Ferrimagnetism and spin canting of Zn57Fe2O4 nanoparticles embedded in ZnO matrix

The structural and magnetic properties of ZnFe2O4 nanoparticles embedded in a nonmagnetic ZnO matrix are presented. X-ray diffractograms and transmission electron microscopy images showed that the resulting samples are composed of crystalline ferrite nanoparticles with average crystallite size (D) = 23.4+/-0.9 nm, uniformly dispersed within the ZnO matrix. Magnetization data indicated a superparamagnetic-like behaviour from room temperature down to T-M similar to 20 K, where a transition to a frozen state is observed. The M(H) curves displayed nearly zero coercive field down to TM, where a sharp increase in the H-C value is observed. The measured saturation magnetization M-S values at 200 and 2 K were MS = 0.028(3) and 0.134(7) mu(B)/f.u. ZnFe2O4, respectively, showing the existence of small amounts of non-compensated atomic moments. Mossbauer measurements at low temperatures confirmed the transition to a magnetically ordered state for T < 25 K, where two magnetically split sextets develop. Whereas these two sextets show strong overlap due to the similar hyperfine fields, in-field Mossbauer spectra clearly showed two different Fe3+ sites, demonstrating that the sample is ferrimagnetically ordered. The two spinel sites are found to behave differently under an external field of 12 T: whereas the moments located at A sites show a perfect alignment with the external field, spins at B sites are canted by an angle alpha(B) = 49degrees +/-> 2degrees. We discuss the significance of this particle structure for the observed magnetic behaviour.