Mark H. Burkhardt

Near edge x-ray absorption fine structure spectroscopy with x-ray free-electron lasers

We demonstrate the feasibility of Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy on solids by means of femtosecond soft x-ray pulses from a free-electron laser (FEL). Our experiments, carried out at the Free-Electron Laser at Hamburg (FLASH), used a special sample geometry, spectrographic energy dispersion, single shot position-sensitive detection and a data normalization procedure that eliminates the severe fluctuations of the incident intensity in space and photon energy. As an example we recorded the {sup 3}D{sub 1} N{sub 4,5}-edge absorption resonance of La{sup 3+}-ions in LaMnO{sub 3}. Our study opens the door for x-ray absorption measurements on future x-ray FEL facilities.

Interplay between intrinsic and stacking-fault magnetic domains in bi-layered manganites

We present a low temperature x-ray photoemission electron microscopy study of the bi-layered manganite compound La1.2Sr1.8Mn2O7 (BL-LSMO) to investigate the influence of stacking faults, which are structurally and magnetically different from the bi-layered host. In BL-LSMO, small magnetic moment persists to T*=300 K, well above the Curie temperature of 120 K (TC). Our magnetic images show that 3D stacking faults are responsible for the T* transition. Furthermore, close to the TC, stacking faults are well coupled to the bi-layered host with latter magnetic domains controlling the spin direction of the stacking faults. Contrary to recent reports, we find that stacking faults do not seed magnetic domains in the host via an exchange spring mechanism and the intrinsic TC of the BL-LSMO is not lower than 120 K.