https://www.ncbi.nlm.nih.gov/pmc/articles/PMC49775/pdf/pnas01090-0405.pdf

“In the microorganisms (10, 11) and fish (15), linear chains of membrane-bound crystals of magnetite (magnetosomes) form structures best described as “biological bar magnets.”

We report here that human tissues possess similar crystals
of biogenic magnetite, with minimum estimates between 5
and 100 million single-domain crystals per gram in the tissues
of the human brain. Magnetic particle extracts from solubi-
lized tissues examined with high-resolution transmission
electron microscopy (TEM) and electron diffraction identify
minerals in the magnetite-maghemite solid solution, with
many crystal morphologies and structures resembling those
precipitated by magnetotactic bacteria and fish.”