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Glossary Terms

Air Gap The portion of a magnetic circuit which does not contain the magnet, or other magnetic materials. This is usually the area where the magnet action takes place. (Examples: The distance between a magnet and its associated catch plate. Also the gap of a magnetic recording head.) The Air Gap is usually filled with something other than air such as plastic, wood, paper, ceramic or other non-magnetic material.

Anisotropy (magnetic) A magnetic sample is said to be magnetically anisotropic when the magnetic properties depend on the axis along which magnetic measurements are made. There are several types of anisotropy. The easiest to understand is shape anisotropy. A nail is much easier to magnetize parallel to its length. All magnetic recording tapes are made using tiny single domain particles having shape anisotropy. Crystal anisotropy arises from very complicated quantum mechanical interactions among the atoms making up a crystalline sample. High coercivity ceramic Barium Ferrite (BaFe) is a material having crystalline anisotropy. BaFe is made of tiny single domain particles which are aligned during manufacture to greatly enhance the magnetic properties of the magnet. Flexible magnets are also loaded with BaFe and are generally isotropic but may also be anisotropic. NdFeB is usually an anisotropic material. The opposite of Anisotropic is Isotropic.

Anisotropy Axis The axis along which an anisotropic magnet is most easily magnetized.

Arisotropy field (HK) The magnetic field Hk needed to magnetize an anisotropic sample to saturation parallel to the hard axis which is perpendicular to the anisotropy axis. Hk and other fields H are in Oersteds (Oe) in cgs units or Amps/meter (A/m) in MKS-SI units. There are 1000/(4∏) or about 80 A/m in one Oe. Generally fields H are vector quantities having both magnitude and direction.

B (Magnetic Induction) The magnetic flux density or flux per unit area at a given point in space. B is specified in Gauss (G) in cgs units or Tesla (T) in MKS-SI units. In general B is a vector quantity having both magnitude and direction.

B-H Curve or Loop A graphical curve showing the relationship between the magnetic flux density B and the magnetic field H for a magnetic material. This is often also referred to as the magnetization curve, or demagnetization curve. It is plotted from a large positive H value to a large negative H value and back to a large positive H value. B is specified in Gauss (G) in cgs units or Tesla (T) in MKS units. H is specified in Oe in cgs units or A/m in MKS-SI units.

An almost never stated but very important assumption in any B-H curve is that B is parallel to the applied field H, and H is the only field that exists. In anisotropic materials or an anisotropic sample, when the field H is not applied parallel or perpendicular to the anisotropy axis, the vectors B and H are usually not parallel. A B-H curve measured in this way is almost meaningless. For example, measuring the B-H loop of a nail with the field H applied at an angle to the length of the nail will give meaningless results. The field H must be applied parallel or perpendicular to the anisotropy axis so that B, M and H are parallel.

Another factor which is often ignored is that the maximum field Hm must be large enough to saturate the sample. This is conveniently determined when the upper parts of the loop are “closed”.

BHmax (Maximum Energy Product) On a B-H graph this is the point at which the product B•H reaches a maximum. The values of B and H at that point are called Bd and Hd. The product Bd•Hd gives BHmax, and is called the Maximum Energy Product. The units are almost always MGOe or 10⁶ erg/cm³. The Grade of a magnet usually incorporates this number. For example: N40 Grade NdFeB has a maximum energy product between 38-41 MGOe. BHmax is the primary indicator of the magnet material “strength”. In general, the higher the BHmax value, the higher the field H the magnet will generate in a particular application.

Br (Remanence) The magnetic flux density remaining in a magnet after the magnetizing field H has been removed. This is often referred to as the remnant induction.

Coercive Force or Coercivity (Hc) The demagnetizing field H necessary to cause a magnet, which has been previously magnetized to its maximum or saturation magnetization Ms, to have zero flux density B. The units of Hc are Oe in cgs units and A/m in MKS-SI u

Curie Temperature (Tc) The temperature at which a magnetic material becomes nonmagnetic. The atomic magnetic dipoles within the magnet are thermally randomized, resulting in a magnetic material which doesn't exhibit magnetic properties. Once heated above Tc and cooled (in zero field H) the magnet is demagnetized.

Diamagnetic A material is said to be diamagnetic when a field induces a magnetization –M in the direction opposite to that of the field H! The diamagnetic magnetization is very small but it is easily measured. Literally everything is diamagnetic to a small degree. The metal Bismuth is the most diamagnetic of all common materials. Chemical vapor deposited Pyrolytic carbon or graphite has the highest diamagnetism of any material.

