Mössbauer studies of spin flop in antiferromagnetic hematite

The antiferromagnetic sublattice magnetizations below the Morin transition temperature T_M≈260°K, are parallel and antiparallel to the trigonal axis of rhombohedral α-Fe₂O₃. Experiments at 80°K, using the Mössbauer technique together with high external magnetic fields on both polycrystalline and single-crystal samples, show that the directions of the sublattice magnetizations flop from along the trigonal axis for the applied field H₀<H_c to perpendicular to the trigonal axis for H₀>H_c; H_c is found to be 67.5±3 kOe. In the polycrystalline sample this behavior is exhibited through the occurrence of a maximum in the width of the outer lines of the Mössbauer spectrum plotted as a function of H₀. In the single crystal, which is oriented so that the gamma rays propagate along the [111] direction, the magnetic transition is clearly observed as a sudden qualitative as well as quantitative change in the appearance of the Mössbauer hyperfine spectrum; below the critical field the Δm=0 lines are the strongest components of the six-line spectrum.