Growth Method to Improve the Resonant Frequency and Magnetic Permeability of FeCo Thin Films

Fabrication of a FeCo thin film with both high magnetic resonant frequency and large permeability for high-frequency device applications is a challenge despite FeCo's high magnetization. Limitations are usually due to relatively high coercivity or low anisotropy. Here, we demonstrate a method for fabrication of FeCo films with both ultrahigh resonant frequency and large permeability, obtained by slightly tuning their nanostructures and reducing their crystal defects during electrodeposition. With a change from a single direct current density to two well-controlled alternating current densities during deposition, the as-synthesized films ( $\text{200}\hbox{-}\text{600} \; \text{nm}$ in thickness) have significantly lower coercivity (down to $\text{8} \; \text{Oe}$ ) and improved anisotropy (up to $\text{96} \; \text{Oe}$ ) and magnetic polarization (up to $\text{2.4} \; \text{T}$ ), which lead to an ultrahigh resonant frequency up to $\text{5.1} \; \text{GHz}$ . Further reduction of the crystal defects and improvement in FeCo atom-filling density through the optimization of the plating solution at lower temperatures and higher concentrations result in resonant frequency and relative permeability improvements up to $\text{6.1} \; \text{GHz}$ and 325 (real part), respectively. Further, FeCo films with only large relative permeability (up to 715) can also be attained through optimization. These nanofilms thus have the potential to be used in gigahertz microwave devices.