Magnet Glossary

Air Gap

Any non-magnetic material sitting between a magnet and the object it attracts — air, paint, plastic, aluminium, an uneven surface, or a coating. It acts as a break in the magnetic circuit and weakens the hold. See Pull-gap Curve for how pull collapses as the gap grows.

Alnico

A family of iron alloys built mainly from aluminium, nickel, and cobalt. Known for high temperature stability and a very high Curie point, but low coercivity — it demagnetizes easily in an open circuit. Common in sensors, pickups, and legacy horseshoe magnets.

Anisotropic

A magnet whose magnetic domains are all aligned in one direction during pressing, giving maximum output. An anisotropic magnet can only be magnetized along that pre-set axis — try any other direction and you get nothing. Sintered neodymium is always anisotropic, which is why it is far stronger than isotropic material.

Annealing

A controlled heat-treatment used to relieve internal stresses in magnetic material and stabilise its magnetic properties. In NdFeB production it is part of the post-sinter tempering that sets final coercivity.

Axially Magnetized

A magnet charged through its thickness, so the poles sit on the two flat parallel faces (a disc magnetized face-to-face is axial). Contrast with diametrically magnetized.

B-H Curve (Magnetization / Demagnetization Curve)

A graph of magnetic flux density (B) against applied field strength (H). Flux density rises with field strength until it reaches saturation and stays flat. The second-quadrant portion is the demagnetization curve every engineer uses to select a grade. B is measured in Gauss or Tesla (10,000 G = 1 T); H in Oersteds.

Bar Magnet

A permanent magnet whose magnetic length is greater than its diameter (or effective width), with north and south poles at opposite ends.

Bi-pole Magnet

A magnet with exactly two poles — one north, one south — the simplest pole configuration. Contrast with multipole magnets used in motor rotors and encoders.

Bonded Magnet

A magnet made by mixing magnetic powder with a polymer binder and then compression- or injection-moulding it, rather than sintering. Bonded NdFeB is weaker than sintered but allows complex net shapes, tight multipole patterns, and very thin walls with no machining.

Br (Residual Induction / Remanence) Gauss / Tesla

The flux density a fully magnetized material retains after the external field is removed, measured in a closed circuit. Higher Br means a stronger magnet. It is the first of the three numbers (with Hcj and BHmax) that fully describe a grade.

Chamfer

A bevelled edge machined onto a magnet to remove the sharp corner. Because sintered NdFeB is brittle, chamfers (or radii) reduce chipping in handling and assembly and protect the coating at the edge. Typical call-out is C0.1–C0.5.

Closed Circuit

An arrangement where ferrous material directly connects the magnet’s north pole back to its south, so flux flows freely and stays inside the circuit. There is no external field — all the magnetism is consumed in the loop. Motors, speakers, and holding assemblies are designed around this idea.

Coating (see Plating)

A surface layer applied to raw NdFeB to stop corrosion and demagnetization. See Plating for the full option list (Ni-Cu-Ni, epoxy, zinc, gold, parylene, PTFE, and more).

Coercivity (Hc / normal coercivity) kOe / kA·m⁻¹

The reverse field needed to drive a saturated magnet’s flux density (B) back to zero. It measures resistance to demagnetization. Don’t confuse it with intrinsic coercivity (Hcj), which is the value that actually governs thermal stability.

Composite Magnet

A magnet blended from magnetic and non-magnetic materials to hit a specific property target or cost point — bonded magnets are the most common composite type.

Countersink

A conical recess machined into a magnet so a flat-head screw can mount it flush. Common on mounting magnets. Because the wall is thinner at the hole, countersunk magnets are more fragile and slightly weaker than the equivalent solid disc.

Curie Temperature (Tc)

The temperature at which a material’s atomic structure changes and it loses all magnetism permanently — the domains randomise and never recover. For NdFeB, Tc is around 310–340°C. Note this is far above the safe working temperature: a magnet is ruined long before it reaches its Curie point.

