**2. Basics of permanent magnets**

The magnetic flux with no energy input defines the uniqueness of permanent magnets. The performance of permanent magnets is estimated based on the magnetization (*M*) and maximum energy product and the magnetic parameter such as (*BH*)max, spontaneous magnetization and coercive forces [7].

In addition, to above indicator, some basic requirements of permanent magnets are mentioned as follows:

• Anisotropy

Shape anisotropy, magneto-crystalline anisotropy and stress anisotropy contribute towards the anisotropy behavior of permanent magnets [8].

• Coercive force/coercivity

High coercivity is the requirement for the development of permanent magnets, which can be achieved by controlling the microstructure of the major constituent phase of the magnetic compound [8, 9].

• Magnetic domains and domain walls

Magnetic domain observations are critical for permanent magnet. The thickness of domain wall and domain wall structure inform the change in magnetic moment and the formation of permanent magnets [10].

• Magnetic hysteresis

This is an important tool for the quantitative analysis of permanent magnet performance. Hard magnetic materials always show the greatest hysteresis [11].

• Recoil permeability:

The recoil permeability describes the steepness of the demagnetization curve in the *B*(*H*) explanation [12].

• Stability

Excellent magnetic performance of permanent magnets materials: Materials must possess large exchange interactions, high spontaneous magnetizations and large magnetic anisotropy [13].

Overall, for a permanent magnet, the material must attribute to high saturation magnetisation (*M*s), Curie temperature (*T*c) and magnetocrystalline anisotropy (MAE).
