Magnetic signature of compositional gradient in exchange-spring bilayer films of CoPt/Co

Abstract

Experimental studies were performed on model bilayers of fully ordered L10 CoPt with thicknesses of 25, 50, or 100 nm, capped by Co of varying thicknesses from 2.6–75 nm. The bilayers were subjected to different annealing conditions; transmission electron microscopy and x-ray diffraction supplied data concerning the crystallographic structure, microstructure, and texture of the constituent phases in the bilayers. The degree of exchange coupling was investigated primarily with magnetic measurement and magnetic force microscopy. Samples subjected to annealing treatments at 300 °C showed evidence of grain growth and increased texture of the hexagonally close-packed (hcp) cobalt layer relative to the as-deposited condition. The degree of exchange coupling in these materials improved only slightly with annealing, as evidenced by both magnetic recoil curve measurement and magnetic force microscopy. Samples subjected to a higher-temperature annealing treatment of 550 °C evidenced dramatic improvement in the magnetic coupling, an improvement attributed to interdiffusion of CoPt and Co and the formation of alloy phases, specifically face-centered-cubic and hcp Co–Pt solid solutions.