4f electrons#
The partly filled inner shell that makes a rare earth a rare earth. Tucked inside the atom, they keep their magnetism in a solid and tie it firmly to the crystal.
Glossary
Glossary
Every technical word on this site, in plain language. Anywhere else on the site, an underlined term shows its short definition when you hover over it or tap it. Here the definitions get room to stretch, and terms that only make sense together share one picture.
No terms match that. Try a shorter word, or a symbol such as Br or Hci.
0–9
A
Aging#
A lower-temperature heat treatment after sintering that rebuilds the grain boundary layer. It's where the coercivity is actually created.
Airgap#
The gap flux has to cross, usually between a motor's rotor and stator. Most of a magnet's effort goes into pushing flux across it.
Alignment#
Lining up every powder particle's easy axis in a strong field before pressing, so the finished magnet is strong in one direction.
Alignment torque#
Torque from a rotor carrying its own field, whether from magnets or a rotor coil, trying to line up with the stator's.
Alnico#
Aluminum, nickel and cobalt with iron. Strong flux and superb heat stability, but easily demagnetized, which is why old magnets are long horseshoes.
Amperes per meter A/m#
SI unit of applied field H, and of coercivity. 1 kA/m ≈ 12.6 Oe.
Anneal#
A controlled heat treatment that relieves stress in a metal. For soft magnetic alloys it is what sets the properties.
Antiferromagnetism#
Neighbors line up exactly opposite each other and cancel. Ordered, but no net magnet to show for it.
Armature#
The winding that carries the main load current, on the rotor in a brushed machine.
Austenitic, ferritic and martensitic stainless#
Stainless steel families by crystal structure. Austenitic (304, 316) isn't magnetic; ferritic and martensitic (430, knife steels) are.
Axial flux#
A motor with flux running parallel to the shaft, built as flat discs side by side. Short and high-torque.
B
Back-EMF#
The voltage a spinning motor generates against its own supply. It rises with speed and sets the top speed for a given voltage.
Bastnaesite#
A rare earth carbonate mineral and the main ore at Mountain Pass and Bayan Obo. Rich in the light rare earths.
Beneficiation#
Crushing, grinding and concentrating ore without changing it chemically. The step that throws away most of the rock.
B-H curve#
The record of one experiment: put a material in a field, turn the field up, down and backwards, and plot what the material does. Its height and width are most of a datasheet.
Everything on a magnet datasheet comes from one experiment. Put a sample in a coil, sweep the applied field H up to saturation, back through zero, all the way negative and back again, and record what the material does. Because the way down is not the way up, the record closes into a loop.
The figure draws that loop two ways. B, in green, is the flux density a magnetic circuit actually sees. J, in amber, is the material's own share: B with the applied field subtracted out. The dashed gray line is empty space, which does exactly what you tell it and nothing more.
Four landmarks do most of the work. The flat top of J is saturation, past which more field buys you only what empty space would give. Where the loop crosses the vertical axis is the remanence, Br, the flux the material keeps with no field applied. Going negative, B reaches zero first, at the normal coercivity Hcb; J holds on longer and only collapses at the intrinsic coercivity Hci. The width of the loop is coercivity, and coercivity is what "hard" means in this business.
A soft material's loop is tall and so thin it is nearly a line, which is how soft iron is drawn here. A permanent magnet's is tall and wide. Only the upper-left quarter of it matters in service, and that quarter has its own entry: the demagnetization curve.
Explained with this figure: Coercivity · Hysteresis · Magnetic field · Magnetization · Remanence · Saturation
Bohr magneton#
The natural unit for an electron's magnetic moment. An iron atom in metal carries a little over two.
Bonded magnet#
Magnet powder held together by a polymer binder, either compression-molded or injection-molded. Weaker than sintered, but molded straight to shape with no machining.
Brushed DC motor#
The oldest design: windings on the rotor, a commutator to switch them, and carbon brushes that wear out.
Brushes#
Carbon blocks that ride on the commutator to feed current. The only consumable part in most motors that have them.
Brushless DC#
A permanent magnet motor with electronic switching in place of brushes. In practice, a PMSM with simpler control.
C
Calcining#
Heating a precipitate in air to drive off water and gases and leave the oxide.
Calciothermic reduction#
Making metal by reacting a rare earth fluoride with calcium at high temperature. The usual route for dysprosium and terbium metal.
