Atomic migration and superexchange interaction in Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/

Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ was studied with X-ray diffraction and Mossbauer spectroscopy. The crystal structure was found to be a cubic spinel with the lattice constant a/sub 0/=8.386/spl plusmn/0.005 /spl Aring/. The Neel temperature was determined to be T/sub N/=755 K for a heating rate of 5 K/min. The Mossbauer spectra consisted of two six-line patterns corresponding to Fe/sup 3+/ at the tetrahedral (A) and octahedral (B) sites. Debye temperatures for A and B sites were found to be 568/spl plusmn/5 K and 194/spl plusmn/5 K, respectively. Atomic migration of Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ starts near 350 K and increases rapidly with increasing temperature to such a degree that 71% of the ferric ions from the A sites moved to the B sites at 550 K. The temperature dependence of the magnetic hyperfine field of Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ was explained by the Neel theory of ferrimagnetism using three superexchange integrals: J/sub A-B/=-29.2 k/sub B/, J/sub A-A/=-21.9 k/sub B/, J/sub B-B/=0.5 kg.