Why Magnet Shape Is Important
A magnet's shape affects how much self-demagnetization occurs inside a particular magnet. This affects the external field produced by the magnet.
Self-demagnetization is an important part of magnet design. However, few people have heard of it. It is really quite simple. The field generated by one part of a magnet affects the other parts of a magnet. We will first look at the self-demagnetization inside of a solid cylinder magnet to see what it is.
For a simple cylinder bar magnet, the magnetic field is as expected. (See the article on bar magnet fields for more details.)
We usually treat bar magnets as solid objects. But in order to understand self-demagnetization, we'll look at this magnet in two parts.
The inner core and outer sleeve of a cylinder magnet
Think of a breaking up a solid cylinder magnet into two parts, an inner core, and an outer sleeve around the core. The inner core is shown in red. The outer sleeve in green.
Now, let's take a careful look at the fields produced by the core and the sleeve.
Field from the cylinder core
This is the field that comes from the core of the cylinder. Notice how it goes backwards through the sleeve.
The field from the red part of the cylinder goes the "wrong" direction through the green part of the cylinder. This means that the field from the core is trying to demagnetize the sleeve of the cylinder.
Field from cylinder sleeve
Similarly, the field from the outer part of the magnet points the "wrong" way through the center. This means the field from the outer part of the magnet is trying to demagnetize the center of the magnet.
Depending on the shape of the magnet, this self-demagnetization can have a serious impact on the magnet's performance.
We'll look at that next.
Three Magnet shapes to look at
Three extremes of cylinder shapes will be looked at.
Two different types of materials will be looked at. Most rare-earth and ferrite materials have a relatively square BH curve. Alnico materials do not.
Materials with square BH curves can be made into flatter shapes. Alnico materials (not having a square BH curve) need to be used in magnets with longer shapes.
Demagnetization Levels inside typical Rare-Earth Material Magnets
These graphs show what can typically be expected with rare-earth and ferrite magnets. The BH curve is shown for a typical rare-earth material. The BH curve for a ferrite will have a similar shape but much lower field values.
The BH curve is color coded to show the part of the curve below the knee (in red), near the knee (in yellow), and in the well-magnetized section (in green). Basically, green is good; red is bad.
Red shows where self-demagnetization can be a problem.
The magnet cross-sections show the demagnetization level inside the 3 magnet shapes. The colors correspond to the BH curve colors.
As can be seen, the longer two shapes are fine. Most of the magnet interior is not seeing much self-demagnetization.
The flatter shape however will have issues with self-demagnetization. This means that this magnet shape probably cannot be fully magnetized. It will probably also have issues with field loss at higher temperatures.
DEMAGNETIZATION LEVELS INSIDE TYPICAL Alnico MATERIAL MAGNETS
These graphs show what might be typically expected for Alnico materials.
Note that the BH curve is plotted to the same scale as the rare-earth one above. Alnico materials have strong interior fields but are very easily demagnetized.
Except for very long shapes, Alnico magnets have issues with demagnetization. Even the longer shape has demagnetization issues at its ends.
Note that many magnet calculators and rules of thumb consider the "effective length" of an Alnico magnet to be about 3/4 of its actual length. These graphs show why this is the case.
As a practical matter, Alnico magnets should be avoided in sensing applications. The required long shape does not always fit into applications very well. Also, they require special handling to avoid demagnetization.
Comments
This article glosses over many technical details. Actual BH curves for materials vary with temperature. They also vary material to material. The BH curves in this article were an approximation to actual curves. You can obtain BH curves from most magnet manufacturers.
Magnets used for sensing applications should be designed to avoid self-demagnetization. One of the keys to success in sensing applications is magnetic consistency. Well designed magnets have fewer issues than poorly designed magnets.
You should note that the articles on this web site are focused on using magnets for sensing applications. The key feature is that magnets are in a magnetic vacuum. Most of the magnetic fields are in free space. I have found that focusing on the fields provides a good conceptual basis for magnet design for sensors.
Many magnet articles you can find on the internet are based on using magnets in magnetic circuits (such as motors, solenoids, or generators). The key feature is that magnets are surround by magnetic materials. Most of the magnetic fields are inside of magnetic materials. Articles dealing with magnetic circuits focus on magnetic flux through the circuit. Magnetic flux is a useful concept for dealing with magnetic circuits.