**3. Properties of rare earth magnets**

The magnetic parameters such as high saturation magnetization (Ms), High remanence (Mr), Very high uniaxial magnetocrystalline anisotropy energy (K1), high coercivity (Hc), High maximum energy product (BH)max and High Curie temperature (TC) decide the performance of permanent magnets. While the parameters like good temperature stability, mechanical strength, machinability, and low cost determines their applications. The key factor maximum energy product (BH)max treated as the figure of merit for characterization of particular grade permanent magnet.

**Figure 7** presents the characterizes B-H loop of the magnets. The shape of the magnet defined the energy product, working point and load line. The mixture of different phases of constituent element in the matrix determines the intrinsic magnetic properties. A good permanent magnet must have a large spontaneous magnetization in zero field (i.e., a high retentivity) and a high coercive force to prevent its being easily demagnetized by an external field. Properties like, density, Curie temperature, Vickers hardness, compress strength, electronic resistivity, bending strength, stretching strength, hot coefficient of expansion and anti-causticity defines the effectiveness of the application of magnets.

Samarium Cobalt based magnets shows exceptional magnetic properties, excellent temperature stability, durability and a high resistance to corrosion or demagnetization [27]. However, 2:17-type SmCo magnets are very sensitive towards the magnetic properties due to the alteration of the magnet composition. For example, if the composition of the Sm2Co17 magnet is changed to Sm (CobalFe0.21Cu0.08Zr0.03)7.6, it affects the coercivity [28]. A step is also observed in the demagnetization curves as shown in **Figure 8**. Further, as observed on this figure, the solution temperature also affects the magnetic order.

The change of composition and materials processing temperature of alloys effect the (BH)max of Sm-Co magnets [29]. Again, SmCo7 phase is always change to 1:5 SmCo phase and 2:17 SmCo due to metastable nature. In order to stable the SmCo7 into TbCu7 structure, another element such as Zr, Hf, Nb, Ti, Ga, etc. are introduced into the matrix to increase its intrinsic magnetic properties [30, 31]. Various element such as Zr are also included for the enhancement of the anisotropy field and the coercivity of SmCo7 powders [17]. The nanostructural modification of the magnet matrix improve the magnetic properties of the magnets.

**Figure 7.** *The B-H loop of a permanent magnet [26].*

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

**Figure 8.** *Demagnetization curves of Sm (CobalFe0.21Cu0.08Zr0.03)7.6 magnets [28].*

NdFeB exhibits highest magnetic properties and maximum performance among all permanent magnets. However, temperature has an effect on the performance of the magnets. It performs very well in lower temperature. These magnets offer the highest energy product, higher remanence, much higher coercivity but lower Curie temperature than other types of magnets. The NdFeB magnets may be demagnetised

**Figure 9.** *Magnetic properties of Sm2Fe17Nx as a function of N content [33, 34].*

by radiation [32]. The microstructure with net shape formability and the exchange coupling between Nd2Fe14B grains manifest better magnetic properties.

Samarium-iron-nitride are considered as high performance and high temperature environment permanent magnets due to its high saturation magnetization value and large magnetocrystalline anisotropy field. Additionally, SmFeN magnets have a theoretical maximum magnetic energy product comparable to that of NdFeB magnets. Sm2Fe17Nx exhibit excellent magnetic properties including high coercivity field and higher Curie temperature. The N content has a crucial influence on the magnetic properties of Sm2Fe17Nx compounds [33, 34]. **Figure 9** shows the change in magnetic properties of Sm2Fe17Nx with the number of N atoms. When the N content x = 6, three N atoms occupy 9e crystal sites, which play an enhanced role in magnetic properties. The other three N atoms occupy half of the 18 g crystalline sites, which weaken the magnetic energy.

Further addition of different metallic elements such as (M = Nb, V, Ta and Co) on Sm2 (Fe. M) 17 also increases the Curie temperature of the alloys which is higher than Sm2Fe17 compounds. However, the Curie temperature and spontaneous magnetization intensity of Sm2(Fe,M)17Nx compound were lower than those of Sm2Fe17Nx. In contrast, for Co, the Curie temperature was increased by nearly 100 K compared to Sm2Fe17Nx to 845 K and the spontaneous magnetization intensity was 1.41 T. Moreover, the Curie temperature and the magneto-crystal anisotropy field increased with the increase of Co content in a certain range. The addition of Cr and Ga significantly increased the HcJ of Sm2(Fe1-xMx)Ny compounds. However, the addition of Zr promoted the amorphization of Sm\_Fe alloy and inhibited the grain growth. The addition of Co increased the Curie temperature but decreased the HcJ because Sm(Fe,Co)2Nx and Sm(Fe,Co)3Nx were easily formed after the addition of Co interstitial compounds, which decompose below 500°C, resulting in the formation of the soft magnetic phase α(Fe,Co), leading to a decrease in the coercivity [35].
