Neodymium Magnet Grade Selection: N35 to N52, Br Remanence, Hci Coercivity and Temperature Coefficient

Overview #

The specification parameter that most procurement teams get wrong when sourcing neodymium magnets from China is not the grade designation — it’s the temperature coefficient of remanence (α(Br)), which determines whether a magnet rated N42 at room temperature still delivers usable flux density at your operating temperature. An N42 magnet with a standard α(Br) of −0.12%/°C loses approximately 12% of its remanence by the time it reaches 100°C — a loss that accumulates silently in motor assemblies and sensor arrays until field performance degrades below threshold. Grade stamping is easy to claim; thermal stability under load is what separates a qualified Chinese supplier from a price-competitive one.

Grade Designations, Remanence Values and What the Numbers Actually Mean #

The NdFeB grade system encodes two critical parameters: remanence (Br) and intrinsic coercivity (Hci). The number following “N” indicates the maximum energy product in MGOe — N35 delivers approximately 35 MGOe, N52 approximately 52 MGOe. The letter suffix encodes the Hci class: no suffix = standard (~12 kOe), H = high coercivity (~17 kOe), SH = super high (~20 kOe), UH = ultra high (~25 kOe), EH = extra high (~30 kOe), AH = highest (~35 kOe).

Most procurement teams treat the grade number as the primary selection variable. The suffix is the variable that actually determines whether the magnet survives your operating environment. An N42 magnet in a 120°C motor housing without an H or SH suffix will irreversibly demagnetize within the first thermal cycle. We have seen this failure mode repeatedly in qualification programs for BLDC motor assemblies sourced from Guangdong suppliers — the initial sample passes room-temperature pull-force testing, and the field failure only surfaces after thermal cycling in the end application.

The table below reflects actual specification ranges per IEC 60404-8-1 and Chinese national standard GB/T 13560, which governs sintered NdFeB magnets in China. Buyers should note that GB/T 13560 allows a Br tolerance of ±30 mT on most grades — wider than the ±20 mT tolerance many Western OEM drawings specify.

Most Western buyers do not realize that the GB/T 13560 standard governing sintered NdFeB in China allows a wider Br tolerance band than ISO 9001-aligned OEM drawings typically specify. A supplier delivering “compliant” material to GB/T may still be out of spec against your engineering drawing — and both parties will be technically correct. This is the single most common source of incoming inspection disputes we see in NdFeB procurement from China.

For related sealing and precision component sourcing where dimensional tolerance class creates similar specification gaps, see our O-Rings and Static Seals category.

Coercivity, Temperature Coefficients and Irreversible Demagnetization Risk #

Coercivity is the parameter that determines whether a magnet retains its magnetization under adverse conditions — elevated temperature, opposing magnetic fields, or mechanical shock. There are two coercivity values on every COA: remanent coercivity (Hcb) and intrinsic coercivity (Hci). Hci is the one that matters for demagnetization resistance. Buyers who only check Hcb are reading the wrong number.

The temperature coefficient of intrinsic coercivity (β(Hci)) for standard NdFeB is approximately −0.55%/°C to −0.65%/°C. This means an N42 magnet with Hci of 1,200 kA/m at 20°C has an effective Hci of approximately 660–720 kA/m at 120°C — a reduction of more than 40%. If your operating point (the intersection of the load line and the demagnetization curve) falls below the knee of the B-H curve at operating temperature, demagnetization is irreversible. No amount of re-magnetization in the field recovers the assembly.

When evaluating Chinese suppliers for NdFeB grades above N45, we always request the full demagnetization curve (B-H curve) at both 20°C and at the rated maximum operating temperature — not just the room-temperature COA values. Suppliers who cannot provide the elevated-temperature B-H curve have almost certainly not characterized the material at temperature, which means their maximum operating temperature rating is a catalog claim, not a measured value.

The α(Br) value for standard NdFeB grades runs −0.09%/°C to −0.13%/°C. For EH and AH suffix grades, α(Br) is typically −0.09%/°C or better, achieved through dysprosium (Dy) or terbium (Tb) substitution in the alloy. This is why high-temperature grades carry a price premium of 25–40% over standard grades at equivalent energy product — the heavy rare earth content is the cost driver, not the manufacturing process.

Qualification testing in our program requires flux density measurement per ASTM A977 before and after a 200-hour thermal soak at the rated maximum operating temperature. Pass threshold: flux density retention ≥ 95% of initial value. Suppliers who cannot meet this threshold on initial sample approval are removed from the qualified vendor list regardless of price.

Dimensional Tolerances, Coating Specifications and Lot Consistency #

Dimensional tolerance is where the gap between Chinese supplier capability and Western OEM requirements most frequently creates production-line problems. Standard sintered NdFeB blanks from Chinese suppliers are typically held to ±0.1 mm on ground dimensions and ±0.2 mm on as-sintered dimensions. If your assembly requires ±0.05 mm, you need to specify ground finish explicitly and confirm the supplier’s grinding capability — not assume it.

Coating selection is driven by the application environment, not by aesthetics. The standard Ni-Cu-Ni triple-layer coating provides corrosion resistance adequate for indoor, low-humidity environments. Salt spray resistance per ASTM B117 for standard Ni-Cu-Ni coating is typically 24–48 hours before first corrosion. For outdoor, marine, or high-humidity applications, specify epoxy topcoat over Ni-Cu-Ni, which extends salt spray resistance to 96–200 hours depending on coating thickness (typically 15–25 µm total).