Cerium and lanthanum#
The most abundant rare earths, and usually the ones a mine has too much of. Cerium sometimes stands in for a little neodymium in low-grade magnets.
Chromatography#
Separating rare earths by passing the solution through a column of resin that holds each element back by a slightly different amount.
Coating#
A protective layer against corrosion, since NdFeB is mostly iron. Nickel-copper-nickel plating is the default; epoxy and others are used where it won't do.
Cobalt iron#
Iron alloyed with roughly half cobalt, sold as Permendur or Hiperco, giving the highest saturation of any practical material. Expensive, and worth it where weight costs more than cobalt.
Coercivity#
How much reverse field it takes to undo a material's magnetization: the width of the loop. When magnet people say "hard," this is what they mean, not mechanical hardness.
Explained with B-H curve, which shares one figure with the related terms.
Cogging#
Lumpy torque felt when you turn an unpowered PM motor by hand, as magnets snap toward stator teeth.
Commutator#
A split copper ring on the shaft that reverses current in each rotor coil as it turns. A mechanical inverter.
Concentrate#
The product of beneficiation: the same minerals as the ore, about ten times more concentrated. Often the last product a mine sells.
Cracking#
Breaking down the mineral with acid or alkali at high temperature so the rare earths can be dissolved.
Curie temperature#
The temperature above which a material loses its magnetic order altogether. For NdFeB, typically around 310 to 320 °C, far above any practical working temperature.
D
d-axis and q-axis#
The two directions controllers use to describe a rotor: along its field (d, direct) and 90° to it (q, quadrature).
Demagnetization#
Losing magnetization, to heat, an opposing field, or the material itself breaking down. Usually recoverable by remagnetizing if the material is intact.
Demagnetization curve#
The upper-left quarter of the loop, where a magnet in service actually lives: pushing flux forward while facing a field that pushes back. It's the only part datasheets publish.
A magnet in service never sees the first quadrant. Its own shape, and whatever the circuit around it does, always push back against it, so it lives in the upper-left quarter of the loop: B still positive, H negative. That quarter is the demagnetization curve, and it is the only part a datasheet bothers to print.
Read the figure from the top left. Br is where the curves meet the vertical axis. Following the green B line down, it reaches zero at Hcb; the amber J line stays flat until the knee, then drops to zero at Hci. The shaded rectangle is the largest B × H that fits under the B curve, which is the energy product, (BH)max.
Where a real magnet sits on that curve is set by the load line, a straight line from the origin whose slope is the permeance coefficient, Pc. A long magnet in a closed steel circuit has a steep load line and a high Pc. A thin disc in open air has a shallow one.
The dashed curves are the same grade, hot (illustrative values for about 150 °C). Heat pulls both curves in, and the knee moves toward the load lines. On the steep line, the operating point stays above the knee, and the loss comes back when the magnet cools. On the shallow line, it falls past the knee, and that loss stays until the magnet is remagnetized. That, in one picture, is why a grade's maximum working temperature only means something for a stated shape.
Explained with this figure: Energy product · Intrinsic coercivity · Knee · Maximum working temperature · Normal coercivity · Operating point · Permeance coefficient
DFARS 252.225-7052#
A US defense procurement rule restricting NdFeB and SmCo magnets, tungsten and tantalum from China, Russia, North Korea and Iran in Defense Department supply. From 1 January 2027 it reaches further back up the supply chain.
Diamagnetism#
A weak repulsion from a field that every material has. In a strong enough field it can levitate small objects, a live frog included.
Die pressing#
Pressing powder between punches in a die while a field aligns it. Field across the press direction (transverse) gives better magnets; along it (axial) is easier.
Dipole#
Anything with a north and a south pole. Every magnet is one, down to a single electron.
Domain#
A region inside a material where the atoms' magnetism already points one way. Magnetizing iron doesn't create magnetism; it gets the domains to agree.
Dysprosium#
A heavy rare earth added to NdFeB to hold coercivity at high temperature. Scarce, and supplied overwhelmingly from China and Myanmar.
E
Earnshaw's theorem#
No fixed arrangement of permanent magnets alone can hold an object in stable levitation. Something has to spin, be actively controlled, or be diamagnetic.
Eddy current separator#
A spinning drum of alternating magnets at the end of a conveyor. It drives eddy currents in aluminum and copper scrap and flicks them off the line, while everything else drops straight down.
