Fluorescence-detected X-ray magnetic circular dichroism of well-defined Mn(II) and Ni(II) doped in MgO crystals: credential evaluation for measurements on biological samples.

L(2,3)-edge X-ray magnetic circular dichroism (XMCD) spectra have been measured for the well-defined dilute Ni(II) and Mn(II) ions doped into a MgO crystal, with sub-Kelvin dilution refrigerator cooling and 2 T magnetic field magnetization. A 30-element Ge array X-ray detector has been used to measure the XMCD for these dilute ions, whose concentrations are 1400 ppm for Ni(II) and 10,000 ppm for Mn(II). Large XMCD effects have been observed for both Ni(II) and Mn(II), and multiplet simulation described the observed spectra. The fluorescence-detected L-edge absorption spectrum and XMCD of Ni(II) in MgO are comparable with both theoretical calculations and the total electron yield measured ions in similar chemical environments, at least qualitatively validating the use of the sensitive fluorescence detection technique for studying XMCD for dilute 3d metal ions, such as various metalloproteins. Sum rule analyses on the XMCD spectra are also performed. In addition, these XMCD measurements have also been used to obtain the sample's magnetization curve and the beamline's X-ray helicity curve. This study also illustrated that bend magnet beamlines are still useful in examining XMCD on dilute and paramagnetic metal sites.