Controlling Crystal Structure and Stoichiometry in Co/Fe Electrodeposited Films for Improved Magnetostriction

Abstract

Magnetostrictive CoFe films were investigated for use as magnetoelastic tags or sensors. The ability to electrodeposit these films enables batch fabrication processes to pattern a variety of geometries while controlling the film stoichiometry and crystallography. In current research looking at CoFe magnetostrictive, giant magnetostriction was achieved using a co-sputtering, annealing, and quenching method [1]. This research focused on maximizing magnetostriction at the optimal stoichiometry range of Co0.7-0.75Fe0.3-0.25 targeting the (fcc+bcc)/bcc phase boundary without being limited to “line of sight” deposition. Other current research has reported electrodeposited CoFe films using a sulfate based chemistry resulting in film compositions that are Fe rich in the range of Co0.3-0.4Fe0.7-0.6 and have problems of co-deposition of undesirables that can have a negative impact on magnetic properties[2, 3]. To obtain the desired compositional range, a chemistry was selected to allow for a higher ratio of Co while maintaining stability and limiting the oxidation of the Fe2+ to Fe3+. As suggested by Osaka et al, Fe(OH)3 is formed and included into the film resulting in a decrease of the saturation magnetic flux density (Bs) value as the Fe cation is oxidized[2]. The use of additives such as saccharin or sodium lauryl sulfate, that could introduce sulfur into the deposited film, were also excluded. This led to a deviation from the traditional sulfate based chemistry used to deposit CoFe alloy thin films and the inclusion of organic molecules like sodium citrate to stabilize deposition. The composition and phase of the deposited films were controlled through the temperature, agitation, concentrations in the electrodeposition chemistry, current density, and duty cycle of the pulsing regime. After initial chemistry characterization to determine the kinetics and mass transfer limitations, samples were plated across a range of current densities and duty cycles onto copper tuning fork substrates that enabled magnetic testing to be performed. The samples were then analyzed with EDS to determine the composition and XRD to look at the texture of the films. Magnetic testing was performed using super conducting quantum interference device measurements (SQUID), as well as visual inspection of the displacement on a deposit stress analyzer as a magnetic field was applied to the films. The magnetostriction was then correlated to EDS and XRD results to identify phase, stoichiometry and the plating parameters to produce giant magnetostriction. Acknowledgements: Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000