How NdFeB Magnets Are Tested: Surface Field, Flux, Magnetic Moment, and Density
Today, we will focus on the principles and methods used for relative magnetic property testing of sintered NdFeB magnets. NdFeB products come in many different shapes and sizes, and while most customers specify the magnetic grade, some also request comparative magnetic performance tests on samples.
Typical customer requests may include:
- checking the surface magnetic field,
- measuring the magnetic flux, or
- verifying the magnetic moment of selected magnets.
To meet these needs, several types of test equipment have been developed:
- Gauss/Tesla meters — used to measure the magnet’s surface magnetic field.
- Fluxmeters — used to measure the magnet’s magnetic flux.
- Helmholtz coils paired with a fluxmeter — used to measure the magnetic moment.
In this article, we will give a brief introduction to each of these instruments and how they are used in evaluating NdFeB magnets.
Measuring the Surface Magnetic Field with a Tesla Meter
Tesla meters come in several types, including Hall-effect Tesla meters, coil-type Tesla meters, and rotating-coil Tesla meters. Among these, the Hall-effect Tesla meter is the most commonly used in permanent magnet testing.
The Hall effect occurs when a thin semiconductor carrying an electric current is placed in a magnetic field that is perpendicular to the current. A voltage—called the Hall voltage—is generated in the direction perpendicular to both the current and the magnetic field. When the Hall element and current remain constant, the Hall voltage is directly proportional to the magnetic field being measured, allowing the instrument to determine magnetic field strength.
When using a Hall-effect Tesla meter to measure the surface field of a magnet, two important points must be observed:
- The plane of the Hall probe must be perpendicular to the magnetic field, meaning the probe surface should lie flat against the magnet pole.
- The magnetic field on an open-circuit magnet surface is not uniform, so measurements must be taken at a consistent, specified position.
Measuring Magnetic Flux with a Fluxmeter
Magnetic flux itself cannot be measured directly. However, based on Maxwell’s equations, it can be calculated by measuring the induced electromotive force in a pickup coil. For this reason, fluxmeters are widely used for comparative flux measurements in permanent magnet quality control.
One limitation is that, for small magnets, the reading can vary depending on the magnet’s position within the sensing coil, which affects repeatability.
Magnets with very high coercivity—especially thin magnets or arc-shaped magnets—are difficult to measure using a Tesla meter or a fluxmeter. Their surface field and flux readings often show large variation and poor repeatability. For these types of products, the only practical method is to measure the magnetic moment.
Measuring Magnetic Moment with a Helmholtz Coil
The magnetic moment of a permanent magnet is typically measured using a Helmholtz coil paired with a fluxmeter. This method offers:
- fast measurement,
- high precision,
- and very small error.
It is especially suitable for thin magnets, arc magnets, and other complex shapes.
Measuring the Density of Sintered NdFeB Magnets
Density is defined as the mass per unit volume of a material, and it is an important quality indicator for sintered NdFeB magnets.
- The theoretical density of the Nd₂Fe₁₄B compound is 7.62 g/cm³.
- The density of a pure ternary sintered NdFeB magnet should typically be 7.50–7.55 g/cm³.
- When small amounts of dysprosium (Dy), terbium (Tb), or other elements are added, the density may increase slightly.
Common Measurement Method: Archimedes' Principle
The most widely used technique is the hydrostatic weighing method based on Archimedes’ principle. Because a sample experiences buoyancy in a liquid, its measured weight decreases when submerged. By measuring:
- its mass in air,
- its mass in the fluid,
- and knowing the fluid’s density,
the volume and therefore the material density can be accurately calculated.
For accurate results, it is recommended to use a fluid with a well-known density, such as:
- anhydrous ethanol (0.989 g/cm³)
- distilled water (1.003 g/cm³)
If you are interested in a more general and comprehensive overview of the testing procedures used for sintered NdFeB magnets, please visit our article Common Testing Procedures for Sintered NdFeB Magnets: A Comprehensive Guide.
These are also the standard quality-control procedures that EPI Magnets applies to every batch of magnets before shipment.