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The most basic and descriptive performance data for a permanent magnet (or any ferro- or ferri-magnetic material) derive not from the magnet’s “strength,” which is traditionally interpreted as mechanical pulling power, but from the so-called B,H curve. In the simplest case, a sample of the material with the physical form of either a right rectangular prism or a right cylinder is placed into a known magnetic test circuit. Sensors measure the externally applied magnetizing/demagnetizing field H, measured in SI units of ampere-turns/metre, and the resulting magnetic flux density B inside of the material, in units of Tesla.
The magnetization M is related to the flux density B by the relationship B = µoH + M where µo is a constant termed the permeability of empty space and H is the external magnetic field. Both B and M are important in defining the expected behavior of a magnet in a particular device. The illustration shows a full four-quadrant magnetization curve for a permanent magnet. The first quadrant represents the initial magnetization curve up to technical saturation, which is the highest level of magnetization achievable in a particular material. The amount of field required to saturate varies significantly among the different classes of magnetic materials.
After magnetizing, the applied field is removed from the test sample and the flux inside the magnet returns to some positive level known as the residual or remanent magnetization, Br. Applying a reverse polarity field will drive the magnet into the second quadrant, which is where a majority of applications operate. The flux density B and magnetization M will decrease in some nonlinear and hysteretic way. The magnetic field required to drive B to zero is termed Hc, the coercive field strength, and represents the amount of magnetic field required in opposition to the magnetization to force the flux density in the magnet to a value of zero. If the H field is then returned to zero, B will return to some value between zero and Br along a “minor loop”; the magnet is still magnetized, although less so than right after it was saturated.
When the H field is decreased beyond Hc until M becomes zero (B is now somewhere down in the third quadrant), the intrinsic coercive field strength Hci is reached. If the H field is now returned to zero, B will return to a very low value and the magnet is essentially demagnetized. When a large enough negative field is applied, the magnetization of the material can be completely reversed. A large enough positive field will again saturate the magnet in the forward direction.
Typically it is not possible to generate a high enough field in real-world test systems to fully saturate or reverse the magnetization of a modern, high-energy magnet. The magnetizing and full reversal, if required, must be done external to the test system.
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