Demagnetization

The loss of a magnet’s external field in an open circuit. It can be caused by heat above the max operating temperature, a strong opposing field, severe mechanical shock, or corrosion. Once sintered neodymium loses its field this way it generally cannot be re-magnetized to full strength.

Demagnetization Curve

The second quadrant of the hysteresis loop — the working part of the B-H curve that shows how flux density falls as an opposing field is applied. Every grade selection starts here, by checking the knee point at your operating temperature.

Density

Mass per unit volume — used to estimate a magnet’s weight from its dimensions. Typical values:

• Sintered Neodymium: ~7.5 g/cm³
• Samarium Cobalt: 8.2–8.4 g/cm³
• Alnico: 6.9–7.3 g/cm³
• Ceramic / Ferrite: ~5 g/cm³
• Flexible: ~3.5 g/cm³

Diamagnetic

A weak form of magnetism that aligns at right angles to an applied field, creating a slight repulsion. All materials are diamagnetic to some degree, but the effect is normally swamped by stronger magnetic behaviour. Ferromagnetic vs paramagnetic vs diamagnetic →

Diameter (OD)

The straight-line measurement across a round magnet through its centre — twice the radius. On rings it is the outer diameter (OD); see also ID (inner diameter).

Diametrically Magnetized

A cylinder or ring magnetized across its diameter rather than through its length, so the poles sit on the curved side faces. Essential for rotor and encoder magnets that need to present poles radially.

Dimensional Tolerance

The permitted size variation from machining. Sintered NdFeB carries a standard tolerance of about ±0.1 mm; tighter ±0.05 mm (and down to ±0.02 mm on precision work) is achievable but adds cost.

Dimensions

The finished size of a magnet including every surface treatment (coating and plating add thickness). The magnetic length is always listed last, e.g. 50×50×100 mm.

Direction of Magnetization

Which axis the poles are charged along — through length, thickness, diameter, axially, radially, or diametrically. It must be specified before manufacture because an anisotropic magnet can only be charged along its pre-set axis.

Domains (Magnetic Domains)

Microscopic regions inside a magnetic material, each with a uniform direction of magnetization. During pressing, a strong field aligns all the domains the same way, which is what gives a finished magnet its strength and anisotropy.

Dysprosium (Dy) & Terbium (Tb)

Heavy rare-earth elements added to NdFeB to raise coercivity and lift the safe operating temperature. They are scarce, expensive, and now export-controlled, which is why modern high-temperature grades use grain boundary diffusion to get the same coercivity with far less Dy/Tb.

Eddy Currents

Swirling currents induced inside a conductive magnet by a rapidly changing field — for example the high-frequency switching inside a brushless motor. They do no useful work; they just become heat and push the magnet toward its knee point. The fix is to segment the magnet into epoxy-insulated slices.

Electromagnet

A magnet whose field is produced by electric current through a coil — usually copper wire wound into a solenoid, often around an iron core. The field exists only while current flows and disappears when it stops. Opposite in behaviour to a permanent magnet.

Epoxy Coating

A polymer coating, often applied over Ni-Cu-Ni plating, that adds corrosion and abrasion resistance for harsher or outdoor environments. Available in black or grey and as a single-layer or composite finish.

Ferrite (Ceramic Magnet)

A low-cost magnet made from iron oxide plus strontium or barium carbonate. Much weaker than neodymium but cheap, highly corrosion-resistant, and stable — the default for cost-driven, lower-performance applications.

Ferromagnetism

The strongest form of magnetism — the only kind producing forces you can feel by hand. Iron, nickel, cobalt, and NdFeB are ferromagnetic. Full explainer →

Flux (Magnetic Flux) Weber / Maxwell

The total number of magnetic field lines flowing from a pole. SI unit is the Weber (Wb); the CGS unit is the Maxwell.

Flux Density (B / Magnetic Induction) Gauss / Tesla

The number of field lines per unit pole area — field lines through each 1 cm×1 cm of pole. Measured in Gauss or Tesla (10,000 G = 1 T). Often quoted as surface field when measured at the pole face.

