Transmission X-Ray Diffraction and Mössbauer Spectroscopy Study of Annealed Magnetoresistive Ag/Ni8iFei9 Multilayers

Abstract

ABSTRACTWe have studied the variation of the structural and magnetic properties of 5-μm thick self-supported Ag/Ni81Fe19 multilayers upon annealing by means of transmission X-ray diffraction and Mössbauer spectroscopy as well as magnetoresistance (MR) and magnetisation measurements.In the virgin state, X-ray experiments show that both Ag and NiFe are (111) textured; the (220) in plane distances are 1.3 % smaller for Ag and 2 % larger for NiFe than the bulk values. Mässbauer spectra consist of a pure magnetic sextuplet, meaning that no non-magnetic layer is present at interfaces. The hyperfine field at 4.2 K is found only 3 % smaller than the bulk value and the Curie temperature is estimated from both Mossbauer and magnetisation at about 550 K.Upon annealing, the (220) Ag distance starts increasing above 250°C, whereas the decrease of the (220) NiFe distance starts at a much lower temperature. The densification of the NiFe layers is accompanied by an increase of the Curie temperature, without any effect on the magnetic moment; both this relaxation and the improved structural quality of the layering are responsible for the increase of the MR ratio from 12 % to 16 % after annealing at 240°C. Although the multilayer structure improves up to 320°C, the appearance of localised defects above 250°C leads to a rapid increase of the remanent magnetisation and to a decrease of the MR ratio.