Look carefully at the longer magnet shape of the low coercivity material in the above graph. They needed long shapes to avoid self-demagnetization to even be magnetized to any degree. The long shapes were bent near the center to bring the poles closer together. Most magnet materials in the rare earth families and ferrite families have high coercivity. This means that they can usually be made in somewhat flatter shapes without problems. In contrast, magnet materials in the Alnico family have very low coercivity. Alnico magnets should have longer shapes to avoid self-demagnetization.įor those who can recall the good old days when many magnets were horseshoe shaped, this is the reason why. Older magnet materials had very low coercivity. If a magnet material is resistant to demagnetization (having a high coercivity), it can be made into flatter shapes without too many problems. If a magnet material is easily demagnetized (having a low coercivity), it should usually be used in longer shapes. The graphs below show illustrations of the cross-section of cylinder magnets with different shapes. The regions in red show where different magnet shapes will probably have significant self-demagnetization problems. What shapes are allowed for a particular material? it has a low coercivity), this can be a serious issue. ![]() it has a high coercivity), this will not be much of an issue. However, if the magnet material is not resistant to demagnetization (i.e. If the magnet material is highly resistant to demagnetization (i.e. This self-demagnetization is affected by the shape of the magnet. Flat magnets (with large poles) have larger self-demagnetization fields. Long magnets (with smaller poles) have smaller self-demagnetization fields. What this means is that the fields from one part of the magnet are pointing in a direction to demagnetize other parts of the magnet! The next two graphs show the cross-section of the cylinder magnet. The top graph shows the field from the outer ring of material. The bottom graph shows the field from the inner core of material. The N and S labels are indicating the magnetization of the entire solid cylinder magnet. Both the inner core and outer ring are magnetized in the same direction.Notice how the field created by the inner core points backwards through the outer ring of the magnet. Similarly, the field created by the outer ring of the magnet points backward through the inner core of the magnet. We will first look at the self-demagnetization inside of a solid cylinder magnet to see what it is. In order to do this, we will think of the single solid cylinder as having 2 parts, an inner core, and an outer ring around the core. We will look separately at the field generated by the inner core material (shown in red) and the outer ring material (shown in green). Edit (Jan-13-2017): See the site for a different version of this article and other articles. Magnet shape affects how much self-demagnetization occurs inside a particular magnet. This affects the external field produced by the magnet.
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