The GMI is essentially due to the penetration-length that is a measure of how deep an ac electrical current can flow inside an electrical conductor. The penetration-length, or skin-depth effect, increases with the square root of the electrical resistivity of the material and it is inversely proportional to the square root of the product of the magnetic permeability and the frequency of the ac electrical current. Thus, in materials with very high values of magnetic permeability, such as soft-ferromagnetic materials, the penetration-length can be much less than the thickness of the conductor even for moderate values of frequencies driving the ac electrical current to near the surface of the material. When an external magnetic field is applied, the size of the magnetic permeability diminishes increasing the penetration of the ac electrical current in the magnetic material. Large variations are observed in both in-phase and out-of-phase components of the magnetoimpedance in applied magnetic fields close to the value of the Earth magnetic field up to few tens of Oersted. For comparison, in normal electrical conductors the effect of the skin-depth becomes important for frequencies in the microwave range only. Despite the dependence of the GMI with the geometry of the electrical conductor and with the external parameters be somewhat complex, there are current theoretical models that allow one to calculate the GMI within some approximations. Beside the dependence of the GMI with the frequency of the ac electrical current there are other sources that contribute to the frequency dependence of the GMI such as the motion of the domain wall and the ferromagnetic resonance.
Measuring experimental set-up
A typical experimental set-up for investigating the GMI in research laboratories is shown below. Essentially, it is required to have an ac electrical current source, a phase sensitive amplifier for detecting the ac voltage across the sample and an electromagnet for applying a dc magnetic field. A cryostat or an oven may be required for measuring the temperature dependence of the GMI.
History
The observation that the impedance of soft-magnetic materials was influenced by the frequency and by small amplitudes of magnetic fields was observed back in the 1930s. However, the pioneering studies were limited to frequencies of a few hundreds of Hz and the changes reported in those works were not large. Only about six decades later this phenomenon was investigated again but this time making use of ac electrical currents with frequencies of hundreds of kHz. Because of the huge variations observed in the magnetic field dependence of the magnetoimpedance it was named giant magnetoimpedance. Due to the high sensitivity of the sensors making use of the GMI they have been used in compasses, accelerometers, virus detection, biomagnetism, among other applications. General overviews on the giant magnetoimpedance effect are presented in refs.
