Self‐reversal of magnetization in oceanic submarine basalts studied with XMCD

International audienceIn oceanic basalts, self‐reversal of magnetization can be produced during extreme low‐temperature oxidation of titanomagnetite by ionic reordering, which leads to Néel N‐type magnetism. Titanomaghemites showing N‐type reversal below room temperature were found in submarine basalts recovered during Ocean Drilling Program (ODP) Leg 197. In order to better understand the mechanism of self‐reversal, we carried out X‐ray magnetic circular dichroism (XMCD) at Fe K‐edge at room temperature and low‐temperature on such a titanomaghemite sample as well as on pure magnetite and maghemite samples. We found that the XMCD spectrum of the N‐type titanomaghemite at 20 K is a mirror image of the XMCD spectrum at 300 K, which shows that the octahedral and tetrahedral subnetworks reverse in this process. Ligand‐field multiplet calculations of XMCD at Fe K‐edge help identify the contributions of the different elements in the measured XMCD spectra. This mechanism could also cause self‐reversal above room temperature, which has important consequences for the reliability of paleomagnetic measurement