Air gap

It is the distance between the working face of the magnet and the part with which it is interacting. Often air, but inclusive of other materials such as coating layer thickness on magnet. If the air gap is enlarged, the interacted strength will be weakened.

Anisotropic Magnet

In manufacturing process, the molecule of magnetic material is aligned by an external magnetic field what process is also called anisotropy or orientation to obtain the higher magnetic value in the direction of anisotropic axis. Except AlNiCo magnet is oriented in the process of heat treatment, all other magnets get anisotropy in the molding process. Magnetizing direction on an anisotropic magnet only could be along the anisotropic axis. Anisotropic magnet will output much stronger flux and remanence than isotropic magnet does?

B-H curve / Hystersis loop

It 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. In practice, if a magnet is operated in a static manner with no external fields present, the normal curve is sufficient for design purposes. When external fields are present, the normal and intrinsic curves are used to determine the changes in the intrinsic properties of the material.

Closed circuit condition

It exists when the external flux path of a permanent magnet is confined with high permeability material.

Coercive Force (Hc)

It is the demagnetizing force required to reduce residual induction (Br) to zero in a magnetic field after magnetizing to saturation.

Curie Temperature (Tc)

It is a temperature point above which the magnet will completely lose its permanent magnet properties. The Curie temperature is dependent on magnet composition.

Demagnetization curve

The second quadrant of the hysteresis loop, it completely characterizes the magnetic properties of the material at a specific temperature. To get correct demagnetization curve, a specific size sample in compliance with relevant standard must be used.

Dimension ratio h:D

It is the ratio of a magnet length 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. On the page with demagnetization curves and the h:D values are indicated, it is possible to read valid remanence for each h:D value. With very low h:D ratio, starting with 0.3, these values are only approximate one. If the magnet is with thin thickness but big length or outside diameter and in a closed magnetic circuit, then magnet will loss force faster when the environment temperature is increased.

Energy product (BdHd)

It indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve.

Flux density

It expresses the concentration of line of force per unit area passing from one pole to the other. Magnet material type, dimension ratio, air gap and position of probe determine the measured value of flux density.

Hd

Is that value of H corresponding to the remanent induction (Bd)

Hysteresis Loop

See B-H curve

Intrinsic coercive force (iHc)

Intrinsic coercive force of a material indicates its resistance to demagnetization. It is equal to the demagnetizing force which reduces the intrinsic induction in the material to zero after magnetizing to saturation. It is used to as a measure of resistance to demagnetization, the higher the figure, the more stable it is.

It is the necessary demagnetizing force to reduce the intrinsic induction in a magnet to zero after magnetizing to saturation.

Irreversible losses

The irreversible losses are defined as partial demagnetization of the magnet, caused by exposure to high or low temperatures, external fields or other factors, and what are not recoverable when the magnet returns to its original temperature or other conditions are lifted. Value for irreversible loss is not definite and is influenced by magnet material, geometric dimension, operating point and its working magnetic circuit.

Some irreversible losses are recoverable by re-magnetization and are not recovered when the temperature returns to its original value. These losses occur when the operating point of the magnet falls below the knee of the demagnetization curve. The qualified design of permanent magnet should have a magnetic circuit in which the magnet operates at an operating point above the knee of the demagnetization curve measured at expected temperature to avoid performance variations along with temperature change.

Some irreversible losses are not recoverable by re-magnetization due to metallurgical changes occur in magnets exposed to very high temperatures.

Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields. The resulting lowering of induction must however be accepted.

Isotropic Magnet

The magnet what not pre-oriented by an external magnetic field is called isotropic magnet. Magnetic values measured in any direction or axis of an isotropic magnet is with no difference. Isotropic magnet could be magnetized with any preferred pattern what is determined by magnetizing yoke.

Keen of the demagnetization curve

It is a point at which the B-H curve ceases to be linear and starts to be down. One magnet draws different B-H curve and has various keen when it is working at different temperature. If the operating point of a magnet falls below the knee, small change of H will result big change of B and the magnet will suffer irreversible loss of flux. The irreversible flux loss can be recovered after re-magnetization.

Keeper

One or several pieces of soft iron that is/are 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.

Mechanical characteristics

In permanent magnet families except bonded magnets and cast AlNiCo magnet are all made by powder metallurgic process, so they are lack of ductility and are very brittle. We strongly recommend careful handle on them and do not design it as structural parts to avoid chip and being damaged. The mechanical characteristic of each class magnet is listed on their section accordingly.

