High Density Nanostructured Soft Ferrites Prepared by High Pressure Field Assisted Sintering Technique.

The synthesis of highly compacted, nanostructured soft magnets is highly desirable due to their promising properties for the development of electronic devices working at frequency higher than 2 MHz. In this work we investigated the potentiality of High Pressure Field Assisted Sintering Technique (HP-FAST). To this aim, we first synthesized soft Mn-Zn ferrite magnetic nanoparticles (MNPs) through an easy-scalable, eco-friendly strategy based on aqueous co-precipitation in basic media, starting from transition metal chlorides. Powder X-ray diffraction (PXRD) and Transmission Electron Microscopy (TEM) analyses evidenced the formation of crystalline nanoparticles with the cubic spinel structure and average crystal size of 7.5 nm. Standard magnetometric measurements showed a saturation magnetization value of ca. 56 emu/g and no magnetic irreversibility at room temperature. The MNPs were then compacted applying an uniaxial pressure over a toroidal shaped die. In order to obtain a material with a density close to the bulk one, the as-prepared green toroids underwent either a classic sintering treatment, obtaining a microstructured system, or to High Pressure Field Assisted Sintering Technique (HP-FAST), which allowed for preserving the nanostructure. The relative permeability and core losses of the toroidal samples were evaluated in the frequency range 1-2 MHz using an in-house built setup. The comparison of the behavior of samples obtained by the two different sintering approaches showed the nanostructured samples had a much smaller relative magnetic permeability (ten times lower than the microstructured sample) and, consequently, higher core losses. However, when samples with similar were compared, a significant decrease of core losses at the larger frequencies was observed. This result suggests HP-FAST is a very promising approach to prepare high density nanostructured soft magnetic materials.