Gauss (G) CGS

The CGS unit of flux density, named after Carl Friedrich Gauss. 1,000 Gauss equals 1,000 field lines per cm² of pole area. 10,000 Gauss = 1 Tesla.

Gauss Meter

An instrument with a Hall probe used to measure surface flux density at a magnet’s pole. Standard QC tool for verifying magnetization and pole position.

Gilbert (Gb)

A CGS unit of magnetomotive force, named after William Gilbert. To convert ampere-turns to Gilberts, multiply by 1.25664.

Grade

The code that defines a magnet material’s composition and magnetic properties. For sintered neodymium it reads N + number + optional letter, e.g. N42SH: “N” for neodymium, “42” for the maximum energy product in MGOe, and “SH” for the temperature class.

Grade Suffix (M / H / SH / UH / EH / AH)

The letters after a neodymium grade number indicate intrinsic coercivity (Hcj) class — and therefore the maximum operating temperature:

• No letter (N): up to ~80°C
• M (Medium): up to ~100°C
• H (High): up to ~120°C
• SH (Super High): up to ~150°C
• UH (Ultra High): up to ~180°C
• EH (Extra High): up to ~200°C
• AH (Abnormal High): up to ~230°C

Grain Boundary Diffusion (GBD)

A process that diffuses heavy rare earth (Dy/Tb) only into the boundaries between grains of a sintered magnet, rather than throughout the whole alloy. This raises coercivity (heat resistance) without dragging down Br — and uses 20–50% less Dy/Tb than traditional bulk alloying. It is how modern high-temperature grades hit their numbers affordably.

Halbach Array

An arrangement of magnets with rotating orientation that concentrates the field on one side and cancels it on the other. Used in high-efficiency motors, MRI, and maglev to get more usable field from the same amount of magnet.

Heavy Rare Earth (HRE / HREE)

The heavier lanthanides — chiefly dysprosium (Dy) and terbium (Tb) — added to NdFeB to raise coercivity and temperature resistance. Scarcer and more tightly controlled than the light rare earths (Nd, Pr). See Dysprosium & Terbium.

Homogeneous Field

A field whose lines are uniform in strength and direction across a region. Hard to achieve with permanent magnets alone; usually engineered with pole pieces, yokes, or Halbach geometry.

Horseshoe Magnet

A bent bar magnet (classically Alnico) with both poles at the open ends. Bringing the poles close together strengthens the hold by forming a near-closed circuit when bridged by ferrous material.

Hysteresis

The lag between a change in the magnetizing force and the resulting change in magnetization. The area of the hysteresis loop reflects how much energy the material stores (hard) or loses (soft).

Hysteresis Loop (M-H Loop)

A four-quadrant graph that traces a material as it is magnetized to saturation, demagnetized, reverse-magnetized, and re-magnetized. Magnetically hard materials (permanent magnets) enclose a large area; soft materials enclose a small one. Reference →

Hysteresis Loss

Energy lost as heat each time the magnetic induction lags behind the magnetizing force through a cycle — a key loss term in transformer and motor core design.

ID (Inner Diameter)

The diameter of the centre hole in a ring or tube magnet. Paired with OD (outer diameter) and thickness to fully define a ring.

Induction (B)

Another name for flux density — field lines per cm² of pole area, measured in Gauss or Tesla. See Flux Density.

Intrinsic Coercivity (Hcj / Hci) kOe

The reverse field needed to permanently demagnetize a magnet — the truest measure of resistance to demagnetization and the value behind the grade suffix. Neodymium has a large gap between Hc and Hcj, meaning far more energy is needed to wipe it out than merely to zero its external field.

Irreversible Losses

Magnetism lost for good after over-heating, a strong opposing field, or shock — the part that does not come back on cooling. Distinct from reversible loss, which recovers. Re-magnetizing can restore reversible loss but not damage from exceeding the max operating temperature.

Isotropic

Material with no preferred direction of magnetism — it can be charged along any axis but is much weaker than anisotropic material. Flexible magnets are usually isotropic; sintered neodymium never is.

