Surface Magnetic Field Measurement of Permanent Magnet Materials

Surface magnetic field measurement (surface magnetism testing) is a core quality control procedure for evaluating the flux density at the pole face of magnetized permanent magnets such as NdFeB, SmCo, ferrite, and AlNiCo. Using calibrated Hall-effect gaussmeters and test procedures aligned with IEC 60404 and GB/T standards, accredited third-party laboratories provide accurate and repeatable surface flux density data expressed in millitesla (mT) or gauss (G) to verify magnetization quality, consistency, and polarity.

Technical Overview

Surface magnetic field measurement, often called surface magnetism testing or surface flux density measurement, quantifies the magnetic flux density present at the pole surface of a magnetized permanent magnet. It is one of the most widely performed magnetic tests in production and incoming inspection because it is fast, non-destructive, and directly reflects whether a magnet has been fully and uniformly magnetized. Results are reported in millitesla (mT) or gauss (G), where 1 mT equals 10 G.

It is important to distinguish surface field from intrinsic material properties. Remanence (Br), coercivity (HcB, HcJ), and maximum energy product ((BH)max) are closed-circuit material characteristics measured with hysteresisgraphs or vibrating sample magnetometers in accordance with standards such as IEC 60404-5 and GB/T 3217. Surface field, by contrast, is an open-circuit quantity: for the same material grade, a tall magnet with a high length-to-diameter ratio will show a much higher surface field than a thin disc, because the geometry determines the permeance coefficient and thus the operating point of the magnet. Surface field measurement therefore complements, but does not replace, closed-circuit magnetic characterization.

• Fast, non-destructive verification of magnetization completeness
• Geometry-dependent indicator of the field available at the working surface
• Expressed in mT or G, typically ranging from a few hundred gauss for ferrite to over 6000 G for high-energy NdFeB assemblies
• Supported by international and national standards including IEC 60404-5, IEC 60404-14, IEC 60404-8-1, and GB/T 3217

Testing Samples

Sintered and Bonded NdFeB Magnets

Neodymium-iron-boron magnets in block, disc, ring, tile, and trapezoid geometries are the most frequently tested samples. Their high energy product makes surface field measurement a sensitive indicator of correct grade, full saturation, and consistent magnetization direction. Both axially and radially magnetized parts are tested, and bonded (compression or injection molded) NdFeB components are evaluated using the same procedures with appropriately scaled acceptance limits.

Samarium Cobalt Magnets

SmCo magnets used in high-temperature aerospace, automotive, and instrumentation applications are tested for surface field to confirm magnetization after saturation in strong pulsed fields. Because SmCo grades exhibit low temperature coefficients of Br (approximately -0.03 percent per degree Celsius), surface field measurement is also used to monitor magnetization stability after thermal cycling.

Ferrite and AlNiCo Magnets

Hard ferrite (strontium or barium ferrite) segments from motor stators and loudspeaker assemblies, as well as AlNiCo rods and horseshoe magnets, are routinely inspected. Lower coercivity AlNiCo materials must be magnetized in-circuit or with keepers, so surface field measurement is often performed on the complete assembly rather than on the bare magnet.

Magnet Assemblies, Rotors, and Multi-Pole Parts

Multipole ring magnets, rotor assemblies with embedded tiles, magnetic couplings, and steel-backed magnetic circuits are tested pole by pole. The test verifies not only peak flux density per pole but also pole count, angular spacing, polarity sequence, and pole-to-pole uniformity, which are critical for motor cogging torque and sensor signal symmetry.

Testing Items

Typical test items covered under a surface magnetism measurement program include the following:

• Peak surface flux density at the geometric center of the pole face
• Maximum surface field values at defined scan lines or edge positions
• Pole-to-pole uniformity and distribution mapping across the magnet surface
• Polarity identification and pole sequence verification for multipole magnets
• Open-circuit magnetic flux and magnetic moment via Helmholtz coil as a complementary measurement
• Comparative checks against specified minimum, maximum, or tolerance-band acceptance criteria

Testing Methods

Hall Effect Gaussmeter Method

The principal technique uses a calibrated Hall-effect gaussmeter (teslameter) with either a transverse probe for flat pole faces or an axial probe for bores, cavities, and interior cylindrical surfaces. The probe is placed in direct contact with the pole face at the geometric center, oriented perpendicular to the surface, and the peak reading is recorded. Because the active Hall element typically sits 0.3 to 0.8 mm behind the probe face and has an active area of roughly 0.5 to 1 mm, the measured value is an effective field at a small standoff distance rather than the true field at the surface. Consistent probe type, contact convention, and positioning are therefore essential for comparable results.

