Magnetoimpedance, ferromagnetic resonance, and low field microwave absorption in amorphous ferromagnets

For many applications, the efficiency of giant magnetoimpedance (GMI) increases as the working frequency increases; the natural limit seems to be ferromagnetic resonance (FMR). However, MI is an essentially different phenomenon than FMR. The latter is a quantum-mechanical phenomenon which should satisfy the Larmor equation, while MI extends continuously from some hundreds of kHz up to the GHz range. A new phenomenon, which appears at very low fields in FMR experiments (by means of measurements around zero field that need a special accessory to compensate the remanence of electromagnets, and accurately measure very low magnetic fields) led to a clear difference between MI and FMR. This microwave interaction, which we call ‘low field absorption’ (LFA), has shown a strong similarity with MI, as far as it is also controlled by the anisotropy field. In this work, we show the start of the splitting between MI and FMR at frequencies ∼ 200 MHz, and the full differentiation between LFA and FMR at 9.4 GHz. We analyze the basic features of LFA and the conditions to be properly compared with GMI. Finally, we present some studies on selected materials.