What Is Magnetism, Really?
Magnetism is a force produced by moving electric charge. In permanent magnets, that "moving charge" is the spin and orbital motion of electrons inside atoms. Most materials have electron spins pointing in random directions, so their magnetic effects cancel out. In ferromagnetic materials — iron, nickel, cobalt, and alloys built around them — electron spins in neighboring atoms naturally align with each other, creating regions of uniform magnetization called magnetic domains.
In a raw, unmagnetized piece of ferromagnetic material, the domains themselves point in random directions and cancel each other out. Magnetizing the material means applying a strong external field that forces the domains into alignment. In a permanent (hard) magnet, the material's crystal structure resists letting those domains rotate back, so the alignment — and the external magnetic field it produces — persists indefinitely.
Poles, Fields, and Flux
Every magnet has a north pole and a south pole — always in pairs. Cut a magnet in half and you get two smaller complete magnets, never an isolated pole. Field lines flow from north to south outside the magnet (and south to north inside it), forming closed loops. The density of those lines — magnetic flux density (B) — is what determines how strongly the magnet acts on nearby materials.
Three practical rules follow from field behavior:
- Opposites attract, likes repel. N-to-S surfaces pull together; N-to-N or S-to-S push apart.
- Field strength falls off fast with distance — roughly with the cube of distance for a small magnet. Doubling the air gap can cut holding force by 80–90%.
- Ferromagnetic materials "complete the circuit." Steel near a magnet gives flux an easy path, which is why a magnet on a steel plate holds far harder than the same magnet on aluminum (which is effectively invisible to it).
A permanent magnet is an energy storage device that never discharges through normal use. Holding, attracting, and repelling do not "use up" a magnet — only heat, opposing fields, radiation, or physical damage degrade it.
Hard vs. Soft Magnetic Materials
High coercivity — keeps its magnetization
NdFeB, SmCo, alnico, hard ferrite. Once magnetized, these resist demagnetization and serve as the field source in a design.
Low coercivity — magnetizes and releases easily
Low-carbon steel, silicon steel, permalloy, soft ferrite. These carry, concentrate, and steer flux — pole pieces, yokes, motor laminations, transformer cores.
Most real magnetic assemblies use both: a hard magnet to generate flux and soft steel to route it where the design needs it. That pairing is the basis of magnetic circuits, covered in Chapter 10.
Units & Quantities You'll Actually Encounter
Magnetics is one of the last fields where CGS units (Gauss, Oersted) and SI units (Tesla, A/m) are still used side-by-side on datasheets. The table below covers every quantity that appears on a magnet specification.
| Quantity | Symbol | CGS Unit | SI Unit | Conversion | What it means in practice |
|---|---|---|---|---|---|
| Flux density | B | Gauss (G) | Tesla (T) | 10,000 G = 1 T | Field strength at a point — what a gaussmeter reads at the magnet's surface. |
| Field strength | H | Oersted (Oe) | A/m | 1 Oe ≈ 79.577 A/m | The magnetizing (or demagnetizing) field applied to a material. |
| Remanence | Br | Gauss | Tesla | — | Flux density remaining in a fully magnetized magnet in a closed circuit. The headline "strength" number. |
| Coercivity | Hcb | Oe | kA/m | — | Reverse field needed to drive B to zero. |
| Intrinsic coercivity | Hcj (Hci) | Oe | kA/m | — | Reverse field needed to truly demagnetize the material. The real measure of demag resistance. |
| Max energy product | (BH)max | MGOe | kJ/m³ | 1 MGOe ≈ 7.958 kJ/m³ | Energy density — the number in the grade (N52 ≈ 52 MGOe). |
| Flux | Φ | Maxwell | Weber (Wb) | 10⁸ Mx = 1 Wb | Total field passing through an area (helmholtz-coil / fluxmeter measurements). |
| Curie temperature | Tc | °C | °C / K | — | Temperature at which ferromagnetism disappears entirely. |
| Max operating temp | Tmax | °C | °C | — | Practical service limit before significant irreversible loss (geometry-dependent). |
Reading surface fields: a gaussmeter on the face of an N52 disc typically reads 3,000–6,500 G depending on geometry — never the 14,800 G Br value. Br is measured in a closed magnetic circuit; open-air surface field is always lower and varies across the pole face. Comparing a datasheet Br to a handheld gaussmeter reading is the single most common spec misunderstanding we see.
