Glossary of Magnet Terms

Below is a glossary of common terms related to magnetic materials, design, and engineering:

Air gap, is a low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.

Anisotropy, Literally means having different properties depending on the inspected direction.  Magnets which are anisotropic, or have an easy axis of magnetization, have their anisotropy developed by two methods: Shape and Magnetocrystalline.

As Area of the air gap, or the cross sectional area of the air gap perpendicular to the flux path, is the average cross sectional area of that portion of the air gap within which the application interaction occurs. Area is measured in sq. cm. in a plane normal to the central flux line of the air gap.

Am Area of the magnet, is the cross sectional area of the magnet perpendicular to the central flux line, measured in sq. cm. at any point along its length. In magnet design, Am is usually considered the area at the neutral section of the magnet.

B Magnetic induction, is the magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.

Bd Remnant induction, is any magnetic induction that remains in a magnetic material after removal of an applied saturating magnetic field, Hs. (Bd is the magnetic induction at any point on the demagnetization curve: measured in gauss.)

Bd/Hd, Slope of the operating line, is the ratio of the remnant induction, Bd, to a demagnetizing force, Hd. It is also referred to as the permeance coefficient, shear line, load line and unit permeance.

BdHd Energy product, indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve; measured in megagauss-oersteds.

(BH) max Maximum energy product, is the maximum product of (BdHd) which can be obtained on the demagnetization curve.

Bis, (or J) Saturation intrinsic induction, is the maximum intrinsic induction possible in a material.

Bg, Magnetic induction in the air gap, is the average value of magnetic induction over the area of the air gap, A; or it is the magnetic induction measured at a specific point within the air gap; measured in gauss.

Bi (or J) Intrinsic induction, is the contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relation is expressed by the equation:

Bi=B-H where: Bi = intrinsic induction in gauss; B = magnetic induction in gauss; H = field strength in oersteds .

Bm, Recoil induction, is the magnetic induction that remains in a magnetic material after magnetizing and conditioning for final use; measured in gauss.

Bo, Magnetic induction, at the point of the maximum energy product (BH)max; measured in gauss.

Br, Residual induction (or flux density), is the magnetic induction corresponding to zero magnetizing force in a magnetic material after saturation in a closed circuit; measured in gauss.

A closed circuit condition exists when the external flux path of a permanent magnet is confined with high permeability material.

Curie Temperature, The transition temperature above which the alloy loses its magnetic properties.  This is not the maximum serviceable temperature, which is usually much lower.

The demagnetization curve is the second (or fourth) quadrant of a major hysteresis loop. Points on this curve are designated by the coordinates Bd and Hd.

Domains, Areas in a magnetic alloy which have the same orientation.  The magnetic domains are regions where the atomic moments of atoms cooperate and allow for a common magnetic moment.  It is the domains which are rotated and manipulated by an external magnetizing field to create a useful magnet which has a net magnetic moment.  In un-magnetized material the domains are un-oriented and cancel each other out.  In this condition there is no net external field.

Eddy currents, are circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as dampening of movement), or may be unwanted consequences of certain designs, which should be accounted for of minimized.

Electromagnet, is a magnet, consisting of solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.

f Reluctance factor, accounts for the apparent magnetic circuit reluctance. This factor is required due to the treatment of H, and H, as constants.

F Leakage factor, accounts for flux leakage from the magnetic circuit. It is the ratio between the magnetic flux at the magnet neutral section and the average flux present in the air gap. F=(B mA m)/(B, A g).

F Magneto motive force, (magnetic potential difference), is the line integral of the field strength, H, between any two points, p1 and p2.
p2
F=? H dl
p1
F = magneto motive force in gilberts
H = field strength in oersteds
dl = an element of length between the two points, in centimeters.

Ferromagnetic material, is a material whose permeability is very much larger than 1 (from 60 to several thousands times 1), and which exhibits hysteresis phenomena.

Flux, is the condition existing in a medium subjected to a magnetizing force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The cgs unit of flux is the Maxwell.

A flux meter is an instrument that measures the change of flux linkage with a search coil.

