Size Distribution and Frustrated Antiferromagnetic Coupling Effects on the Magnetic Behavior of Ultrafine Akaganeite (β-FeOOH) Nanoparticles

The magnetic properties of low dimensional materials of several iron oxyhydroxide phases, such as akaganeite (beta-FeOOH) or lepidocrocite (gamma-FeO(OH)), remain poorly explored, probably due to their specific preparation as single crystalline phase requires special conditions owing to their structural instability. In the present work, ultrafine akaganeite nanoparticles were prepared by the hydrolysis of FeCl3 solutions at room temperature induced by the presence of NaOH. The resulting product was characterized by several analytical techniques. Structural investigations using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) revealed that the sample was mainly constituted by rather-equiaxial akaganeite nanocrystals with mean diameter of 3.3 +/- 0.5 nm. In addition, a small amount of rodlike akaganeite particles with 23 +/- 5 nm in length and 5 +/- 1 nm in width was also detected. The study of the respective dependences of the dc magnetization and the ac susceptibility on temperature and exciting magnetic field revealed complex magnetic relaxation processes, high coercivity values at low temperature, and exchange bias effect. These results have been tentatively explained considering size distribution effects and the presence of superparamagnetic and spin glass-like contributions arising from the frustration of the antiferromagnetic order owing to surface effects and an insufficient filling of the akaganeite channels with Cl anions.