  High frequency [1-500 MHz] measurements of the magnetoimpedance (MI) of
glass-coated Co$_{69.4}$Fe$_{3.7}$B$_{15.9}$Si$_{11}$ microwires are carried
out with various metal-to-wire diameter ratios. A twin-peak, anhysteretic
behaviour is observed as a function of magnetic field. A maximum in the
normalized impedance, $\Delta Z$/$Z$, appears at different values of the
frequency $f$, 125, 140 and 85 MHz with the corresponding diameter ratio $p$ =
0.80, 0.55 and 0.32. We describe the measurement technique and interpret our
results with a thermodynamic model that leads to a clearer view of the effects
of $p$ on the maximum value of MI and the anisotropy field. The behavior of the
real and imaginary components of impedance is also investigated; they display a
resonance that becomes a function of the DC field $H_{DC}$ for values larger or
equal to $H_{K}$ the circumferential anisotropy field for each $p $ value.
These results are interpreted in terms of a rotation model of the outer shell
magnetization.