Keeper

A steel bar or disc placed across a magnet’s poles to close the circuit and preserve its field — essential for low-coercivity Alnico, optional for neodymium (sometimes used to make handling and air freight safer by containing stray field).

Knee Point (Knee Temperature)

The point on the demagnetization curve where it bends sharply downward. Above the knee, the magnet recovers fully when conditions return to normal; below it, a small extra push causes large, irreversible flux loss. The knee shifts as temperature rises, so a grade that is safe at 20°C can fall below its knee at 120°C.

Laminations

Thin, insulated layers of magnetic (usually electrical-steel) material stacked to build a motor or transformer core. Breaking the core into layers interrupts eddy currents and cuts core losses. The same principle applied to the magnet itself is called segmentation.

Load Line (Permeance Coefficient)

A line on the demagnetization curve representing a magnet’s operating point, set by its shape and magnetic circuit. Where it crosses the curve tells you the working flux density. Its slope is the permeance coefficient.

L/D Ratio (Length-to-Diameter)

The ratio of a magnet’s magnetic length to its diameter — a quick proxy for its permeance coefficient. A higher L/D ratio means a higher, more stable operating point. Once length exceeds diameter the magnet works near its optimum, and adding more length gives only small gains.

Magnetic Anisotropy

The directional dependence of a material’s magnetic properties — the reason an anisotropic magnet has one easy axis of magnetization and is far stronger along it.

Magnetic Axis

The single direction along which an anisotropic magnet’s domains are aligned — and the only direction it can be magnetized. Set during pressing and fixed for the life of the part.

Magnetic Circuit

The complete path flux follows from north to south, through magnetic material and any air gaps. Motors, generators, sensors, and holding assemblies are all designed as magnetic circuits to channel flux efficiently. Reference →

Magnetic Length

The dimension that runs along the magnetic axis — always listed last in a magnet’s dimensions. A bar charged through its length might read 50×50×100 mm.

Magnetomotive Force (mmf) Ampere-turns

The magnetic “drive” produced by current through a coil — equal to current multiplied by number of turns. More current or more turns means more mmf and a stronger field.

Material

The physical composition of a magnet. The five main families:

• Neodymium (NdFeB) — strongest; the workhorse for motors and electronics
• Samarium Cobalt (SmCo) — high temperature & corrosion resistance
• Alnico — very stable, high Curie point, low coercivity
• Ferrite / Ceramic — cheap, corrosion-proof, lower strength
• Flexible — bonded into rubber sheet/strip

Maximum Energy Product (BHmax) MGOe

The primary single-number indicator of a magnet’s strength — the product of remanence (Br) and coercivity, equal to the largest B×H rectangle under the second-quadrant curve. For neodymium, the grade number is the BHmax in MGOe (N42 = 42 MGOe). Higher BHmax means more field from less magnet. Typical maxima: Neodymium up to 52, SmCo up to 32, Alnico ~5.5, Ceramic ~3.5, Flexible ~2 MGOe.

Maximum Operating Temperature (Tmax)

The highest temperature a grade can run at before suffering irreversible loss. Below it, the field weakens with heat but fully recovers on cooling; above it, the loss is permanent. Standard neodymium tops out near 80°C; high-suffix grades reach ~230°C.

Mega Gauss Oersteds (MGOe) CGS

The CGS unit of maximum energy product (BHmax). The grade number on neodymium is its MGOe value.

Monopole

A hypothetical magnet with only one pole. None has ever been found — every real magnet has both a north and a south, and flux always flows between them.

Multipole Magnetization

Charging several alternating N-S pole pairs onto one magnet — standard for motor rotors, ring encoders, and couplings. The pole count and pattern are set by a custom magnetizing fixture.

NdFeB (Neodymium Iron Boron)

The strongest commercial permanent-magnet material, an alloy of neodymium (Nd), iron (Fe), and boron (B) forming the Nd₂Fe₁₄B crystal. The basis of every neodymium grade.

