Figure of merit - a numerical value, feature or characteristic which can be used to categorise the performance of a material, device or process.¹⁾²⁾

Magnetisation curves for Ferrotron and Fluxtrol materials³⁾, with different values of permeability

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Figures of merit are useful for comparison of specific feature in engineering applications. There can be several such characteristics, depending on the point of view, or the required physical performance required in a given device.

For example, ferromagnetic materials can be characterised by:

• saturation - higher values allow generation of larger magnetic forces in an electromagnet
• permeability - higher values mean stronger ability to concentrate magnetic flux, which is useful for reducing no-load current in transformers, or increasing sensitivity of certain magnetic sensors
• remanence and coercivity - higher values are more important in applications such as permanent magnets, but much lower values of coercivity are required for soft magnetic materials
• mechanical strength - larger values permit operation under higher forces or rotational speeds
• specific power loss - lower values can be used for designing more efficient devices
• thermal conductivity - higher values allow more efficient cooling
• Curie temperature - higher values permit operation at higher temperatures
• electrical resistivity - higher values reduce power loss due to eddy currents under AC excitation
• cost - important for commercial applications, in which the price of the final product is crucial for a commercial success

There is no single material which is “the best” for all applications. Specific devices require unique combinations of many such figures of merit, so that the device can be designed, manufactured, made available commercially, and operated reliably, as well as recycled at the end of life, or after a failure.

• Order of magnitude