Current Advances in Nanocrystalline Rare Earth Based Modern Permanent Magnet

Abstract

Modern permanent magnets are the essential materials for many industries and technologies. All strong permanent magnets that contain rare earth element have wide range of application. Different processing technique, material and manufacturing methods are used to produce different types of rare earth magnets. New developments and improvement in properties are proposed based on the addition of nanocrystalline materials to address the effectiveness of rare earth magnets which is beneficial for different application. This chapter reviews the fundamental aspect and progress of rare earth modern magnet along with the need of essential key factor for future development of cost-effective rear earth permanent magnets.

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, 2, 3, 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, 6, 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 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.

Figure 1.

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

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.

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 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, 13, 14, 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, 13, 14, 15].

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].

Figure 3.

Unit cell of the hexagonal SmCo₅ [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.

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.

Figure 4.

Crystal structure of Nd₂Fe₁₄B magnets [18].

2.3 Sm Fe N magnets

SmFeN magnets belong to the group of R₂Fe₁₇N_x 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 R₂Fe₁₇N_x compounds, Sm₂Fe₁₇N_x 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 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 rare-earth 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 c-axis. The Sm₂Fe₁₇N_x [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%.

Figure 6.

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

3. Properties of rare earth magnets

The magnetic parameters such as high saturation magnetization (M_s), High remanence (M_r), Very high uniaxial magnetocrystalline anisotropy energy (K₁), high coercivity (H_c), High maximum energy product (BH)_max and High Curie temperature (T_C) 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.

Figure 7.

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

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 (Co_balFe_0.21Cu_0.08Zr_0.03)_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.

Figure 8.

Demagnetization curves of Sm (Co_balFe_0.21Cu_0.08Zr_0.03)_7.6 magnets [28].

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.

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 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. Sm₂Fe₁₇N_x exhibit excellent magnetic properties including high coercivity field and higher Curie temperature. The N content has a crucial influence on the magnetic properties of Sm₂Fe₁₇N_x compounds [33, 34]. Figure 9 shows the change in magnetic properties of Sm₂Fe₁₇N_x 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.

Figure 9.

Magnetic properties of Sm₂Fe₁₇N_x as a function of N content [33, 34].

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)₁₇N_x compound were lower than those of Sm₂Fe₁₇N_x. In contrast, for Co, the Curie temperature was increased by nearly 100 K compared to Sm₂Fe₁₇N_x 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 H_cJ of Sm₂(Fe_1-xM_x)N_y 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 H_cJ because Sm(Fe,Co)₂N_x and Sm(Fe,Co)₃N_x 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].

4. Manufacturing process

The manufacturing process is very crucial for production of good permanent magnet to achieve excellent magnetic properties. Even in an opposing magnetic field, a permanent magnet will exhibit magnetic characteristics. The favorable magnetic characteristics such as Curie temperature, coercivity and anisotropy are achieved from the mixtures of the constitute matrix designed for the magnets through structural and thermodynamic modifications. Few known manufacturing processes are used for the fabrication of permanent magnets.

Bonding is the common process used for manufacturing og Ne-Fe-B and Sm-Co magnets. The main processing routes such as calendaring, injection molding, extrusion, and compression bonding are used for manufacturing of most bonded magnets [36]. Other emerging magnet manufacturing processes are extrusion, additive manufacturing, spark plasma sintering, shock compaction, and thermomagnetic processing [37]. Furthermore, rare earth magnets fabricated by rapidly solidified process shows better magnetic properties than conventual processed route. The kinetics and thermodynamics of different processing routes such as melt spinning, atomization and melt extraction also determines the quality and characteristics of the magnets.

Figure 10 shows the standard process steps for manufacturing of the Sm-Co magnets. The steps include alloy preparation, powder production, particle alignment, pressing, sintering, heat treatment, machining, and finally magnetizing. SmCo magnets indicates high temperature stability, which can operate in conditions up to 500°C without suffering any magnetization loss.

Figure 10.

Manufacturing process steps for Sm-Co based magnets [37, 38].

The magnets alloy composition and microstructure are the critically important for the processing of NdFeB magnets. A well-defined sintering treatment at a suitable temperature result in high density final product magnets. The basic process step for the manufacture of NeFeB magnet is shown in Figure 11.

Figure 11.

The manufacturing process steps for the Nd-Fe-B-based magnets [37, 39].

Mostly sintering, polymer bonding and hot deformation are used as fabrication routes for manufacture of NdFeB-based bulk magnets. In recent days melt quenching nanocrystalline material are used as bonded and hot deformed components and sintering microcrystalline powder.

The SmFeN compounds are thermodynamically metastable and begin to decompose above 600°C, making traditional sintering or hot-pressing methods unsuitable for manufacturing magnets. However, bonded magnets can be produced from SmFeN alloy powders. The alloy powders can be prepared using traditional powder metallurgy methods or by using melt spinning or mechanical alloying techniques to improve powder properties. The preparation of Sm₂Fe₁₇N_x compound magnetic powder is divided into two steps: the first is to prepare the single-phase Sm₂Fe₁₇ compound, and the second is to nitride the Sm₂Fe₁₇ compound to produce Sm₂Fe₁₇N_x. The methods of preparing Sm₂Fe₁₇ compound include rapid quenching, reductive diffusion, powder metallurgy and hydrogenation disproportionation. The reduction diffusion method is used for synthesis of Sm-Fe-N powders. The synthesized powders are used in injection molded or compression bonded magnets. Further, the Low temperature milling process are successfully implemented for fabricating the high-performance Sm–Fe–N bonded magnets [40].

