some Mg2+ ions can enter in Td sites of the spinel structure. Because at 0K the magnetic moment of a spinel ferrite can be calculated according to the colinear model of Neel by:
Sum of magnetic moments of ions located in Oh sites - Sum of magnetic moments of ions located in Td sites,
the total magnetic moment at saturation can be increased if the second term related to Td ions is decreased (i.e. when there are some ions without magnetic moment).