The Four Permanent Magnet Families
Nearly every permanent magnet in production today belongs to one of four material families. Choosing between them is a trade among strength, temperature capability, corrosion resistance, and cost.
| Property | NdFeB | SmCo | Alnico | Ferrite (Ceramic) |
|---|---|---|---|---|
| (BH)max | 33–55 MGOe | 16–33 MGOe | 1.4–7.5 MGOe | 1–4.5 MGOe |
| Br | 11.7–14.8 kG | 8.7–11.6 kG | 6.7–13.5 kG | 2.2–4.1 kG |
| Intrinsic coercivity | High–Very High | Very High | Very Low | Moderate |
| Max operating temp | 80–230 °C (grade-dep.) | 250–350 °C | 450–550 °C | ~250 °C |
| Curie temp | 310–370 °C | 700–825 °C | ~800 °C | ~450 °C |
| Corrosion resistance | Poor — must be coated | Good | Good | Excellent |
| Relative cost (per unit energy) | $$ (best value) | $$$$ | $$$ | $ |
| Mechanical character | Hard, brittle | Very brittle | Hard, tough, castable | Brittle, abrasive |
| Best for | Maximum strength per size/weight | High temp + harsh environments | Very high temp, stable-with-temp sensors | Cost-driven, high-volume, wet environments |
When each family wins
The default choice for modern designs
The strongest commercial magnet material, roughly 10× the energy product of ferrite. When the design goal is maximum force in minimum space — EV traction motors, sensors, medical devices, robotics, holding systems — NdFeB is almost always the answer. Its weaknesses (corrosion, temperature sensitivity) are managed with coatings and grade selection, covered in Chapters 5, 8, and 9.
Heat, chemistry, and radiation resistance
Slightly weaker than NdFeB but stable to 250–350 °C, inherently corrosion-resistant (usually needs no coating), radiation-tolerant, and with a very low temperature coefficient. The pick for aerospace, downhole, defense, and high-temperature sensor applications where NdFeB grades run out of headroom.
Extreme temperatures and stability
Usable to ~500 °C with the most stable output-vs-temperature of any family — which is why it persists in instrumentation, guitar pickups, and legacy sensors. Its Achilles' heel is very low coercivity: alnico can be demagnetized by its own field if removed from its magnetic circuit, or by modest external fields.
Cost and corrosion immunity
The cheapest magnet material by far and completely immune to rust — it's already an oxide. Weak per unit volume, so parts are bulky, but for high-volume consumer products, separators, and wet environments where size doesn't matter, ferrite remains the volume leader worldwide.
Start with NdFeB. Move to SmCo if operating temperature, chemical exposure, or radiation rules NdFeB out. Move to ferrite if cost dominates and volume is unconstrained. Choose alnico only for >350 °C service or when temperature stability of output is the spec.
Neodymium (NdFeB) In Depth
NdFeB magnets are built on the Nd₂Fe₁₄B intermetallic compound, discovered independently by General Motors and Sumitomo in 1983–84. The tetragonal crystal structure of this phase has extremely high magnetocrystalline anisotropy — each crystal strongly "prefers" to be magnetized along one axis — which is what makes both very high remanence and high coercivity possible in the same material.
Commercial NdFeB is not just neodymium, iron, and boron. Producers tune the alloy with additional elements:
- Dysprosium (Dy) / Terbium (Tb) — heavy rare earths substituted into the crystal lattice (or diffused along grain boundaries) to raise intrinsic coercivity for high-temperature grades. The dominant cost driver in H/SH/UH/EH grades.
- Praseodymium (Pr) — routinely blended with Nd (as "didymium," PrNd) with minimal performance impact.
- Cobalt (Co) — raises Curie temperature and improves corrosion behavior.
- Aluminum, copper, gallium, niobium — grain-boundary refinements for coercivity and manufacturability.
