Block Magnets

Specification

Grade range

N35–N52 standard; M, H, SH, UH, EH suffixes for elevated temperature (e.g. N42SH, N38UH, N35EH)

BHmax

263–422 kJ/m³ (33–53 MGOe), grade dependent. N35 ≈ 263–287 kJ/m³ (33–36 MGOe); N42 ≈ 318–342 kJ/m³ (40–43 MGOe); N52 ≈ 398–422 kJ/m³ (50–53 MGOe)

Remanence (Br) at 20 °C

1.17–1.22 T (N35) rising to 1.43–1.48 T (N52). Reversible temperature coefficient of Br −0.11 to −0.13 %/°C

Intrinsic coercivity (Hcj)

≥955 kA/m (N, 12 kOe) · ≥1114 kA/m (M, 14 kOe) · ≥1353 kA/m (H, 17 kOe) · ≥1592 kA/m (SH, 20 kOe) · ≥1990 kA/m (UH, 25 kOe) · ≥2388 kA/m (EH, 30 kOe)

Max operating temperature

80 °C (N) · 100 °C (M) · 120 °C (H) · 150 °C (SH) · 180 °C (UH) · 200 °C (EH). Guide values at a permeance coefficient near 2 — the real limit depends on geometry and applied field, not the suffix alone

Dimensional tolerance

±0.1 mm typical on general machined dimensions; ±0.05 mm on precision-ground faces; ±0.02 mm on request with cost and lead-time impact. As-sintered (unmachined) ±2–5%

Coatings

NiCuNi 10–25 µm (default) · Zn 5–15 µm · epoxy 15–25 µm · NiCuNi+epoxy · Ni+Au · Parylene · phosphate passivation for potted assemblies only

Size range and magnetisation

≈1 × 1 × 0.5 mm up to ≈150 × 100 × 50 mm single-piece; larger footprints tiled. Magnetised through thickness, length, width or on a diagonal — axis must be nominated on the drawing

Typical applications

• Linear motors and voice-coil actuators — blocks tiled along a magnet way, where flat pole faces and consistent piece-to-piece Br keep force ripple predictable
• Magnetic separators, drum and plate separators — block arrays behind a stainless face plate, typically N42 or N42H, pulling ferrous contamination from bulk material flow
• Surface-mounted rotor segments in servo, spindle and traction motors — flat blocks on low pole-count and prototype rotors; arc segments derive from the same sintered stock
• Holding, clamping and welding fixtures — blocks in a steel yoke, giving far higher holding force per unit volume than ferrite in the same envelope
• Sensor and encoder targets — small blocks under about 10 mm, magnetised through thickness to trigger Hall or reed devices at a defined switch point
• Magnetic couplings and eddy-current brakes — alternating-polarity blocks around a hub, usually SH or UH to cover rotor self-heating

Engineering notes

• Sintered NdFeB is a rare-earth alloy, not a ceramic. Ferrite (ceramic) blocks are a genuinely different product — roughly a third of the remanence and about a tenth of the energy product, but better corrosion resistance, better cost and a positive intrinsic coercivity coefficient with temperature (ferrite demagnetises at low temperature, not high). If your application is cost-driven and space is not tight, ferrite may be the correct answer; do not reach for NdFeB by reflex. Be aware that some catalogue listings, including TT's own live block page, mislabel these blocks as ceramic — the data here is NdFeB.
• The per-grade temperature limits above assume a permeance coefficient around 2. A thin block magnetised through its short axis sits at a low working point and will show irreversible loss well below the quoted maximum, especially against an opposing coil field. Specify your geometry, ambient and applied field rather than a temperature alone, and expect to step up a grade (N42 → N42H) for thin sections. Separately, the reversible Br drift of about −0.12 %/°C bites in metrology and sensor work long before any permanent loss appears.
• Corrosion is the dominant field-failure mode. Bare NdFeB oxidises and eventually powders in humid or salt service, so the coating is part of the design life, not decoration. NiCuNi is the sensible default at roughly 24–72 h neutral salt spray; epoxy or NiCuNi+epoxy suits damp or chemically aggressive environments. No coating survives a chipped edge — specify a chamfer or radius on handled parts, and raise hydrogen-bearing or acidic environments before RFQ.
• The material is brittle and weak in tension — roughly 75–80 MPa tensile against about 1000–1100 MPa compressive, with fracture toughness near 2 MPa·m^0.5 — so it cracks under bending, impact and thermal shock. Blocks must be held in compression or bonded, and must never carry structural load. Sintered NdFeB is also electrically conductive, so segmentation may be needed to control eddy-current loss at high electrical frequency. Machining is done unmagnetised — no meaningful rework is possible after magnetisation.
• Grade inflation is common across the supply base, N52 especially, where stock often measures closer to N50. Ask for hysteresigraph data on the actual lot rather than a datasheet reprint, and put acceptance limits on Br and Hcj if the design has little margin. Heavy rare-earth content (Dy/Tb) drives H/SH/UH/EH pricing and is volatile; grain-boundary-diffusion grades can reach equivalent Hcj with less Dy and are worth asking about on cost-sensitive high-temperature parts.