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

Abstract

The magnetic structure of goethite (?-FeOOH) has been determined by M?ssbauer and neutron powder diffraction techniques using both natural and synthetic samples. The antiferromagnetic structure, which exists below the N?el point of (130?2) ?c, is collinear and the magnetic and chemical unit cells are the same; a = 4?587, b = 9?937 and c = 3?015 ?, 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 4?2 ?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 N?el point is qualitatively explained by the existence of residual magnetic order.