Magnetic circuit

Magnetic circuit is consisted of permanent magnets, pole tips, air gap and leakage fields.

Magnetic field strength (H) / Magnetizing or demagnetizing force

It 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.

Magnetic flux

It 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 over an area.

Magnetic induction (B)

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

Maximum Energy Product ( (BH)max)

Is the maximum product of (BdHd) which can be obtained on the demagnetization curve. Using magnet with greater (BH)max value can reduce the magnet volume if the application is given the same task and other conditions remain unchanged. The unit of (BH)max is kJ/m3 or MGOe

Maximum Working Temperature

This is the maximum temperature to which the magnet may be exposed with no significant long range instability and structural changes. It is valid for magnets with sufficient large ratios of h:D and free from happening of demagnetization because of dimension change. On bonded magnet, it is also dependent on the working temperature of binder.

In the range of working temperature, the remanence and coercive field strength will back to its original value when the temperature returns to departure point. In case of out of this working temperature range, rapid reduction of magnetic strength can occur and new temperature coefficient has to be reckoned with. There are several factors determine the working temperature of individual magnet: ratio of dimension h:D and mechanical or chemical strain.

Orientation direction

It also called as “axis”, “easy axis” or “angle of inclination”. The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties is called orientation direction.

Open circuit condition

It exists when a magnetized magnet is by itself with no external flux path of high permeability material.

Operating point

The location on a demagnetizing curve with specific flux density (B) and field strength values (H) is called operating point. The longer the magnet is in the direction of anisotropic axis, the nearer the operating point is at remanence (Br). In the closed magnetic circuit the operating point corresponds approximately to Br. This is determined by the temperature and operating conditions as well as the geometry of the magnet.

On account of the temperature coefficient which is basically negative with rare earth magnets, the operating point is of the great significance at higher temperature. Magnet with low operating point and / or opposing magnetic fields can, however suffer persistent loss of magnetization caused by reduction of coercivity at high temperature. To avoid performance variations along with temperature change, the qualified design of permanent magnet should have a magnetic circuit in which the magnet operates at an operating point above the knee of the demagnetization curve measured at expected temperature.

Permeability

It is used to express the ability of induction to pass through a material and demonstrate various relationships between magnetic induction (B) and the field strength (H). For some permanent magnets, its permeability is only marginally better than air.

Permeance (P)

It is the reciprocal of the reluctance (R), measured in maxwells per gilbert.

Permeance coefficient (Pc)

It is the ratio of the remanent induction (Bd) to a demagnetizing force (Hd), what is also referred as the operating slope, slope of the operating line, shear line, load line or B/H of the magnet in open circuit condition. It is an important data in magnet material selection to ensure optimal design.

Pole pieces/counter-plates

The other elements use to construct an efficient holding magnet. Both components are mild steel or iron and the purpose of the pole pieces is to collect the flux from the pole faces and concentrate this to the point of attachment with the counter-plate. The counter-plate in effect completes a magnetic circuit.

Recoil Permeability

It is the average slope of the recoil hysteresis loop, also known as a minor loop.

Reluctance (R)

It is the quantity that determines the magnetic flux resulting from a given magneto-motive force, somewhat analogous to electrical resistance.

Remanent induction (Bd)

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.)

Residual induction / flux density / Remanence (Br)

Is the magnetic induction corresponding to zero magnetizing force in a magnetic material after saturation in a closed circuit.

Return path

Steel or ferrous elements in a magnetic circuit which form a low reluctance path for the magnetic flux is called return path.  

Reversible losses

Reversible losses are defined as partial demagnetization of the magnet, caused by exposure to high or low temperatures, can be recovered when the magnet returns to its original temperature.

Reversible temperature coefficients

It 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 is usually expressed as the percentage change per unit of temperature.

RoHS

It is the abbreviation of “The Restriction of the Use of Certain Hazardous Substance in Electrical & Electronic Equipment”. According to European Community directive 2002/95/EC of 27th January 2003, the content of hazardous material of Pb, Cd, Hg, Cr6+, PBDEs and PBBs in all parts should be controlled within specified PPM. This directive is effective from 1st July 2006.

Slope of the operating line (Bd/Hd)

See permeance coefficient

Stabilization

To prevent magnetic flux change caused by later external influences, treatments on a magnet with defined temperature or being intentionally exposed in a magnetic field are called stabilization.