced saturation magnetization. The effects of collective dipolar interaction on the magnetic properties are also studied with the as-prepared powder sample. The apparent magnetic anisotropy is seriously reduced with the presence of dipolar interaction. The switching volume is determined by the analysis on the magnetic measurements both with and without the dipolar interaction effect. There is a discrepancy in the value of switching volume determined by the two different analysis methods. Possible reasons are discussed.