Eddy currents#
Loops of current a changing field stirs up inside a conductor. In a motor they're wasted heat (hence laminations); in a recycling line they're the whole trick.
Electromagnet#
A field made from electric current, usually in a coil around an iron core. Switch off the current and the field goes with it.
Energy product (BH)max#
The largest value of B × H on the demagnetization curve. A rough measure of how little magnet a design can get away with. Higher is stronger.
Explained with Demagnetization curve, which shares one figure with the related terms.
Exchange interaction#
The quantum effect that makes neighboring spins in iron, cobalt and nickel line up with each other. It's far stronger than the plain magnetic pull between them, and it's why ferromagnets exist.
Export controls#
Government licensing required before certain materials, equipment or know-how can leave a country. China requires licenses for several rare earths, their magnets and related technology.
F
Faraday's law#
A changing magnetic flux through a loop drives a voltage around it. Every generator, transformer and induction motor runs on it.
Feedstock#
The material a process takes in, whether oxide, metal, alloy or scrap.
Ferrimagnetism#
Neighbors line up opposite each other but don't fully cancel, leaving a net moment. Ferrite magnets work this way.
Ferrite#
Iron oxide fused with strontium or barium, also called a ceramic magnet. Weak, cheap, rustproof, rare-earth-free, and the most produced magnet in the world by tonnage.
Ferromagnetism#
Strong, spontaneous magnetic order, with neighboring moments lined up parallel. Iron, cobalt, nickel, and gadolinium if it's cold enough.
Field weakening#
Running current that opposes the magnets' field to hold back-EMF down at high speed. It costs efficiency and pushes the magnets toward demagnetization.
Flotation#
Separating minerals by making the wanted ones stick to air bubbles and float off, while waste rock sinks.
Flux Φ#
The amount of magnetic field passing through a surface you choose: flux density times area. Measured with a search coil wrapped around that surface, and read in volt-seconds (webers) or maxwell-turns.
Three names, three instruments, and most of the confusion in magnet specs comes from treating them as one.
Flux density (B) is the field at a point. You read it with a gaussmeter, which is a Hall probe on a stick, and the number depends on exactly where the probe sits. Move it a millimeter off the surface and the reading drops, which is why two honest people can measure the same magnet and disagree.
Flux (Φ) is how much field passes through a surface you choose. Wind a search coil around the region you care about, pull the magnet out (or the coil off), and an integrating fluxmeter adds up the voltage the coil saw. The answer comes out in volt-seconds. Search coils tend to be built in-house for one application, and there is no certificate that turns one shop's reading into another's.
Total flux describes the whole magnet. Pass it through a Helmholtz coil, a matched pair of coils with a uniform zone between them, and the same kind of fluxmeter gives you volt-seconds again. The difference is the coil's calibrated coefficient, which converts that reading into maxwell-centimeters. That conversion is what makes a Helmholtz number reproducible on any certified coil, and it's why total flux is the figure that turns up in incoming-inspection specs.
So two of the three measurements start from the same raw reading. They end up in different units because of calibration practice, not physics. (Strictly, maxwell-centimeters is a unit of magnetic moment, which is why physics texts call the same number the moment.)
Explained with this figure: Flux density · Fluxmeter · Gaussmeter · Helmholtz coil · Search coil · Total flux
Flux barriers#
Air slots cut into a rotor to block flux along one axis and steer it along another.
Flux density B#
How much magnetic flux passes through a given area: what a sensor, a steel part or a motor actually sees. Read at a point with a gaussmeter, in tesla (SI) or gauss (CGS).
Explained with Flux, which shares one figure with the related terms.
Fluxmeter#
An instrument that integrates the voltage a coil produces while the flux through it changes, and reports volt-seconds. Search coils and Helmholtz coils are both read with one.
Explained with Flux, which shares one figure with the related terms.
G
Gadolinium#
A rare earth that is ferromagnetic only below about room temperature. Used in MRI contrast agents; a curiosity for magnets.
Gauss G#
CGS unit of flux density. 10,000 gauss = 1 tesla. Still common on US datasheets and gaussmeters.
Gaussmeter#
A handheld instrument, usually a Hall probe, that reads flux density at one point. The reading depends heavily on exactly where, and how far from the surface, you hold it.
Explained with Flux, which shares one figure with the related terms.
Grade#
The NdFeB grade number is the minimum energy product in MGOe. N52 is near the top of what sintered NdFeB can do.
