CHEMICAL REMANENT MAGNETIZATION DURING GAMMA-FEOOH PHASE-TRANSFORMATIONS

We have studied chemical remanent magnetization (CRM) accompanying the phase transformations lepidocrocite (gammaFeOOH) --> maghemite (gammaFe2O3) --> hematite (alphaFe2O3) in a field of 50 muT at a series of 13 temperatures from 200 to 650-degrees-C. CRM intensity peaked after the 250-degrees-C run as paramagnetic lepidocrocite transformed to ferrimagnetic maghemite of single-domain size. After the 275, 300 and 350-degrees-C runs, CRM intensity decreased about an order of magnitude, probably because of the formation of antiphase domains with oppositely coupled magnetizations in the maghemite. A second CRM peak after the 400-degrees-C run is thought to reflect the growth of hematite on antiphase boundaries, breaking the negative coupling and allowing maghemite antiphase domains to align individually with the field. After runs at 450-degrees-C and above, CRM intensities were very low and the CRM consisted of two vectors, sometimes reversed in direction to each other and always at a large angle to the field. The explanation seems to lie in perpendicular coupling of the magnetic sublattices of maghemite and hematite.