The magnetic structure and hyperfine field of goethite ($\alpha$-FeOOH)

Abstract

The magnetic structure of goethite (α-FeOOH) has been determined by Mössbauer and neutron powder diffraction techniques using both natural and synthetic samples. The antiferromagnetic structure, which exists below the Néel point of (130±2) °c, is collinear and the magnetic and chemical unit cells are the same; a = 4587, b = 9937 and c = 3015 Å, space group Pbnm. The iron atoms related by the b-glide plane are coupled ferromagnetically and are antiferromagnetically coupled to the remaining iron atoms. The spins lie parallel to the c axis, and the major axis of the electric field gradient was found to lie in the ab plane, making an angle of about 40° with the b axis. No evidence was found for the existence of more than a single hyperfine field in the temperature range between 42 °K and room temperature: the value of 504 kG at low temperatures is discussed in relation to the covalency of a number of ferric compounds. The exchange interactions in the oxyhydroxide are related to the magnetic structure; the observed behaviour of the susceptibility above the Néel point is qualitatively explained by the existence of residual magnetic order.