Collective Superspin Glass State of Interacting Cobalt Ferrite Nanoparticles

In this article, magnetic and structural characteristics of CoFe
₂
O
₄
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) × 10
⁵
J/m
³
, anisotropy field μ
₀
H
_A
= 1.80-5.80 T, or magnetic moment per formula unit μ = 3.45(5)μ
_B
[for CP(60)] and μ = 3.50(5)μ
_B
[for NM(18)] were also determined and discussed.