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