Why does zinc substitution increase the saturation magnetization of some spinel ferrites?

In spinel ferrites, the saturation magnetization is determined by the ferrimagnetic alignment of metal cations across two distinct lattice sites: tetrahedral sites and octahedral sites. These sublattices possess opposing magnetic spins, and the net magnetization is the result of the incomplete cancellation of these moments.12

Zinc substitution increases the saturation magnetization of these materials because zinc cations ($Z{n}^{2+}$) have a strong preference for occupying tetrahedral sites. By occupying these sites, the zinc cations force additional trivalent iron cations ($F{e}^{3+}$) to migrate to the octahedral sites. This redistribution alters the balance of the opposing sublattices, resulting in less cancellation of magnetic spins and, consequently, a greater net saturation magnetization.1