Interdiffusion in bilayer CoPt/Co films: potential for tailoring the magnetic exchange spring

The magnetic and microstructural properties and magnetic domain configuration of exchange-spring model bilayer samples based on L1(0)-type magnetically-hard CoPt and magnetically soft Co were examined and analyzed. Bilayers of varying thicknesses and annealing conditions were examined with room-temperature and elevated-temperature SQUID magnetometry, X-ray diffraction, transmission electron microscopy and magnetic force microscopy. While lower-temperature (300 degreesC less than or equal toT less than or equal to 400 degreesC) annealing treatments produced little change in the domain configuration, it did produce subtle change in the microstructure and a noticeable increase in the degree of exchange coupling of the bilayers. Higher-temperature (T = 550 degreesC) annealing treatments produced profound changes in all parameters: the magnetic reversal behavior, the remanence ratio and the magnetic domain configuration. These changes were accompanied by distinct changes in the bilayer phase constitution and proportions of hard and soft phases produced by interdiffusion of the Co and CoPt layers which altered the overall anisotropy and associated magnetic behavior of the system. To support these conjectures micromagnetic modeling of different conditions of the bilayer properties showed that changing the relative proportions of the hard and soft layers could indeed lead to changes in the magnetic behavior similar to those observed in the experimental systems. Both the experimental and modeling-derived results of this work demonstrate that the bulk technical properties of a hard/soft magnetic nanocomposite material depend on the relative proportions of the phases present, the degree of exchange coupling across the interface between those phases as well as on the physical and magnetic properties of those phases. Changing the physical properties of the phases in systematic ways allows the magnetic properties of the ensemble to be tailored. (C) 2001 Elsevier Science B.V. All rights reserved.