Sintered vs. Bonded NdFeB
The same chemistry ships in two very different product forms, and choosing between them is a fundamental fork in any design. (See our full sintered vs. bonded comparison guide for a deeper treatment.)
| Attribute | Sintered NdFeB | Bonded NdFeB |
|---|---|---|
| Process | Pressed powder, sintered ~1050 °C, ground to size | Magnet powder in polymer binder — compression or injection molded |
| (BH)max | 33–55 MGOe | 5–12 MGOe |
| Geometry freedom | Simple shapes; complex features via grinding (cost adds up) | Net-shape complex geometry, thin walls, molded-in features, insert molding onto shafts |
| Tolerances as-formed | ±0.05 mm typical (ground) | ±0.05–0.1 mm as-molded, no grinding |
| Magnetization patterns | Mostly single-axis; multipole difficult | Excellent — multipole, true radial rings, complex patterns easy |
| Mechanical behavior | Brittle | Less brittle, machinable, crack-tolerant |
| Typical uses | Traction motors, holding, couplings, speakers, anywhere max force matters | Sensor rings, encoders, small BLDC/stepper rotors, fuel pumps |
Where radial magnetization fits: bonded NdFeB rings are the workhorse for true radially-magnetized rings and multipole encoder targets, because the isotropic powder can be magnetized in any pattern after molding. Sintered radial-ring technology exists (radially-oriented sintered rings) and delivers much higher output for motor applications — this is a core Radial Magnets specialty. Chapter 7 covers magnetization directions in detail.
Decoding NdFeB Grades
An NdFeB grade like N42SH encodes two independent things:
- The number (35–55) is the maximum energy product in MGOe — the "strength" axis. N52 stores roughly 48% more energy per unit volume than N35.
- The letter suffix (none, M, H, SH, UH, EH, AH) is the intrinsic coercivity class — the "temperature resistance" axis. Higher suffixes carry more Dy/Tb, cost more, and give up a little Br.
| Suffix | Hcj (min) | Max op. temp* | Typical use case |
|---|---|---|---|
| (none) N | ≥12 kOe | 80 °C | Room-temperature holding, consumer products, fixtures |
| M | ≥14 kOe | 100 °C | Warm environments, enclosed electronics |
| H | ≥17 kOe | 120 °C | Industrial motors, automotive cabin |
| SH | ≥20 kOe | 150 °C | Motors under load, underhood automotive |
| UH | ≥25 kOe | 180 °C | EV traction motors, aerospace actuators |
| EH | ≥30 kOe | 200 °C | High-demand traction, downhole, defense |
| AH | ≥35 kOe | 230 °C | The extreme end — specialty high-temp designs |
*Rated maximum operating temperatures assume a favorable magnetic circuit (permeance coefficient ≈ 2). Thin magnets in open circuits demagnetize at much lower temperatures — see Chapters 8 and 10.
| Grade | Br (kG) | Hcj (kOe) | (BH)max (MGOe) | Notes |
|---|---|---|---|---|
| N35 | 11.7–12.2 | ≥12 | 33–36 | Economy workhorse |
| N42 | 12.8–13.2 | ≥12 | 40–43 | The best strength-per-dollar sweet spot for most designs |
| N45 | 13.2–13.7 | ≥12 | 43–46 | Step up when space is tight |
| N52 | 14.2–14.7 | ≥11 | 49.5–52.5 | Near the practical ceiling; 80 °C limit, premium price |
| N55 | 14.5–15.0 | ≥11 | 52–55 | Bleeding edge, limited availability and geometry |
| N42SH | 12.8–13.2 | ≥20 | 40–43 | N42 strength with 150 °C capability |
| N38UH | 12.2–12.6 | ≥25 | 36–39 | Classic EV/motor spec — 180 °C class |
| N35EH | 11.7–12.2 | ≥30 | 33–36 | 200 °C class for the harshest duty cycles |
Within sintered NdFeB, strength and temperature resistance pull against each other. You can buy N52 or 200 °C capability — not both. The highest-energy grades top out around N48–N52 in the plain class, dropping to roughly N33–N38 by the EH class. Specify the coercivity class from your real worst-case temperature and demagnetizing field, then take the highest N-number available in that class.
Buyer beware: "N52" claims on uncontrolled marketplaces frequently test at N35–N42. Grade verification requires a hysteresisgraph (BH tracer) on a sample, or at minimum Helmholtz-coil flux comparison against a certified reference. Reputable suppliers provide material certifications and, for regulated industries, full PPAP documentation.
The BH Curve: Reading a Magnet's DNA
Every magnetic property on a datasheet comes from one measurement: the hysteresis loop. A sample is driven through a full cycle of applied field (H) while its flux density (B) is recorded. The second quadrant of that loop — where the magnet fights a demagnetizing field — is the demagnetization curve, and it's the part engineers actually use.
The four numbers that matter
- Br (remanence) — where the normal curve meets the B axis. Sets the ceiling on flux the magnet can deliver. Higher Br → higher surface field and pull force for the same geometry.
