Application of soft magnetic materials
Iron-Silicon Alloys
These alloys are used for transformer cores and are known as electrical steels. In the power industry electrical voltage is almost always AC and at low frequency, 50-60Hz. At these frequencies eddy currents are generated in the transformer core. Alloying the Fe with Si has a large marked effect on the electric resistivity of the material, with an increase of a factor of 4 for 3wt%Si. Silicon also has the benefit of reducing the magnetostriction (i.e. length change on magnetisation) and the magnetocrystalline anisotropy. In addition, the material is used in the form of laminations, typically 0.3 to 0.7mm thick. The addition of too much silicon makes the material extremely brittle and difficult to produce, giving a practical limitation of 4wt% to the amount of Si that can be added. Recently, a technique has been developed to produce laminations with >6wt% Si, by a SiCl4 chemical vapour deposition treatment to enrich the laminations with Si after forming the laminations. Typically most electrical steels will contain between 3 and 4 wt% Si.
Amorphous and Nano-Crystalline Alloys
These materials can be produced in the form of a tape by melt-spinning. The alloys consist of iron, nickel and/or cobalt with one or more of the following elements: boron, carbon, phosphorous and silicon. They have extremely low coercivity, an order of magnitude less than standard Fe-Si, and consequently lower hysteresis losses. However, they have relatively low magnetisation and are not suitable for high current applications. They do find a market in low current applications and specialised small devices where they can compete with Ni-Fe.
Instead of casting the alloy onto a rotating wheel to produce tapes it is also possible to squirt a stream of molten alloy into a bath of water or oil to produce amorphous wires of typically 50mm thick. These wires show a very square hysteresis loop with large changes in magnetisation at low field, making them ideal for sensing and switching.
Recently there has been much interest in nano-crystalline material, which is produced by annealing the amorphous material. These alloys can be single phase but are usually comprised of nano-sized grains, in the range 10-50nm, in an amorphous matrix. They have relatively high resistivity, low anisotropy and good mechanical strength.
Nickel-Iron Alloys
These alloys, known as permalloy, are extremely versatile and are used over a wide range of compositions, from 30 to 80wt%Ni. Over this composition range the properties vary and the optimum composition must be selected for a particular application. The high Ni content alloys have high permeability; around 50wt%Ni have high saturation magnetisation and low Ni content have a high electrical resistance.
There are special grades of Ni-Fe alloys that have zero magnetostriction and zero magnetic anisotropy, such as mumetal which is produced by a careful heat treatment and minor additions of Cu and Cr. These alloys have extremely high permeable, up to 300000 and intrinsic coercivity as low as 0.4Am-1.
Soft Ferrites
At high frequency metallic soft magnetic materials simply cannot be used due to the eddy current losses. Therefore, soft ferrites, which are ceramic insulators, become the most desirable material. These materials are ferrimagnetic with a cubic crystal structure and the general composition MO.Fe2O3, where M is a transition metal such as nickel, manganese or zinc.
MnZn ferrite, sold commercially as ferroxcube, can be used at frequencies up to 10MHz, for example in telephone signal transmitters and receivers and in switch mode power supplies (also referred to as DC-DC converters). For these type of application the driving force to increase frequency is to allow miniaturisation.
Additionally, part of the family of soft ferrites, are the microwave ferrites, e.g. yttrium iron garnet. These ferrites are used in the frequency range from 100MHz to 500GHz, for waveguides for electromagnetic radiation and in microwave devices such as phase shifters.