Modification of the Structure and Magnetic Properties of Cobalt-Doped Ferrihydrite Nanoparticles Under Heat Treatment

Nanoparticles of antiferromagnetic materials acquire the magnetic moment due to the surface effects and structural defects. According to the Neel hypothesis, magnetic moment μ _P of a particle containing N magnetically active atoms with magnetic moment J can be estimated as μ _P ∼ J · N ⁿ or μ _P ∼ V ⁿ , where V is the particle volume. Numerous studies of the magnetic properties of ferrihydrite 5Fe₂O₃⋅9H₂O and ferritin revealed a value of n ≈ 1/2 for this material, in which Fe atoms have the octahedral surrounding of anions. We investigate the effect of low-temperature annealing of cobalt-doped ferrihydrite nanoparticles on their average size and magnetic properties. Using the Mössbauer spectroscopy study, we demonstrate that doping with Co makes Fe atoms enter the anion tetrahedra which leads to an increase in the exponent n〉1/2 in the expression μ _P ∼ J · N ⁿ .