Grade suffix#
Letters after the number raise the coercivity and the rated temperature, roughly: none 80 °C, M 100, H 120, SH 150, UH 180, EH 200, AH 220. Usually bought with heavy rare earth.
Grain#
A single crystal within a solid metal. In a sintered magnet each grain is a few microns across and should hold its magnetization independently.
Grain boundary#
The thin layer between grains. In NdFeB it's rich in neodymium, and its job is to keep each grain magnetically isolated from its neighbors.
Grain boundary diffusion#
Coating a finished part with dysprosium or terbium and heating it, so the heavy rare earth travels in along the grain boundaries, where it does the most good, instead of through the grains.
Grain-oriented and non-oriented steel#
Grain-oriented sheet is rolled so its crystals line up one way, ideal for transformers. Non-oriented sheet works equally in every direction, for motors where the field rotates.
Green body#
The pressed, aligned powder before sintering: about 70% dense and fragile enough to crumble in your hand.
H
Halbach array#
Magnets arranged with their directions rotating step by step, which strengthens the field on one side and nearly cancels it on the other.
Helmholtz coil#
A matched pair of coils spaced one radius apart, with a uniform zone between them. Pass a magnet through, read the fluxmeter, apply the coil's calibrated coefficient, and you have total flux in maxwell-centimeters.
Explained with Flux, which shares one figure with the related terms.
Hot-deformed magnet#
NdFeB made by hot-pressing and squashing very fine-grained powder instead of sintering it. Its fine grains need less heavy rare earth for the same heat resistance.
Hund's rule#
Electrons filling a partly empty shell spread out with their spins parallel before they start pairing up, which leaves some atoms with a net moment.
Hydrogen decrepitation#
Exposing alloy flake to hydrogen, which the grain boundaries absorb and swell on, cracking the flake into coarse powder along exactly those boundaries.
Hydrogen processing of magnet scrap#
Using hydrogen to crumble whole scrap magnets into powder, the same way hydrogen decrepitation breaks up cast flake. The usual first step of the short loop.
Hysteresis#
The material's response depends on its history: the curve going up is not the curve coming down. For a permanent magnet, that lag is the whole point.
Explained with B-H curve, which shares one figure with the related terms.
I
I²R loss#
Heat from current flowing through winding resistance: current squared times resistance. Also called copper loss.
IE efficiency classes#
International efficiency ratings for line-connected motors, from IE1 (standard) to IE5 (ultra-premium), set by IEC 60034-30-1. IE6 is not yet part of that standard.
Induction motor#
An asynchronous machine: its rotor has to run slightly slower than the field, because that difference is what induces its current.
Intrinsic coercivity Hci#
The reverse field at which the material's own magnetization collapses. The real measure of how hard a magnet is to demagnetize, and what the grade suffix (SH, UH, EH) tracks.
Explained with Demagnetization curve, which shares one figure with the related terms.
Inverter#
Power electronics that synthesize AC at whatever frequency and voltage the motor needs, sold as a variable-frequency drive. Often half the system cost.
Ionic clay#
Clay with rare earths held loosely on the particle surfaces, so they wash off with a salt solution and no rock needs crushing. The world's main source of dysprosium and terbium.
IPM rotor#
Interior permanent magnet: a rotor with the magnets buried in slots inside the steel. Adds reluctance torque and protects the magnets; the usual EV traction choice.
Iron loss#
Heat in the steel from hysteresis and eddy currents as the field changes direction. Also called core loss.
Iron nitride#
A candidate rare-earth-free magnet material. Promising on paper; its performance in commercial magnets is still being proven.
Isostatic pressing#
Sealing aligned powder in a flexible bag and squeezing it with fluid from every side. The best alignment, at the cost of throughput.
Isotropic and anisotropic magnets#
An anisotropic magnet had its grains aligned during manufacture and is much stronger along that one direction. An isotropic magnet can be magnetized any way, and is weaker for it.
J
Jet milling#
Grinding powder by firing it at itself in high-speed jets of nitrogen, until each particle is about one grain. Nothing touches the powder but gas.
K
Keeper#
A soft iron bar laid across a magnet's poles to close the circuit, historically shipped on alnico magnets to keep them from demagnetizing themselves.
Kerf#
The width of material a saw blade or wire destroys with every cut.
Knee#
The point where the demagnetization curve turns sharply down. Run a magnet past it and the loss is permanent until it is remagnetized.