North Pole

The pole that points toward Earth’s geographic North. (Because opposites attract, Earth’s geographic North is technically a magnetic south — a north-seeking pole.)

NMR (Nuclear Magnetic Resonance)

A technique exploiting the magnetic properties of atomic nuclei to identify chemical structure — and the physics behind MRI imaging, which relies on very high-field magnets.

Oersted (Oe) CGS

The CGS unit of magnetic field strength (H), named after Hans Christian Ørsted. Used to express coercivity and the output of magnetizers. Closely paired with Gauss.

Open Circuit

A magnet not attached to ferrous material, so its flux must return through air. Because air carries flux poorly, a magnet produces less Gauss in open circuit than in a closed one — which is the condition most catalogue ratings assume.

Orientation

The physical location and direction of a magnet’s poles — through length, thickness, diameter, axially, radially, or diametrically. Synonymous in practice with direction of magnetization.

Permeability

How readily a material carries magnetic flux compared with free space. Ferromagnetic metals have very high permeability and become magnetized in a field; soft versions lose it when the field is removed, hard versions keep it.

Permeance Coefficient (Pc / B/H)

A number describing a magnet’s operating point set purely by its shape and circuit — the slope of the load line. A low Pc (thin, flat magnet) sits lower on the demag curve and demagnetizes more easily; a high Pc (long, slender magnet) is more stable.

Permanent Magnet

A solid that produces its own persistent field because the material is magnetized — no current required. Opposite of an electromagnet. Permanent Magnet 101 →

Piezoelectric

Materials that generate a charge under mechanical stress and deform under an applied field. Distinct from magnetism, but often mentioned alongside it in sensor and actuator design.

Plating (Coating)

The surface finish that protects raw NdFeB from corrosion and demagnetization. The default is triple-layer nickel-copper-nickel (Ni-Cu-Ni). Other options include:

• Nickel (Ni), Zinc (Zn), Tin (Sn), Gold (Au), Chrome
• Epoxy, Parylene C, PTFE (Teflon), Everlube, rubber
• Titanium / Titanium Nitride (TiN), phosphate / zinc-chromate passivation
• Composite stacks: Ni-Cu-Ni + epoxy / parylene / PTFE / rubber

Polarity

A magnet’s orientation with respect to its poles — which face is north and which is south, normally 180° apart. Like poles repel; opposite poles attract.

Pole

The area of greatest field strength in a given direction — either north-seeking or south-seeking. Every magnet has at least one of each.

Powder Metallurgy (Sintering Route)

The production route for sintered NdFeB: strip-cast alloy is hydrogen-decrepitated and jet-milled into fine powder, aligned and pressed in a magnetic field, then sintered and tempered into a dense, anisotropic block before machining and coating.

Pull-gap Curve

A plot of holding force against air gap. Pull follows an inverse-square law with distance. High-field-gradient magnets clamp hardest in direct contact but fade fastest across a gap; low-field-gradient magnets do the opposite — their curves cross when plotted together.

Pull Strength (Holding Force) kg / N

The maximum force needed to pull a magnet straight off a thick, flat, clean steel surface in full contact. Steel grade, surface finish, thickness, and pull angle all change the real value — published figures are best-case.

Rare-earth Metals

The lanthanide group, plus scandium and yttrium. In magnets the key players are neodymium, praseodymium, samarium, dysprosium, and terbium. Despite the name they are reasonably abundant — “rare” refers to the difficulty of finding concentrated, economic deposits.

Remanence (Br)

The magnetism left in a material after the magnetizing field is removed. Identical in meaning to residual induction (Br) — the higher it is, the stronger the magnet.

Reluctance

A magnetic circuit’s opposition to flux — the magnetic analogue of electrical resistance. Air gaps add reluctance, which is why even a small gap weakens a circuit.

Repelling

The force that pushes two like poles apart (N–N or S–S). Their fields try to flow the same way, collide, and push back.

RoHS / REACH

EU directives restricting hazardous substances (RoHS) and registering chemicals of concern (REACH/SVHC). Compliant plating and documentation are required for magnets sold into European supply chains.

