**2. Structure of the rare earth magnets**

The three family of the rare earth magnets are (i) Sm-Co Magnets (SmCo5, Sm2Co17), (ii) Nd FeB magnets (specially Nd-Fe14B) and Sm-Fe-N magnets. These *Current Advances in Nanocrystalline Rare Earth Based Modern Permanent Magnet DOI: http://dx.doi.org/10.5772/intechopen.114227*

rare earth permanent magnet materials possess uniaxial crystal structure (tetragonal, hexagonal, or rhombohedral) and the magnetization along the crystallographic symmetry axis. Detail crystal structure are as follows,

#### **2.1 Sm-Co magnets**

Sm-Co magnet is a rare earth magnet consists of two basic elements samarium and cobalt. The phase composition and structure define the basic magnetic behavior of the magnets. The sintered 1:5 type SmCo magnets have hexagonal crystal structure or 2:17 type SmCo magnets have a rhombohedral crystal structure [11]. **Figure 2** presents the crystal structure of SmCo5, SmCo7 and Sm2Co17. The SmCo5 have the hexagonal CaCu5-type crystal structure [12–15]. SmCo5 structure belongs to the space group P6/mmm while Sm2Co17 (P63/mmc) is Th2Ni17-type hexagonal or Th2Zn17-type rhombohedral (R3̅m) structure. SmCo5 is a layer-by-layer configuration along the c-axis [16].

**Figure 2.**

*Crystal structure of (a) SmCo5 and (b) SmCo7 (c) rhomohedral Sm2Co17 (d) heaxagonal Sm2Co17 [12–15].*

**Figure 3.** *Unit cell of the hexagonal SmCo5 [17].*

Typical unit cell of Sm-Co5 is shown in **Figure 3**. Alternative Sm-Co layers and Co form the hexagonal structure. The lattice constants of stable bulk SmCo5 are

a = 5.00 Å and c = 3.96 Å and of Sm2Co17 are a = 8.36 Å and c = 8.15 Å [17].

The composition of distinct phases, their elemental profiles, and the process parameters optimization is necessary for the creation of perfect structure. The coherent crystal symmetry along with perfect phase separation is the key to achieve the technical realization of the unique magnets. The formation of such unique structure consisting of phases produce Sm-Co magnets with high Curie temperature for the high temperature operation.
