Giant magnetoimpedance

Giant Magnetoimpedance (GMI) is a physical effect that expresses the large variation in the electrical impedance that occurs in some materials when subject to an external magnetic field. It should not be confused with Giant Magnetoresistance that is a totally different physical phenomenon.

The phenomenology of the GMI

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 (ribbons, wires, multilayers meander-likes) and with the external parameters be somewhat complex, there are current theoretical models that allow one to calculate the GMI within some approximations.[1][2] 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.[3]

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.[4]

Typical experimental set-up for measuring the GMI (inset) and GMI data for a FeZr alloy at 150 K (From ref. 4).

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.[5][6] 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.[7] Because of the huge variations observed in the magnetic field dependence of the magnetoimpedance it was named giant magnetoimpedance.[8] 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.[9][10][11]

In-phase component of the GMI measured for a piece of an amorphous ferromagnetic material (From ref. 7)

References

  1. ↑ Machado; et al. (1996-04-15). "A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft‐ferromagnetic alloys". Journal of Applied Physics. 79 (8): 6558–6560. doi:10.1063/1.361945. ISSN 0021-8979.
  2. ↑ Panina; et al. (1995-03-01). "Giant magneto-impedance in Co-rich amorphous wires and films". IEEE Transactions on Magnetics. 31 (2): 1249–1260. doi:10.1109/20.364815. ISSN 0018-9464.
  3. ↑ Machado; et al. (1999). "Surface Magnetoimpedance Measurements in Soft-Ferromagnetic Materials". Physica Status Solidi A. 173: 135.
  4. ↑ Ribeiro; et al. (2016-09-05). "GMI in the reentrant spin-glass Fe90Zr10 alloy: Investigation of the spin dynamics in the MHz frequency regime". Applied Physics Letters. 109 (10): 102404. doi:10.1063/1.4962534. ISSN 0003-6951.
  5. ↑ HARRISON, E. P.; TURNEY, G. L.; ROWE, H. "Electrical Properties of Wires of High Permeability". Nature. 135 (3423): 961–961. doi:10.1038/135961a0.
  6. ↑ Harrison, E. P.; Turney, G. L.; Rowe, H.; Gollop, H. (1936-11-02). "The Electrical Properties of High Permeability Wires Carrying Alternating Current". Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 157 (891): 451–479. doi:10.1098/rspa.1936.0208. ISSN 1364-5021.
  7. ↑ Machado; et al. (1994-05-15). "Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe4.6Si15B10". Journal of Applied Physics. 75 (10): 6563–6565. doi:10.1063/1.356919. ISSN 0021-8979.
  8. ↑ Panina; et al. (1994-11-15). "Giant magneto‐impedance and magneto‐inductive effects in amorphous alloys (invited)". Journal of Applied Physics. 76 (10): 6198–6203. doi:10.1063/1.358310. ISSN 0021-8979.
  9. ↑ Knobel, M.; Vazquez, M.; Kraus, L. (2003). Buschow, K.H.J., ed. Giant Magnetoimpedance in Handbook of magnetic materials. The Netherlands: Elsevier. pp. 497–564. ISBN 0-444-51459-7.
  10. ↑ Knobel, M.; Pirota, K. R. (2002-04-01). "Giant magnetoimpedance: concepts and recent progress". Journal of Magnetism and Magnetic Materials. Proceedings of the Joint European Magnetic Symposia (JEMS'01). 242–245, Part 1: 33–40. doi:10.1016/S0304-8853(01)01180-5.
  11. ↑ Phan, Manh-Huong; Peng, Hua-Xin (2008-02-01). "Giant magnetoimpedance materials: Fundamentals and applications". Progress in Materials Science. 53 (2): 323–420. doi:10.1016/j.pmatsci.2007.05.003.
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