- Hcb (normal coercivity) — reverse field that drives net B to zero. Relevant to circuit calculations.
- Hcj (intrinsic coercivity) — reverse field that destroys the material's own magnetization. This is the number that predicts survival against heat and opposing fields.
- (BH)max (energy product) — the largest B×H rectangle under the normal curve; energy density in the working gap. The grade number.
Load lines and the operating point
A magnet in a real device doesn't operate at Br. Its geometry and surrounding circuit define a load line whose slope is the permeance coefficient (Pc); the intersection of that line with the demagnetization curve is the operating point. Long magnets in closed steel circuits have steep load lines (high Pc, operating near Br). Thin, unshielded discs have shallow load lines (Pc < 1) and operate far down the curve — which is why thin magnets are the first casualties of heat.
Everything in Chapters 8 (temperature) and 11 (demagnetization) comes back to one design rule: keep the operating point above the knee of the intrinsic curve at the worst-case temperature.
Magnetization Directions & Patterns
A magnet's shape and its magnetization direction are independent specifications — and getting the direction wrong is one of the most common (and expensive) drawing errors in the industry. Sintered NdFeB is anisotropic: its orientation axis is fixed during pressing, before sintering, and cannot be changed afterward. You can only magnetize it along the axis it was built with.
Axial & through-thickness
The default for discs, cylinders, rings, and blocks: poles on the two flat faces. Simple to orient, simple to magnetize, maximum availability. If a drawing doesn't specify direction, this is what you'll get — which is exactly why direction should always be specified.
Diametral
Poles on opposite sides of the curved surface of a cylinder or ring. Used in couplings, rotary sensors, and stirrers. A diametrally magnetized cylinder rolling on a table will always come to rest with its poles horizontal.
True radial vs. "pseudo-radial"
In a true radially magnetized ring, flux exits (or enters) uniformly around the entire outer diameter, with the return pole on the inner diameter — a seamless 360° pattern with no gaps or joints. This requires either radially-oriented sintered material (pressed with a radial orientation field) or isotropic bonded material magnetized in a radial fixture. The common workaround — gluing discrete arc segments around a hub — produces a segmented (pseudo-radial) assembly with flux dips at every joint, tolerance stack-up, and adhesive bond lines that become failure points at temperature.
- Uniform 360° flux — no cogging signature from joints
- One part: no segment assembly labor or adhesive risk
- Balanced rotor by construction; better at speed
- Cleaner sensor signals for encoders and resolvers
- Flux dips and harmonic content at each joint
- Assembly tolerance stack and balancing operations
- Adhesive limits temperature and lifetime
- Sometimes still the right call for very large diameters
True radial rings are a core specialty at Radial Magnets — for PM motor rotors, torque couplings, magnetic bearings, and sensor targets where field uniformity drives performance.
Multipole & special patterns
- Multipole (radial or axial) — alternating N-S sectors around a ring or face; the basis of encoder targets, BLDC rotors, and pole-count-matched sensor systems. Pole counts from 2 to 100+ are practical in bonded material.
- Halbach arrays — segments arranged with rotating magnetization to concentrate nearly all flux on one side of the array. Field on the strong side increases ~40%+ while the back side nearly cancels; used in motors, maglev, beam physics, and self-shielding assemblies.
- Multipole-on-one-face, unipole, and custom patterns — achievable in isotropic bonded material with fixture magnetization; always worth a manufacturability conversation before the drawing is frozen.
Drawing rule: always dimension the magnetization direction relative to a physical datum, state anisotropic vs. isotropic material, and specify whether parts ship magnetized or unmagnetized. "N42 ring" is not a complete specification.
Temperature Behavior
Heat is the number-one field failure mode for NdFeB. Three distinct things happen as a magnet warms up, and they're often confused:
Recovers on cooling
Output drops smoothly with temperature and returns completely when the magnet cools. For NdFeB, Br falls ~0.11–0.12% per °C; Hcj falls a much steeper ~0.5–0.6% per °C.
Recovers only by re-magnetizing
If falling Hcj lets the operating point slide below the knee, domains flip and stay flipped. The magnet is permanently weaker until it's re-magnetized at full saturation.
Permanent, unrecoverable
Extended exposure near or above the Curie temperature (310–370 °C for NdFeB) causes metallurgical change. No amount of re-magnetizing restores the material.
