Introductory Chapter: Modern Permanent Magnets – Basics and Applications

1. Introduction

Permanent magnets are highly magnetized functional hard materials, which do not lose magnetism over time due to the generation of magnetic field by the internal structure of the material itself [1, 2, 3]. These modern permanent magnets are made from a “cocktail” of minerals which can include iron, neodymium, samarium, cobalt and nickle. Normally, the functionality of the permanent magnets depends on the intrinsic properties of the base compound. It is commonly known that permanent magnets are source of magnetic field. The composition, extrinsic properties such as microstructure and processing method of materials are define the magnetic properties of the permanent magnets [4, 5, 6].

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).

3. Types of modern permanent magnets and materials

Modern permanent magnets are manufactured from different types of materials and systematic variations of compounds using different process. Based on composition variations in compounds, sub-lattice substitutions, crystallization of amorphous deposits and structure, permanent magnets are classified.

Generally, five different types of permanent magnets are known and produced for different purposes [14, 15, 16]. They are as follows:

1. Aluminium-nickel-cobalt (Alnico)

2. Ferrite

   These are M-type Sr- and Ba-hexaferrites

3. Samarium-cobalt (SmCo)

   Further in the Sm-Co family, again two types of permanent magnet such as SmCo₅ and Sm₂(Co,Fe,Cu,Zr)₁₇ type are based on the variation of compositions and substitution of different elements.

4. Neodymium-iron-boron (NdFeB) magnets

   Similarly, NdFeB magnets are classified into sintered NdFeB (main phase is Nd₂Fe₁₄B), hot-deformed NdFeB magnets, bonded NdFeB produced using melt spun powder and bonded NdFeB magnets made from hydrogenation-disproportionation-desorption-recombination powder.

5. SmFeN

   SmFeN magnet, also known as Sm₂Fe₁₇N₃, is a potential option for next generation of permanent magnetic material.

4. Key challenges of modern permanent magnets

Design and development of permanent magnets have challenges for the intrinsic and extrinsic magnetic effects due to mechanical, thermal and chemical parameters [1]. The challenges like metallurgical processing, microstructural changes and phase stability understanding are important for the development of good permanent magnets [17, 18]. The basic properties of the magnets such as remanent magnetization and coercivity are dependent on microstructure [7]. The parameters like room temperature (BH)_max, M_s and H_c, J diminish rapidly with increasing temperature. The fine grain size also has effect on the coercivity at high temperature [19]. Further, the packaging of magnet in operating environment should be thermodynamically stable, so that magnets will be physically stable (resistant to corrosion, decomposition or oxidation). Again, challenges differ from one another on the selection of material system, which show simultaneous high magnetization and high coercivity at any given time and environment.

5. Application of modern permanent magnets

Application of permanent magnets are many folds, starting from mechanical instruments such motors, generators, sensors, loudspeakers, bearings and clutches, automobiles to missiles, electrical to mechanical energy conversion, traveling wave tubes, transmission and distribution of electric power, microwave communications, and data storage [20]. Permanent magnets are a very crucial component of numerous industries and markets such as transportation, industrial, residential/commercial, consumer electronics, defence, IT/PC, wind energy and medical [21]. Permanent magnets are also important for military application, which add functionality to jet fighter engines and electronic countermeasure, missile and satellite communication systems. Many energy-efficient appliances, electric vehicles and direct-drive wind turbines also use permanent magnets [1, 22, 23].

Based on the application of permanent magnets, different materials are used for the manufacture of suitable permanent magnets, which can work at high temperature and harsh environment keeping the strength of the magnet intact.

6. Future of modern permanent magnets

The permanent magnet’s role is to provide a magnet field in a substance or vacuum for specific applications. Development of material and technology improved the appropriate need, functionality and flexibility of permanent magnets. Research progress in nanomaterial [24, 25, 26] and nanotechnology area meet the future demand of permanent magnet and miniaturization of devices. Further new development in rare earth permanent magnets opens the field of future permanent magnet markets, which plays an important role in a carbon neutral society. Understanding of cost-effective practical manufacturing process and material incorporation for effective production of permanent magnets contribute to a brighter future for permanent magnet. Following consideration can be taken for the development of new modern permanent magnets, which can be commercialized for future markets [27, 28, 29].

1. Improvement of magnet properties

   1. Enhance the existing magnetic material types: manipulation of crystal structure, increased the coercivity and H.

   2. Materials modification for specific requirement: addition of new materials or change of composition of existing materials.

   3. Improve the magnet manufacturing technology.

   4. Develop user friendly magnets.

2. New magnetic materials for modern magnets

   Search for new magnetic materials having high Curie point, easy-axis-type crystal anisotropy and high saturation magnetization. Nanomaterials combine with the used permanent materials may be an option for new ferromagnetic materials.

3. Manipulation of permanent magnet properties and calculation of theoretical limit

   Look for high H magnetic materials, then theoretical calculation can be done ignoring coercive force and anisotropy. It is reported that rare earth addition additions in transition-metal alloys can improve the magnetic properties. Similar avenue can be search for the development of new permanent magnets.

4. Sensitivity of magnet performance

   Processing conditions affect magnet manufacturing costs. Cost structures determine the commercialization process. Therefore, proper care should be taken staring from synthesis pathways to application. New tools, technique, experimental method and computational work can provide detail description about the performance of the magnets.

7. Summary

Field of permanent magnets is diverse. Proper research and attention create a large demand in this area for the development of variety of materials as per the application requirements. The existing permanent magnets with superior performance and development of new permanent magnets are the future of the new devices and industry.