Atomic migration in MgFe2-xCrxO4

Mg-Cr ferrite has been studied with Mossbauer spectroscopy, x-ray diffracti on, and vibrating sample magnetometer. The crystal structure for this syste m is spinel structure, and the lattice constant is in accord with Vegard's law. The Mossbauer spectra consist of two six-line patterns corresponding t o Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Curie temperatu re decreases linearly with Cr concentration, suggesting the superexchange i nteraction Fe(A)-O-Fe(B) link is stronger than that for the Fe(A)-O-Cr(B) l ink. Debye temperatures for the A and B sites of MgFe1.9Cr0.1O4 are found t o be theta(A)=515 +/- 5 K and theta(B)=265 +/- 5 K, respectively. Atomic mi gration of MgFe1.9Cr0.1O4 starts near 350 K and increases rapidly with incr easing temperature to such a degree that 50% of the ferric ions at the A si tes have moved over to the B sites by 600 K. The temperature dependence of both the magnetic hyperfine field and magnetization of MgFe1.9Cr0.1O4 is ex plained by the Neel theory of ferrimagnetism using three superexchange inte grals: J(A-B)=-17.9k(B), J(A-A)=1.9k(B), and J(B-B)=7.1k(B). (C) 2000 Ameri can Institute of Physics. [S0021-8979(00)40808-X].