Diameter Width specification (across the flat face) of one of our rare earth magnets which is round in shape.

Diametrical A cylindrical magnet which is magnetized across the diameter (width) is said to be diametrically magnetized or diametrical. If you have two diametrically magnetized discs or rods they will attach side to size at 0 & 180 degrees forming a figure 8 or OO

Dimensional Tolerance The design tolerance of a magnet. Magnets are produced in lots. Lot-to-lot, the finished dimensions will be within this tolerance. Within a specific lot, the part- to-part dimensional variance is usually much smaller than the quoted dimensional tolerance.

Dimensions The finished size, including all surface treatments (coatings, plating, etc).

Dipole Moment (Magnetic) The magnetic dipole moment m of a magnet is given by the product m = MV where M is the magnetization and V is the volume of the magnet. The units are emu in cgs units and Am² in MKS-SI units.

Electron Dipole Moment (µe) All magnetic materials are magnetic because the atoms making up the material contain electrons which have the quantum mechanical property called “Spin” and a basic electron magnetic dipole moment µ_b~ 9.2*10^-21 emu called a Bohr magneton.

Flux (F) The magnetic flux Φ through an area is the integral (or sum) of B times the area. Φ is measured in Maxwell's (Mx) in cgs units and Webers (Wb) in MKS units.

Gauss (G) The Gauss, abbreviated as G, is the cgs unit of measurement of the magnetic flux density B (also known as the "magnetic induction"). It is named after the famous German mathematician and physicist Carl Friedrich Gauss. One Gauss = 1 Maxwell per cm²

Gauss Meter An electronic device which measures magnetic fields H using a Hall Effect Sensor. Since it really measures H in Oersteds, not B in Gauss, it should really be called an “Oersted Meter” but, unfortunately, it never is!

Grade A list of specifications which define the magnetic properties of a given magnetic material. (See Grade Chart)

H (Magnetic Field Strength) A measure of the magnetizing field. H may arise from an electrical current or from another permanent magnet. The units of H are Oe in cgs units or A/m in MKS units. The earth’s magnetic field is about 0.6 Oe.

Hcr (Remanent Coercivity) Hcr is the demagnetizing field needed to cause the remanent magnetization Mr to be zero after the sample is saturated. Mrs is the remanent magnetization in zero field after the sample is saturated.

Height The distance between the flat areas of the magnet, usually measured face to face. Height is usually expressed as a measurement of magnets which are wider then they are long. Longer cylindrical magnets would have their height refered to as length.

ID Inner Diameter of a magnet. In a ring or tube type magnet this would be the width/diameter of the center hole.

Intrinsic Coercivity (Hci)

The demagnetizing field H necessary to cause a magnet which was previously saturated to have zero magnetization M. The units of M are emu/cm³ in cgs units and A/m in MKS-SI units. 1000 A/m = 1 emu/cm³. In general Hc and Hci are quite different with Hci usually being much larger than Hc.

The relation between B, M and H in MKS-SI units is B = µ₀(H + M), where µ₀is “the permeability of free space” µ₀= 4∏*10^-7,and B = H + 4∏iM in cgs units. Strictly speaking these are vector equations but generally H and M, and hence B, are parallel and can be considered to be scalars.

Isotropic An isotropic magnet has the same properties when measured parallel to any axis. During the manufacturing process the material can be aligned or oriented in any direction. NdFeB is an anisotropic material. Rubberized magnets are often Isotropic, and are magnetized in North-South bands in a special very high field fixture.

Surprisingly, the magnetization pattern in flexible magnets is usually “one sided”. Nearly all of the field comes out on one side of the sheet! The flexible sheet is uniformly magnetized in a special pattern where the vertical and horizontal fields add on one side and cancel on the other. This doubles the holding force.

Length The measured distance along the magnet from end to end. On a disc magnet (where length < diameter) this same measurement would be refered to as the thickness.

M-H Loop

A graph of the magnetization M plotted vs. H. The differences between B-H and M-H loops have generated a great deal of confusion. This is not surprising because many text books contain important errors! The two loops will be discussed in some detail below. The M-H loop tells one more about the basic properties of the magnetic material than does the B-H loop.

