Mössbauer study of aluminum-substituted hematites

A Mössbauer spectroscopic study is reported on a series of aluminum-substituted hematites, α(Fe 1- cAl c) 2O 3, with c up to 0.32. These samples were prepared by heating aluminum-substituted goethites, αFeOOH, at 500°C. X-ray line broadening gives particle dimensions of ≃200 Å to >1000 Å. Heating the samples to 900° improves crystallinity but reduces the maximum obtainable c to ≃0.15. At 77 K for c⩽0.04 the magnetic structure is antiferromagnetic with the spins aligning close to the (111) axis. For c⩾0.08 and for all compositions at 298 K the spins are perpendicular to (111) and for this weakly ferromagnetic phase the supertransferred hyperfine field is (5±1) kOe per magnetic neighbor. Samples with 0.04⩽ c<0.08 at 77 K have both magnetic phases present in varying proportions. The magnetic field at 298 K varies with aluminum content according to the 1/3-power law for the reduced sublattice magnetization. The asymmetric shape of the spectra for c⩾0.17 has been accounted for by a model based on the molecular field approximation with nearest neighbor superexchange interaction strength Jnn≃15 K.