Surface magnetic properties and magnetoimpedance of Co-rich amorphous and nanocrystalline (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ ribbons with oxide layer formed by long-term exposure to air

The surface magnetic properties and magnetoimpedance (MI) of amorphous and nanocrystalline (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ melt-spun ribbons with x$=$ 0, 0.025, 0.05 {\&} 0.1 was investigated. A 540\textdegree C heat treatment for 1 hour under a 2 T transverse field formed a large volume fraction of nanocrystalline phases in the ribbons, in addition to a well-defined transverse anisotropy indicated by x-ray diffraction and magneto-optical Kerr effect microscopy. After the heat treatment, the ribbon samples were exposed to open air for an extended period of time producing a visible oxide layer on the surfaces. High frequency magnetoimpedance measurements in the driving frequency range of 1-1000 MHz were made to characterize the potential impact of the surface oxide layer on the ac magnetization process. Unique field-dependent behavior of the real and imaginary components of the MI was found in nanocrystalline ribbons with higher Co content (x $\ge $ 0.05), showing multiple peaks above 50 MHz driving current.