Control of surface morphology and internal structure in magnetite microparticles: from smooth single crystals to rough polycrystals

Abstract

Magnetite particles in the micrometer range have been obtained by the oxidative aging of ferrous hydroxide with KNO₃. The surface morphology and the number of crystallites that constitute each particle can be controlled by adjusting the Fe²⁺ excess in the reaction media. Thus, for a relatively low [Fe²⁺]_Exc we obtained smooth polyhedral single-crystal particles, whereas for larger [Fe²⁺]_Exc the particles had rough surfaces and a raspberry-like appearance due to their polycrystalline nature. The differences in the surface morphology of the particles are intimately related to the differences in the internal structure, which are the outcome of particular growth mechanisms. These mechanisms of particle formation can therefore also be controlled and can be qualitatively explained in terms of the interparticle electrostatic interactions after the initial nucleation. Magnetic properties were also connected to the internal structure of the particles. Because of the relatively large size of the crystalline domains, magnetization reversal took place by magnetic domain wall motion and all the particles we obtained were magnetically soft at room temperature. At 5 K the more complex structure of the rough particles resulted in a larger coercivity.