Effect of surface composition observed by Auger electron spectroscopy on magnetization and magnetostriction of NiFe and NiFeRh thin films

The saturation magnetization, 4πMs, and the magnetostriction constant, λs, of Ni81Fe19, Ni81Fe19/Ta, and Ni72Fe17Rh11/Ta thin films were studied as a function of film thickness before and after annealing at 250 °C in vacuum for 2 h. For films of thickness t<200 Å, 4πMs and λs were found to be strongly dependent on film thickness with an even larger variation after annealing. For all films, at t=60Å, 4πMs decreases by 20%-30% compared to bulk value. The value of λs is in the range of 10-6 for all films and shows a complex behavior, including a change of sign, depending on film composition as well as Ta capping. Auger depth profiling of uncapped films has shown the presence of an Fe-rich surface oxide layer whose composition changed and thickness increased after annealing. The composition and thickness of the Ta-film interface layer in the Ta-capped films did not change with annealing. The presence of such layers, at the film surface, Ta-film interface, and film-substrate interface, of different composition and magnetic properties from the film bulk, can explain the observed behavior of 4πMs and λs.