Explained with Demagnetization curve, which shares one figure with the related terms.
L
Lamination#
A thin, insulated sheet of electrical steel. Stacked into a core, laminations break up the paths eddy currents would otherwise take.
Lanthanides#
The fifteen elements from lanthanum to lutetium, which fill the 4f shell one electron at a time and are chemically almost identical as a result.
Leaching#
Dissolving the rare earths out of a solid into solution, leaving the rest behind.
Light and heavy rare earths#
A split by atomic weight, and where the line falls depends on who is drawing it. In magnets, "heavy" in practice means dysprosium and terbium.
Load angle#
How far the rotor trails the rotating field. Torque peaks at 90°; push past that and the machine pulls out of step.
Lodestone#
Naturally magnetized magnetite, the iron ore in which people first noticed magnetism.
Long loop#
Recycling magnets by dissolving them back into solution and re-separating the rare earths, then making metal and alloy again. Costlier, but the output is as good as new material.
Lorentz force#
The force a magnetic field exerts on a moving electric charge. It's the reason a current-carrying wire in a magnet pushes sideways, and the reason motors turn.
M
Machining#
Slicing and grinding sintered blocks to finished size with diamond tooling. Typically costs about a quarter of the block.
Magnet alloy#
The magnet's recipe in metal form: neodymium, iron, boron and a set of minor additions, melted together under vacuum.
Magnetic circuit#
The path flux takes through magnets, steel and air, analyzed much like an electric circuit, with reluctance playing the part of resistance.
Magnetic field H#
The field you apply, usually by running current through a coil. Think of it as the drive. Measured in amperes per meter (SI) or oersteds (CGS).
Explained with B-H curve, which shares one figure with the related terms.
Magnetic moment#
How strong a tiny magnet something is, whether an electron, an atom or a whole bar. It is what lines up in a field.
Magnetization J#
The material's own contribution to the field, with the applied field subtracted out: J = B − μ₀H. Datasheets in CGS write the same thing as 4πM.
Explained with B-H curve, which shares one figure with the related terms.
Magnetizing#
Magnets are made unmagnetized and given their field last, by a short, very strong pulse of current through a coil. It also restores a magnet that has been knocked down.
Magnetizing current#
The current an induction motor's stator draws just to build the field, since nothing in its rotor brings one along.
Magnetocrystalline anisotropy#
A crystal's preferred direction of magnetization. Turning it away from that easy axis takes energy, and that resistance is where a permanent magnet's coercivity comes from.
Maximum working temperature#
A grade's rated temperature. The rating assumes a particular magnet shape; a thin magnet in open air starts losing ground well before it.
Explained with Demagnetization curve, which shares one figure with the related terms.
Maxwell's equations#
The four equations that describe all of classical electricity and magnetism. Notably, they include no magnetic charge.
Meissner effect#
A superconductor expels magnetic field from its interior, which lets it levitate a magnet stably.
Metallization#
Turning a rare earth oxide back into metal by stripping the oxygen off. The step between chemistry and metallurgy.
MGOe#
Mega-gauss-oersteds, the CGS unit of energy product. 1 MGOe ≈ 7.96 kJ/m³. The "52" in N52 is MGOe.
MGU-K and MGU-H#
Formula 1 hybrid units: the MGU-K recovers energy from braking; the MGU-H recovered it from the turbocharger and was dropped from the 2026 rules.
Microstructure#
The arrangement of grains and phases inside a metal, visible only under a microscope. In a magnet it decides nearly everything.
Mixed rare earth carbonate#
An intermediate holding all the rare earths together, not yet separated. Common product of ionic clay operations and of mines without their own separation plant.
MOFCOM#
China's Ministry of Commerce, which administers its export controls.
Molten salt electrolysis#
How neodymium metal is made: oxide dissolved in a bath of molten fluoride salts, with current driving metal to one electrode and oxygen off the other.
Monazite#
A rare earth phosphate mineral, often recovered from mineral sands. It carries thorium, which makes processing a regulatory exercise as well as a chemical one.
Monopole#
A hypothetical isolated north or south pole. Allowed by some theories, never observed.
MTPA#
Maximum torque per amp: a control strategy that splits current between alignment and reluctance torque to get the most torque from each amp.
Mu-metal#
A nickel-iron alloy that magnetizes at the faintest field, used for magnetic shielding. Its properties come from a final anneal and are easy to ruin by handling.
