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].