PHENOMENONS

The following is a brief description of many of the phenomenons that can occur within anomalies under the influence of alternating magnetic fields. A number of electromagnetic phenomenons that were considered to have no possibility of use, such as those that only occur on superconductors, have been left out. A number that have still been included, such as those that related to heat gradients, are recognised as not being likely to be of use with metal detectors. Most of these have been included as related phenomenons such as those that dissipate heat should have effects that are detectable. Also, as can happen in research, the effect that is least likely to be useful can end up being the most useful.

Some of the descriptions are excerpts from Wikipedia and most of the phenomenons have links to web pages giving more information.   A number of the descriptions refer to Magnetic states of material.  These are described on our Magnetic States page at http://humanise.org/demining/electromagnetic-states

Intermodulation Distortion
http://en.wikipedia.org/wiki/Intermodulation
This form of distortion normally occurs when two sine waves of frequencies f₁ and f₂ are present at the input, resulting in the creation of several other frequency components, whose frequencies include (f₁+f₂), (f1-f2), (2f₁-f₂), (2f₂-f₁), and generally (mf₁ ± nf₂) for integer m and n. Generally the size of the unwanted output falls rapidly as m and n increase.

A linear system cannot produce intermodulation. If the input of a linear time-invariant system is a signal of a single frequency, then the output is a signal of the same frequency; only the amplitude and phase can differ from the input signal. However, non-linear systems generate harmonics, meaning that if the input of a non-linear system is a signal of a single frequency, then the output is a signal which includes a number of integer multiples of the input frequency. Intermodulation normally occurs when the input to a non-linear system is composed of two or more frequencies.

Each of these frequency components will have a different amplitude and phase, which depends on the specific non-linear function being used, and also on the amplitudes and phases of the original input components.

Fluxgate Effect
This effect changes the the B-H curve or hysteresis loop response of a ferromagnetic material when there is both a static magnetic field and an Alternating magnetic field. A significant effect can occur with only a very weak static field. If you apply a static field, you shift the operating point on the hysteresis or B-H curve. It shifts to the left or right along the H (applied field) axis depending on the direction of applied field. The material spends more time saturated in one field direction so the magnetization is an asymmetric wave. This adds even harmonics to the results which are easily detected. The principle is used in fluxgate magnetometers, fluxgate compasses and Electronic Article Surveillance (EAS).

Magnetostriction
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. Magnetostrictive materials can convert magnetic energy into kinetic energy

From: http://books.google.com.au/books?id=Y774DLQf-eIC&pg=PA298&lpg=PA298&dq=M...

When a object made of ferromagnetic substance is placed in a magnetic field produced by a current carrying coil, the object not only gets magnetised but also shortens in length irrespective of the direction of magnetisation due to magnetostriction. In an alternating magnetic field of frequency f, the magnetostriction frequency would be double the AC frequency, i.e. 2f.

However, when the specimen is a permanent magnet, or is currently magnetised due to another DC field, and small variations due to an applied AC field passing through the coil are superimposed on it, only magnetostriction oscillations of frequency f are generated in the medium.

From: http://en.wikipedia.org/wiki/Magnetostriction
Internally, ferromagnetic materials have a structure that is divided into domains, each of which is a region of uniform magnetic polarization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate, both these effects causing a change in the material's dimensions. The reciprocal effect, the change of the susceptibility of a material when subjected to a mechanical stress, is called the Villari effect. Two other effects are related to magnetostriction: the Matteucci effect is the creation of a helical anisotropy of the susceptibility of a magnetostrictive material when subjected to a torque and the Wiedemann effect is the twisting of these materials when a helical magnetic field is applied to them. The Villari Reversal is the change in sign of the magnetostriction of iron from positive to negative when exposed to magnetic fields of approximately 40000 A/m (500 oersted).

Villari Effect
The change in magnetic properties (change in magnetic permeability) of a ferromagnetic material in response to the presence of stress in the ferromagnetic material. This is the inverse of magnetostriction.

Wiedemann Effect
The Wiedemann effect is the twisting of these materials when a helical magnetic field is applied to them. This is normally seen as a twist in a current-carrying ferromagnetic wire or tube when placed in a longitudinal magnetic field. Also known as circular magnetostriction.

The Matteucci Effect
the Matteucci effect is the inverse of the Wiedemann effect.

The Villari Reversal
The Villari Reversal is the change in sign of the magnetostriction of iron from positive to negative when exposed to magnetic fields of approximately 40000 A/m (500 oersteds). If an iron rod is stretched when weakly magnetized, its magnetic induction will be increased whereas stretching the same rod in a strong field its magnetic induction will be decreased. The point where stretching (or compression) does not affect the intensity of magnetization is called the Villari reversal point.

