Conversion electron Mossbauer spectroscopic study of ion-beam mixing at Fe-Mo interface

Abstract

Ion beam induced atomic mixing at Fe-MO interface has been studied by using the technique of conversion electron mossbauer spectroscopy (CEMS). Use has been made of a thin (~50 Å) layer of Fe⁵⁷ isotope (enriched to 95.4%) at the interface to obtain the mossbauer information selectively from this region. A noninterface sensitive measurement has also been performed to reveal the magnetic hyperfine interactions in the entire region of the iron overlayer. It is shown that a deposition induced reaction between the molybdenum substrate (having a thin coating of native oxide) and the Fe⁵⁷ layer renders a graded nature to the interface, which is transformed upon ion bombardment (100 keV Kr⁺, dose ~10¹⁶ ions/cm²) into a disordered alloy. The dominant nonmagnetic component corresponding to the interface of the ion beam mixed sample happens to be a quadrupole doublet, which represents the presence of Fe⁵⁷ atoms in Fe²⁺ charge state. The appearance of this contribution is attributed to formation of an oxygen coordinated ternary compound in the interface layer during ion bombardment. On the basis of the comparison of the results of the interface-sensitive and non-interface-sensitive studies it is established that the inclusion of oxygen in the ion mixed sample is mainly confined to near interface region and that the region of the overlayer contains a metastable alloy of the binary Fe-Mo system. The influence of thermal annealing at various temperatures between 200°C and 500°C on the ion beam mixed state has also been studied by monitoring the changes in the hyperfine interaction parameters. It is shown that the oxygen-incorporated regions of the disordered alloy retain the nonmagnetic Fe²⁺ charge state subsequent to annealing at 500°C, while other regions lead to precipitation of α Fe and Fe₂Mo phases.