Ferromagnetic material properties
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The article Ferromagnetic material properties is intended to contain a glossary of terms used to describe (mainly quantitatively)
ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
materials, and
magnetic core A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, in ...
s.


Terms

;
Hysteresis loop Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
: Induction B as function of field strength H for H varying between Hmin and Hmax; for ferromagnetic material the B has different values for H going up and down, therefore a plot of the function forms a loop instead of a curve joining two points; for perminvar type materials, the loop is a "rectangle" (''Domain Structure of Perminvar Having a Rectangular Hysteresis Loop'', Williams, Goertz, ''Journal of Applied Physics'' 23, 316 (1952); in fact it is rectangle if B-μ0H is used instead B on the plot); ;
Remanence Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
, Br; "induction which remains": After magnetization to saturation, a value of induction B in the material in a closed magnetic circuit without external field H; the point where hysteresis loop crosses B axis; ;
Coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
, Hc: After magnetization to saturation, a value of field strength H at which induction B in the material becomes 0; the point where hysteresis loop crosses H axis; ;
Maximum energy product In magnetics, the maximum energy product is an important figure-of-merit for the strength of a permanent magnet material. It is often denoted and is typically given in units of either (kilojoules per cubic meter, in SI electromagnetism) or (meg ...
, (BH)max: Largest possible field of a rectangle on the hysteresis loop plot, which has two edges on the B and H axes, and a vertex on the hysteresis loop in the second quadrant (B positive, H negative); range from below 1J/m3 for some soft materials (permalloy, 3E4 ferrite), to above 400kJ/m3 for hard ones (
neodymium magnet A hard_disk_drive.html"_;"title="Nickel-plated_neodymium_magnet_on_a_bracket_from_a_hard_disk_drive">Nickel-plated_neodymium_magnet_on_a_bracket_from_a_hard_disk_drive_ file:Nd-magnet.jpg.html" ;"title="hard_disk_drive_.html" ;"title="hard_disk_d ...
s); ;Magnetic viscosity: When an external field H is changed, and then kept at a new value, the induction B first changes almost at once, then some smaller change of B follows in a time; for a permanent magnet typically the time dependence is B(t) = B(t0) − S·ln(t/t0), where t is time since H change, t is some reference time, and S is a constant of the process (but not of the material, as it varies with magnitude of the H and its change); a theory describing this kind of time dependency was developed by
Louis Néel Louis Eugène Félix Néel (22 November 1904 – 17 November 2000) was a French physicist born in Lyon who received the Nobel Prize for Physics in 1970 for his studies of the magnetic properties of solids. Biography Néel studied at the Lycée ...
(''J. de Phys. et Radium'', 11, 49 (1950)) and by Street and Wooley (A Study of Magnetic Viscosity, ''Proc. Phys. Soc.'' A62. 562 (1949)).


Formulae

To describe a soft
ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
material for technical use, the following parameters are specified: ;(Relative) permeability: Ratio of induction B in the material caused by some field H to an induction in a vacuum in the same field; it is a dimensionless value, as it is ''relative'' to a vacuum permeability; :;Initial permeability, \mu_i: The ratio for small magnetization of initially demagnetized material: \mu_i = \frac for very small H; :;Incremental permeability, \mu_\Delta: The ratio of change of induction in the material to a change of induction in a vacuum due to the same field change, when the change is superimposed to some constant field: \mu_\Delta = \frac; :;Amplitude permeability, \mu_a: The ratio of induction in the material to an induction in a vacuum for larger magnetization: just \mu_a = \frac; :;Maximum incremental/amplitude permeability: The maximal value of the incremental/amplitude permeability on the hysteresis curve; ; Saturation induction: Induction B for large (enough for \mu_\Delta to become small), but reasonable H; ;
Resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
, \rho: Specific resistance, as for usual resistive materials, important because of eddy currents; SI units, ohm-metres (Ω·m); ;
Mass density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
: Mass per unit volume, as for usual materials; ;Temperature factor of the permeability, \alpha_F: Defined as \alpha_F = \frac by IEC133, and as \alpha_F = \frac by IEC367-1; ;
Curie point In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
(or Curie temperature): A temperature, above which the ferromagnetic material becomes a paramagnet; more in
ferromagnetism Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
; ; Tangent of loss angle: Ratio of a resistance (R) to a reactance (2 \pi fL) of a coil on a core without a gap (\mu_e = \mu_i - otherwise it must be scaled), assuming the resistance is result of losses in the magnetic material; the angle describes a delay between B in the material versus H; measured for sinusoidal magnetic field of frequency f; usually specified as \frac\tan(\delta) \times 10^6 ;Disaccommodation factor, D_F: It is a measure of material permeability variation after demagnetization, given by a formula D_F = \frac, where \mu_1, \mu_2 are permeability values, and t, t are time from demagnetization; usually determined for t = 10min, t = 100min; range from 2×10−6 to 12×10−6 for typical MnZn and NiZn ferrites; ;Hysteresis constant, \eta_B ;DC sensitivity constant, \beta_F


Magnetic core parameters

;Core constant, C1: Sum of l/A along magnetic path; l is length of a part of the path, A is its cross-section. The summation of the
magnetic path length Magnetic path length (MPL) is the effective length of a closed magnetic loop inside a magnetic core made of ferromagnetic material which may be also gapped. MPL is relevant in transformer and inductor design and more generally in all kinds of mag ...
s of each section of the magnetic circuit divided by the square of the corresponding magnetic area of the same section; ;Core constant, C2: Sum of l/A along magnetic path; ;Effective length of a magnetic path, le; ;Effective cross-section, Ae; ;Effective volume: V_e = \frac; ;Effective permeability: \mu_e = \frac For a magnetic circuit constructed with an air gap or air gaps, the permeability of a hypothetical homogeneous material which would provide the same reluctance; (these "effective" above are sizes of a toroid core made from the same material which has the same magnetic properties as the core); ;Minimum cross-section, Amin; ;Inductance factor, AL: Inductance of one-turn coil, in nH (note inductance L = ALn2, n is number of turns) Inductance of a coil on a specified core divided by the square of the number of turns. (Unless otherwise specified the inductance test conditions for inductance factor are at a flux density ~10 gauss); ;Turns factor, \alpha: Number of turns for 1{{nbspmH (note \alpha^2 A_L = 1000000); These parameters used e.g. in Philips' handbook and Magnetic Materials Producers Association "Soft Ferrites, A Users Guide".Magnetic Materials Producers Association "Soft Ferrites, A Users Guide", MMPA SFG-98, 1998


See also

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Ferrite (magnet) A ferrite is a ceramic material made by mixing and firing large proportions of iron(III) oxide (Fe2O3, rust) blended with small proportions of one or more additional metallic elements, such as strontium, barium, manganese, nickel, and zinc. They ...
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Cobalt Ferrite Cobalt ferrite is a semi-hard ferrite with the chemical formula of CoFe2O4 (CoO·Fe2O3). The substance can be considered as between soft and hard magnetic material and is usually classified as a semi-hard material. Applications It is mainly use ...


References

Ferromagnetism