The magnetic property of $\mathrm{Fe}{}_3\mathrm{O}{}_4$ (magnetite) is

In $\mathrm{Fe}{}_3\mathrm{O}{}_4$ the magnetic moments of the ions are aligned in parallel and antiparallel directions in unequal numbers, giving a net moment. This is ferrimagnetism; on heating above its Curie temperature it becomes paramagnetic.