materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

Giant Magnetoimpedance (GMI) is the effect that occurs in some materials where an external magnetic field causes a large variation in the

electrical impedance In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of Electrical_resistance, resistance and Electrical_reactance, reactance in a electrical circuit, circuit. Quantitatively, the impedan ...

of the material. It should not be confused with the separate physical phenomenon of

Giant Magnetoresistance Giant magnetoresistance (GMR) is a quantum mechanics, quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Alb ...

The phenomenology of the GMI

GMI is caused by the penetration length that is a measure of how deep an ac electrical current can flow inside an

electrical conductor In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow of negatively c ...

. The penetration length (also known as the skin-depth effect) increases with the square root of the electrical resistivity of the material and is inversely proportional to the square root of the product of the

magnetic permeability In electromagnetism, permeability is the measure of magnetization produced in a material in response to an applied magnetic field. Permeability is typically represented by the (italicized) Greek letter ''μ''. It is the ratio of the magnetic ...

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 current near the surface of the material. When an external magnetic field is applied, the size of the permeability diminishes, increasing the penetration of the current in the magnetic material. Large variations are observed in both in-phase and out-of-phase components of the magnetoimpedance for applied magnetic fields close to the value of the

Earth magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magne ...

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 fact that the dependence of the GMI on the geometry of the

(ribbons, wires, multilayers meander-likes) and external parameters is somewhat complex, there are theoretical models that allow calculation of the GMI to within some approximations. Beside the dependence of the GMI on the frequency of the 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.

Experimental measurement

A typical experimental set-up for investigating the GMI in research laboratories is shown below. It includes an alternating

current source A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it. A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources fed ...

, a phase sensitive amplifier for detecting the ac voltage across the sample and an

electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire (likely copper) wound into a electromagnetic coil, coil. A current through the wire creates a magnetic ...

for applying a dc magnetic field. A

cryostat A cryostat (from ''cryo'' meaning cold and ''stat'' meaning stable) is a device used to maintain low cryogenic temperatures of samples or devices mounted within the cryostat. Low temperatures may be maintained within a cryostat by using various ...

or an

oven upA double oven A ceramic oven An oven is a tool that is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been use ...

may be required for measuring the temperature dependence of the GMI. GMIsetup

Several experimental measurements were also performed to characterize the long-term stability and the thermal drift of the GMI, which were supported by a theoretical model describing the physical modeling of the sensing element.

History

The observation that the impedance of soft-magnetic materials is influenced by the frequency and amplitudes of applied magnetic fields was first observed in the 1930s. These initial studies were limited to frequencies of a few hundreds of Hz and the changes of impedance reported in those works were not large. Starting in the 1990s, this phenomenon was investigated again, this time making use of currents with frequencies of hundreds of

kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...

. 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 using the GMI effect, they have been used in

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...

es,

accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...

s, virus detection, biomagnetism, among other applications. Firts GMI data

References

{{Reflist Electromagnetism