Helmholtz Coil and Fluxmeter Method

For a more geometry-stable assessment of total magnetization, the sample is withdrawn from a Helmholtz coil pair connected to a fluxmeter, and the integrated voltage yields the magnetic flux or dipole moment. This method, related to the draw-out approach described in IEC 60404-14, is less sensitive to small probe placement errors and is preferred for lot acceptance of identical parts, while surface field measurement remains valuable for locating localized magnetization defects.

Procedural Controls and Traceability

Reliable measurement requires zeroing the probe in a zero-gauss chamber or by using the probe zero function away from magnetic sources, verifying calibration with reference magnets traceable to national metrology institutes, and compensating for temperature. The Hall probe sensitivity and the magnet output both drift with temperature, so testing is performed in a controlled environment, typically 23 plus or minus 5 degrees Celsius, with instruments warmed up per manufacturer instructions. Positioning fixtures that repeat probe placement within 0.1 mm significantly improve repeatability, which is otherwise often limited to plus or minus 2 to 5 percent for freehand measurement.

Related Closed-Circuit Characterization

When a full material qualification is required, surface field testing is supplemented by closed-circuit measurement of the demagnetization curve using a hysteresisgraph or vibrating sample magnetometer in accordance with IEC 60404-5, GB/T 3217, and ASTM A977/A977M, and by comparison against material grade specifications in IEC 60404-8-1.

Applications

Surface magnetic field measurement is applied across the entire permanent magnet value chain, from material production to end-product verification:

• Motor and generator manufacturing: rotor and stator magnet pole strength, pole count, and magnetization orientation checks for traction motors, servomotors, and BLDC rotors
• Automotive systems: sensor magnets for position and speed detection, EPS motor magnets, and magnetic couplings
• Consumer electronics and acoustics: speaker magnets, vibration motor magnets, magnetic latches, and wireless charging magnetic arrays
• Industrial and separation equipment: magnetic grates, drums, pulleys, and filters where effective surface field determines separation efficiency
• Medical and instrumentation devices: magnetic couplings, rotors in pumps, and magnetic assemblies requiring verified field strength and polarity
• Incoming quality control and failure analysis: verification of supplier magnetization, detection of partially magnetized or demagnetized parts, and post-thermal-exposure stability assessment

FAQ

What is the difference between surface field and remanence (Br)?

Remanence is an intrinsic, closed-circuit material property measured on fully saturated, closed-circuit specimens, while surface field is an open-circuit, geometry-dependent value measured at the pole face of the actual part. A thin N42 disc may show only 2000 to 3000 G at its surface even though the material Br is about 13,000 to 13,200 G, because the open-circuit operating point reduces the external field.

Why do different gaussmeters give different readings on the same magnet?

Differences arise from probe active area, the standoff distance between the sensing element and the probe face, calibration accuracy, alignment errors, and temperature effects. To obtain comparable data, always specify the probe type, contact convention, and measurement position in the test plan.

Should the probe touch the magnet surface?

Standard practice is to place a thin transverse probe in light contact with the geometric center of the pole face, held perpendicular to the surface. Because the field decays rapidly with distance, even a 0.5 mm air gap can reduce the reading noticeably, so a consistent contact convention must be documented and followed.

Can surface field measurement confirm that a magnet is fully saturated?

It is a strong screening indicator: a partially magnetized part will show a depressed and often non-uniform surface field. However, for definitive lot acceptance, surface field data are best combined with Helmholtz coil flux measurement or closed-circuit hysteresisgraph testing against the specified material grade.

What accuracy and repeatability can be expected?

Calibrated Hall instruments typically offer instrument accuracy around plus or minus 1 to 2 percent of reading. Practical repeatability depends mainly on positioning; with fixtures, repeatability better than plus or minus 1 to 2 percent is achievable, whereas freehand measurement commonly varies by several percent.

How are the units converted?

One tesla equals 1000 mT, and one tesla equals 10,000 G, so 1 mT equals 10 G. For example, a surface field of 450 mT corresponds to 4500 G.