Because Hcj collapses so much faster than Br, temperature ratings are really coercivity ratings. And because the safety margin also depends on the operating point, the same grade has different real temperature limits in different shapes. A thick N42 cylinder (Pc ≈ 2) may be fine at 80 °C; a thin N42 disc (Pc ≈ 0.5) can take measurable permanent loss at 60 °C.
| Material | α(Br) %/°C | β(Hcj) %/°C | Curie temp |
|---|---|---|---|
| NdFeB (sintered) | −0.11 to −0.12 | −0.50 to −0.60 | 310–370 °C |
| SmCo (2:17) | −0.030 to −0.035 | −0.15 to −0.30 | 800–825 °C |
| Alnico 5 | −0.02 | +0.01 (rises!) | ~860 °C |
| Ferrite | −0.18 to −0.20 | +0.2 to +0.4 (rises) | ~450 °C |
Ferrite's cold-weather trap: unlike NdFeB, ferrite's coercivity falls as it gets colder — ferrite magnets in motors can demagnetize in sub-zero starts. NdFeB actually gets stronger and more robust in the cold (down to ~135 K, below which some grades experience a spin-reorientation effect).
Designing for temperature — the checklist
- Establish true worst-case magnet temperature — including self-heating from eddy currents in motors, not just ambient.
- Compute the permeance coefficient of the magnet in its circuit (thin/open = low Pc = less margin).
- Include any demagnetizing fields (stator reaction fields in motors are large).
- Select the coercivity class (M/H/SH/UH/EH) so the operating point stays above the knee at worst case, with margin.
- Then maximize the N-number within that class.
- For designs near the edge: consider grain-boundary-diffusion grades (high Hcj with less Dy), SmCo, or a thermal stabilization bake that pre-ages the parts.
Our interactive temperature derating tool models Br and Hcj vs. temperature for common grades.
Coatings & Corrosion Protection
Sintered NdFeB corrodes readily — the neodymium-rich phase at the grain boundaries oxidizes preferentially, and in humid environments an unprotected magnet will bloom white oxide, swell, and eventually crumble along grain boundaries. Virtually all commercial NdFeB ships coated. Coating selection is driven by environment, adhesive plans, wear, and regulatory requirements.
| Coating | Typical thickness | Corrosion | Notes & best use |
|---|---|---|---|
| Ni-Cu-Ni (standard) | 15–25 µm | Good (dry/indoor) | The industry default: hard, bright, wear-resistant. Not sufficient for salt spray or long-term immersion. Nickel surface is a poor epoxy bond surface without prep. |
| Epoxy (over Ni-Cu) | 15–30 µm | Very good | Best all-around outdoor/humidity choice; black finish. Softer surface — avoid sliding wear. |
| Zinc (Zn) | 8–20 µm | Moderate | Low cost, sacrificial protection, bonds well with adhesives. Dull gray; can white-rust in wet service. |
| Ni-Cu-Ni + Au | +0.1–0.5 µm Au | Very good | Biocompatible-friendly and cosmetic; common in medical devices and wearables. |
| Parylene C | 5–25 µm | Excellent | Conformal vapor-deposited polymer; pinhole-free, biocompatible (medical implant class), covers complex geometry. Delicate surface. |
| PTFE | 20–100 µm | Very good | Low-friction, chemically inert, easy to clean — food processing and lab magnets. |
| Phosphate | 1–3 µm | Minimal (temporary) | Short-term protection for magnets that will be potted/overmolded immediately. |
| Everlube / Al-based | 8–20 µm | Excellent (salt fog) | Aerospace/defense specifications; strong salt-spray performance. |
| Rubber/plastic overmold | 0.5–3 mm | Excellent | Impact protection + waterproofing + grip surface; mounting magnets, fixtures. |
Selection quick-guide
- Indoor, dry, general purpose: standard Ni-Cu-Ni.
- Gluing into an assembly: zinc or epoxy coat (or abrade/prime nickel); validate the bond, not just the coating.
- Outdoor / humidity / condensation: epoxy over nickel, or overmold.
- Medical / body contact: parylene C or gold over nickel; confirm ISO 10993 biocompatibility needs with your supplier.
- Salt fog / marine / defense: aluminum-based spec coatings, thick epoxy systems, or hermetic encapsulation (welded stainless or titanium cans for implants and downhole).
- High temperature: verify the coating's rating separately — many epoxies give out before an SH-class magnet does.
Coatings and tolerances: coating thickness is part of the finished dimension. A ±0.05 mm part with 20 µm of plating per side consumes most of that tolerance band. Specify whether drawing dimensions apply before or after coating — and remember that SmCo and ferrite usually need no coating at all, which sometimes settles a materials decision by itself.
