Depth-Sensitive Grazing Incidence Crystallography: From Atomic to Mesoscopic Scale In-plane Structures*

A high brilliance neutron source, the second target station at the Spallation Neutron Source at ORNL, provides unique conditions for experiments on small thin-film samples with complex structures for wave vector transfer extended from the total reflection region to inverse interatomic distances. The latter scales are accessible with grazing incidence diffractometry with its extreme sensitivity to near surface and interfacial phenomena, as well as to nuclear and magnetic scattering length density distributions across the thickness of thin film heterostructure. Such sensitivity is absent in conventional diffractometry. The solution to access the in-plane atomic scale structures with the depth sensitivity implemented in the M-STAR reflectometer design [1] is a new option specialized for measurements of the grazing incidence lateral diffraction.  This option will combine diffractometry with reflectometry using the same beam-forming and polarization set up, the advantages of ToF mode, as well as the depth-sensitivity.  In view of the remarkable possibilities provided by the GID, it is also important to note that out-of-plane magnetization is not accessible in conventional reflectometry. This property is essential for studying systems with perpendicular anisotropy, as well as with an anti-ferromagnetic structure. Another important property is that the coherence length for GID reduces in any direction to the value of the order l/Dw (where w is azimuthal angle), which is much smaller than the characteristic size of ferromagnetic domains. This means that the GID is kind of local probe for domain magnetization: the diffraction intensity of each individual domain adds up incoherently. A feasibility of GID to study multi-domain state in ferromagnetic thin film heterostructure was demonstrated in the pilot experiments. It has confirmed theoretical expectations and sufficient luminosity of the method.