In general, a sintered magnet process is used for manufacturing of anisotropic high remanence regular shaped magnet of large volume. However, hot pressed or die-upset magnet are suitable for net-shape formability and powder stability.

4.1 Improvement in properties

The magnetic properties of permanent magnet alloys are always sensitive to the microstructure, and the addition of rare earth elements mainly increases the magnetic crystal anisotropy of the material. The crystal anisotropy is an intrinsic factor for the strong coercivity of rare earth-transition group alloys which enhance the properties of magnets. The Curie temperature, coercivity and magnetization of rare-earth permanent magnets are the primary properties of interest for application. Making intermetallic with suitable doping with metals and non-metals atom for Fe or inserting interstitials atoms that can change lattice site and can significantly increase in Curie temperature. Grain boundary diffusion process is an emerging technology implemented to improve the coercivity of Nd-Fe-B magnets with low rare earth consumption. Figure 12 shows the coercivity enhancement mechanism by grain boundary optimization. In this process, the grain boundary phase is modified to increase the decoupling of exchange interactions for coercivity enhancement.

Figure 12.

The schematic diagram of the coercivity enhancement mechanism by GB optimization [41].

It is also reported that the properties of rare earth magnets can be improved through microstructure modification [42]. Figure 13 shows the mechanism of microstructural modification for the enhancement of coercivity. The coercivity depends on the intrinsic magnetic properties and the microstructure of the magnets. Generally, a demagnetizing field is easily forms around the hard magnetic grains in a magnet which reduces the coercivity of the magnet.

Figure 13.

Coercivity enhancement by microstructural modification [41, 42].

Further, the grain surface and the grain boundary are the weak regions in the magnet, where the magnetization reversal starts [43]. Microstructure modification will be performed to retard the magnetic reversal and increase the coercivity. This can be done by the enhancement of the anisotropy field by compositional modification. The defect on the surface of the grain will be reduced through smoothing of grain boundary or optimizing the grain boundary phase distribution. This process will improve the coercivity of the magnet. Grain boundary diffusion process is under test and research for improving the performance rare earth permanent magnets.

Further, introduction of other rare earth elements can modify the intrinsic magnetic properties of magnets. For example, Dy in Nd–Fe–B permanent magnet provide enough coercivity at elevated temperatures by controlling the nanostructures of the materials [44]. The size of the particle alters the magnetic properties of the magnets. The domain structure and chemistry of respective phases of the heterogenous nanostructure can change the coercivity of the magnetic materials. Now a day machine-learning techniques are also used for designing magnetic materials with reduced rare earth components by integrating different physical model [45]. The advancement and improvement in properties of rare-earth will be further done with the addition of nanocrystalline materials.

5. Applications

Permanents magnets have many applications in everyday life from basics to high profile. The specific application of the magnets also depends on the magnetic performance of the materials at a given temperature and cost. Modern permanent magnets are important for academic research, defense applications along with energy technologies. The rare earth magnets have wider range of application in our daily life to modern technology and devices [4].

Bonding applications are common for SmCo magnets. For high temperature application specially in the field of Space, aeronautics and electric vehicle, Sm–Co-based alloys magnets are used [11, 46]. The most powerful Neodymium Rare Earth Magnets has versatile application which includes motors, sensors, mobile phones, computer hard drives and data storage industry [47, 48, 49, 50], washing machines, refrigerators. NdFeB magnets markets are the automobile industry and energy sector. SmFeN magnets also have potential applications in electric motors, generators, actuators, magnetic bearings, and other high-performance magnetic devices. Their high coercivity field and high saturation magnetization make them suitable for use in motors and generators that require high torque and power density.

6. Key challenges of rare earth permanent magnets

The performance and properties of magnets varies from magnets to magnet and batch to batch manufacturing. Design and development of suitable processing technique without losing the intrinsic magnetic properties is always a challenge. Further, consumption of critical rare earth elements for high coercivity low-cost magnets, increasing scarcity, price fluctuation and supply instability are the key factor. The cost ratio of rare earth permanent magnet is another challenge. Further the challenges and limitations associated with rare earth magnets are particularly regarding the availability of key rare earth elements.

7. Summary

Rare earth magnets play critical role in many advanced technologies. The rapid growth demand for rare earth magnet is placing increased stress on the availability of key rare earth elements. The improvements to manufacturing processes can form near-net-shape product with reduce materials losses and total machining costs. SmFeN magnets can be an alternative to NdFeB magnets for certain applications. SmFeN can be a new generation magnet after the optimisation of the chemical composition of the elements, the preparation of alloy powder, the coercivity mechanism and the preparation of high-performance magnets with enhanced magnetic properties. A lot of research needs to be done for development of new rare-earth magnetic material magnets for high-temperature applications. Additional experiments or analyses can be conducted to further investigate the development of new rare earth magnetic materials for high temperature applications.