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{{Expert-subject|physics|date=May 2009}}
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The article '''Ferromagnetic material properties''' is intended to contain a glossary of terms used to describe (mainly quantitatively) [[Ferromagnetism|ferromagnetic]] materials, and [[magnetic core]]s.
 
==Terms==
*'''[[magnetic hysteresis|hysteresis loop]]''' - induction B as function of field strength H for H varying between H<sub>min</sub> and H<sub>max</sub>; 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, ''J. of Applied Physics'' '''23''', 316 (1952); in fact it is rectangle if B-µ<sub>0</sub>H is used instead B on the plot);
*'''[[remanence]]''' B<sub>r</sub> - "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]]''' H<sub>c</sub> - 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''' (BH)<sub>max</sub> - 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/m<sup>3</sup> for some soft materials (permalloy, 3E4 ferrite), to above 400kJ/m<sup>3</sup> for hard ones ([[neodymium magnet]]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(t<sub>0</sub>)-S*ln(t/t<sub>0</sub>), where t is time since H change, t0 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]] (''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 [[ferromagnetism|ferromagnetic]] material for technical use, the following parameters are specified:
 
*(relative) '''[[Permeability (electromagnetism)|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'' is the ratio for small magnetization of initially demagnetized material: <math>\mu_i=\frac{B}{\mu_{0}*H}</math> for very small H;
**''incremental permeability'' is 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: <math>\mu_\Delta=\frac{\Delta B}{\mu_{0}*\Delta H}</math>;
**''amplitude permeability'' is the ratio of induction in the material to an induction in a vacuum for larger magnetization: just <math>\mu_a=\frac{B}{\mu_{0}*H}</math>;
**''maximum incremental/amplitude permeability'' is the maximal value of the incremental/amplitude permeability on the hysteresis curve;
 
*'''[[Saturation (magnetic)|saturation induction]]''' - induction B for large (enough for <math>\mu_\Delta</math> to become small), but reasonable H;
 
*'''[[resistivity]]''' <math>\rho</math> - specific resistance, as for usual resistive materials, important because of eddy currents; unit = <math>\Omega*m</math>;
 
*'''[[mass density]]''' - mass per unit volume, as for usual materials;
 
*'''temperature factor''' of the permeability is defined as <math>\alpha_F=\frac{\mu_\theta-\mu_{ref}}{\mu_{ref}^2*(\theta-\theta_{ref})}</math> by IEC133, and as <math>\alpha_F=\frac{\mu_\theta-\mu_{ref}}{\mu_\theta*\mu_{ref}*(\theta-\theta_{ref})}</math> by IEC367-1;
 
*'''[[Curie point]]''' (or Curie temperature) - a temperature, above which the ferromagnetic material becomes a paramagnet; more in [[ferromagnetism]];
 
*'''[[Loss tangent|tangent of loss angle]]''' - ratio of a resistance (R) to a reactance (<math>2*Pi*f*L</math>) of a coil on a core without a gap (<math>\mu_e=\mu_i</math> - 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 <math>{\frac{tan\delta}{\mu_i}}*10^6</math>
 
*'''disaccomodation factor''' - it is a measure of material permeability variation after demagnetization, given by a formula <math>D_F=\frac{\mu_1-\mu_2}{(\mu_1)^2*log(t2/t1)}</math>, where <math>\mu_1,\mu_2</math> - permeability values, t1,t2 - time from demagnetization; usually determined for t1=10min, t2=100min; range from 2*10<sup>−6</sup> to 12*10<sup>−6</sup> for typical MnZn and NiZn ferrites;
 
*'''hysteresis constant''' <math>\eta_B</math> - ?
 
*'''d.c. sensitivity constant''' <math>\beta_F</math> - ?
 
==Magnetic core parameters==
*'''core constant C<sub>1</sub>''' - 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 lengths of each section of the magnetic circuit divided by the square of the corresponding magnetic area of the same section;
*'''core constant C<sub>2</sub>''' - sum of l/A^2 along magnetic path;
*'''effective length''' of a magnetic path l<sub>e</sub>;
*'''effective cross-section''' A<sub>e</sub>;
*'''effective volume''' <math>V_e=\frac{{C_1}^3}{{C_2}^2}</math>;
*'''effective permeability''' <math>\mu_e=\frac{C_1}{\Sigma \frac{l}{\mu A}}</math> 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''' A<sub>min</sub>;
*'''inductance factor''' A<sub>L</sub> - inductance of one-turn coil, in nH (note inductance L=A<sub>L</sub>*n<sup>2</sup>, 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''' <math>\alpha</math> = number of turn for 1mH (note <math>\alpha^2*A_L=1000000</math>);
 
These parameters used e.g. in Philips' handbook <ref>Philips' handbook ''Components and materials'', Part 4a, Nov 1978, said in accordance with IEC401 and IEC125</ref> and Magnetic Materials Producers Association "Soft Ferrites, A Users Guide".<ref>Magnetic Materials Producers Association "Soft Ferrites, A Users Guide", MMPA SFG-98, 1998</ref>
 
==References==
<references/>
 
Note: possibly there are some errors - need verification from another source; see discussion page.
 
[[Category:Ferromagnetic materials]]

Latest revision as of 22:19, 22 July 2014

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