Determination of magnetic anisotropy distribution and anisotropy constant of manganese spinel ferrite nanoparticles

SQUID magnetometry and Mössbauer spectroscopy has been combined to quantitatively characterize the superparamagnetic relaxation of MnFe₂O₄ nanoparticles with a mean size of 7.4 nm. The key parameter for understanding and controlling the superparamagnetic properties of nanoparticles, the anisotropy constant K has been determined as 0.056 J/cm³ for this nanoparticulate sample, which is more than 20 times higher than bulk MnFe₂O₄ materials. The large enhancement of magnetic anisotropy in these nanoparticles suggests the possibility of strong interactions between the magnetic core and shell of the nanoparticles. The quantitative characterization of the 7.4 nm MnFe₂O₄ nanoparticles also shows the preexponential factor τ₀ as 0.95 × 10^-10 s. This τ₀ value is in a good agreement with the theoretical value. Such an agreement is possibly as the result of minimal interparticle interaction in MnFe₂O₄ nanoparticulate sample.