Glossary
Up to the middle of nineteenth century magnetism was considered a physical phenomenon separated by the electric ones. But the processes that rules the magnetism are strongly tied up to the electric phenomenon by mean of the Maxwell equations. For a best understanding of magnets behaviour we bring here some technical terms: they are presented in a precise form, also comprehensible to who draws near for the first time to this matter.
B
(vedi Induzione Magnetica)
Air gap
Distance from a non magnetic material laid between the poles of a magnetic circuit: it can be seen as a obstacle to the magnetic flux flow, so, if no useful to the device, it should be minimized. To increase the magnetic induction and its homogeneity, it is necessary to minimize the air gap.
Anisotropy
It is the preferred direction along which a magnet can be oriented some magnets have high anisotropy (anisotropic magnets), some others not (isotropic). The anisotropic orientation can arise from the production process but in many cases it is due to the material structure. In anisotropic magnets the best properties can be obtained along the anisotropy direction.
B
(vedi See Magnetic Induction)
Barium
Chemical elements of 2nd group (alkaline earthy). The most important material for magnetism is barite: it is added as barium carbonate to iron oxide to produce barium BaFe12O19(barium ferrite).
(B•H)max
It is the maximum energy product in a permanent magnet: more in detail, it is the maximum of the BH values in 2nd quadrant of hysteresis loop. BH product represents the energy density per volume unit: in general we can state that increasing the (B • H)max value we get a decrease of the magnet volume useful to obtain the same performance.
Calibration (magnetic)
Specific magnetizing and demagnetizing procedure by which it is possible to minimize permanent magnet tolerances. For specific applications it is needed to tune the magnetic flux in more strict range than usual (ex. In specific electric motors, sensors, electromechanical relays).
Coercive strength
It is the value of magnetizing field H during whose action the induction B of a ferromagnetic material previously magnetized to saturation, was set to 0. The coercive strengths jHc (intrinsic) and bHc (extrinsic) can by found: this distinction is technically important for magnets with high coercive strength. You can have the coercive strength jHc from the J hysteresis loop, while Hc is got from B hysteresis loop. The SI unity of coercive strength is given in KA/m or in Oersted.
Curie Temperature
The temperature at which a ferromagnetic material becomes paramagnetic and looses its magnetic properties. Name derived from M.me Curie, physicist and chemist of early ’900.
Demagnetization
Decrease of magnetization condition (that is of B) in a magnet which also means decrease of its performances, by mean of strength field H opposed to its magnetization direction: to get a complete demagnetization it is required a oscillating strength field. A partial or total demagnetization can also happen at very high (or low for ferrite magnets) temperature: it will be a partial and irreversible demagnetization when temperature is between max working temperature and Curie temperature, total if Curie temperature is passed Max working temperature is linked to material properties, to magnet and magnetic circuit geometry while Curie temperature is characteristic of the material itself.
Demagnetization curve
Is the part of hysteresis loop in 2nd quadrant, where H value is negative. The main properties deducible from this curve are Br (remanence), Hc (coercive force) and maximum Energy product. The DIN 17410 definition consists in 2 numbers, where the first one is the minimum value of energy product, the second the minimum coercive force divided by 10, in SI unit system. For ex. 28/26 means: (B • H)max> 28 kj/m3 , jHc> 26 • 10 – 260 KA/m.
Density (weight)
Density weight in Kg/m3, in g/cm3 or Kg/dm3 (1 g/cm3 = 1 Kg/dm3 =103 Kg/m3 )
Energia immagazzinata (B•H)
Moltiplicando i valori di induzione B e della intensità di campo H si ottiene una grandezza legata alla densità di energia per unità di volume. Vedasi anche valore (B • H)max.
Factor of demagnetization (N)
It is a factor mainly depending on magnet geometry and expresses the possibility to get good performance from the magnet geometry or magnetic circuit. If the working point of magnet is linked with the origin of coordinates system B-H, a straight line is obtained. The demagnetisation factor N is linked to such line and it is a non-dimensional quantity, assuming values between 0 (closed magnetic circuit) and 1 (magnetic circuit completely open). It’s tied up to the permeance from the relation: P=1−1/N
Ferromagnetism
Materials with permeability much higher than 1. Their behaviour can be described as joined action of magnetic domains, each with its magnetic elementary moment. Permanent magnet as permeability slighly higher than 1, while soft ferromagnetic materials have relative permeability much higher than 1 (from 10^2 to 10^6).
Flux density
Flux density of the magnetic induction B. In case B is crossing uniformly surface A, it’s equal to the ratio between the flux and the cross section area A.
Fluxmeter
Measuring instrument of the magnetic flux: the measure is obtained by integrating electronically or digitally the voltage at the measuring coil.
Flux useful
Part of the total magnetic flux that passes through a useful gap of a magnetic circuit; the remaining part of the total flux is said “leakage flux”.
Gaussmeter
Also called magnetometer. Instrument to measure the flux density: it usually works with the Hall effect in semiconductor of the sensor. It directly shows the density of magnetic flux, without movement of the measuring probe.
H
Intensity of the magnetic field: this property establishes how a material is magnetically strained. H tightly depends on the current circulating into a winding, set around or in proximity of the material submitted to H.
Hard ferrite
Barium ferrite, strontium ferrite or lead, with chemical formula MeO • 6Fe2O3 where MeO represents a metallic oxide. All the permanent magnets in hard ferrite are hexagonal for ex. BaO • 6Fe2O3 or SrO • 6Fe2O3.
