NdFeB Rare Earth Magnet Specification: N35-N52 Grade, Br Remanence and Hci Coercivity Data

Overview #

The specification parameter that most procurement teams get wrong when sourcing NdFeB rare earth magnets from China is not the grade designation — it’s the coercivity class. An N42 magnet from two different Chinese suppliers can have identical remanence (Br) values and still perform completely differently in a motor application above 80°C, because intrinsic coercivity (Hci) within the same grade can vary by more than 200 kA/m depending on dysprosium (Dy) content and sintering process control. Grade alone does not tell you what you need to know. Coercivity class — H, SH, UH, EH, or AH — is the parameter that determines whether your magnet survives operating temperature, and it is the first thing to specify on your purchase order.

NdFeB Grade Structure: Br, Hci, and the Coercivity Class System #

The NdFeB grading system used by Chinese manufacturers follows GB/T 13560 and aligns broadly with IEC 60404-8-1, but the alignment is not exact. The grade number (35 through 52) refers to the maximum energy product (BHmax) in MGOe — N35 delivers a minimum BHmax of 33 MGOe, N52 delivers a minimum of 50 MGOe. What the grade number does not encode is the coercivity class, which is appended as a suffix: no suffix = standard (Hci ≥ 955 kA/m), H = high coercivity (Hci ≥ 1114 kA/m), SH = super high (Hci ≥ 1353 kA/m), UH = ultra high (Hci ≥ 1592 kA/m), EH = extremely high (Hci ≥ 1990 kA/m), and AH = the highest commercial class (Hci ≥ 2388 kA/m).

Most Western buyers do not realize that the GB/T standard governing NdFeB in China allows tolerance bands on Br and Hci that are wider than what many motor and sensor OEMs specify on their engineering drawings. A “compliant” Chinese magnet may pass GB/T 13560 and still fall outside your application’s demagnetization safety margin at elevated temperature. This is not a quality failure — it is a specification gap that happens at the sourcing stage, before a single magnet is shipped.

The table below compares six representative grades across the parameters that matter for procurement qualification. Values are minimum guaranteed figures per standard datasheet; actual production lots should be verified against COA and incoming inspection data.

The energy product and Br values scale together — higher grade numbers require tighter control of Nd₂Fe₁₄B phase purity and grain boundary diffusion. The coercivity suffix, by contrast, is primarily controlled by heavy rare earth (HRE) additions: dysprosium (Dy) and terbium (Tb). N42SH and N42UH have identical Br floors but require meaningfully different HRE loading, which directly affects raw material cost and supply chain exposure to HRE price volatility.

For pump and valve seal assemblies that use magnetic coupling, the coercivity class is the primary selection driver — not the grade number. We have seen procurement teams specify N42 without a coercivity suffix for magnetic drive pump applications running at 130°C, then experience partial demagnetization within 6 months of commissioning. The fix required a full magnet replacement with N42SH — at three times the original unit cost, plus downtime.

Magnetic Property Verification: What the COA Must Show and What to Test Incoming #

A supplier COA for NdFeB should report, at minimum: Br (mT), Hcb (kA/m), Hci (kA/m), BHmax (kJ/m³), and the measurement temperature (standard is 20°C ± 2°C). If the COA reports only Br and BHmax — which is common from tier-2 Chinese suppliers — reject it and request the full four-parameter report. Hci is the parameter most frequently omitted, and it is the one most likely to be out of specification in a substituted or downgraded batch.

In our supplier qualification program, we require three consecutive batch COAs before recommending volume commitment. The reason is lot-to-lot consistency, not initial sample performance. We have qualified suppliers whose first-article samples passed all four magnetic parameters, then delivered production lots where Hci dropped by 180 kA/m — still within the grade’s minimum, but below the application’s demagnetization threshold at operating temperature. The trigger was a reduction in Dy content at the alloy stage, which a standard dimensional and hardness check would not catch.

Incoming inspection protocol for NdFeB magnets should include:

• Flux measurement per ASTM A977: measure open-circuit flux on a sample of ≥5 pieces per lot using a calibrated fluxmeter. Reject if mean flux deviates more than ±3% from the approved first-article value.
• Coercivity spot-check: use a permeameter or hysteresisgraph per IEC 60404-5 on ≥2 pieces per lot. Reject if Hci falls below the grade minimum or below your application-specific threshold, whichever is higher.
• Dimensional verification: NdFeB magnets are typically ground to tolerance after sintering. Verify against drawing tolerance — Chinese suppliers commonly hold ±0.05 mm on ground faces and ±0.1 mm on unground faces, but this must be confirmed in the purchase specification, not assumed.

Most procurement teams over-specify coating thickness and under-specify the parameter that actually drives field performance in assembly: flux uniformity across the magnet face. For motor and sensor applications, a ±5% flux variation across the pole face can cause torque ripple or signal offset that no amount of coating specification will prevent.

The coating itself deserves a separate line on the purchase specification. Standard NiCuNi electroplating provides adequate corrosion resistance for most indoor applications, but in humid or chemically aggressive environments, epoxy coating or Parylene C should be specified. Salt spray resistance per ISO 9227 should be required at ≥96 hours for NiCuNi and ≥240 hours for epoxy-coated grades. We have seen NiCuNi-coated magnets from Chinese suppliers fail salt spray at 48 hours — not because the coating was thin, but because the nickel layer had pinholes from inadequate pre-treatment of the sintered substrate.

