From conventional presintering to spark plasma presintering: A comparative study on phase stability, densification, and magnetic properties of spinel ferrites

Presintering is commonly used to improve the density and properties of ceramics; however, its prolonged thermal treatment undermines the time- and energy-saving advantages of spark plasma sintering (SPS). This study investigates whether conventional pressureless presintering (CPP) can be omitted or replaced by an integrated spark plasma presintering (SPP) approach in the fabrication of spinel ferrites, and establishes correlations between the presintering method and the resulting densification, phase composition, oxygen vacancy concentration, and magnetic properties. Li 0.5 Fe 2.5 O 4 (LF), NiFe 2 O 4 (NF), Li 0.4 Zn 0.2 Fe 2.4 O 4 (LZF), and Ni 0.4 Zn 0.6 Fe 2 O 4 (NZF) were prepared via two routes: CPP followed by SPS, and SPP followed by SPS. For LZF and NZF, SPP led to higher final density and microhardness (∼4.9 g cm −3 , ∼1000 HV 1 ), whereas LF and NF showed lower densification due to insufficient activation (<4.5 g cm −3 , <900 HV 1 ). X-ray photoelectron spectroscopy (XPS) revealed that SPP reduced the Fe 2+ /Fe 3+ ratio relative to CPP, owing to the low oxygen partial pressure and short dwell time that suppress lithium loss and oxygen vacancy formation. Raman spectroscopy confirmed lower α-Fe 2 O 3 retention in the conventionally presintered (CPP-ed) samples. Saturation magnetization ( M s ) was consistently higher for the spark plasma presintered (SPP-ed) samples, with increases of more than 10%. This enhancement is attributed to the lower Fe 2+ content, reduced porosity, and minimal carbon contamination in the SPS-ed samples. These findings demonstrate that CPP can be successfully replaced by SPP stage for Zn-doped spinel ferrites, offering a simplified processing route with comparable or improved magnetic and mechanical properties. For non-Zn compositions, further optimization of SPP parameters is required to achieve equivalent performance.