Permeability measurement on ferromagnetic thin films from 50 MHz up to 18 GHz

The needs for permeability measurements on ferromagnetic materials at high frequencies are driven by the development of broad-band recording systems. However, measurements < 500 MHz on ferromagnetic films are difficult. Wide band permeability measurements are reported only up to a few GHz. Measurements at higher frequencies can be performed using resonant cavities, as it is done in ferromagnetic resonance, but only at discrete frequencies. In contrast, we present a technique for measuring high frequency permeability of ferromagnetic films over a wide band. It works in a frequency range (50 MHz to 18 GHz or higher) that allows direct observation of the gyromagnetic resonance even on non-saturated samples, and can yield permeability in different direction for anisotropic films. In our technique a surface of approx0.5 dm exp 2 of the ferromagnetic material has to be wound to obtain proper sample geometry. As a consequence, our technique is restricted to coatings deposited on thin flexible substrates and to thin self supported ribbons produced by rapid solidification. Comparisons with another measurement technique working up to 3 GHz, and with calculated gyromagnetic response, yield a satisfactory agreement. The limitations of our method are discussed. Spectroscopic permeability measurements are reported for several alloys.