Magnetic properties of needle-like α-FeOOH and γ-FeOOH nanoparticles

We produced needle-like goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) nanoparticles and investigated their magnetic properties. Both goethite and lepidocrocite nanoparticles were found to have an orthorhombic structure with lattice constants of a = 4.642, b = 9.945, and c = 3.059 Å and a = 3,388, b = 12.517, and c = 3.067 Å, respectively, which were consistent with their corresponding bulk values. We observed Neel temperatures at 252 and 53 K for goethite and lepidocrocite nanoparticles, respectively, which were lower than their corresponding bulk values. We found that the magnetic properties of goethite and lepidocrocite nanoparticles were quite different. In addition, we observed unusual magnetic behaviors in both goethite and lepidocrocite nanoparticles. These include an additional phase transition near 12.5 K and unusual temperature dependences of both M _r and H _c in goethite nanoparticles and hysteretic magnetizations, unlike those in antiferromagnetic materials, in lepidocrocite nanoparticles. We think that these unusual magnetic behaviors are related to the defect structures, such as vacancies in goethite nanoparticles and splits and snaps in lepidocrocite nanoparticles, observed in the transmission electron microscope micrographs.