In a first step, we shall review the basic concepts of X-ray magnetic and dichroic spectroscopies such as X-ray Magnetic Circular Dichroism (XMCD), X-ray Natural Circular Dichroïsm (XNCD), X-Ray Natural Linear Dichroic (XNLD) spectroscopies with special attention to XMCD. We shall explore two types of applications, a first one related to molecular magnetism and a second one to paleomagnetism. Finally, we might address the question of optical activity in the X-ray range. Magnetic materials interfaced with superconductors reveal new physical phenomena with potential for quantum technologies. We have investigated submonolayers of single molecule magnets (SMM) deposited on a superconducting lead surface [1,2]. The SMMs can be a tetrairon(iii) propeller-shaped molecule [3,4] or a terbium(III) bis(phthalocyaninato) complexe . These material combinations reveal a strong influence of the superconductor on the spin dynamics of the single molecule magnets (SMM). By applying XMCD at sub-Kelvin temperatures, it is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime.
In oceanic basalts, self-reversal of magnetization can be produced during extreme lowtemperature oxidation of titanomagnetite by ionic reordering, which leads to Neel 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 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 measurements .
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