N
Nameplate capacity#
The output a plant is designed for. Actual production is usually lower, and in a ramp-up, much lower.
Nd₂Fe₁₄B#
The crystal compound inside an NdFeB magnet that does the magnetic work. Everything else in the alloy is there to support it.
NdFeB#
Neodymium-iron-boron, the strongest permanent magnet in commercial production. Mostly iron by weight, which is why it rusts.
NdPr#
Neodymium and praseodymium, produced together as one product, also called didymium. NdPr oxide is the benchmark price for magnet-grade rare earths.
Neodymium#
The rare earth that puts the "Nd" in NdFeB. Typically about a quarter to a third of the magnet by weight, together with praseodymium.
Normal coercivity Hcb#
The reverse field at which the net flux density B reaches zero. It can never exceed the intrinsic coercivity, and on a datasheet it's the less important of the two.
Explained with Demagnetization curve, which shares one figure with the related terms.
O
OEM#
Original equipment manufacturer: the company that builds the finished product, such as the car or the wind turbine, and buys magnets or motors to put in it.
Oersted Oe#
CGS unit of applied field H. 1 kOe ≈ 79.6 kA/m. Coercivity is often quoted in kOe.
Offtake#
A buyer's commitment to purchase a share of a future plant's output. Often what makes the project financeable.
Operating point#
Where on its demagnetization curve a magnet actually sits in a given design, set by its shape and the steel and air around it.
Explained with Demagnetization curve, which shares one figure with the related terms.
Ore body#
The mineralized rock worth mining. Hard-rock rare earth ores run a few percent rare earths at best.
Oxide#
A metal combined with oxygen. In this business it usually means the traded form of a separated rare earth, a fine powder; grades and prices are quoted on an oxide basis.
P
Paramagnetism#
A weak attraction to a field that disappears when the field goes away. Aluminum is mildly paramagnetic; you'll never feel it with a hand magnet.
Pauli exclusion principle#
The rule that no two electrons in an atom can share the same state. It's why electrons pair up with opposite spins, and why most atoms cancel out.
Permanent magnet#
A hard magnetic material: one that keeps its magnetization once the applied field is gone, and takes a large reverse field to undo. "Hard" refers to coercivity, not mechanical hardness.
Permeability#
How readily a material carries flux compared with empty space. Soft iron's runs to thousands; a sintered neodymium magnet's is barely above one.
Permeability of free space μ₀#
The constant that turns applied field into flux density in empty space, about 4π × 10⁻⁷ T·m/A. The reference line every material is measured against.
Permeance coefficient Pc#
The slope of the load line. A long magnet in a closed steel circuit has a high Pc and sits safely above the knee; a thin disc in open air has a low one.
Explained with Demagnetization curve, which shares one figure with the related terms.
PM-assisted SynRM#
A synchronous reluctance motor with cheap magnets, often ferrite, dropped into the flux barriers to boost torque and power factor.
PMSM#
Permanent magnet synchronous machine: a synchronous motor whose rotor field comes from permanent magnets. The default for EVs and most high-efficiency drives.
Pole#
Where flux leaves or re-enters a magnet. They always come in pairs; cut a magnet in half and you get two magnets, not two poles.
Pole count#
How many north-south pairs the field has around the machine. More poles means slower rotation at the same supply frequency, and more torque per amp.
Pole piece#
Shaped soft iron that steers and concentrates flux from a magnet or coil to where it's needed.
Post-consumer scrap#
Magnets recovered from products at end of life: hard drives, motors, speakers. Scattered, mixed and hard to collect, which is the real problem.
Post-industrial scrap#
Scrap generated inside a plant: swarf, broken parts, off-spec blocks. Known chemistry, known location, and mostly already recycled.
Power density#
Power or torque per kilogram (or per liter) of machine. The numbers that decide whether a motor fits an aircraft.
Praseodymium#
Neodymium's chemical near-twin. It substitutes for neodymium in magnets and is usually separated and sold with it.
Precipitation#
Adding a chemical that makes the separated rare earth drop out of solution as a solid.
Production quota#
China's annual caps on rare earth mining and smelting/separation, issued in batches to state-approved groups.
Pull force#
The force needed to pull a magnet straight off a thick, flat steel plate. It's a property of the setup, not the material: thinner steel, paint or a gap all cut it sharply.
R
Radial flux#
The conventional layout: flux crosses the gap outward from the shaft, rotor inside a cylindrical stator.
