EXAFS Study of the Local Order in Metastable Cobalt and Iron Films

The knowledge of the crystallography of metastable cobalt and iron films prepared by deposition on suitable substrates is the key for the understanding of their unusual magnetic properties; their local structure is determined precisely by SEXAFS. This technique allows a measure of the asymmetry of the first shell radial distribution function in monometallic cobalt and iron films prepared by vapor condensation on a low temperature amorphous substrate. In both cases the measured profile allows to conclude the existence of true amorphous monometallic samples and compares well with the calculated one assuming a random packing of hard spheres. Following the evolution of the local order with increasing temperature we show that the cobalt amorphous films crystallize abruptly at a temperature dependent on their thickness, while the iron amorphous films crystallize slowly. Moreover EXAFS has been used to investigate the local structure of f.c.c. iron films which have puzzling magnetic properties. These metastable films grown on Cu(100) have a structure strongly dependent on their temperature of growth: Both the short range order and the amplitude of the thermal vibrations measured at room temperature are higher in films prepared at higher temperatures. These different crystallographic structures can be the origin of the conflicting results produced in the study of their magnetic properties.