Permanent magnets are present everywhere in modern life and essential for our way of living. The highest performance magnets are made with rare-earth metals, but with the increasing price and scarcity of these metals, efforts have been made in synthesizing high performance rare-earth free magnets. Hexaferrites are of great interest because of high stability, high anisotropy and favorable cost/performance ratio.
This work focuses on the W-phase hexaferrite which excels with its high anisotropy constant, Curie temperature, and a higher saturation magnetization compared to other hexaferrites. This is believed to be caused not just by super-exchange coupling across one oxygen (Fe-O-Fe) but super-super-exchange across two oxygens (Fe-O-O-Fe). The unit cell is rather complex with dimensions of a=b= 5.91 Å and c= 32.75 Å.
In this work, SrZn2Fe16O27 was synthesized by a sol-gel autocombustion synthesis method using metal nitrates, citric acid and concentrated ammonia to prepare the precursor gel. The gel was sintered at four different sintering temperatures (1000, 1100, 1200, and 1300 ˚C) for two hours in a conventional oven. Almost phase pure SrZn2Fe16O27 was formed at temperatures above 1200 ˚C resulting in a significant increase of the saturation magnetization from 47 Am2/kg to 72 Am2/kg. The sample heated at 1000 ˚C has a coercivity of 292 kA/m, which was decreased to 6 kA/m when the SrZn2Fe16O27 was formed.
The coercivity is strongly dependent on particle size and it is reasonable that the powder sintered for 2 h at 1200 and 1300 °C have become multidomain particles due to particle growth resulting in a very limited coercivity.