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-aver aging technique well suited for the quantitative investigation of martensit ic transformations at surfaces. Both premartensitic behavior as well as the fully developed surface relief can be studied due to an inherent sensitivi ty to roughness ranging from the nanometer to the micrometer scale. We disc uss the information content of the method, exemplified by data from Ni-37.5 at.%Al and Ni2MnGa single crystals and polycrystalline Ni-49.2at.%Ti with t he martensite induced by cooling. For Ni-37.5at.%Al and Ni2MnGa, the surfac e relief is manifest in the total ref;lection regime as a second critical a ngle, indicative of mesoscopic relief heights and low apparent surface cove rage. Temperature-dependent electron density profiles are derived for the N i-37.5at.%Al surface relief from fits to the data. The Ni2MnGa relief is sh own to build up in two distinct steps 40 K apart, with no analogue in the b ulk. By contrast, the Ni-19.2at.%Ti spectra mirror consistently the bulk be havior and at low temperatures show only a strong exponential decay of the reflectivity below the critical angle. (C) 1999 Elsevier Science S.A. All r ights reserved.