Modulation of magnetic properties at the nanometer scale in continuously graded ferromagnets

Ferromagnetic alloy materials with designed dopant composition depth profiles provide an efficient route for the control of magnetism at the nanometer length-scale. In this regard, cobalt-chromium and cobalt-ruthenium alloys constitute powerful model systems given that they exhibit easy to tune magnetic properties such as saturation magnetization MS and Curie temperature TC while preserving their crystalline structure in a wide composition range. In order to demonstrate this materials design potential, we have grown a series of graded Co(1-x)Cr(x) and Co(1-w)Ru(w) (10-10) epitaxial thin films, with x and w following predefined doping profiles. Structural analysis measurements verify the epitaxial nature and crystallographic quality of our entire sample sets, which were designed to exhibit in-plane c-axis orientation and thus an in-plane easy magnetic axis to suppress magnetostatic domain generation. Temperature and field-dependent magnetic depth profiles have been measured by means of polarized neutron reflectometry. In both investigated structures, TC and MS are found to vary as a function of depth in accordance with the predefined compositional depth profiles. Our Co(1-w)Ru(w) sample structures, which exhibit very steep material gradients, allow the determination of the localization limit for compositionally graded material, which we find to be of the order of 1 nm. The Co(1-x)Cr(x) systems show the expected U-shaped TC and MS depth profiles, for which these specific samples were designed. The corresponding temperature dependent magnetization profile is then utilized to control the coupling along the film depth, which even allows for a sharp onset of decoupling of top and bottom sample parts at elevated temperatures.