**2.2 Nd-Fe-B magnets**

The rare earth element Neodymium along with Iron and boron alloy with Nd2Fe14B have tetragonal structure permanent magnets. This tetragonal Nd2Fe14B belongs to the space group P42/mnm. The lattice parameters values are *a* = 8.818(2) and *c* = 12.236(5) Å. This NdFeB shows large coercivities and energy product [18].

**Figure 4** shows the crystal structure of Nd2Fe14B magnets which is tetragonal in nature [19]. This tetragonal crystal structure of NdFeB are resistance to demagnetization and high uniaxial magnetocrystalline anisotropy attributing to its high magnetic strength.

#### **2.3 Sm Fe N magnets**

SmFeN magnets belong to the group of R2Fe17Nx intermetallic compounds where R represents a rare earth element, and x is typically between 2 and 3. These compounds shows rhombic dodecahedron or hexagonal structure, with nitrogen occupying octahedral interstitial sites. Among all R2Fe17Nx compounds, Sm2Fe17Nx is the most promising permanent magnet material due to its uniaxial (c-axis) anisotropy. **Figure 5** presents the Sm2Fe17N3 (a) rhombohedral and (b) disordered hexagonal structure.

**Figure 4.** *Crystal structure of Nd2Fe14B magnets [18].*

*Current Advances in Nanocrystalline Rare Earth Based Modern Permanent Magnet DOI: http://dx.doi.org/10.5772/intechopen.114227*

**Figure 5.** *Structure of samarium iron nitride (a) rhombohedral and (b) hexagonal [20].*

At room temperature, Sm2Fe17 compound is a Th2Zn17 type structure [21, 22]. The Th2Zn17 type crystal structure is the basic types of crystal structures of rareearth permanent magnetic compounds. The spatial stereo diagram of the Th2Zn17 type crystal structure is shown in **Figure 6**, which belongs to the rhombic square crystal system (or tripartite crystal system) with the space group R3m, and a single cell contains three Sm2Fe17 molecules. There are 57 atoms in the unit a cell, 6 Th (or R) atoms occupy the c-site, 9 out of 51 Zn (or Co, Fe, etc.) atoms occupy the d-site, 18 occupy the f-site, 18 occupy the h-site, and 6 occupy the c-site. There are two large interstitial positions in this structure: one is an octahedral interstitial position, located at the 9e crystal site, on the atomic plane containing rare earth atoms; the other is a 3b crystal site located between two rare earth atoms along the

**Figure 6.** *Single cell structure of 2:17: N compound [23, 24].*

c-axis. The Sm2Fe17Nx [21, 25]-type compounds has the same structure as Sm2Fe17, with an increase in both a and c, and an increase in the volume of the single cell by about 6%.
