Magnetic structure of thin films and multilayers as seen by nuclear resonant scattering of synchrotron radiation

Grazing incidence nuclear resonant scattering experiments performed on thin films and multilayers combine the sensitivity of Moessbauer spectroscopy to hyperfine interactions (HFI) with the depth information yielded by reflectometric methods. With the advent of high luminosity synchrotron radiation (SR) sources, efficient Moessbauer reflectometry (MR) has become possible. The talk reviews the principles and applications of this technique (Synchrotron Moessbauer Reflectometry, SMR) to the investigation of the magnetic structure of thin films and multilayers. The full information on the depth profile of HFI is contained in the time- and angle-dependent reflectivity of the stratified sample (time differential SMR). Time integral SMR, a rapid method similar to neutron reflectometry gives considerably less detailed information. The strong angle dependence of the penetration depth of SR close to the critical angle of the non-resonant total external reflection results in an inherent depth selectivity of SMR down to about 2 m. This can be used for depth selective phase analysis as it is demonstrated on partially oxidised 57 Fe films. In FePd films of perpendicular magnetic anisotropy the presence of surface closure domains is investigated with the same method. Monolayer depth resolution is achieved using selective deposition of the resonant isotope. With this technique, a non-collinear magnetic structure is found in fcc Cu/Co/Fe/CoCu films. Pure nuclear superreflections and their suppression in high external magnetic field are observed in antiferromagnetically coupled Fe/FeSi and Fe/Cr multilayers. In low magnetic fields finite-stacking effects are seen and the bulk-spin-flop transition is observed in coupled multilayers of fourfold anisotropy. Off-specular scattering in the same systems reveals a history-dependent antiferromagnetic domain structure. (author)