Effect of milling-induced disorder on the lattice parameters and magnetic properties of hematite

SILVANA JACQUELINE STEWART; R. A. BORZI; E. D. CABANILLAS; G. PUNTE; R. C. MERCADER

Journal of Magnetism and Magnetic Materials

Año: 2003 vol. 260 p. 447 - 454

0304-8853

A ball-milling treatment in air for 30 min is enough to reduce anhydrous bulk hematite (a-Fe2O3) grains to nanometric sizes. For milling times, tm, of 15, 30 min, 1 and 2 h, the crystals suffer an anisotropic lattice dilation, which is more pronounced for the smaller average grain sizes attained. M.ossbauer and susceptibility results show that the process alters the effective Morin temperatures, TM. The transition occurs less sharply than for the non-milled materialand spreads over a maximum extent of 50K for tm=30 min. The susceptibility data indicate that the TM for the fraction of material undergoes the transition shifts toward values not lower than 235 K. For tm=10 h the transition is suppressed down to 12 K. Our results do not allow distinguishing bulk from surface regions of the grains. In addition, the disorderbrought about by the milling reduces the magnetic response in the weak ferromagnetic state of a-Fe2O3. The modification of TM seems to depend mainly on the anisotropic dilation of the unit cell that affects the whole grain and it is related to the nanometric grain sizes achieved.