Geometry, Magnetic Circuits & Pull Force
Two magnets of identical grade and volume can differ several-fold in useful output depending on shape and circuit. Geometry decides where the magnet sits on its demagnetization curve and how much flux reaches the working gap.
Shape families and what they're for
The general-purpose workhorse. Rods (L > D) reach farther; thin discs are efficient in closed circuits but vulnerable in open air.
Flat pole faces for holding, linear arrays, and Halbach construction. Easy to fixture and machine.
Shaft-mounted rotors, couplings, sensor targets, speaker motors. Direction options: axial, diametral, radial, multipole.
Motor rotor and stator segments; assembled into cylinders around a hub when true radial rings aren't used.
Self-aligning couplings, valves, and sensing. Field of a uniformly magnetized sphere is a perfect dipole.
A magnet in a steel cup: the steel steers nearly all flux to one face, multiplying holding force up to ~4–5× the bare magnet on that face.
What actually determines pull force
Published pull forces are measured under ideal conditions: magnet in direct, flat contact with a thick, clean, ground low-carbon steel plate, pulled straight off. Every real-world deviation reduces force:
| Factor | Effect |
|---|---|
| Air gap (paint, dirt, plating, paper) | Steep loss — a 0.1–0.2 mm paint layer can cost 20–40% of holding force on a small magnet. |
| Steel thickness | Thin sheet saturates and can't carry the flux; below ~3–6 mm (magnet-size-dependent) force drops sharply. |
| Steel alloy | Low-carbon 1006/1008/1018 ≈ ideal. Hardened, high-carbon, and some tool steels carry noticeably less. Austenitic stainless (304/316) is essentially non-magnetic. |
| Surface condition | Rough, rusty, or curved surfaces reduce contact area → reduce force. |
| Loading direction | Shear (sliding) resistance is far lower than tensile pull-off — often only 15–30% of rated pull, set by friction. |
| Temperature | Per Chapter 8 — output falls as the magnet heats. |
Size holding applications at 2–3× calculated worst-case load minimum — more if shear loading, vibration, or shock is present. Use our pull force calculator for first-pass numbers, then validate with samples in the real assembly.
Magnetic circuit thinking
Treat flux like current in a circuit: the magnet is the source, air gaps are large resistors, and soft steel is low-resistance wire. Three consequences:
- Backing a magnet with steel effectively mirrors it — the flux that would have leaked out the back is redirected forward, boosting gap field and raising the magnet's own Pc (protecting it).
- Two magnets in attraction through the workpiece (a "sandwich" or channel circuit) can multiply holding force well beyond the sum of the parts.
- Minimize total gap. Every millimeter of non-magnetic material in the flux path is force thrown away.
How Magnets Lose Strength (and How to Prevent It)
Under ordinary conditions a modern NdFeB or SmCo magnet loses a negligible fraction of its magnetization over decades — long-term ambient aging is typically well under 1% per decade for a properly specified part. When magnets "die," one of these five mechanisms is responsible:
- Heat — the dominant cause. Irreversible loss when the operating point crosses the knee at temperature (Chapter 8).
- External opposing fields — a stronger opposing field (from another magnet, a magnetizer/demagnetizer, a motor fault event, MRI environment, or welding cables) can drive the material past its intrinsic coercivity locally or entirely.
- Corrosion — grain-boundary attack physically destroys material; force loss follows structural loss (Chapter 9).
- Mechanical damage — chips and cracks remove material and create local flux distortion; severe shock can also cause minor domain disturbance (a small effect in modern high-coercivity material, but real for alnico).
- Radiation — significant gamma/neutron doses demagnetize NdFeB progressively; SmCo is markedly more tolerant and is the standard choice for accelerator and space applications.
Myth check: ordinary use — holding, attracting, repelling, even repeated attach/detach cycles — does not wear a magnet out. Dropping a modern NdFeB magnet does not meaningfully demagnetize it (though it will likely chip or shatter, which is its own problem). And magnets do not "recharge" by stacking them together.
Stabilization: pre-aging on purpose
Precision applications (sensors, instrumentation, aerospace) often specify thermal stabilization: parts are deliberately heat-soaked or field-knocked slightly past their expected worst case before shipment, taking the small irreversible loss up front. The parts then hold calibration through service because the loss they would have taken in the field has already happened.
