Heat-generation behavior of Fe3O4 particles in AC magnetic fields: analysis of microstructures through tilting†

Abstract

Magnetic field-dependent magnetization of highly crystalline Fe₃O₄ magnetic nanoparticles has been carried out to understand surface canting structures at low and room temperatures. The exchange bias (H_EB) values of ∼18 to 27 Oe at 300 K for three samples prepared from different precursors are observed; and a decrease in value is obtained when the samples are measured at 5 K. However, with a decrease in temperature, coercivity (H_c) increases. This is related to an increase in the percentage of magnetization in the core of the particles and a decrease in the percentage of antiferromagnetic contribution on the surface of the particles when the sample is cooled from 300 K to 5 K. At low concentrations of magnetic particles and low power of the alternating current (AC) magnetic field, heat generation from the three samples is found to vary. In the case of high concentrations of magnetic particles and high power of the AC magnetic field, heat generation from the samples is almost the same. This is due to the saturation of AC magnetic field absorption by the magnetic nanoparticles at such high power and high concentration, irrespective of the samples. These findings are very interesting. The microstructures of the magnetic nanoparticles are studied through tilting from 0° to 14°. Agglomeration, non-agglomeration, porosity, etc. can be distinguished. From dark field (DF) and bright field (BF) images, we were able to resolve many mysterious microstructures, without which many properties would be interpreted wrongly.