Clear definitions of the terms you meet on magnet datasheets and quotations, written by magnet engineers and backed by manufacturer data and textbooks.
The quantities follow common usage in the literature [11], [12] and the IEC specification for magnetically hard materials [13]. Numbers marked with a bracket come from the references at the end of the page.
A permanent magnet made from aluminium, nickel and cobalt (with iron and other additions), produced by casting or sintering. Its great strength is temperature stability: manufacturers give maximum operating temperatures of 450 to 550 °C depending on grade [4]. Its coercivity is low, so it is easily demagnetised by opposing fields and is usually made long in the direction of magnetisation.
See also: Intrinsic coercivity (Hcj) Maximum operating temperature Permeance coefficient (Pc)
A magnet whose crystals were aligned in a magnetic field during pressing, so it has a preferred direction of magnetisation that is fixed at manufacture. It gives a higher energy product than an isotropic magnet of the same material, but it can only be magnetised along that direction. Sintered NdFeB and SmCo magnets are anisotropic; ferrite and bonded magnets are made in both forms [11], [12].
See also: Direction of magnetisation Bonded magnet Sintered magnet
A magnet made from magnetic powder mixed with a binder such as rubber, plastic or resin. It can be moulded or extruded into complex shapes, thin sections and flexible forms with good tolerances, at the cost of a lower magnetic output than a fully dense sintered magnet of the same material.
See also: Flexible magnet Sintered magnet Anisotropic magnet
A thin protective layer on the surface of a magnet. Sintered NdFeB corrodes easily and is almost always coated, most often with nickel-copper-nickel; epoxy, zinc and other finishes are also used. The coating affects corrosion resistance, appearance and, slightly, dimensions and weight. It does not change the magnetic grade.
See also: Neodymium (NdFeB) magnet Pull force (holding force)
The reverse magnetic field needed to bring the flux density B in the magnet to zero, that is, the point where the demagnetisation curve crosses the H axis. It is also called normal coercivity and is always smaller than or equal to the intrinsic coercivity Hcj.
The temperature above which a ferromagnetic material loses its permanent magnetisation. For sintered NdFeB it is about 310 to 340 °C [1]; for SmCo it is much higher, about 700 to 850 °C depending on the type [3]. A magnet must be used far below its Curie temperature: the usable temperature is limited much earlier by demagnetisation.
See also: Maximum operating temperature Knee of the demagnetisation curve
The second-quadrant part of the B–H hysteresis loop, running from remanence Br on the B axis to coercivity on the H axis. It shows how the flux density falls as a reverse field is applied and is the key chart in magnet design. Manufacturers publish it at several temperatures, and the curve for the exact grade should always be used for a final design.
See also: Knee of the demagnetisation curve Operating point and load line Remanence (Br)
The direction in which a magnet is magnetised, for example through the thickness, across the diameter, through the length, radially or with several poles on one face. For anisotropic magnets it is fixed by the alignment during pressing and cannot be chosen freely afterwards, so it must be specified when ordering.
See also: Anisotropic magnet Halbach array
A ceramic permanent magnet, usually based on strontium ferrite (SrFe₁₂O₁₉). It is low in cost, resistant to corrosion and an electrical insulator, with a lower energy product than rare-earth magnets. Unlike other magnets its coercivity falls as it gets colder, so low temperatures, not high ones, are its particular risk [5], [7].
See also: Intrinsic coercivity (Hcj) Reversible temperature coefficient (α, β) Flexible magnet
The magnetising field, produced by electric currents or by other magnets. The SI unit is ampere per metre (A/m) and the CGS unit is the oersted (Oe), with 1 Oe = 1000/4π A/m ≈ 79.577 A/m. Coercivity values are given in these units, as kA/m or kOe.
See also: Flux density (B): tesla and gauss Coercivity (Hcb) Intrinsic coercivity (Hcj)
A bonded ferrite magnet in rubber or plastic, supplied as strips, tapes, sheets or rolls, plain or with a self-adhesive or PVC-laminated face. It is easy to cut and bend and has a low magnetic output. Its temperature limit is set by the binder and the adhesive rather than by the magnetic powder; manufacturer figures for tapes and strips are around 70 °C as a tested limit [6].
See also: Bonded magnet Ferrite (ceramic) magnet Maximum operating temperature
The strength of the magnetic field expressed as flux per unit area. The SI unit is the tesla (T) and the CGS unit is the gauss (G), with 1 T = 10,000 G. A surface field measured with a gaussmeter is not the same as the remanence Br: it depends on the shape of the magnet and on the distance of the probe.