Piezoresistance
http://en.wikipedia.org/wiki/Piezoresistive_effect
The piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. In contrast to the piezoelectric effect, the piezoresistive effect only causes a change in resistance; it does not produce an electric potential. The piezoresistive effect of metal is only due to the change of the sensor geometry resulting from applied mechanical stress. Despite this rather small value compared to piezoresistive effect of other materials, metal piezoresistors are successfully used as strain gages in a wide range of applications.

Hall Effect
http://en.wikipedia.org/wiki/Hall_effect
The Hall effect is the production of a potential difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.

The Thermal Hall Effect
http://en.wikipedia.org/wiki/Thermal_Hall_effect
The thermal Hall effect is the thermal analog of the Hall effect. Here, a thermal gradient is produced across a solid instead of an electric field. When a magnetic field is applied, an orthogonal temperature gradient develops. For conductors, a significant portion of the thermal current is carried by the electrons.

Righi-Leduc Effect
The Righi-Leduc Effect describes the heat flow resulting from a perpendicular temperature gradient and vice versa, transverse to an electric current in a conductor and a magnetic field perpendicular to the current.

Maggi-Righi-Leduc Effect
The Maggi-Righi-Leduc effect describes changes in thermal conductivity when placing a conductor in a magnetic field.

Anomalous/Extraordinary Hall Effect
In ferromagnetic materials and paramagnetic materials in a magnetic field, the Hall resistivity includes an additional contribution, known as the Anomalous Hall Effect (or the Extraordinary Hall effect). This effect depends directly on the magnetization of the material, and is often much larger than the ordinary Hall effect. Note that this effect is not due to the contribution of the magnetization to the total magnetic field. Although a well-recognized phenomenon, there is still debate about its origins in the various materials.

The Corbino Effect
The Corbino effect is a phenomenon similar to the Hall effect, but a disk-shaped metal sample is used in place of a rectangular one. A radial current through a circular disc subjected to a magnetic field perpendicular to the plane of the disk, produces a "circular" current through the disk.

Nernst Effect
http://en.wikipedia.org/wiki/Nernst_effect
The Nernst Effect (also termed first Nernst-Ettingshausen effect) is a thermoelectric (or thermomagnetic) phenomenon observed when a sample allowing electrical conduction is subjected to a magnetic field and a temperature gradient normal to each other. An electric field will be induced normal to both.

Ettingshausen effect
http://en.wikipedia.org/wiki/Ettingshausen_effect
The Ettingshausen Effect is a phenomenon that affects electric current in a conductor when a magnetic field is present. The result of the phenomenon is that a potential difference is induced normal to both the direction of the magnetic field and the current. Alternately, a temperature gradient is induced. This is the reverse process to the Nernst Effect is also known as the second Nernst-Ettingshausen effect.

Pyroelectric Effect
http://en.wikipedia.org/wiki/Pyroelectricity
Pyroelectricity is the ability of certain materials to generate a temporary electrical potential when they are heated or cooled. The change in temperature slightly modifies the positions of the atoms within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a temporary electric potential, although this disappears after the dielectric relaxation time. Pyroelectricity should not be confused with thermoelectricity, where a fixed, non-uniform temperature profile gives rise to a permanent electrical potential difference.

Electrocaloric effect
http://en.wikipedia.org/wiki/Electrocaloric_effect
The electrocaloric effect is where a material shows a reversible temperature change under an applied electric field

Thermoelectric Effects
http://en.wikipedia.org/wiki/Thermoelectric_effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely (and, thermodynamically speaking, reversibly) when a voltage is applied to it, it creates a temperature difference. The term refers collectively to the Seebeck effect, Peltier effect, and the Thomson effects.

Seebeck Effect
The Seebeck effect is the conversion of temperature differences directly into electricity. This thermoelectric effect occurs where a junction of dissimilar metals produces an electric current when exposed to a temperature gradient.

Peltier effect
The Peltier Effect is the calorific effect of an electrical current at the junction of two different metals. When a current is made to flow through the circuit, heat is evolved at the upper junction, and absorbed at the lower junction .

Thomson Effect
The Thomson effect describes the heating or cooling of a current-carrying conductor with a temperature gradient.

Negative Thomson Effect
In metals such as cobalt, nickel, and iron, which have a cooler end at a higher potential and a hotter end at a lower potential, when current moves from the hotter end to the colder end, it's moving from a low to a high potential, there is an absorption of heat. This is called the negative Thomson effect.

Piezoelectric Effect
Piezoelectric effect is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric potential in response to applied mechanical stress.
This is a reversible effect. That is, an electric potential applied across the same material will create a mechanical stress in the material.

Skin Effect
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.

