Ferrimagnetism and spin canting of ZnFe2O4 nanoparticles embedded in ZnO matrix

The structural and magnetic properties of ZnFe2O4 nanoparticles embedded in a non-magnetic ZnO matrix are presented. X-ray diffractograms and Transmission Electron Microscopy (TEM) 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 behavior from room temperature down to T_{M} ~ 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 M_S = 0.028(3) and 0.134(7) muB/f.u. ZnFe2O4 respectively, showing the existence of small amounts of non compensated atomic moments. Mössbauer 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 Mössbauer 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 = 49(2)°. We discuss the significance of this particle structure for the observed magnetic behavior.

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