Superparamagnetic Transition and Local Disorder in CuFe2O4 Nanoparticles

We present X-ray diffraction (XRD), M\"ossbauer spectroscopy (MS) and d.c. magnetization measurements performed on ball-milled CuFe2O4 samples. The average particle size <d> was found to decrease to the nanometer range after t=15 min of milling. Room temperature M\"ossbauer data showed that the fraction of particles above the blocking temperature TB increases with milling time, and almost complete superparamagnetic samples are obtained for <d> = 7(2) nm. Magnetization measurements below TB suggest spin canting in milled samples. The values of saturation moment mu_S reveal that site populations are slightly affected by milling. M\"ossbauer resonant intensities are accounted for on the basis of local disorder of Fe3+ environments, and the development of sample inhomogeneities of CuxFe3-xO4 composition.