In our qualification program, we have seen suppliers pass initial sample approval with correctly coated magnets and then deliver production batches with coating thickness below specification — the most common trigger being a switch to a lower-cost plating subcontractor at volume. A standard COA does not include coating thickness measurement. Specify coating thickness as a mandatory incoming inspection parameter with a minimum of 10 µm Ni layer and require the supplier to include XRF coating thickness data with each shipment lot.

Lot-to-lot Br consistency is the variable that most procurement teams under-specify. We routinely see Chinese NdFeB suppliers with lot-to-lot Br variation of ±40–50 mT on nominally identical grade material — well within GB/T tolerance but problematic for sensor calibration or motor winding balance. If your application is sensitive to flux variation, specify a tighter Br window (e.g., ±20 mT) in the purchase order and require statistical process control (SPC) data across a minimum of three consecutive production lots before volume qualification.

For procurement of other precision components where lot-to-lot consistency drives assembly yield, see our Precision Fasteners and Power Transmission category.

Practical Guidance for Buyers #

When sourcing neodymium magnets from China, the first specification to request from suppliers is not the grade designation — it’s the full B-H demagnetization curve at your operating temperature, not just at 20°C. Most buyers ask for a room-temperature COA and assume the grade suffix handles the rest. It does not. A supplier who cannot provide the elevated-temperature B-H curve has not characterized the material under the conditions that determine whether your assembly will fail.

The most common sourcing mistake we see is specifying grade (e.g., N42H) without specifying the Br tolerance window. GB/T 13560 allows ±30 mT on Br for most grades. If your sensor or motor design was calibrated to a Br of 1,300 mT and the delivered lot measures 1,270 mT — both values within GB/T tolerance — your assembly yield drops without any supplier non-conformance on paper.

Before committing to volume order, require the following: (1) flux density retention test per ASTM A977 after 200-hour thermal soak at rated maximum operating temperature, with ≥95% retention as the pass threshold; (2) SPC data for Br across three consecutive production lots; (3) XRF coating thickness report with minimum 10 µm Ni layer per lot. Suppliers who push back on items 2 or 3 are telling you something important about their process control capability.

What to Specify in Your Purchase Order — Checklist #

• Grade designation with suffix: e.g., N42SH — not just “N42”
• Br range (tightened): specify ±20 mT maximum, not the GB/T default ±30 mT
• Hci minimum: state the kA/m value explicitly, not just the suffix class
• α(Br) maximum: e.g., ≤ −0.11%/°C for standard grades
• Maximum operating temperature: state the actual application temperature, not the catalog maximum
• Dimensional tolerance: specify ground or as-sintered; state tolerance in mm (e.g., ±0.05 mm on critical dimensions)
• Coating type and minimum thickness: e.g., Ni-Cu-Ni, minimum 10 µm Ni layer, XRF report required per lot
• Salt spray requirement: state hours per ASTM B117 (e.g., 48h minimum, 96h for humid environments)
• Flux density retention test: ≥95% after 200h thermal soak at operating temperature per ASTM A977
• Lot traceability: require heat number and compounder identity on COA
• SPC data: Br mean and standard deviation across minimum 3 consecutive lots before volume release
• AQL level: specify ASTM E2234 or equivalent, AQL 1.0 for critical dimensions, AQL 2.5 for visual/coating

Frequently Asked Questions #

Q1: What is the most important parameter to verify on a neodymium magnet COA beyond the grade designation?

A: Intrinsic coercivity (Hci) at operating temperature — not room-temperature Hci, and not Hcb, which is the value more commonly reported and easier to pass.

Q2: How do I select between N42H, N42SH and N42UH for a motor application running at 130°C continuous?

A: At 130°C continuous, N42H (rated to 120°C) is marginal and should be rejected for this application. N42SH is rated to 150°C with Hci ≥ 1,592 kA/m and is the minimum appropriate grade. N42UH (rated to 180°C, Hci ≥ 1,990 kA/m) provides a safety margin if the thermal model has uncertainty. The decision should be made from the B-H curve at 130°C, not from the catalog temperature rating — request the curve from the supplier before specifying. Reference IEC 60404-8-1 for the full grade classification framework.

Q3: What is the most common quality failure mode when sourcing NdFeB magnets from Chinese suppliers at volume?

A: This is where most sourcing decisions go wrong: raw material substitution at the compounder level between sample approval and production volume. The trigger is almost always a rare earth price spike — the supplier switches to a lower Dy or Tb content alloy to protect margin. The grade stamp does not change. The Hci drops by 10–20%, which a standard incoming hardness check will not catch. Require lot-level compounder identity on the COA and spot-test Hci on every third production lot.

Q4: What certifications and test documentation should I require before approving a Chinese NdFeB supplier?

A: Require the following before volume approval: full B-H demagnetization curve at 20°C and at rated maximum operating temperature; flux density retention data per ASTM A977 (≥95% after 200h thermal soak); XRF coating thickness report; and SPC data for Br across three consecutive production lots. ISO 9001 certification is a baseline requirement, not a quality guarantee — we have qualified and disqualified ISO 9001-certified suppliers based on the above data.

Q5: Is N52 the best grade for maximum performance in a compact assembly?

A: Not necessarily. N52 has the highest energy product but the lowest coercivity suffix class (standard, rated to 80°C) and the tightest dimensional tolerances to achieve — yield rates on N52 are lower, which drives up unit cost and lot-to-lot variability. For most compact motor or sensor assemblies operating above 80°C, N48M or N45H delivers better total system performance at lower procurement risk.

Published by sinoraw.com Technical Team | Request a sourcing consultation