Download Small Dictionary of Magnetism

Small Dictionary
of Magnetism
We have collected most important terms and definitions on the topic of magnetism and magnets.
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Alnico
An alloy of aluminum, nickel, and cobalt, Alnico magnets have been popular since the
1930s. Alnico magnets are available in cast and sintered forms. Extensively used in
rotating machinery, Alnico magnets exhibit high Br, but low Hc, and BHmax in the 5 MGOe
range. They are the most temperature stable magnets of all the different types, and may
be used with no significant losses up to about 500 C. They are relatively easily
demagnetised, resisting oxidation well. Energy product ranges from 1.4 MGOe to about
7.5 MGOe.
Anisotropy
The characteristic of a substance for which a physical property, varies in value with the
direction. Anisotropic magnets are exposed to a high magnetic field, in a determined
direction, during the sintering process. In this magnetic field direction, higher magnetic
values are obtained than in a transversal direction.
Coercive
Force
The value of demagnetising force that reduces residual induction to zero. The maximum
coercive force, as measured on a saturated magnet, is proportional to the remanent flux
density, calculated in Oersteds or KiloAmps per meter (kA/m).
Coercivity
The resistance of a magnetic material to demagnetisation. It is equal to the value of H that
cancels the magnetic induction, and is calculated in Oersteds or KiloAmps per meter
(kA/m).
Named after Pierre Curie. It is the temperature above which ferromagnetic materials
Curie
become paramagnetic, losing substantially all of their permanent magnetic properties. It
Temperature normally depends on the chemical composition of the magnetic material.
Demagnetisa The part of a hysteresis loop which is found in the second or fourth quadrant, and which
defines the main magnetic properties of a magnet. The points on the curve are designated
tion Curve
by the coordinates Bd and Hd.
Density
Measures specific weight, indicated in gr/cm³ or kg/dm³.
Diamag
netism
Diamagnetism is the property of an object or material which causes it to create a magnetic
field in opposition to an externally applied magnetic field. Unlike a ferromagnet, a
diamagnet is not a permanent magnet. Diamagnets were first discovered when Sebald
Justinus Brugmans observed in 1778 that bismuth and antimony were repelled by
magnetic fields. The term diamagnetism was coined by Michael Faraday in September
1845, when he realized that every material responded (in either a diamagnetic or
paramagnetic way) to an applied magnetic field.
Energy
Product
Calculated in megaGauss-Oersteds, MGOe, or kJ/m³ , it is the energy that a magnetic
material can supply to an external magnetic circuit when operating at a point on its
demagnetisation curve.
Ferrite
(Ceramic)
Composed of Barium or Strontium Ferrite, these materials exhibit good resistance to
demagnetization, and are the lowest cost materials available today. Sintered Ferrite
magnets are hard and brittle and are extensively used in consumer products. You are
probably familiar with ferrite magnets, often used on the back of refrigerator magnets.
Energy product ranges from 1.1 MGOe to about 4.5 MGOe.
Ferromag
netism
Ferromagnetism is the basic mechanism by which certain materials (such as iron) form
permanent magnets, or are attracted to magnets. Ferromagnetism is the only type of
magnetism that creates forces strong enough to be felt, and is responsible for the
common phenomena of magnetism encountered in everyday life.
Flux / Flux
Density
It is the magnetic field, represented conceptually as “magnetic lines of force”, calculated in
Gauss or Tesla.
Gauss
Named after Friedrich Gauss. Unit of measure of magnetic induction, B, in the CGS
system. See Tesla.
1 G = 10-4 T
1 kiloGauss = 1000 Gauss
1 mT = 10 G
1 T = 1000 mT
Holding
Force
This means the strength of a magnet or magnetic system. Holding force is measured as
the force you need to detach a magnet from an optimal surface. It is usually given in kgs
or in Gauss.
