Introduction
You're reviewing a supplier datasheet. It lists a surface field of 4,000 Gauss. But your materials reference cites a different value for remanence. Did the supplier make an error — or are you comparing two fundamentally different measurements?
Neither is wrong. You're looking at two distinct magnetic properties that are routinely conflated — even by experienced engineers. Understanding the difference between surface field and remanence (Br) determines whether you select the right magnet, design an accurate magnetic circuit, or misread a quality inspection result.
What Is Remanence (Br)?
Remanence, denoted Br, is an intrinsic material property. It describes the magnetic flux density a material retains after an applied external field is removed and measured under closed-circuit, ideal conditions.
• Unit: millitesla (mT) or tesla (T)
• How it's measured: A hysteresisgraph (permeameter) fully saturates the sample in a controlled field, removes that field, and records the residual flux density at zero applied field — the point where the demagnetization curve crosses the B-axis.
• What it reflects: The fundamental magnetic energy of the material, independent of geometry or surface treatment.
Remanence is a material constant. A grade N35 NdFeB magnet carries a Br of approximately 1,170–1,210 mT regardless of whether it's a disc, ring, or block. Changing the shape does not change Br.
Use Br when: designing magnetic circuits, running FEA simulations, selecting material grades, or comparing fundamentally different magnet alloys (e.g., NdFeB vs. SmCo vs. ferrite).
What Is Surface Field?
Surface field (sometimes called surface flux density) is an external measurement. It quantifies the magnetic field intensity at a specific point on or near the magnet's physical surface under open-circuit conditions.
• Unit: Gauss (Gs) or tesla (T)
• How it's measured: A calibrated gaussmeter (Hall-effect probe) is placed at a defined contact point on the magnet surface, and the instrument reads field strength directly.
Unlike Br, surface field is heavily dependent on measurement conditions. The same magnet can yield significantly different readings based on:
Practical example: A sintered N35 NdFeB disc, Ø14 × 2 mm, produces approximately 1,830 Gauss at its geometric center but exceeds 2,300 Gauss near its outer edge — a 26% difference on the same magnet face.
Use surface field when: evaluating pull force for a specific assembly, performing incoming inspection on production batches, or validating real-world field strength at a defined working distance.
Why High Remanence Doesn't Guarantee High Surface Field
This is the source of most specification confusion. Consider the analogy of a structural column: the steel's yield strength (an intrinsic material property) tells you about fundamental capacity — but the load at any given point depends on cross-section, length, and boundary conditions.
The same logic applies to magnets:
• A thin disc with high Br may produce a lower surface field than a thicker block of lower-grade material — purely due to geometry.
• A magnet with a nickel-copper-nickel coating will read lower surface field than an uncoated equivalent, even with identical Br.
• Two magnets with equal surface field readings may differ substantially in Br — and perform very differently in closed magnetic circuits.
High Br does not guarantee high surface field. High surface field does not confirm high material quality. Both measurements are necessary for a complete specification.
Which Measurement Should You Use? A Decision Framework
How Each Parameter Is Measured in Practice
Measuring Remanence: The Hysteresisgraph
A hysteresisgraph (permeameter) generates a full B-H demagnetization curve for the sample. The Br value is read directly from the curve at H = 0. This test requires the magnet to be fully saturated prior to measurement and is typically conducted in a metrology lab or by the material supplier.
This is the specification value you see in grade datasheets (e.g., N35, N42, N52 for NdFeB; YXG-24H for SmCo).
Measuring Surface Field: The Gaussmeter
A gaussmeter with a Hall-effect probe is placed at a defined contact point on the magnet surface. Measurements should specify:
• Probe type (axial vs. transverse)
• Contact point (center, edge, or defined offset)
• Presence of any coating or gap
Without these conditions documented, surface field measurements from different sources are not directly comparable.
FAQ
Q: Is the surface field value on a supplier datasheet the same as remanence?
A: No. Surface field is an external measurement taken at a specific point on the magnet surface using a gaussmeter. Remanence (Br) is an intrinsic material parameter measured under closed-circuit laboratory conditions using a hysteresisgraph. They reflect different physical quantities and will always differ numerically.
Q: Which magnet specification should I use for FEA magnetic circuit simulation?
A: Use remanence (Br) and coercivity (Hc or Hcj) from the material grade datasheet. Surface field measurements are geometry- and condition-dependent and cannot be used as direct simulation inputs.
Q: Why does surface field vary across the same magnet face?
A: Flux density is not uniform across a magnet's surface. Edge effects cause field lines to concentrate near the perimeter, producing significantly higher readings than at the geometric center. For an N35 NdFeB disc (Ø14 × 2 mm), center-to-edge variation can exceed 25%.
Q: Can I compare two different magnet materials using surface field alone?
A: Not reliably. Surface field is influenced by geometry, coating, and measurement position. For material-to-material comparison — such as NdFeB vs. SmCo vs. ferrite — always compare remanence (Br) values from standardized grade datasheets under equivalent test conditions.
Q: Does a thicker coating reduce a magnet's effective surface field?
A: Yes. Any non-magnetic layer between the magnet surface and the measurement point — including nickel, epoxy, or stainless steel enclosures — attenuates the measured field. The attenuation increases with coating thickness and the permeability of the intervening material.
Ready to Specify the Right Magnet for Your Application?
Whether you're sourcing NdFeB grades for a sensor assembly or validating surface field for production inspection, the right specification starts with knowing which measurement to trust.
Talk to an applications engineer to confirm the correct Br grade, geometry, and coating specification for your design — before you commit to tooling or volume orders.
Post time: Apr-02-2026