Rare earth elements#
Seventeen metals: the fifteen lanthanides plus scandium and yttrium. Not especially rare in the crust; rarely concentrated enough to mine, and hard to separate from each other.
Recoil#
How a magnet behaves once it has been knocked down: it moves back and forth along a straighter, lower line rather than retracing its original curve.
Reluctance#
The magnetic version of resistance: how hard it is to push flux down a given path. Air has a lot; steel has very little.
Reluctance torque#
Torque from shaped iron twisting to give flux the shortest steel path. No magnet needed, but weaker for the same size.
Remanence Br#
The flux density a material keeps once the applied field is back to zero. In soft iron it's a nuisance; in a permanent magnet it's the product.
Explained with B-H curve, which shares one figure with the related terms.
Retention sleeve#
A carbon-fiber or metal sleeve wrapped over surface-mounted magnets so they don't leave the rotor at high speed.
Reversible and irreversible loss#
A reversible loss disappears when the magnet cools back down. An irreversible one stays until the magnet is remagnetized.
Rotating field#
A field of constant strength that sweeps around the stator bore, made by three-phase current with no moving parts. The trick the whole industry is built on.
Rotor#
The part of a motor that turns.
S
Saliency#
A rotor having a much easier magnetic direction and a much harder one. Without it there is no reluctance torque.
Samarium#
The rare earth in samarium-cobalt magnets. Comparatively plentiful in light rare earth ores.
Saturation#
The point where every domain is already lined up and the material has nothing left to give. More applied field adds only what empty space would.
Explained with B-H curve, which shares one figure with the related terms.
Scandium#
Counted as a rare earth by convention; geologically it keeps different company, and it plays no part in magnets.
Search coil#
A coil wound around the part or region you care about and read with an integrating fluxmeter. Usually built in-house for one job and not independently certified, so readings are reported in volt-seconds rather than converted.
Explained with Flux, which shares one figure with the related terms.
Self-heating powder#
Fine magnet powder with so much surface area that it oxidizes and warms on contact with air. Kept under protective gas from milling to sintering.
Separation#
Splitting a mix of rare earths into individual elements or pairs. Because they are chemically near-identical, this is the hard, expensive, most concentrated step in the chain.
Short loop#
Recycling magnets back into magnets without returning to individual elements, typically by hydrogen-processing the scrap into powder and re-sintering it. Cheaper, but you get back the alloy you put in.
Silicon iron#
Iron with a few percent silicon, also called electrical steel, which cuts eddy-current losses. Rolled into thin sheet, it is the core of nearly every transformer and motor.
Sintered magnet#
A magnet made by pressing fine powder and fusing it in a furnace into a fully dense block. The highest-performing route, and the one how-magnets-are-made walks through.
Sintering#
Heating the green body in a vacuum until the grain boundary phase melts and pulls the grains together into a dense block, shrinking it about 15% in each direction.
Slip#
How far an induction rotor lags the field, usually a few percent at full load. It is the operating principle, not a defect, and the price is rotor heat.
SmCo#
Samarium cobalt, the first rare-earth magnet family. Not as strong as NdFeB at room temperature, but far more stable when hot, and more corrosion-resistant.
SmCo 1:5 and 2:17#
The two SmCo families, named for their atom ratios: SmCo₅ and Sm₂Co₁₇. The 2:17 type is stronger and holds up to higher temperatures.
Soft magnetic material#
A material that carries a field strongly while one is applied, then lets go. It routes flux rather than supplying it: transformer cores, motor laminations, shields.
Solvent extraction#
The industry's standard separation route: the solution is contacted, stage after stage, against an organic liquid that prefers some elements very slightly. Hundreds of stages turn a small preference into a clean split.
Spin#
A built-in property of the electron that makes it act like a tiny magnet. Despite the name, nothing is literally rotating.
Spin-orbit coupling#
The link between an electron's spin and its motion around the nucleus. In rare earths it's strong, and it locks the magnetization to the crystal lattice.
SPM rotor#
Surface permanent magnet: a rotor with the magnets glued to its outside surface. Simple and strong, but the magnets need holding on at speed.
Squirrel cage#
An induction rotor made of conducting bars shorted by end rings. No magnets, no connections, nothing to wear.
Stator#
The stationary part around (or beside) the rotor, carrying the windings.
Stepper motor#
A motor that moves in fixed steps rather than spinning freely. The hybrid type adds a magnet and is what almost everyone means by "stepper."
