What is Remanence in Magnetics?
Remanence is the remaining magnetic field found in the material after the applied magnetic field is reduced to zero. Remanence is derived from the same word as “remainder” or “remaining” so we can think of Remanence as the magnetic field that remains in the material after the outside magnetic field has been released. Remanence is also known as Retentivity. Remanence is best understood in the context of Permeability.
The Meaning of Remanence in Hard- and Soft Magnetic Materials
Remanence has different implications in hard magnetic materials than it has in soft magnetic materials. Hard magnetic materials retain a high degree of permanent magnetism after release of the applied field, while soft magnetic materials more easily lose their magnetic energy after release of the applied field.
When releasing the applied magnetic field from a hard magnetic material, it retains a very large amount of its saturation magnetization, but when the applied field is released from a soft magnetic material, it retains a lower percentage of its saturation magnetization.
This can be seen by looking at the upper right side of the hysteresis loop. The hysteresis loop on the left side of the graph in Figure 1 shows that as we go from the far right of the graph and start moving to the left, the external field is released and the magnetization declines so that when Bo =0 the material reaches its remaining magnetic value. We call this value Remanence, and for the hard magnetic material depicted in the leftmost drawing of Figure 1, the Remanence is a very high fraction of the total value.
The hysteresis loop on the far right side of the graph in Figure 1 shows that as we go from the far right of the graph and start moving to the left, the Remanence of this soft magnetic material is a much lower fraction of the total value than it is for the hard magnetic material. The Remanence value is the place on the M axis where the hysteresis loop crosses the M line going from the upper right to the lower left.
Saturation Magnetization is the material’s maximum magnetic strength in the presence of an external magnetic field. It is achieved when the driving line approaches -but doesn’t cross- the horizontal Magnetization line.
Hard magnetic materials require a much larger coercive force (Coercivity) to demagnetize them than do soft magnetic materials, so naturally, designers use soft magnetic materials in situations where it is advantageous to switch polarities quickly and often, and they use hard magnetic materials as a permanent magnet source. This is why we see soft magnetic materials like silicon steel, iron-nickel, and iron in motor windings and transformers where magnetic fields are constantly changing.
Similarly, hard magnetic materials, such as Neodymium, Samarium Cobalt, and Ferrite are used in components that require hard magnetic materials. Hard magnetic materials have high Remanence and high Coercivity.
Since the coercive force is much lower for soft magnetic materials, it makes sense to use them for applications where the polarity will be reversed often. The lower Coercivity of soft magnetic materials results in lower losses during operation, so they see a lot of usage in transformer cores and motor windings.
Magnetic Remanence of Neodymium Magnets
Remanence in Neodymium magnets is similar in principle to Remanence of other hard magnetic materials, but the actual values are quite a bit higher. The properties can be best understood by viewing the hysteresis curves of hard- and soft magnetic materials and comparing them.
The Remanence value of Neodymium magnets is typically around 1-1.3 Tesla (10-13kGauss), which is roughly 3X’s that of ferrite magnets.
The hysteresis graph for a hard magnetic material maintains high Remanence after the external field is released (see Figure 2). This is easily seen by the gentle slope of the top line -the saturation line -the horizontal line at the top of the graph. The slope of the saturation line only begins to steepen as the applied field is reversed in the upper left quadrant of the graph.
The hysteresis graph for a hard magnetic material tends to be wide, because a hard magnetic material requires a bigger magnetic field to magnetize it, so the x axis (labeled B, Applied Magnetic Field) values get bigger.
As the external field (B) is released, see how gentle the slope is as we go back from right to left and approach zero (see Figure 3). The slope does not steepen until the value of (B) begins to go to negative values. Once the magnetization reaches zero, the slope becomes very vertical -more vertical in a hard magnetic material than in a soft magnetic material.
Another factor to note is that for soft magnetic materials, the slope of the hysteresis loop as it crosses the (B) axis is very steep (see Figure 4) when compared to that of a hard magnetic material. This is because soft magnetic materials lose much more of their magnetism as the external field is released compared to hard magnetic materials.
The slope for hard magnetic materials is much less steep until the applied magnetic field begins to push the magnetization towards zero. When the magnetization of the material reaches zero, the strength of the demagnetization signal (negative H) gives us the value for Coercivity of that material
About Tengye: Tengye Magnetic supplies finest permanent magnet materials and custom magnetic assemblies to world industry. We work closely with our customer from designing stage to mass production.
