Superparamagnetic blocking and magnetic interactions in nanoferrihydrite adsorbed on biomineralized nanorod-shaped F3S4 crystallites

A composite based on nanorod-shaped greigite (Fe₃S₄) crystallites with adsorbed ferrihydrite (Fe₂O₃ ‧ nH₂O) nanoparticles has been synthesized. The synthesis has been performed by biomineralization of the bacterial wall of a sulfate-reducing Desulfovibrio sp. A2 bacteria. The phase composition of the synthesized composite has been investigated by X-ray powder and electron diffraction, as well as Fourier-transform infrared, extended X-ray absorption fine structure, and Mössbauer spectroscopy. The magnetic measurement data have shown that the sample under study contains two magnetic phases: multidomain nanorod-shaped greigite and ultrasmall ferrihydrite nanoparticles. The constant atomic fraction of the greigite crystalline phase in the range of 4–300 K (~20%) revealed by Mössbauer spectroscopy is indicative of a blocked magnetic moment of nanorod-shaped Fe₃S₄. It is shown that nanorod-shaped Fe₃S₄ crystallites are strongly magnetically bound with adsorbed Fe₂O₃ ‧ nH₂O (E_int ~ 1200k_B) nanoparticles. This significantly slows down the superparamagnetic relaxation of the magnetic moments of ferrihydrite nanoparticles. Therefore, the blocking temperature noticeably increases and attains, according to the Mössbauer spectroscopy data, a value of T_B = 140 K (the magnetic measurements yield T_B = 72 K). The processes of superparamagnetic blocking of the magnetic moments of ferrihydrite nanoparticles manifest themselves in the evolution of the magnetic properties of the investigated sample (a significant increase in the coercivity and remanent magnetization). In support of the Mössbauer spectroscopy data, a sufficiently high superparamagnetic blocking temperature has been established, which discloses the effect of magnetizing of ferrihydrite nanoparticles by coarser greigite formations, analogously to the effect of interparticle magnetic interactions.