Magnetic The atoms of all magnetic materials have two properties. They have a net quantum mechanical spin and a magnetic moment. In other words, they are tiny magnets. They also have a quantum mechanical coupling energy between the atoms that is minimized when the magnetic moments are parallel. Hence the magnetic moments of adjacent atoms add. The metals iron, nickel, and cobalt are the only pure materials that are magnetic at room temperature but there are literally thousands of compounds that are strongly magnetic at room temperature. For example chromium dioxide CrO₂ and manganese arsenide MnAs are strongly magnetic.

Magnetic Circuit Generally a more or less circular construction containing a magnet and soft magnetic materials to concentrate the field of the magnet in an air gap in the circuit.

Magnetic Domain All magnetic materials are magnetic because the quantum mechanical coupling energy between adjacent electron spins is minimized when the spins are parallel. The larger the volume of such magnetized regions the larger is the external magnetic field energy. At some critical volume the external field energy is larger than the quantum mechanical coupling energy and the volume breaks up into two or more magnetic domains separated by a domain wall.

Magnetic Field Strength (H) A measure of the magnetic field. This may be induced by an electric current or a permanent magnet. The units are Oe in cgs units and A/m in MKS units.

Magnetic Induction (B) The magnetic flux density or flux per unit area at a given point. The units are Gauss in cgs units or Tesla in MKS units.

Magnetization (M) A measure of the magnetic strength or dipole moment per unit volume of a magnetic material. The units are emu/cm³ in cgs units or A/m in MKS units.

Magnetized When a magnetic material is induced with a current (using an electromagnet or another permanent magnet) it is said to be magnetic. Magnets can be delivered in the magnetized or unmagnetized state, depending on the customers needs.

Material The primary composition of magnets. NdFeB magnets are composed of Nd (Neodymium), Fe(Iron) and B (Boron). The MSDS for NdFeB products is available here.

Material Grade A list of specifications which define the magnetic properties of a given magnetic material. (See Grade Chart)

Maximum Energy Product (BHmax) On a B-H graph this is the point at which the product B•H reaches a maximum. The values of B and H at that point are called Bd and Hd. The product Bd•Hd gives BHmax, and is called the Maximum Energy Product. The units are almost always MGOe or 10⁶ erg/cm³. The Grade of a magnet usually incorporates this number. For example: N40 Grade NdFeB has a maximum energy product between 38-41 MGOe. BHmax is the primary indicator of the magnet material “strength”. In general, the higher the BHmax value, the higher the field H the magnet will generate in a particular application.

Maximum Operating Temperature (Tmax) The Maximum Operating Temperature for a given magnetic material. Due to geometry in the magnetic circuit, a specific magnet may sustain irreversible losses somewhat below this temperature.

Maximum working temperature (Tm) The maximum temperature that a magnet can have before its magnetic properties are degraded. NdFeB magnets have fairly low Tm values. Tm is always less than but sometimes nearly equal to Tc.

Maxwell The cgs unit of measurement for magnetic flux. One Maxwell = 1 line of Flux.

MGOe (Mega Gauss Oersteds) Unit of measure typically used in stating the maximum energy product for a specific magnetic material.

North Pole In magnetic terms this is the specific pole of the magnet which “seeks” the earths Geographic North Pole. The Earths geographic North poles is actually has a magnetic South polarity, thus causing no end of confusion. In therapeutic applications a magnets North Pole is considered the “positive energy” source and is the side which is generally thought to relieve pain.

Oersted The CGS unit for magnetic field strength. It was named after the famous Danish physicist/chemist Hans Christian Oersted. 1 Oersted is the strength of the magnetic field when the force operating on a unit magnetic pole is 1 dyne. 1Oe=1000/4π A/m. (A/m is the MKS unit of magnetic field strength)

Orientation How a magnet’s poles are established with regards to the magnets geometry. See the Magnetic Orientation page for more details.

Permeability µ Defined by the expression B = µH. In vacuum or air in cgs units µ = 1. Unfortunately, in MKS-SI units in air µ = µ₀ = 4pi*10^-7.

Permeance Coefficient (Pc) In a magnetic circuit, a magnet will operate at a specific point on its extrinsic demagnetization (B-H) curve. This is related to the magnets physical geometrical design as well as the magnetic circuit in which it operates. This is also referred to as the load line or operating point. Pc = Bd/Hd

Plating The surface treatment as applied to the finished magnet material, usually as a means of inhibiting oxidation. This is most commonly a combination such as Nickel (Ni), Copper (Cu), Gold (Au), Tin(Sn), Zinc(Zn), Epoxy or ABS Plastic. The majority of NdFeB magnets we sell are Ni-Cu-Ni plated.

