Remanence, coercivity, intrinsic coercivity and maximum magnetic energy product are the four most common parameters for measuring the performance of magnetic materials. In addition, we often see two other parameters in finished magnets – surface magnetism and magnetic flux (also called magnetic flux).
Surface magnetism
Surface magnetism is simply understood as the magnetic induction intensity of the surface, which is defined as the magnetic flux passing through a unit area, so it is also called surface flux density. The unit is Gauss Gs or Tesla T. The measuring instrument is a Gauss meter or a test instrument with a Hall chip.
Surface magnetism refers to a measurement point, or rather, a value of a small measurement area, and the magnetic field distribution of a magnet is variable, so the surface magnetism at different points is generally different. For application areas that require the use of spatial magnetic fields, the surface magnetism or the magnetic induction intensity value of a specified point is usually taken as an important technical requirement.
Magnetic field simulation diagram
Surface magnetism is related to the aspect ratio of the magnet (the ratio of the height to the diameter of the magnet, where the height or thickness is assumed to be the magnetization direction of the magnet). The larger the aspect ratio, the higher the surface magnetism, that is, the larger the surface area perpendicular to the magnetization direction, the lower the surface magnetism; the larger the size in the magnetization direction, the higher the surface magnetism. In addition, the Hall elements on the Gaussmeters of different manufacturers are different, and the surface magnetism measured for the same magnet is also slightly different.
Magnetic flux Φ
The definition of magnetic flux physics is the number of magnetic lines of force passing through a certain area perpendicularly. We simply understand it as a physical quantity that measures the overall magnetic magnitude. The unit is Wb, expressed by Φ, Φ=B×S, B represents the magnetic induction intensity, S represents the magnetic pole area, that is, the magnitude of the magnetic flux depends on the magnetic pole area and the magnetic induction intensity.
The instrument for measuring magnetic flux is a fluxmeter. With the Helmholtz coil, it can not only measure the magnetic flux, but also calculate its magnetic moment, because the measured value of the magnetic flux will change with the parameters of the fluxmeter and the Helmholtz coil. Magnetic flux and magnetic moment are more like the difference between weight and mass. Weight is affected by the gravitational constant. The weight of the same object on Earth and Mars is different, but the mass is the same. Affected by the number of turns of the coil, the magnetic flux of the same magnet measured by different fluxmeters and coils may be different, but the magnetic moment must be the same.
When the magnet is in an open circuit state, the actual residual magnetism value Bdi (also called the intrinsic magnetic flux density) corresponding to the working point can be calculated by converting magnetic flux into magnetic moment, Bdi=Φ*coil constant/magnet volume.
Remanent magnetism, surface magnetism and magnetic flux are three concepts that are very easy to confuse. Let’s clarify them here:
Remanent magnetism is the essential property of the material. Under the premise of no self-demagnetization, the residual magnetism of a magnet is unchanged. It is determined by the product raw material formula and preparation process. The test is carried out in a completely closed circuit state
Surface magnetism is the magnetic induction intensity value of the measured position (a small area) when the magnet or magnetic component is in an open circuit or semi-open circuit state. Surface magnetism is a directional vector. The surface magnetic data on different surfaces of the magnet vary greatly. We usually talk about the surface magnetic value perpendicular to the magnetic pole surface. The surface magnetism of a single magnet is up to half of the residual magnetism. Note that it is a “single magnet”. In some magnetic components and magnet arrays, the surface magnetism of the magnet can be improved through special magnetic circuit design, and its value can even exceed the residual magnetism.
Magnetic flux is the magnetic magnitude of the entire magnet measured by the coil test method. Usually, magnetic components are not suitable for testing magnetic flux. Magnetic flux also has a default direction requirement. Of course, the total magnetic flux value can also be measured using a three-dimensional Helmholtz coil in actual measurement. Special attention should be paid to the test direction when measuring both surface magnetism and magnetic flux.
Extended knowledge – magnetic circuit
Magnetic circuit refers to a component with a specific working air gap magnetic field formed by combining one or more permanent magnets and magnetic conductive materials in a certain shape and size. Magnetic conductive materials play a role in controlling the flow direction of magnetic flux, increasing local magnetic induction intensity, preventing or reducing leakage, and improving the mechanical strength of the entire component in the magnetic circuit.
The magnetic state of a completely independent single magnet (without magnetic conductive materials and other permanent magnetic materials to change the direction of the magnetic circuit) is usually called an open circuit state. The most typical example is to place a magnet on a platform with no magnetic attraction at all, without any magnetic conductive materials or other magnets around it. At this time, the magnet is in an open circuit state.
If the magnetic path of a magnet is completely locked in a built-in space and does not show magnetism to the outside, then the magnet is in a closed circuit state. The most typical example is a magnet that is measuring the demagnetization curve, which shows almost no magnetism to the outside.
Except for the open circuit and closed circuit states, the rest can be called semi-open circuit.