The Hysteresis Loop & Testing Permanent Magnet Materials

A permanent magnet refers to a material that remains magnetic even once the applied magnetic field is removed. Used in speakers, motors and generators, sensors, and actuators, permanent magnets have to work well in the particular environment required by the application. Adverse conditions, such as the presence of other magnetic fields, or temperature sensitive parameters, affect performance of the material.

Although permanent magnets remain magnetic, they possess characteristics that make them difficult to work with. Hysteresis refers to a delayed response to a stimulus that is applied, such as an applied magnetic field (H) with delayed magnetization or flux density (B).  The hysteresis loop tells us information about the magnetic properties of a material, showing the relationship between the induced magnetic flux density (B) and the applied magnetic force (H).

Examining the size and shape of the hysteresis loop tells us what kind of material we have. All the parameters one sees on a data sheet are based on this. With permanent magnets, one wants the hysteresis loop to be as wide as possible: high Hci and high Br. There are really only four materials that meet the requirements: alnico, ferrite, SmCo (samarium-cobalt), and NdFeB (neodymium).

The loop begins with a look at the saturation magnetization (Ms), or the maximum amount of magnetization (M) one can get out of a particular material, even if more magnetic field were applied. When the applied magnetic field is reduced, one finds there is still some magnetization that remains. This is called remnant magnetization (Br). Remnant magnetization is a fundamental characteristic of permanent magnets.

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To thoroughly test a material, it is necessary to go negative in field. The field required to reduce the magnetization to zero, often referred to as ‘resistance to magnetization,’ is typically indicated as Hci. Hci helps one to understand how the material will operate in an adverse environment, such as when other magnetic fields are around, or with temperature sensitive parameters.

The major hysteresis loop refers to a material that has been completely saturated. You want to use a permanent magnet that has been fully saturated. The major hysteresis loop gives you the complete information; however, it is very difficult to get the entire loop because there are aren’t enough magnetic fields so usually what is measured is the region between remnant magnetization (Br) and Hci – the region where most magnets operate and where the interesting data lies.

For more detailed insight, watch the archived webinar hosted by the MCMA Understanding Magnetic Technology - Magnetics 101, which features Dr. Stan Trout, a leading authority on magnetics. The webinar provides a brief review of magnetic theory and ferromagnetism and introduces the basic properties of permanent magnets. The information in this post was extracted from Dr. Trout’s presentation.

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