See also: Magnetic flux (Φ) Field strength (H): A/m and oersted Remanence (Br)
The name of a magnet grade, for example N42 or N42SH. For neodymium the number is roughly the maximum energy product in MGOe and the letters, if any, give the coercivity and temperature class. Grade names are widely used across manufacturers, but exact properties differ between suppliers, so the datasheet of the actual supplier should always be checked.
See also: Grade suffix (M, H, SH, UH, EH, AH) Maximum energy product (BH)max
The letters after the number of a neodymium grade. In order of increasing intrinsic coercivity they are: none (standard N), M, H, SH, UH, EH and AH. Typical maximum operating temperatures are about 80, 100, 120, 150, 180, 200 and 220 °C respectively [1]. These are reference values for a magnet at a favourable working point, and very high-energy grades inside a suffix are rated about 10 K lower. For continuous service see the Temperature Selector.
See also: Grade (N35, N42, …) Intrinsic coercivity (Hcj) Maximum operating temperature
An arrangement of magnets whose directions of magnetisation rotate step by step, so the field is concentrated on one side of the array and, ideally, cancelled on the other. It is used in some motors, magnetic couplings, linear tracks and laboratory magnets.
See also: Direction of magnetisation Magnetic circuit
Dysprosium and terbium are added to NdFeB to raise its intrinsic coercivity, which makes the high-temperature suffixes possible. They are relatively scarce and costly, so higher-coercivity grades usually cost more than standard grades.
See also: Intrinsic coercivity (Hcj) Grade suffix (M, H, SH, UH, EH, AH)
The reverse field needed to bring the magnetisation of the material itself to zero. It is the true measure of a material's resistance to demagnetisation and the main property that sets the temperature capability of a grade: a higher Hcj allows higher working temperatures. The suffix letters of neodymium grades indicate classes of increasing Hcj.
See also: Coercivity (Hcb) Grade suffix (M, H, SH, UH, EH, AH) Heavy rare earths (Dy, Tb)
A permanent loss of magnetisation that does not come back when the magnet cools or the opposing field is removed. It occurs when the operating point falls below the knee, for example through heat or an opposing field. Losses of this kind can often be restored by re-magnetising; losses caused by metallurgical change or corrosion cannot.
See also: Knee of the demagnetisation curve Reversible temperature coefficient (α, β) Time-dependent (long-term) loss
A piece of soft iron or steel placed across the poles of a magnet to close its magnetic circuit. It raises the permeance coefficient, helps protect the magnet against losses during storage and shipping, and reduces the stray field.
See also: Magnetic circuit Permeance coefficient (Pc)
The point where the demagnetisation curve bends from a nearly straight line into a steep drop. While the operating point stays above the knee, changes are reversible; if it falls below the knee, part of the magnetisation is lost permanently. As temperature rises the knee moves up the curve, which is why hot, thin magnets are at risk.
See also: Demagnetisation curve Operating point and load line Irreversible loss
The path followed by the magnetic flux through the magnet, the air gaps and any steel parts. A closed circuit of steel with small gaps gives the magnet a high permeance coefficient and makes it more resistant to demagnetisation; a magnet in open air has a low one.
See also: Permeance coefficient (Pc) Keeper Operating point and load line
The total magnetic field passing through a given area. For a uniform field perpendicular to the area, Φ = B × A. The unit is the weber (Wb) in SI and the maxwell (Mx) in CGS, with 1 Wb = 10⁸ Mx. A coil or a sensor responds to flux, not to flux density alone.
The largest product of B and H found on the demagnetisation curve. It is the usual single figure for comparing the strength of magnet materials and is given in MGOe or kJ/m³ (1 MGOe ≈ 7.9577 kJ/m³). For an ideal straight demagnetisation curve it equals Br²/(4μ₀). The number in a neodymium grade name is roughly this value in MGOe: N42 is about 42 MGOe, or about 334 kJ/m³. A high energy product says little about temperature resistance, which is set mainly by coercivity.
See also: Remanence (Br) Intrinsic coercivity (Hcj) Grade (N35, N42, …)
A manufacturer's guideline figure for the highest temperature at which a grade can be used. It is not an exact limit: it depends on the shape of the magnet and its circuit, and it does not state for how long the magnet may stay at that temperature [9], [10]. For continuous 24-hours-a-day service a margin below the figure is needed because of time-dependent losses [8]. Our Temperature Selector applies this approach.
