Temperature dependence of the circular magnetic x-ray dichroism in the Invar alloy Fe₇₂Pt₂₈

The method of spin-dependent absorption has been used to examine the temperature dependence of the circular magnetic x-ray dichroism (CMXD) of the (disordered) Invar alloy fcc Fe₇₂Pt₂₈. At the Fe K edge the difference of the transmission for the reversed magnetization direction was determined with an accuracy of 10^-5. By comparing the dichroic effects at 110 and at 410 K evidence was found for a temperature-dependent change of the magnetic absorption profile different from the pure amplitude reduction due to the reduced magnetization. It indicates a change of the spin splitting in the low-lying unoccupied p states. The polarization-averaged absorption spectrum shows also a small variation between 4 and 11 eV above the threshold indicating that the p-like electron density drops when the sample is heated. The experimental CMXD profiles are compared with spin-polarized band-structure calculations for ordered Fe₃Pt and discussed within the frame of the model of high-spin-low-spin transitions in the Invar regime.