Permanent Magnet Materials:

In Section 2A, we introduced the three most common classes of commercial permanent magnet material by way of their anisotropy mechanism and the resulting demagnetization curves. Now, we similarly describe two new less common materials, which nevertheless are useful for some niche applications. The most recent rare earth powder to be introduced is samarium-iron-nitride, which was the result of a search for alternatives to the very popular neodymium-iron-boron, but which unfortunately uses a rare earth element which is considerably less plentiful. Another attempt to circumvent "Nd₂Fe₁₄B" was to reduce the amount of Nd by various means, though the resulting modified neodymium-iron-boron magnets also have reduced intrinsic coercivities. So, in this Section, we introduce:

samarium-iron-nitride | modified neodymium-iron-boron

Samarium-iron-nitride

Newer developments in "rare earth" magnets have concentrated on incorporating Fe rather than the more costly transition metal Co. Several rare earth elements (R) have been combined with Fe in R₂Fe₁₇ compounds, but they all have very low potential operating temperatures. The first breakthrough was the discovery that the addition of boron yields a ternary compound with strong uniaxial magnetocrystalline anisotropy, and a much improved operating temperature for the neodymium-iron-boron compound "Nd₂Fe₁₄B". In 1990, it was first reported that the absorption of nitrogen could also improve the anisotropy, magnetization and operating temperature, the most promising being the samarium-iron-nitride compound "Sm₂Fe₁₇N₃". While its properties can theoretically surpass those of Nd₂Fe₁₄B, the processing of Sm₂Fe₁₇N₃ into magnet is far more complex.

There are several different routes by which the base Sm₂Fe₁₇ alloy can be produced, but it must be milled into powder prior to the "nitrogenation" step, during which its useful magnetic properties are developed. For example, mechanical alloying, rapid quenching or HDDR (described in Section 2A) followed by milling will produce powder which has an extremely fine microstructure as illustrated again here in diagram (c). The resulting intrinsic coercivity is roughly inversely proportional to the grain size. If it were possible to mill to a grain size of under 1µm, then the resulting powder would be anisotropic. However, Sm₂Fe₁₇N₃ is highly unstable after nitrogenation, which restricts the amount of subsequent processing it can withstand: a grain size of around 30µm is more typically achieved, and further heat treatment steps above about 250^oC have to be avoided.

In practice, only bonded magnets are made from Sm₂Fe₁₇N₃, which are more easily made in isotropic form than anisotropic. Magnetization provides alignment of the magnetic moments in diagram (c) above, which are then pinned at the grain boundaries to provide the material's intrinsic coercivity as illustrated in diagram (b) (as was described for Sm₂Co₁₇ in Section 2A). Injection-molded anisotropic Sm₂Fe₁₇N₃ has been made with a room temperature energy product of around 13 MGOe, but its demagnetization characteristics (below) show a deterioration of H_ci with increasing temperature which is similar to Sm₂Co₁₇ (NOTE: intrinsic curves are shown solid in this diagram, while B vs. H curves are shown dashed).

The practical difficulties in producing powder and then magnets from nitrogenated Sm₂Fe₁₇ limit the applications for which it is viable. To make matters worse, the alloy naturally contains significant amounts of alpha-Fe (as described in the next Section), so one can either accept the accompanying degradation in H_ci, or elect to perform long homogenization treatments to make the alloy predominantly single-phase, or suppress the alpha-Fe by the addition of other elements.

Modified Neodymium-iron-boron

Reducing the amount of Nd in the "Nd₂Fe₁₄B" compound has the obvious advantage of lowering cost by virtue of less rare earth element being used, though we no longer have the best combination of magnetic and thermal properties for the magnet. Such modified neodymium-iron-boron has found niche application in two particular forms:

1: Nanocomposites

"Nd₂Fe₁₄B" contains approximately 11.8 atomic % of Nd, which is equivalent to around 30% by weight. Reducing the Nd content closer to 20% by weight allows the introduction of a soft magnetic phase, such as Fe₃B or alpha-Fe. It is possible to form these phases in neodymium-iron-boron using the rapid quenching process, though as mentioned in Section 2A this powder is inherently isotropic. In earlier Sections we have described how a compound for use as a permanent magnet material is designed to achieve its highest possible intrinsic coercivity. Deviating from the preferred "Nd₂Fe₁₄B" composition results in a reduction in intrinsic coercivity, but the material retains a high saturation magnetization thanks to its increasing Fe content. This is illustrated in the intrinsic demagnetization curves (in red) and the "normal" B vs. H curves (in blue) for a typical powder shown below (NOTE: the equivalent characteristics for a magnet made from such a powder will exhibit magnetization which is reduced by the volume fraction of magnetic particles).

A nanocomposite having a steeper demagnetization curve than that which occurs in a normal "Nd₂Fe₁₄B" magnet can be advantageous for certain applications. Furthermore, while deviating from the "Nd₂Fe₁₄B" composition causes a reduction in intrinsic coercivity, it is generally easier to magnetize a nanocomposite magnet.

2: Ferrite-Neo Hybrids

A second method of reducing the amount of Nd in the compound is simply to blend "Nd₂Fe₁₄B" itself with another suitable magnet powder, such as the ferrite M_n(Fe₂O₃). Processing methods for such a "Ferrite-Neo" blend are obviously somewhat limited, most commonly only into bonded magnets, which means that rapidly quenched "Nd₂Fe₁₄B" powder is used and the resulting magnets are isotropic. The higher the proportion of ferrite in the compound, the lower will be the magnet's magnetization, but the intrinsic coercivity is also affected by the mixture. The magnetic characteristics are similar to those for magnets made using nanocomposites, except that these can be tailored somewhat in a "Ferrite-Neo" by adjusting the blend. Examples of the demagnetization curves for Arnold's commercial injection-molded "Ferrite-Neo" magnet materials are shown below.

From our discussion on "Change in Coercivity" in Section 3A, while the intrinsic coercivity of ferrite magnets increases with temperature, that of neodymium-iron-boron decreases with temperature. A "Ferrite-Neo" hybrid may therefore be blended to achieve almost zero temperature coefficient for the intrinsic coercivity if this is desirable for the application.

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