Hysteresis
It is the tendency of a magnetic material to retain its magnetization, causing the graph of
magnetic flux density versus magnetizing force to form a loop. The area of the loop
represents the difference, or hysteresis loss, measured at low frequency, between energy
stored and energy released per unit volume of material per cycle.
Intrinsic
Coercivity
Value measured in Oersteds or kA/m, which indicates a material’s resistance to
demagnetisation. The maximum value is obtained after the material has been saturated
(fully magnetised).
Irreversible
Loss
This occurs when a magnet is fully or partially demagnetised due to exposure to high or
low temperatures, or other external demagnetising fields or factors. By remagnetising the
magnets, these losses can be solved.
Isotropy
A magnet is isotropic when its properties are independent to its orientation. This means
that the particles are not oriented and are randomly arranged. It has no preferred direction
of magnetic orientation, which allows magnetisation in any direction. Usually, magnets are
anisotropic: they have a preferred direction of magnetic orientation because the material
has been oriented during its processing.
Magnetic
It is the magnetising or demagnetising force, measured in Oersteds, which determines the
Field Strength ability of an electric current or a magnetic body to induce a magnetic field at a given point.
Magnetic
Induction
Magnetic induction, B, is the magnetic field induced by an applied field, H, measured in
Gauss. It is composed of two parts. One part results from the laid down field, the other
from the matter.
Magnetising
Field
When a magnetic field is applied to a material, it is magnetised.
Maximum
The maximum temperature of exposure that a magnet can resist without an instability or
Operating
structural changes being caused to its properties.
Temperature
North Pole
The magnetic pole which attracts the geographic North Pole of the earth.
Neodymium
Iron Boron
A class of Rare Earth material, available in both sintered and bonded forms. Major
characteristics: the most powerful (highest energy product) class of magnet material
commercially available today. High Br, relatively high Hc, high BHmax, lower in cost than
Samarium Cobalt, temperature sensitive, and susceptible to oxidation. With careful
design, some grades will function up to a maximum of 210 C. Energy product ranges
from 1.0 MGOe to about 48 MGOe.
Oersted
Named after Hans Christian Ørsted. The unit of magnetic field strength, H, in the cgs
electromagnetic system. One oersted x 0.08 = 1 kA/m.
Saturation
It is the maximum value of magnetisation, referred to the decrease of permeability with
increasing magnetisation force. It is the flux density of maximum value magnetisation.
Highest obtainable magnetic polarisation of a magnet. In an inductor, it corresponds to a
decrease of inductance with current.
Remanent or Also called remanence or residual flux, is the remaining flux density in a magnetic material
Residual Mag after an applied magnetic field has been removed or reduced to zero. Br is expressed in
Tesla (T), Millitesla (mT) or Gauss (G).
netism, Br
Samarium
Cobalt
A class of Rare Earth material, available in both sintered and bonded forms. Major
characteristics: High Br, high Hc, relatively high BHmax, highly resistant to oxidation. Most
grades function well up to about 350 C. Energy product ranges from 18 MGOe to about
32 MGOe.
Sintering
Process in which after exposing pieces to high temperatures, and being pressed, they
decrease in size and become homogeneous. Temperatures for ferrites: approximately
between 1200ºC and 1250ºC. Temperatures for rare earth magnets: approximately
between 1050ºC and 1200ºC.
South Pole
The south pole of a magnet is attracted towards the south geographic pole of the earth.
Changes in temperature produce reversible changes in magnetic properties. When a
Temperature magnet has been exposed above or below a critical temperature, sometimes due to the
materials magnetic charecteristics, an irreversible cos can be produced, however these
Coefficient
characteristics can be recovered by re-saturating.
Tesla
Named after Nikola Tesla. The Tesla is the SI derived unit of magnetic flux density,
commonly denoted as B, (which is also known as "magnetic field"), defined by Faraday's
Law. A Tesla equals a volt-second per square meter per turn, which is the same as 10.000
Gauss. 1 Tesla = 1Vs/m2 or 10.000 Gauss.
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