Hysteresis cycle
It can be produced by tracing the induction B versus magnetic strength field H, first with H positive (magnetization loop-1st quadrant of hysteresis loop), then switching to H negative (2nd quadrant loop, in demagnetization loop). It can be performed for B or J.
Irreversible losses
Losses of the magnet properties to high temperature, not recoverable by returning to the initial temperature (normally this is the room temperature). For ferrite the remanence decays at low temperatures.
Magnetic flux
Flux of the magnetic induction B through a surface A. If B is homogenous across A ihen is equal to B*A, otherwise it is the mathematical integral of B on surface A. Unity of measure in SI system: 1 Wb (Weber) = 1 Vs (Volt*second).
Magnetic Induction (B)
It’s the property that shows the state of magnetization of a magnetic material: its main original definition is founded on the effect that an
induction field equal to 1 Wb/m2 creates on a metallic conductor crossed by current. Unity Tesla (T=Wb/m2). B = μo H + j.
Magnetic polarization (J)
Also called intrinsic induction. Contribution of the material to the flux density: J = B — μo H.
Magnetic pole
Surface of permanent magnet from which the magnetic flux goes out or enters: it can be Norh or South.
Magnetization
To magnetize a magnet, an external strength field some times higher its coercive strength is employed. The duration of the magnetization is generally very brief, from few microseconds to some seconds.
Maxwell
Measurement unity of the magnetic flux in the GCS system (see also the magnetic flux) and it is equal to 10−8 Tesla m2.
Oersted
Measurement unit of magnetic strength field H in GCS. It comes from the name of Danish Physicist Hans Christian Oersted.
Preferential direction of magnetization
The direction of magnetization along which the magnet reaches the best values in terms of Br, Hc and BHmax (see also anisotropy) and it is due to the magnetic preorientation or structure of the material. Often in magnets with circular symmetry it is axial. In magnets with rectangular section it is often the minimum thickness. In segments of arc is the diametrical direction (that is the parallel lines to the diameter) or radial.
Permeability (μ)
It is the ratio between the magnetic induction B and the magnetic field H. It can be explained as a sort of magnetic “transmissibility” or “conductibility”. In vacuum it is a constant: µo = 1,256 H/ms (T / A/m). In diamagnetic materials µr <1, while in paramagnetic materials µr>1; in ferromagnetic material µr >> 1 (from 102 to 106). It is often used the relative permeability , defined as µr = µ/µ o = B/µo H.
Permeance
Ratio betweeng the induction B and the product between o and H: in a magnetic circuit the higher is the permeance module, the closer is the magnetic circuit (no flux leakages).
Ratio h/D
Ratio between the height of a magnet and its diameter; it is a very important ratio especially in open circuit magnets (without soft ferromagnetic parts). In demagnetization curves the values h/D can be plotted, so that for every h/D value the induction can be computed at the working point. On this ratio depends the magnetic performances of a magnet of given size.
Recoil permeability (μp)
Permanent magnet permeability in second quadrant of hysteresis loop.
Remanence or residual induction (Br)
It’s the value of induction Br in a closed circuit magnetic material (no air gaps), when H is equal to 0. In SI is given in tesla (T), in millitesla (mT); in cgs is given in Gauss(G).
Reversible
Reversible o repeatable. A magnet behaviour is thermally reversible when it returns to its original properties after a heating or cooling process.
Saturation
Condition in which the induction B increases in the material with an inclination equal to o, as consequence of an increase of H; after reaching the condition of saturation, the magnet can get the maximum values of magnetic properties.
Sintering
Process of high pressure powder compacting at high temperature, with the purpose to get material compacted and homogenized. The temperatures of Sintering are, approximalety: for hard ferrite around 1200°C – 1250°C, for rare earths magnets around 1050°C – 1200°C.
Stabilization (magnetic)
To stabilize a magnet it is possible to raise it to a defined temperature, near to the maximum working temperature of the magnet or to dip it in an oscillating magnetic field and produce light demagnetization of the same magnet. This should prevent possible small demagnetisation, produced by inside and external factors.
Stored energy (B•H)
Multiplying the values of induction B by the intensity of field H we get a range tied up to the energy density for volume unit. See also the (B • H)max value.
Strontium
Chemical element of the II group (alkaline-earthy metals). it comes from the minerals stronzianites and celestine. The strontium is added in the form strontium carbonate instead of barium carbonate and confers to hard ferrite magnets a particularly elevated coercive strength.
Susceptibility (magnetic)
It describes the dependence between magnetization and magnetic field. The relation with the Magnetization is: M = X • μt H e μr = X+1
Temperature coefficient
In magnetic materials the variation of residual induction or coercive force may be related with temperature changes: it is a very important parameter as magnets are very sensitive to temperature value.
Tesla
Measuring unit of the magnetic flux density , also called magnetic induction. 1 T = 1 Vs/m2 = or 10.000 Gausses. It comes from the name of the Serbian physicist Nicola Tesla (1856-1943).
Weber
Measuring unit of the magnetic flux 1 Wb = 1 V s = 108 Maxwell. Name derived from Mr. miby Wilhelm Weber
Working temperature
The most elevated temperature at which a magnet can be maintained without irreversible flux losses. The working temperature also depends on the magnetic circuit in which the magnet is employed: it produces more easily effects of demagnetization in case of low permeance, that is in case of open magnetic circuit. The most unfavourable case will be therefore a single magnet with very small L/D ratio (for instance 0.3 or smaller).
Working point
Point of the demagnetization curve that gives the values of the flux density B and of the coercive field H in the working point. The greater is the length of the magnet in the direction of magnetization, the closer is B to Br. In a closed magnetic circuit, the working point corresponds to the value of Br.
