Achieving Complete Magnetization in Saturated Samarium Cobalt Magnets: What is the Required Magnetization Force?
In order to achieve maximum energy output from a , it must be saturated – meaning fully magnetized, even if the magnet is stabilized later through the use of heat or “pulse magnetization”. The amount of magnetization force required to achieve saturation depends on the coercivity of the magnetic material, and to a lesser extent on the physical characteristics of the magnet and the components to which it can be affixed during magnetization.
As a general rule, saturation of a magnet requires applying a peak magnetic field that is 2-2.5 times the intrinsic coercivity. For example, a 20,000 Oersted Hci will require at least 40,000 Oersted to achieve saturation. In the case of a magnet that is connected to a conductive anchor, eddy currents will be established in the material, creating a reverse magnetic field during the extremely short pulse of magnetization. This prevents the magnetization flux from fully penetrating the conductor or even the magnet itself, and reduces the magnetic field seen by the magnet, sometimes also reducing the magnetic flux path (direction). In these situations, equipment manufacturers will need to adjust the LC (inductance-to-capacitance ratio) of the excitation circuit to extend the duration of the excitation pulse. This extended pulse will produce more heat, slowing the rate of magnetization production, and requires a careful compromise to be reached.
Required Magnetic Field
Bonded rare-earth magnets represent another problem. The magnetic powder and nonmagnetic binder separate, and the nonmagnetic binder will degrade the flux penetration force during pulses, requiring a higher magnetic field than what the previous rule predicts. For example, an MQP-B powder with an Hci of 10,000 Oersted will require at least 30,000 Oersted to achieve saturation. Most magnet manufacturers should be able to provide you with a curve of peak applied plus induced field versus induction saturation percentage, Hci, or maximum flux output percentage for various magnet types. The following table lists the approximate fields necessary to achieve at least 98% of maximum output for various magnet types. These are generic and are influenced by fixturing and LC circuits, among other things.
Stabilization and Calibration
Permanent magnets have very low losses unless they are driven beyond certain critical parameters at which point they have “irreversible losses”. If remagnetization restores all of the original magnetic output, then they have suffered no irretrievable or structural loss. When only irreversible losses are present and result in unacceptable differences between initial operation and subsequent use, the magnet can almost always be “preconditioned” so that little or no additional loss is observed during use. (This assumes that the best material is already chosen and extraordinary methods must be applied.)
If losses expected in use stem from elevated temperatures, then thermal stabilization is recommended. If losses are due to demagnetizing stresses, then magnet stabilization is necessary.
Changes in magnetic output (flux output) are inevitable. Most applications can tolerate normal fluctuations in flux output. For those requiring extremely tight tolerance, the magnet can be “calibrated” to the desired output by partially pulse magnetizing the magnet. This is typically a magnetic calibration process, but on occasion a heat calibration is used — uniformity at high temperatures is essential.
For example, one approach is that thermal adjustment can be performed in an open or fixed load circuit. For applications that require stable output over a wide temperature range, many customers consider using RECOMA STAB, a grade of SmCo where the change in output per degree C through a range of -40 to +200 oC is less than 50 ppm.