Salt Spray Test (SST)

An accelerated corrosion test (per ASTM B117) where coated magnets sit in a salt-fog chamber for a rated number of hours. It verifies that the plating will survive humid or marine conditions — a standard acceptance check for outdoor and automotive parts.

Samarium Cobalt (SmCo)

A rare-earth magnet that is slightly weaker than neodymium but far more heat- and corrosion-resistant, working up to ~250–350°C and often used uncoated. The go-to when temperature or corrosion rules out NdFeB.

Saturation

The state where a material is fully magnetized and applying more field produces no further increase in flux density. A grade only delivers its rated Br once it has been magnetized to saturation.

Segmentation (Laminated Magnet)

Slicing a rotor magnet into thin, epoxy-insulated pieces to break up eddy-current loops. This cuts rotor-magnet losses by roughly 40–70% and keeps the magnet well below its knee point in high-frequency motors.

Shear Force (Sliding Resistance)

The force resisting a magnet sliding sideways across steel — roughly one-fifth of the vertical pull (friction coefficient ~0.2). A magnet on a vertical steel wall slips once the load reaches about 20% of its rated pull. Rubber coatings raise friction and resist sliding far better.

Sintering

The high-temperature step that fuses pressed NdFeB powder into a dense, solid block just below its melting point, followed by tempering to set coercivity. Sintered magnets are the strongest neodymium type — far stronger than bonded — but must be machined and coated afterward because they emerge oversized and brittle.

Single Domain Particle

A particle too small to contain a domain wall, making it a tiny but very stable magnet. The basis of magnetic recording media.

Soft Magnet

Material that magnetizes and demagnetizes easily (low coercivity) — used for cores in transformers, inductors, and motor laminations, where you want the field to switch with the current.

South Pole

The pole that seeks Earth’s geographic South (which carries a magnetic north polarity). The partner to every north pole.

Stacking

Combining magnets to increase net pull by raising the effective L/D ratio. Five discs stacked are much stronger than one — until length exceeds diameter, after which extra height adds little.

Superconducting Magnet

An electromagnet wound from superconducting wire that carries current with zero resistance, producing extremely high fields. Used in MRI, NMR, and research magnets.

Surface Field (Surface Gauss) Gauss

The flux density measured with a gauss meter right at the pole face. A useful QC and comparison figure — though it is not the same as pull strength, which also depends on the steel and circuit.

Temperature Coefficient (α / β)

The factor describing how flux falls as temperature rises (recovered on cooling, provided Tmax isn’t exceeded). Typical reversible loss of Br per °C: Neodymium ~0.11%, Samarium Cobalt ~0.03%, Alnico ~0.02%, Ceramic ~0.2%.

Temporary Magnet

A material magnetic only while in an external field or current — soft iron is the classic example. Loses its field the moment the source is removed.

Tesla (T) SI

The SI unit of flux density, named after Nikola Tesla. 1 Tesla = 10,000 Gauss.

Thread

A threaded feature for mounting. Neodymium itself is too brittle to thread, so it is encased in or bonded to a threaded carrier (e.g. a steel pot or housing) instead.

Tunneling Magnetoresistance (TMR)

An effect where the resistance across a thin junction between ferromagnetic layers depends on their relative magnetization — the basis of modern hard-disk read heads and TMR magnetic sensors.

Undulator

A periodic array of magnets that makes a charged-particle beam wiggle, producing intense radiation. Used in synchrotron and free-electron-laser facilities.

Vacuum Permeability (μ₀)

A physical constant for the magnetic permeability of free space — the reference against which a material’s relative permeability is measured.

Vortex Magnet

A magnet with a swirling, vortex-like field pattern, used in specialised research and niche sensing applications.

Weber (Wb) SI

The SI unit of magnetic flux. One Weber equals one Tesla over one square metre.

Yoke

A piece of soft-magnetic material that links a magnet’s poles to form a closed, low-reluctance path — concentrating flux where it’s needed in motors, holders, and sensor assemblies.