Severe reduction of Ni–Zn ferrites during consolidation by Spark Plasma Sintering (SPS)

• Nanostructured ferrites are obtained at 600°C by 10 min.
• Magnetic properties change significantly.
• These changes are due to reductive conditions in SPS.
• Metallic Ni precipitated from the spinel and are replaced by ferrous ions.
• This leads to an increase in Ms and Tc.

NiZn ferrites of composition Zn0.7Ni0.3Fe2O4 were synthesized by the polyol method and subsequently consolidated into fine grain, high density ceramics by means of Spark Plasma Sintering at 600 °C for 10 min, under vacuum and applying a uniaxial pressure of 80 MPa. Their saturation magnetization at room temperature exhibited a value far larger (~71 emu/g) than the value corresponding to the bulk ferrite (~50 emu/g), and their Curie point showed also an increase of about 210 K, as compared with the bulk value. These results, together with the presence of a small amount of metallic Ni, are interpreted in terms of a departure of Ni from the spinel phase and an associated reduction of ferric to ferrous cations to compensate for Ni loss.