Temperature dependence of the exchange stiffness in FePd(001) thin films: Deviation from the empirical law a (T) MS2 at intermediate temperatures

We study the temperature dependence of the exchange stiffness A (T) of a thin FePd(001) film, which exhibits well-ordered magnetic stripe domains. We employed two different methods: (i) applying an exact micromagnetic model of the nucleation point and (ii) a detailed analysis of the temperature dependence of the saturation magnetization. The experimental data needed as input for the theoretical model are the stripe domain width and the nucleation field, and these were obtained as a function of temperature using soft x-ray magnetic scattering and superconducting quantum interference device (SQUID) magnetometry, respectively. The temperature independence of the domain width is very important from the point of view of the energetics of the FePd film, i.e., the number of magnetic domains remains constant per unit area. From this experimental finding we infer that the exchange stiffness must vary as a function of the temperature. We show that the A (T) dependence obtained with the two procedures are consistent but different from the phenomenological law A (T) MS2 (T), normally assumed to be valid in the temperature range examined (T=50-250 K), an intermediate temperature range for which there is no known expression of the magnetization as a function of the temperature. We have also investigated the Fe3s core level and valence band of FePd using hard x-ray photoemission spectroscopy. The Fe3s spectra exhibit negligible changes in the temperature range investigated, while small changes occurring over large energy scales are observed in the valence-band spectra. Based on the lack of consistency of the magnetometry and scattering results compared to local-moment theories, these results favor an itinerant magnetism picture for FePd.