Polarity The magnets magnetic orientation with regards to its poles. All magnets will have both a North pole and a South pole, and most magnets will have their N and S poles 180 deg apart. Various polarity configurations can be found on our Magnetic Orientation page.

Pole The area on the magnet which has the greatest magnetic dipole concentration. Most magnets will have two poles (180 degrees apart), but other types also have multiple poles. Each pole is either North facing or South facing. See North Pole for more information.

Pull strength The amount of force the magnet can support when a load is applied normal to the surface of Steel. The test results shown on the site are near ideal (max) results, where the magnet and metal have full direct contact on both surfaces. Inferior Steel, abnormal surface condition or angle of pull (which is not normal to the surface) will result in a lower resulting value. All results shown are from calibrated certified force meters, with varying capacities. Meters are calibrated to +/-0.5% accuracy. The max pull information we provide is for comparison purposes only (with other available products). All tests are run with actual material samples, and the average results in a series of measurements are recorded. If you have a specific application or need, please contact support who can guide you though the magnet selection process.

Rare Earth Metal In the periodic table of elements the category Lanthanides describes the elements referred to as Rare Earth Metals. The most common elements in this category used in magnet production are Neodymium, Samarium and Dysprosium. The majority of magnets sold by Amazing Magnets are composed of Nd, Fe, and B, which make up the strongest type of permanent magnets commonly available.

Remanence (Bd) The magnetic flux density remaining in a specific magnetic circuit after the saturation (i.e. magnetizing) force has been removed. This is often referred to as remanent induction.

Remanent Magnetization Mr The magnetization remaining at zero field.

Remnant Field (Hd) The maximum demagnetizing field remaining in a magnetic circuit after saturation.

Residual Induction (Br) Is the maximum remanent magnetic flux density for a given magnetic material. It is referred to as the Material Gauss.

Saturation Magnetization Ms The maximum magnetization for very high field H.

Single Domain Particle A particle that is so small there is no room for a magnetic domain wall. Hence the particle is a tiny but very strong magnet. All magnetic recording tapes are made using such particles.

South Pole In magnetic terms this is the specific pole of the magnet which “seeks” the earths Geographic South Pole. The Earths geographic South pole actually has a magnetic North polarity, thus greatly confusing the issue. In therapeutic applications the magnets South Pole is considered the “positive energy” source and is the side which is generally not put toward the skin.

Surface Field The maximum field H which can be measured on the surface of the magnet. Measurements are generally made using a Gauss meter (Oersted meter) with a Hall Effect sensor probe.

Surface Gauss The particular measurement of a magnetic field when measured at the surface of the magnet. The measured results found on our site, list the maximum surface gauss as measured by a calibrated, NIST certified gaussmeter. If you need to know the Gauss level at a specific distance please send an note to support.

Susceptibility X Defined by the expression M = X H. It is given by X = (µ-1)/(4pi) where µ is the magnetic permeability.

Tc (Curie Temperature) The temperature at which the magnetic material will be completely demagnetized. Magnetic dipoles within the magnet are completely randomized, resulting in a magnetic material which doesn't exhibit external magnetic properties. Achieving or exceeding this temperature may cause permanent damage to the magnets physical or structural properties.

Tesla (T) The MKS-SI derived unit of magnetic field B (also known as "magnetic flux density" or "magnetic induction"). The unit was named in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla. One Tesla is equal to one Weber per square meter. 1 Tesla is equal to 10,000 Gauss.

Thickness The measured distance between the magnets flat faces. On a rod or bar magnet (where length >= diameter) this same measurement would be refered to as the length.

Thread The specification of the fastner feature. Amazing Magnets sells both Metric and English threaded parts. Note: NdFeB magnetic material itself can't be threaded as it is too brittle.

Tmax (Maximum Operating Temperature) The Maximum Operating Temperature for a given magnetic material. Due to geometry in the circuit having a high demagnetization field Hd, a specific magnet may sustain Irreversible Losses below this value.

Vibrating Sample Magnetometer VSM A device used to measure the magnetic properties of magnetic materials. Typically it is used to measure the M-H Loop.

Weight The magnets weight as measured by a precision scale.

Width