See also: Grade suffix (M, H, SH, UH, EH, AH) Time-dependent (long-term) loss Permeance coefficient (Pc)
A magnet based on the compound Nd₂Fe₁₄B. It has the highest energy product of the commercial permanent magnets. It is sensitive to heat (see grade suffixes) and to corrosion, so it is normally coated. Its Curie temperature is about 310 to 340 °C [1].
See also: Grade suffix (M, H, SH, UH, EH, AH) Coating (plating) Curie temperature (Tc)
The load line is a straight line from the origin whose slope is given by the permeance coefficient. Where it crosses the demagnetisation curve is the operating point (Bd, Hd) of the magnet in its circuit. When temperature or an opposing field changes, the operating point moves; if it crosses below the knee, the loss is irreversible.
See also: Permeance coefficient (Pc) Knee of the demagnetisation curve Demagnetisation curve
A number that describes how the geometry of the magnet and its circuit load it: Pc = −Bd/(μ₀·Hd). A tall, narrow magnet, or a magnet in a closed steel circuit, has a high Pc; a thin, wide magnet in open air has a low Pc. A low Pc places the operating point lower on the demagnetisation curve, so the magnet is more easily demagnetised by heat [9].
See also: Operating point and load line Magnetic circuit Knee of the demagnetisation curve
The force needed to pull a magnet straight off a thick, flat, clean steel plate, usually given in kgf, lbf or N and measured under ideal conditions. In use it is lower because of air gaps, coatings, rough, thin or curved surfaces, the steel grade and temperature. The force needed to slide the magnet sideways (shear) is usually much lower than the direct pull force.
See also: Coating (plating) Flux density (B): tesla and gauss
The flux density that remains in a magnet after the magnetising field has been removed, measured in a closed circuit. It is the highest value on the demagnetisation curve and is given in tesla or kilogauss. It sets the potential of a magnet, but the field actually found in air depends on shape and distance.
See also: Flux density (B): tesla and gauss Maximum energy product (BH)max Demagnetisation curve
The percentage change per kelvin of remanence, α(Br), and of intrinsic coercivity, β(Hcj), that is recovered when the magnet returns to its starting temperature. Typical values: sintered NdFeB α ≈ −0.11 %/K and β ≈ −0.60 %/K [1], [2]; SmCo₅ α ≈ −0.05 %/K and Sm₂Co₁₇ α ≈ −0.035 %/K [3]; ferrite α ≈ −0.18 %/K, while its coercivity rises with temperature [5]. The values are a linear approximation and apply only if the operating point stays above the knee.
See also: Irreversible loss Knee of the demagnetisation curve Maximum operating temperature
A rare-earth magnet in two families: SmCo₅ (1:5) and Sm₂Co₁₇ (2:17). It has high temperature stability and good corrosion resistance; it is more brittle and usually more costly than neodymium. Manufacturers quote maximum operating temperatures of about 250 °C for SmCo₅ and 300 °C for Sm₂Co₁₇, with a 350 °C type available [3].
See also: Maximum operating temperature Curie temperature (Tc) Neodymium (NdFeB) magnet
A magnet made by pressing magnetic powder, aligned in a field if anisotropic, and sintering it at high temperature into a dense solid, which is then ground or cut to size. Sintered NdFeB, SmCo and ferrite give the highest magnetic properties of their materials. They are hard and brittle, and chipping is a risk in handling.
See also: Bonded magnet Anisotropic magnet
A change in the crystal anisotropy of Nd₂Fe₁₄B at about −138 °C (135 K). Below it the flux of a sintered NdFeB magnet can fall by up to about 14 %, and the loss is recovered when the magnet warms again [7]. It matters mainly for cryogenic applications.
See also: Neodymium (NdFeB) magnet Reversible temperature coefficient (α, β)
A slow loss of flux in sintered NdFeB that continues at constant elevated temperature, roughly in proportion to the logarithm of time. In a doctoral study, the extrapolated 30-year loss stayed below 1 % only below a critical temperature that rose with coercivity (about 80 to 220 °C for four materials at a permeance coefficient of 1.1). Above it the losses grew with temperature [8].
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Important notice: The definitions and typical values on this page are provided for general information. They are simplified, may differ between manufacturers and standards, and do not constitute a specification, an offer or a performance guarantee. They are not binding on Rami Technics. Please check the datasheet of the exact grade and supplier before taking technical or commercial decisions.