Fringing fields are leakage flux particularly associated with edge effects in a magnetic circuit.

The gauss is the unit of magnetic induction, B, in the cgs electromagnetic system. One gauss is equal to one maxwell per square centimeter.

A gauss meter is an instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

The gilbert is the unit of magneto motive force, F, in the cgs electromagnetic system.

H Magnetic field strength, (magnetizing or demagnetizing force), is the measure of the vector magnetic quantity that determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point; measured in oersteds.

Hc Coercive force of a material, is equal to the demagnetizing force required to reduce residual induction, Br to zero in a magnetic field after magnetizing to saturation; measured in oersteds.

Hci Intrinsic coercive force of a material, < indicates its resistance to demagnetization. It is equal to the demagnetizing force which reduces the intrinsic induction, Bi, in the material to zero after magnetizing to saturation; measured in oersteds.

Hd, is that value of H corresponding to the remnant induction, Bd; on the demagnetization curve, measured in oersteds.

Hmv, is that value of H corresponding to the recoil induction, B,; measured in oersteds.

Ho, is the magnetic field strength at the point of the maximum energy product (BH)max; measured in oersteds.

Hs, Net effective magnetizing force, is the magnetizing force required in the material, to magnetize to saturation measured in oersteds.

A hysteresis loop is a closed curve obtained for a material by plotting (usually to rectangular coordinates) corresponding values of magnetic induction, B, for ordinates and magnetizing force, H, for abscissa when the material is passing through a complete cycle between definite limits of either magnetizing force, H, or magnetic induction. B.

Irreversible losses are defined as partial demagnetization of the magnet, caused by exposure to high or low temperatures external fields or other factors. These losses are recoverable by remagnetization. Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields.

J, see Bi Intrinsic induction.
Js, see Bis, Saturation intrinsic induction.

A keeper is a piece (or pieces) of soft iron that is placed on or between the pole faces of a permanent magnet to decrease the reluctance of the air gap and thereby reduce the flux leakage from the magnet. It also makes the magnet less susceptible to demagnetizing influences.

Keepers, A keeper is a high permeability material, typically mild steel, which is installed on a magnet or magnetic assembly to reduce the reluctance of the magnetic circuit.  This reduces the overall leakage fields generated by the magnet or magnetic assembly.  Keepers are typically installed to help the magnet or magnetic assembly resist demagnetization during handling, transportation, or storage.  Keepers are typically found on Alnico magnets and Alnico magnetic assemblies.

Knee of the demagnetization curve is the point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. Alnico 5 exhibits a knee at room temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without re-magnetization.

Leakage flux is flux, ø, whose path is outside the useful or intended magnetic circuit; measured in maxwells.

lg Length of the air gap, is the length of the path of the central flux line of the air gap; measured in centimeters.

lm Length of the magnet, is the total length of magnet material traversed in one complete revolution of the centerline of the magnetic circuit; measured in centimeters.

lm/D Dimension ratio, is the ratio of the length of a magnet to its diameter, or the diameter of a circle of equivalent cross-sectional area. For simple geometries, such as bars and rods, the dimension ratio is related to the slope of the operating line of the magnet, BdHa.

Load line is a line drawn from the origin of the demagnetization curve with a slope of B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see permeance coefficient.

Magnetic Assemblies, A combination of materials, magnetic and non-magnetic, which form a particular solution. Incorporates a permanent magnet as the flux generator and usually relies on mild steel to conduct the flux to the workface. Allows for better means of mounting-tapped holes, threads, press fits, etc.

Magnetic circuit, an assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, and electrical currents.

Magnetic Length, The physical length of the magnet dimension which corresponds to the direction the magnet is magnetized.  This may or may not be the magnet’s orientation direction.

The major hysteresis loop of a material is the closed loop obtained when the material is cycled between positive and negative saturation.

The maxwell is the unit of magnetic flux in the cgs electromagnetic system. One maxwell is one line of magnetic flux.

The neutral section of a permanent magnet is defined by a plane passing through the magnet perpendicular to its central flux line at the point of maximum flux.

North pole, is the pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often the best to clarify by using “north seeking pole” instead of “north pole” in specifications.