Magnetocaloric Effect
http://en.wikipedia.org/wiki/Magnetic_refrigeration
The Magnetocaloric effect (MCE, from magnet and calorie) is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. This is also known as adiabatic demagnetization by low temperature physicists, due to the application of the process specifically to effect a temperature drop. For example, gadolinium and some of its alloys demonstrate a magnetocaloric effect whereby its temperature increases when it enters a magnetic field and decreases when it leaves the magnetic field. Praseodymium alloyed with nickel (PrNi5) has such a strong magnetocaloric effect that it's allowed scientists to approach within one thousandth of a degree of absolute zero. The effect was discovered in pure iron in 1881.

Electrostriction
http://en.wikipedia.org/wiki/Electrostriction
Electrostriction is a property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. Electrostriction is caused by the presence of randomly-aligned electrical domains within the material. When an electric field is applied to the dielectric, the opposite sides of the domains become differently charged and attract each other, reducing material thickness in the direction of the applied field (and increasing thickness in the orthogonal directions. Reversal of the electric field does not reverse the direction of the deformation.

Magnetoresistance
http://en.wikipedia.org/wiki/Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. This effect was later called ordinary magnetoresistance (OMR).

William Thomson (better known as Lord Kelvin) first discovered the ordinary magnetoresistance in 1856. He did experiments with pieces of iron and discovered that the resistance increases when the current is in the same direction as the magnetic force and decreases when the current is at 90° to the magnetic force. He then did the same experiment with nickel and found that it was affected in the same way but the magnitude of the effect was bigger. This effect is referred to as anisotropic magnetoresistance (AMR).

Magnetoimpedance Effect
The magnetoimpedance effect (MI) consists in the change of the total impedance (real and imaginary components) of a magnetic conductor under a DC applied magnetic field, when a high-frequency AC current flows through it. Note that this is a different property to that of Magnetoresistance.

Magnetocapacitance
Magnetocapacitance is a property of some dielectric or insulating materials and/or metal-insulator-metal heterostructures that exhibit a change in the value of their capacitance when an external magnetic field is applied to them. Magnetocapacitance can be an intrinsic property of some dielectric materials, such as multiferroic compounds like BiMnO3[1], or can be a manifest of properties extrinsic to the dielectric but present in capacitance structures.

Permeability Effects
http://en.wikipedia.org/wiki/Permeability_(electromagnetism)
In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. In general, permeability isn't a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a nonlinear medium, the permeability can depend on the strength of the magnetic field. Permeability as a function of frequency can take on real or complex values. In ferromagnetic materials, the relationship between B and H exhibits both non-linearity and hysteresis: B is not a single-valued function of H[2], but depends also on the history of the material.

Dielectric Relaxation
http://en.wikipedia.org/wiki/Dielectric_relaxation
Dielectric relaxation is the momentary delay (or lag) in the dielectric constant of a material. This is usually caused by the delay in molecular polarization with respect to a changing electric field in a dielectric medium (e.g. inside capacitors or between two large conducting surfaces). Dielectric relaxation in changing electric fields could be considered analogous to hysteresis in changing magnetic fields (for inductors or transformers). Relaxation in general is a delay or lag in the response of a linear system, and therefore dielectric relaxation is measured relative to the expected linear steady state (equilibrium) dielectric values.

Ferroelectric Effect
http://en.wikipedia.org/wiki/Ferroelectric_effect
Most materials are polarized linearly with external electric field. Nonlinearities are insignificant. This is called dielectric polarization. Some materials, known as paraelectric materials, demonstrate a more pronounced nonlinear polarization. The electric permittivity, corresponding to the slope of the polarization curve, is thereby a function of the external electric field. In addition to being nonlinear, ferroelectric materials demonstrate a spontaneous polarization. Such materials are generally called pyroelectrics. The distinguishing feature of ferroelectrics is that the direction of the spontaneous polarization can be reversed by an applied electric field, yielding a hysteresis loop.

Exchange Bias Effect
The coercivity, also called the coercive field, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. If an antiferromagnetic solid is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the exchange bias effect.

Barkhausen Effect
http://en.wikipedia.org/wiki/Barkhausen_effect
The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. It's caused by rapid changes of size of magnetic domains. A slow, smooth increase of a magnetic field applied to a piece of ferromagnetic material, such as iron, causes it to become magnetized, not continuously but in minute steps.

A coil of wire wound on the ferromagnetic material can demonstrate the sudden, discontinuous jumps in magnetization. The sudden transitions in the magnetization of the material produce current pulses in the coil. These can be amplified to produce a series of clicks in a loudspeaker. This sounds as crackle, complete with skewed pulses which sounds like candy being unwrapped, or a pine log fire. Hence the name Barkhausen noise. Similar effects can be observed by applying only mechanical stresses (e.g. bending) to the material placed in the detecting coil.

The material magnetizes neither gradually nor all at once, but in fits and starts
The amount of Barkhausen noise for a given material is linked with the amount of impurities, crystal dislocations, etc. and can be a good indication of mechanical properties of such a material.