Grazing incidence X-ray reflectivity: a new experimental approach to the martensitic surface relief

X-ray specular reflectivity is presented as a highly sensitive, sample-averaging technique well suited for the quantitative investigation of martensitic transformations at surfaces. Both premartensitic behavior as well as the fully developed surface relief can be studied due to an inherent sensitivity to roughness ranging from the nanometer to the micrometer scale. We discuss the information content of the method, exemplified by data from Ni–37.5at.%Al and Ni2MnGa single crystals and polycrystalline Ni–49.2at.%Ti with the martensite induced by cooling. For Ni–37.5at.%Al and Ni2MnGa, the surface relief is manifest in the total reflection regime as a second critical angle, indicative of mesoscopic relief heights and low apparent surface coverage. Temperature-dependent electron density profiles are derived for the Ni–37.5at.%Al surface relief from fits to the data. The Ni2MnGa relief is shown to build up in two distinct steps 40 K apart, with no analogue in the bulk. By contrast, the Ni–49.2at.%Ti spectra mirror consistently the bulk behavior and at low temperatures show only a strong exponential decay of the reflectivity below the critical angle.