We have shown earlier [1,2] that magnetic linear dichroism in x-ray absorption is a valuable tool for the study of antiferromagnetic oxides, such as NiO and LaFeO3, and of their coupling to ferromagnets. Here, we will present new results, showing that magnetic linear dichroism contrast also arises from magnetically ordered metal films and can be used in domain imaging with X-ray Photoemission Electron Microscopy (X-PEEM). The figure below shows XMLD (left) and XMCD (right) images of 3 nm Fe grown on NiO(001). Different domain orientations are visualized by different colors. Note the correspondence of bright domains in the XMLD image and black/white domains in the XMCD image. This finding opens the door for the investigation of the magnetic structure of technologically important, metallic antiferromagnets, such as Mn alloys.

Models of exchange bias put different emphasis on certain features in the magnetic microstructure of the interface between the ferromagnet and the antiferromagnet. Some stress the role of a) planar or b) vertical domain walls whereas others emphasize the role of c) uncompensated interface spins. Using X-ray Photoemission Electron Microscopy and X-ray Spectroscopy using X-ray Magnetic Dichroism, we will show that all three play a role, although planar walls only appear in soft antiferromagnets like single crystalline NiO [1]. Lateral walls enhance bias as will be shown in a statistical analysis of the domain-size-dependent bias field in LaFeO3 [2]. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L[Formula: see text] edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L[Formula: see text] edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.

When materials are exposed to X-ray pulses with sufficiently high intensity, various nonlinear effects can occur. The most fundamental one consists of stimulated electronic decays after resonant absorption of X-rays. Such stimulated decays enhance the number of emitted photons and the emission direction is confined to that of the stimulating incident photons which clone themselves in the process. Here we report the observation of stimulated resonant elastic (REXS) and inelastic (RIXS) X-ray scattering near the cobalt L3 edge in solid Co/Pd multilayer samples. We observe an enhancement of order 106 of the stimulated over the conventional spontaneous RIXS signal into the small acceptance angle of the RIXS spectrometer. We also find that in solids both stimulated REXS and RIXS spectra contain contributions from inelastic electron scattering processes, even for ultrashort 5 fs pulses. Our results reveal the potential and caveats of the development of stimulated RIXS in condensed matter.