How Sintered NdFeB Magnets Are Made
Understanding the process explains most of the material's quirks — why it's brittle, why orientation is fixed early, why tight tolerances mean grinding, and why every step matters for quality.
- Alloying & strip casting — raw Nd, Fe, B and additives are vacuum induction melted and strip-cast into thin flakes with a controlled microstructure.
- Hydrogen decrepitation & jet milling — hydrogen embrittles the alloy so it crumbles; jet mills reduce it to ~3–5 µm single-crystal-scale powder under inert gas (the powder is pyrophoric).
- Aligning & pressing — powder is pressed in a die while a strong orienting field aligns every particle's easy axis. This is the moment the magnetization direction is permanently set. Radially-oriented rings require specialized radial-field pressing.
- Sintering & heat treatment — compacts are vacuum-sintered at ~1030–1080 °C to full density (~7.5 g/cm³), then precision heat-treated to optimize the grain-boundary phase that produces coercivity.
- Machining — sintered blocks are diamond-ground, sliced, and honed to final dimensions. NdFeB is too hard and brittle for conventional cutting; EDM and grinding are the standard methods. This is where tolerances (typically ±0.05 mm) and surface finish are created — and a meaningful share of part cost.
- Coating — per Chapter 9.
- Magnetizing — a capacitor-discharge magnetizer delivers a pulse of roughly 2–3× Hcj (often 30+ kOe) through a fixture that defines the pole pattern. Parts can ship magnetized or unmagnetized (for customers who magnetize after assembly).
- Inspection — dimensional checks, coating adhesion/thickness, and magnetic verification: Helmholtz coil flux, surface Gauss mapping, hysteresisgraph sampling; PPAP-level documentation (process flow, PFMEA, control plan, MSA, SPC) for automotive and medical programs.
Every dimension on a sintered NdFeB part is a grinding operation. Opening a non-critical tolerance from ±0.02 mm to ±0.1 mm can cut part cost meaningfully; so can designing around standard shapes. Send us the drawing before it's frozen — DFM feedback is free, tooling changes aren't.
Handling & Safety
Strong magnets are genuinely hazardous in ways that surprise people used to refrigerator magnets. Two 2-inch N52 blocks can slam together from several inches apart with enough force to break fingers and shatter both magnets, throwing sharp fragments.
- Pinch/crush: keep fingers clear of the approach path; slide magnets apart sideways rather than pulling directly.
- Medical implants: keep strong magnets at least 30 cm (12") from pacemakers, ICDs, and insulin pumps — fields can alter device modes.
- Eye protection when handling large magnets — impact fragments are sharp and fast.
- Swallowing hazard: two or more swallowed magnets attract through intestinal walls and cause life-threatening injury. Keep all small magnets away from children; seek immediate medical care if ingestion is suspected.
- Keep away from credit cards, mechanical watches, hearing aids, CRTs, and magnetic media.
- Modern phones/laptops (SSD-based) are largely tolerant, but magnetometer/compass functions can be disturbed nearby.
- Store with keepers or in attracted pairs with spacers, in original packaging, away from heat.
- NdFeB is brittle: don't machine, drill, or grind outside proper equipment — powder is flammable and sparks can ignite it.
Shipping regulations
Magnetized material is regulated in air freight (IATA). Packages must be tested for external field; above 0.00525 G at 4.5 m (15 ft) they're forbidden from air transport, and above 0.002 G at 2.1 m (7 ft) they ship as Magnetized Material, UN2807 with hazard labeling. Proper packaging uses attracted pairs, steel shunting, and distance — one of many reasons bulk magnets ship in specific carton arrangements. Ground freight is far less restrictive.
Applications by Industry
Roughly speaking, permanent magnets do six jobs: convert electrical to mechanical energy (motors, actuators, speakers), convert mechanical to electrical (generators, energy harvesters, microphones), hold, couple through a barrier, sense position/speed, and separate materials. Here's how those map to the verticals we serve.
