Brief History Of Soft Magnetic Materials

Ever since Michael Faraday demonstrated electromagnetic induction in 1831, there has been a continuing evolution of soft magnetic materials. Faraday's natural choice of core material was iron, which has the highest room temperature M_s of any element in addition to a large μ_r, and fairly low H_c. However, even in a simple material comprised of a single element there was room for considerable improvement.

It was discovered that annealing iron not only improved its mechanical properties but also decreased its coercivity through stress relief, making it better suited for use in inductive applications. Seeking even better performance, scientists and engineers looked for ways to improve upon the properties of soft iron.

In 1900, Robert Hadfield, a metallurgist from England, invented nonoriented silicon steel by adding up to 3% silicon to iron and increasing its electrical resistivity (p) while also increasing μ_r. American metallurgist Norman Goss invented grainoriented silicon steel in 1933 by promoting grain growth along a crystalline direction of low anisotropy, increasing μ_r, even further . Even today, silicon (or electrical) steels account for a major share of the global soft magnet market because of their high M_s and relatively low cost .

The most common applications for silicon steel are large-scale transformers (grain-oriented silicon steel) and electrical machines (isotropic nonoriented silicon steel is preferred for rotating machines), where its economical price is a huge benefit.

However, a low (-, 0.5 μohm.m) makes silicon steels lossy at high frequency. Recently, electrical steel manufacturers have developed a path to increase the silicon content of their steel to 6.5% using a chemical vapor deposition (CVD) process . This approach increases to 0.82 μΩ.m but still leaves other materials as better choices for high-frequency power electronics and high rotational speed electrical machines.

In the 1910s, Gustav Elmen at Bell Laboratories experimented with nickel-iron alloys and discovered the nickel-rich (78%) permalloy composition . A major advantage of permalloy is its high μ_r, (up to 100,000). Nickel-iron alloys are still used in some specialty inductive applications today but are not common in power electronics and electrical machines because they have high eddy current losses, and the addition of nickel decreases M_s. With the addition of a small amount of molybdenum (2%) to permalloy, molypermalloy powder (MPP) can be produced . MPP is used to fabricate the lowest loss powder cores .

In the late 1940s magnetically soft ferrites were invented by J. L. Snoek . These materials are competitive because of their very high electrical resistivities (10 - 10⁸ μohm.m), which make them effective at suppressing eddy current losses.

Additionally, because they are produced with ceramic processing techniques and abundant materials, ferrite parts can be produced at a very low cost. The high

and affordability of soft ferrites keeps these materials in high demand for inductive applications, including those at high frequency. In fact, their share of the global market in soft magnets is second only to silicon steel . They do suffer from a relatively low M_s. (nearly a quarter of that of silicon steel), which limits the energy density of inductive elements containing a ferrite core.

In 1967, a new class of materials, amorphous alloys, were invented . By the mid 1970s, interest in iron and cobalt based amorphous alloys was surging and they began finding their way into applications. Through the elimination of any long range order, coercivity is substantially reduced in these alloys.

In 1988, researchers at Hitachi included Nb and Cu additives and added an annealing step to the production of amorphous alloys to produce small and closely spaced crystallites of iron or cobalt (on the order of 10 nm in diameter) within a matrix of amorphous material . This was the inception of the nanocrystalline soft magnetic alloys. The formation of isolated transition metal crystallites reduced the eddy current losses of these materials in comparison to amorphous alloys. Both amorphous and nanocrystalline alloys are gaining market share in high-frequency power electronics and electrical machines today because of their low losses and competitive M_s.

Despite a higher initial cost than silicon steel, these advanced alloys can reduce the total lifetime costs of power electronics and electrical machines, thanks to reduced losses.

In the early 1990s, powder cores (also known as soft magnetic composites or SMCs) gained acceptance in some soft magnetic applications . These materials combine magnetic particles, anywhere between approximately 1 to 500 gm in diameter, and either coat or mix them with an insulating material before consolidating with high pressures (MPa to even GPa pressures).

Heat can also be applied either during or after densification to improve magnetic properties. The magnetic particles are most often Fe powders but can also consist of alloys such as MPP (mentioned earlier), Fe-P, Fe-Si, or Fe-Co. Because of the insulating and non-magnetic matrix phase, these materials have a distributed air gap that limits their μ_r to a range of 100 to 500. However, the insulating matrix also boosts their

(10^-3 to 10^-1 µohm•m), reducing eddy current losses.

SMCs can also be pressed into more complex final geometries without the need of any machining (net-shaping), which can substantially reduce manufacturing costs. Their isotropic nature, low cost, and the ability to net-shape complex parts have made SMCs fairly successful in rotating electrical machines .

The brief history of soft magnetic materials described above is by no means exhaustive. Instead, our intent is to focus on materials that have been and will continue to be competitive for the fabrication of soft magnetic components in high-frequency power electronics and electrical machines. Performance metrics such as M_s and core loss are extremely important. However, because soft magnetic parts will need to be used in large quantities, the importance of cost cannot be neglected. For this reason, soft ferrites still remain a competitive core material at high frequency. Because of their excellent performance at high frequency, the amorphous and nanocrystalline alloys will certainly continue to be key materials. Although silicon steels still make up a majority of the global market for soft magnetic materials, their primary applications are in large transformers operating at 50 or 60 Hz and slow rotational speed electrical machines.