The oersted is the unit of magnetic field strength, H, in the cgs electromagnetic system. One oersted equals a magneto motive force of one gilbert per centimeter of flux path.

An open circuit condition exists when a magnetized magnet is by itself with no external flux path of high permeability material.

The operating line for a given permanent magnet circuit is a straight line passing through the origin of the demagnetization curve with a slope of negative Bd/Hd. (Also known as permeance coefficient line.)

The operating point of a permanent magnet is that point on a demagnetization curve defined by the coordinates (BdHd) or that point within the demagnetization curve defined by the coordinates (BmHm).

Orientation direction, is the direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties. Also known as the “axis,” “easy axis,” or “angle of inclination.”

An oriented (anisotropic) material is one that has better magnetic properties in a given direction.

Paramagnetic material, is a material having a permeability slightly greater than 1.

A permeameter is an instrument that can measure, and often record, the magnetic characteristics of a specimen.

P Permeance, is the reciprocal of the reluctance, R, measured in maxwells per gilbert.

Pole pieces, are ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.

R Reluctance, is somewhat analogous to electrical resistance. It is the quantity that determines the magnetic flux, ø, resulting from a given magneto motive force, F.
Where: R=F/ø
R=reluctance, in gilberts per maxwell
F=magneto motive force, I gilberts
Ø=flux, in maxwells.

Return path, are conduction elements in a magnetic circuit, which provide a low reluctance path for the magnetic flux.

Saturation, A condition where the increase in applied external field yields no increase in induction.  When this condition is met, all of the elementary magnetic moments have the same alignment.  This condition is important in permanent magnet alloys and in Ferromagnetic alloys.  Magnet alloys must always be magnetized to saturation.  The magnet may not be used at this level, but before conditioning and stabilization the magnet must always first be magnetized to saturation.  Usually saturation should not be exceeded in Ferromagnetic alloys which comprise the yoke or return path elements of a magnetic circuit.  If ferromagnetic elements are saturat ed there will be flux leakage in the system and a redesign should be considered.

A search coil is a coiled conductor, usually of known area and number of turns, that is used with a flux meter to measure the change of flux linkage with the coil.

Sintered, A sintered magnet is comprised of a compacted powder which is then subjected to a heat treat operation where the full density and magnetic orientation is achieved.

Stabilization, is exposure of a magnet to demagnetizing influences expected to be encountered in use in order to prevent irreversible losses during actual operation. Demagnetizing influences can be caused by high or low temperatures, or by external magnetic fields.

Tc, Curie temperature, is the transition temperature above which a material loses its magnet properties.

T max Maximum service temperature, is the maximum temperature to which the magnet may be exposed with no significant long-range instability or structural changes.

Reversible temperature coefficients are changes in flux which occur with temperature change. These are spontaneously regained when the temperature is returned to its original point.Magnetic saturation of a material exists when an increase in magnetizing force produces no increase in intrinsic induction.

The temperature coefficient is a factor which describes the reversible change in a magnetic property with a change in temperature. The magnetic property spontaneously returns when the temperature is cycled to its original point. It usually is expressed as the percentage change per unit of temperature.

An unoriented (isotropic) material has equal magnetic properties in all directions.

Vg Air gap volume, is the useful volume of air or nonmagnetic material between magnetic poles; measured in cubic centimeters.

Weber, is the practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric current during an interval of 1 second, will induce in this circuit an electromotive of force of 1 volt.

µ permeability, is the general term used to express various relationships between magnetic induction, B, and the field strength, H.

µre recoil permeability, is the average slope of the recoil hysteresis loop. Also known as the minor loop.

ø magnetic flux, is a contrived but measurable concept that has evolved in an attempt to describe the “flow” of a magnetic field. Mathematically, it is the surface integral of the normal component of the magnetic induction, B, over an area, A.
ø = ??B • dA
where:
ø = magnetic flux, in maxwells
B = magnetic induction, in gauss
dA = an element of area, in square centimeters
When the magnetic induction, B, is uniformly distributed and is normal to the area, A, the flux, ø = BA.