Stoner criterion#
The condition for a metal's shared electrons to split into more spin-up than spin-down on their own. It's the band-theory explanation of why iron, cobalt and nickel are ferromagnetic.
Strip casting#
Pouring molten alloy onto a spinning, water-cooled copper wheel so it freezes in a fraction of a second into thin flake. Book molding, an older route, casts thick ingots instead.
Superconductor#
A material with zero electrical resistance below a critical temperature. Superconducting coils make the strongest steady fields, including MRI magnets.
Swarf#
The sludge of fine particles from sawing and grinding magnets. Plentiful, oily, oxidized, and self-heating when it dries.
Switched reluctance machine#
A motor with toothed iron on both rotor and stator and windings switched in sequence. Rugged and magnet-free, but noisy and lumpy.
Synchronous machine#
A machine whose rotor turns at exactly the speed of the rotating field.
Synchronous reluctance machine#
A synchronous motor with an iron rotor shaped by flux barriers and no magnets at all, running on reluctance torque.
T
Tailings#
What is left after beneficiation: finely ground waste rock, stored at the mine, and most of the mass that was dug up.
Temperature coefficient#
How much Br or Hci drops per degree of warming. For sintered NdFeB, typically about −0.12%/°C for Br and around −0.5%/°C for Hci.
Terbium#
Dysprosium's more potent and more expensive sibling. Less of it does the same job.
Tesla T#
SI unit of flux density. A strong neodymium magnet measures a few tenths of a tesla at its surface; a hospital MRI runs at 1.5 to 3.
Tetrataenite#
An ordered iron-nickel crystal found naturally in meteorites, where it forms over millions of years of slow cooling. Making it on a factory timescale is the research problem.
Thorium#
A mildly radioactive element that rides along in most rare earth ores, monazite especially. Regulators classify it as NORM, naturally occurring radioactive material.
Three-phase#
Three separate windings fed with alternating current shifted 120° apart in time. Together they add up to a rotating field.
Tolling#
Processing someone else's material for a fee, without taking ownership of it.
Torque#
Twisting force on a shaft. Power is torque times speed, so a slow machine needs a lot of torque to do the same work.
Torque ripple#
Fluctuation in torque as the rotor turns. Felt as vibration, heard as noise.
Total flux#
The flux of a whole magnet, measured by passing it through a Helmholtz coil. The coil's calibrated coefficient turns the reading into maxwell-centimeters, so results match from one certified coil to another.
Explained with Flux, which shares one figure with the related terms.
Trapezoidal and sinusoidal drive#
Two ways to shape the current fed to a brushless motor: simple block steps (trapezoidal) or smooth waves (sinusoidal), which are quieter and smoother.
TREO#
Total rare earth oxide: how mines report grade, as if every rare earth in the rock were converted to oxide.
TRL#
Technology readiness level: a 1-to-9 scale for how far a technology has come, from a principle on paper (1) to proven in operation (9).
Tundish#
A small trough that meters molten metal from the crucible onto the casting wheel.
U
Universal motor#
A brushed motor with its field wound in series, so it runs on AC or DC. The sound of every vacuum cleaner and router for a century.
Unpaired electron#
An electron without a partner of opposite spin to cancel it out. Atoms with unpaired electrons are the raw material of magnetism.
Upstream, midstream, downstream#
Rough stages of the chain: mining and concentrating (upstream); separation, metal and alloy (midstream); magnets and the products they go into (downstream).
V
Vacuum induction melting#
Melting metal inside a vacuum by inducing current in it, which keeps oxygen and nitrogen out of a very reactive melt.
W
Winding#
Copper wire wound into the stator or rotor slots to carry the current that makes the field.
Wound field machine#
An externally excited synchronous machine: the rotor field comes from a coil fed with current instead of from magnets. Rare-earth-free, at the cost of rotor losses.
X
Xenotime#
An yttrium phosphate mineral, comparatively rich in the heavy rare earths.
Y
Yield#
The share of input that ends up as saleable product. In magnets it is lost a little at almost every step, and most heavily at the mine and the grinding wheel.
Yttrium#
Counted as a rare earth though it isn't a lanthanide. Mostly used in phosphors, ceramics and coatings, not magnets.
Work with Peculiar Materials
Technical and financial diligence on magnetics, magnet manufacturing, and the rare‑earth supply chain — from oxide to finished magnet.