**1. Introduction**

Permanent magnets are critical components for design of modern devices in many technological aspects. Modern permanent magnets made of rare earth magnetic materials play important role in clean energy sector and climate economy products such as electric vehicles, consumer electronics, wind turbines, military products, phones, refrigerators, weapons, missiles and jets [1–4]. Rare earth permanent magnets consist of some rare earth elements (Sm, Nd, Pr, Dy, Ce, etc.) form critical magnetic structures. Most of the research approaches have been dedicated towards synthesis of permanent rare earth materials, its characterization, modeling, and processing of materials [5–7]. Such materials have specific magnetic order parameter which define its physical properties for potential applications. These properties are extremely important for our daily life: exploiting the benefit of mentioned promising order parameters (coercivity, spontaneous magnetization, Curie temperature), it is possible to develop and design efficient technological devices for computer and office automation, household appliances, and magnetic data storage, position sensors, actuators, micromechanical system and medical application [4].

It is widely known that Sm-Co magnet was the first developed powerful rare-earth magnets and rapid advances was made on the improvement of the performance of Sm-Co magnets [8]. This was followed by the development of stronger and cheaper Nd-Fe-B rare earth magnets [4, 9]. In recent years, SmFeN magnets have gained

#### **Figure 1.**

*The magnetization characteristics of some rare earth magnets (Ref: https://www.advancedmagnets.com/ what-are-rare-earth-permanent-magnets-repms/.*

attention due to their high-temperature stability and excellent magnetic properties [10]. SmFeN magnets are also known as third-generation permanent magnets and are considered to be an alternative to NdFeB magnets for certain applications.

**Figure 1** presents the magnetization characteristics of the mostly used rare earth magnets (NdFeB and Sm-Co) with respect to non-rare-earth magnets. Nowadays Nd-Fe-B magnets received most attention in the market due to their cheap price and excellent room temperature magnetic properties. However, for high temperature and next generation application, high Coercivity is the prime requirement. The anisotropy field of the magnets can also be enhanced due to heavy rare earth magnets. The inclusion of heavy rare-earth magnets increases the cost of the magnets and reduces the remanence value. Sm-Co rare earth magnets have high curie temperature and high thermal stability. This magnet is the best candidate for high temperature applications. Similarly, Sm-Fe-N magnets which have higher anisotropy has low stability at high temperature which limits its practical application in high tech industries and energy transition technologies.

Therefore, continuous improvement of permanent magnet in terms of performance, efficiency and application is a technological challenge. The development of advanced permanent magnets is always a requirement which can provide high efficiency and reliability, low cost, outstanding magnetic properties and low maintenance for applications. This chapter reviews the fundamental aspect and progress of the rare earth modern magnet which highlight the essential key factor for future development of cost-effective permeant magnets.
