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

Abstract

A ball-milling treatment in air for 30 min is enough to reduce anhydrous bulk hematite (α-Fe 2O 3) grains to nanometric sizes. For milling times, t m, 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. Mössbauer and susceptibility results show that the process alters the effective Morin temperatures, T M. The transition occurs less sharply than for the non-milled material and spreads over a maximum extent of 50 K for t m=30 min. The susceptibility data indicate that the T M for the fraction of material undergoes the transition shifts toward values not lower than 235 K. For t m=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 disorder brought about by the milling reduces the magnetic response in the weak ferromagnetic state of α-Fe 2O 3. The modification of T M 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.