Temperature Stability, Demagnetization Risk, and Grade Selection for Elevated-Temperature Applications #

The reversible temperature coefficient of Br (α) for sintered NdFeB is approximately −0.11%/°C to −0.13%/°C, and the reversible temperature coefficient of Hci (β) is approximately −0.55%/°C to −0.65%/°C. These are not marketing figures — they are the physical basis for maximum operating temperature limits. At 150°C, a standard-coercivity N42 magnet has lost roughly 9% of its room-temperature Br and more than 35% of its room-temperature Hci. Whether that Hci loss causes irreversible demagnetization depends on the permeance coefficient (Pc) of the magnetic circuit — a parameter that must be calculated for the specific assembly geometry, not read from a datasheet.

When evaluating Chinese suppliers for elevated-temperature NdFeB grades (SH, UH, EH), we always request the demagnetization curve (B-H curve) at the application operating temperature, not just at 20°C. Most tier-1 Chinese magnet manufacturers can provide this data. Tier-2 suppliers typically cannot — and that gap is a qualification disqualifier for any application above 100°C.

The relationship between HRE content and coercivity class has a direct cost implication. Terbium (Tb) is approximately 3–4× more expensive per kilogram than dysprosium (Dy) as a coercivity enhancer, but Tb is more efficient — a smaller addition achieves the same Hci improvement. For AH-class magnets (Hci ≥ 2388 kA/m), Tb grain boundary diffusion (GBD) process is the dominant production method among leading Chinese manufacturers, and it commands a significant price premium over conventional Dy-doped grades. Buyers sourcing AH-class magnets at prices comparable to SH-class should treat that as a red flag for HRE content substitution.

For sensors and detection systems that rely on NdFeB for bias magnets or encoder rings, the relevant specification is not just Hci — it is the irreversible flux loss after thermal cycling. Require suppliers to provide thermal aging data: flux loss after 100 hours at the application maximum temperature, measured per IEC 60404-8-1. The acceptable threshold for most precision sensor applications is ≤2% irreversible flux loss after the first thermal cycle.

Practical Guidance for Buyers #

When sourcing NdFeB magnets from China, the first specification to request from suppliers is the full four-parameter magnetic COA — Br, Hcb, Hci, and BHmax — measured at 20°C. Most buyers ask for grade designation and coating spec. Grade designation alone does not tell you the coercivity class, and coercivity class is what determines whether the magnet survives your operating temperature.

The most common sourcing mistake we see is specifying a grade number without a coercivity suffix for applications above 80°C. An N42 without a suffix is rated to 80°C maximum operating temperature. If your application runs at 120°C and you receive N42 instead of N42H, the magnets will appear to function initially — and then demagnetize progressively over weeks or months of operation. By the time the failure is visible, the root cause is buried in a purchase order that never specified the coercivity class.

Before committing to volume order, require three things: (1) a full four-parameter COA for three consecutive production lots, not just the first-article sample; (2) a demagnetization curve at your application operating temperature if that temperature exceeds 100°C; and (3) salt spray test results per ISO 9227 at the appropriate duration for your coating specification — 96 hours minimum for NiCuNi, 240 hours for epoxy. These three documents will disqualify more than half of tier-2 Chinese suppliers before you spend a dollar on tooling or first-article samples.

Frequently Asked Questions #

Q1: What is the difference between Hcb and Hci on a NdFeB COA, and which one matters for demagnetization resistance?

A: Hci (intrinsic coercivity) is the parameter that determines demagnetization resistance — Hcb (coercive force of B) is always lower and is not the relevant figure for elevated-temperature stability. Always verify Hci against the coercivity class minimum, not Hcb.

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

A: At 130°C, N42H (Hci ≥ 1114 kA/m, rated to 120°C) is marginal and should not be used without a permeance coefficient calculation confirming adequate demagnetization margin. N42SH (Hci ≥ 1353 kA/m, rated to 150°C) is the minimum safe selection for continuous 130°C service. N42UH provides additional margin and is appropriate if temperature excursions above 130°C are possible. Verify against the B-H curve at operating temperature per IEC 60404-8-1, not just the datasheet maximum temperature rating.

Q3: What is the most common quality failure when sourcing NdFeB from Chinese tier-2 suppliers?

A: This is where most sourcing decisions go wrong: Hci drops between first-article approval and production volume because the supplier reduced dysprosium content at the alloy stage. The threshold to watch is a deviation of more than 150 kA/m from the approved first-article Hci value — that level of drop is not random variation, it is a raw material substitution.

Q4: What certifications and test documentation should I require before placing a volume order?

A: Require a full four-parameter magnetic COA per GB/T 13560 or IEC 60404-8-1 for each production lot, salt spray test results per ISO 9227 for the coating specification, and — for applications above 100°C — a demagnetization curve at operating temperature. RoHS compliance documentation per the EU RoHS Directive is also required for any magnets entering EU-bound assemblies, as NdFeB alloys can contain restricted substances depending on the additive package.

Q5: Is a higher grade number (e.g., N52 vs. N42) always better for a given application?

A: No. Higher grade numbers mean higher Br and BHmax, but they also mean lower achievable coercivity class — N52 is only available in standard coercivity, which limits maximum operating temperature to 80°C. For elevated-temperature applications, N42SH or N48UH will outperform N52 in service life even though N52 has higher room-temperature energy product.

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