Collective Superspin Glass State of Interacting Cobalt Ferrite Nanoparticles

In this article, magnetic and structural characteristics of CoFe2O4 nanopowders with mean diameter 17.8(4) and 63.7(2.4) nm [denoted as NM(18) and CP(60), respectively] were studied by applying X-ray diffraction method (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). It was established that for NM(18), the nanoparticle size corresponds to one crystallite while CP(60) nanoparticle forms a multi-crystallite system which means certain kind of averaging of magnetic moments. Magnetic analysis was performed according to the ZFC-FC protocol in the temperature range of 10-380 K, and magnetization curves M(H) were recorded at the magnetic field up to 7 T. It was shown that, independently on nanoparticle size distribution, the magnetic response of both nanopowders is mainly determined by superspin glass type interaction of magnetic moments corresponding to a single crystallite. In addition, basic magnetic characteristics such as anisotropy constant K-1 = (3.9-12.5) x 10(5) J/m(3), anisotropy field mu H-0(A) = 1.80-5.80 T, or magnetic moment per formula unit mu = 3.45(5)mu(B) [for CP(60)] and mu = 3.50(5) mu(B) [for NM(18)] were also determined and discussed.