Structural disorder in the high-energy milled magnesium ferrite

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mossbauer spectroscopy. It is f ound that the nanostructural state of the milled MgFe2O4 is characterized b y a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites i n the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4 ) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distr ibution curves of mechanically treated samples show additional components a t smaller magnetic fields. The weight of the B(6) field decreases with incr easing milling time, and the B(5) field becomes the most probable hyperfine field component in the distribution curve of the mechanically activated sa mples. The degree of inversion in MgFe2O4 is calculated from the probabilit ies of the different [B]-site surroundings as well as from the Mossbauer su bspectral areas. Excellent agreement is obtained in the two independent pro cedures for the determination of the cation distribution. This enables us t o separate from the [B]-site magnetic field distribution profile the contri bution arising from the mechanically induced "new" nearest-neighbor (A)-sit e configuration with n=3 nearest (A)-site iron neighbors. Taking into accou nt the nanoscale nature of the mechanically activated MgFe2O4, the observed spin canting, which increases with increasing milling time, is attributed to the noncollinear spin structure of the near-surface atoms. In strongly a ctivated ferrite, the magnetic hyperfine splitting breaks down totally and the Mossbauer spectrum is dominated by a superparamagnetic relaxation effec t. (C) 2000 American Institute of Physics. [S0021-8979(00)05322-6].