The largest NdFeB market
- Traction motors (IPM rotors — typically N35–N42 UH/EH segments)
- Dozens of auxiliary motors: pumps, HVAC, seats, wipers, steering
- Rotor position sensing, speed sensor rings (multipole bonded)
- IATF 16949 / PPAP supply chains; heavy focus on Dy reduction
Precision, biocompatibility, traceability
- Surgical tools, magnetic couplings in blood pumps, hearing implants
- Drug delivery, MRI-adjacent hardware (careful field management)
- Parylene/gold coatings, ISO 13485 quality systems, full lot traceability
Temperature, radiation, and sourcing rules
- Actuators, TWTs, gyros, generators, guidance sensors
- SmCo prevalence for temperature/radiation margins
- DFARS 225.7018 / NDAA-compliant sourcing of magnet material
Motion, sensing, holding, separation
- Servo and stepper rotors, magnetic couplings for sealed pumps
- Encoder rings and proximity targets, magnetic chucks and fixtures
- Conveyor separators, robotic end-effectors, magnetic bearings
Consumer electronics (speakers, haptics, closures, wireless-charging alignment), energy (wind generators), and appliances round out the demand picture — but the four verticals above are where specification rigor matters most, and where we focus.
Frequently Asked Questions
What's the strongest magnet I can buy?
Commercially, sintered NdFeB grade N52 (with N55 in limited shapes) — around 52–55 MGOe energy product and Br near 14.8 kG. But "strongest grade" rarely means "strongest solution": geometry, circuit design, and temperature class usually matter more than the last few MGOe. An N42 in a steel cup outperforms a bare N52 of the same size several times over.
Do magnets wear out or lose strength over time?
Not from normal use. Properly specified NdFeB/SmCo lose a small fraction of a percent over years at stable conditions. Real losses come from heat beyond the grade's capability, opposing fields, corrosion, or physical damage — all preventable by specification (Chapters 8–11).
Can I re-magnetize a magnet that got weak?
If the loss was thermal or field-induced (not structural or corrosion), yes — a full-saturation pulse in a magnetizer restores original performance. It requires fields of 2–3× the material's Hcj, so it's a supplier/service operation, not something achievable by stacking other magnets on it.
What's the difference between "radial" and "diametral" magnetization?
Diametral means one N pole and one S pole across the diameter — two poles total. True radial means flux exits uniformly around the entire OD with the return path on the ID — the poles are the cylindrical surfaces themselves, 360° around. They're completely different field patterns for completely different jobs, and confusing them on a drawing is a common, costly error.
Will a magnet damage my phone, laptop, or credit cards?
Credit card magstripes, hotel keys, and mechanical watches: yes, keep magnets away. Modern phones and SSD laptops tolerate ordinary exposure well (they contain magnets themselves), but strong magnets will confuse compasses and can trigger sleep sensors. Traditional spinning hard drives are more vulnerable in principle, though the platters are well shielded.
How close can a magnet be to a pacemaker?
Device manufacturers commonly recommend keeping strong permanent magnets at least 15–30 cm (6–12 inches) from implanted pacemakers and defibrillators, because magnetic fields can switch these devices into magnet mode. For workplace design involving strong magnets, treat 30 cm as a floor and consult the device manufacturer's guidance.
Why is my magnet's measured surface field lower than the Br on the datasheet?
Br is a closed-circuit material property; surface field in open air is always lower and depends on geometry. A thin disc might read 25–35% of Br at its face center; a long rod reads more. Both can be perfectly in-spec. Verify grade with flux (Helmholtz coil) or a hysteresisgraph, not a handheld surface reading.
Are neodymium magnets restricted or hard to source?
Rare-earth supply is geographically concentrated, and heavy rare earths (Dy/Tb) have seen export-control volatility — which is why defense work carries DFARS/NDAA sourcing requirements and why buyers increasingly qualify dual sources and design for reduced Dy. Our rare-earth supply chain whitepaper covers procurement strategy in depth.
Magnetized vs. unmagnetized shipment — which should I order?
If you assemble magnets into products, unmagnetized parts are easier and safer to handle, glue, and fixture — then magnetize after assembly (requires a fixture designed for your part). If you can't magnetize in-house, order magnetized and design handling around it. High-pole-count patterns are typically magnetized by the supplier either way.
Can you machine or drill a hole in a neodymium magnet?
Not with ordinary shop tools. Sintered NdFeB is hard, brittle, and its grinding dust is flammable; machining is done unmagnetized, with diamond tooling or EDM, under coolant, by suppliers equipped for it. The right approach is to specify the hole in the part drawing and let it be ground in during production.
Put This Knowledge to Work
Whether you're at the napkin-sketch stage or holding a frozen drawing, our engineering team can help you land on the right material, grade, direction, coating, and tolerance scheme — and back it with samples and full quality documentation.
This guide is provided for general engineering education. Validate all designs with samples and testing.