Why is magnetite strongly magnetic while hematite is only weakly magnetic?

The difference in magnetic behavior between magnetite and hematite arises from their distinct crystal structures and the arrangement of their iron ions.

Magnetite: Ferrimagnetism

Magnetite (Fe${}_3$O${}_4$) is a ferrimagnetic material. Its structure is an inverse spinel, containing both iron(II) (Fe${}^{2+}$) and iron(III) (Fe${}^{3+}$) ions. Within the crystal lattice, the magnetic moments of the iron ions align in two sublattices that point in opposite directions (antiparallel). Because the magnetic moments of these sublattices are unequal in size, they do not cancel each other out completely, resulting in a strong net magnetic moment. This allows magnetite to act as a natural magnet.1234

Hematite: Weak Magnetism

Hematite (Fe${}_2$O${}_3$) contains only iron(III) (Fe${}^{3+}$) ions and is the fully oxidized form of iron oxide. Its crystal structure is tightly packed and layered, similar to corundum. In hematite, the magnetic moments of the iron ions are arranged such that they largely cancel each other out, leading to only weak magnetism compared to the ferrimagnetic alignment found in magnetite.132