Bat
Microbats make use of magnetoreception, in that they have a high sensitivity to the Earth's magnetic field, as birds do. Microbats use a polarity-based compass, meaning that they differentiate north from south, unlike birds, which use the strength of the magnetic field to differentiate latitudes, which may be used in long-distance travel. The mechanism is unknown but may involve magnetite particles.
Magnetite
Biomagnetism is usually related to the presence of biogenic crystals of magnetite, which occur widely in organisms. These organisms range from bacteria (e.g., "Magnetospirillum magnetotacticum") to animals, including humans, where magnetite crystals (and other magnetically-sensitive compounds) are found in different organs, depending on the species. Biomagnetites account for the effects of weak magnetic fields on biological systems. There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).