**1. Introduction**

### **1.1 Magnetic materials**

Magnetic materials play a crucial role in the progress of industrial development and scientific growth. They are constantly used in power generation and transmission, electronic devices, analog and digital data storage, medical devices, magnetic therapy and drug delivery, sensors and scientific equipment, etc. Functional magnetic materials are materials with unique physical properties, which can be affected when subjected to an applied excitement such as magnetic field. They are considered as the smart materials of the future. A material can be applied in magnetic refrigerators when a change in the entropy across its magnetic ordering temperature occurs. This functionality of a magnetic material has huge possibility to be used as an alternative cooling technology and it is based on magnetocaloric effect (MCE), which is reversible temperature change in a magnetic material when a variable magnetic field is applied. This functionality additionally offers the prospect of a compact, highly efficient, and environment-friendly alternative to the most commonly used vapor-compression-based freezing system. The main challenges are the availability of high magnetocaloric materials in large quantities exhibiting large MCE at room temperature in a reasonable magnetic field as well as low hysteretic losses.

Magnetic nanoparticles have been the focus of research because of their interesting properties, which doubtless may see use in data storage and processing, spintronics, catalysis, drug delivery, magnetic resonance imaging (MRI), environmental studies, etc. These materials show uncommon magnetic behavior compared with bulk materials, principally because of their surface/interface effects, electronic charge transfer, and magnetic interactions. The local magnetic properties with the size scale of nanometers play the key role in the microstructure-magnetic properties interplay in permanent magnets as **Figure 1** illustrates. The typical phenomena related to nanoscale structures are the increased relevance of surface effects, defects, and the existence of new phases. Therefore, these phenomena can be utilized in developing new magnetic nanoparticles.

### *1.1.1 Hard magnetic materials*

Several permanent magnet materials were discovered within the past century. Techniques to effectively manufacture these magnets have been shown [2]. Device designs using such magnets in different active and inactive applications have been fruitfully exploited. The energy product of permanent magnets has been improved, commencing from ≈1 MGOe for steels, increasing to ≈3 MGOe for hexagonal ferrites, and finally peaking at ≈56 MGOe for neodymium-iron boron magnets during the previous few years. With this, almost 90% of the limit for the energy density, (BH)max, (based on the Nd2Fe14 B phase) can be attained in commercially produced sintered Nd-Fe-B grades. The historic development, spanning about 100 years, of such permanent magnets is shown in **Figure 2**.

However, the search for novel hard magnetic compounds with higher remnant magnetization has, to some extent, settled and no more breakthrough is noticeable. On the other side, only a modest number of ternary and quaternary systems have been explored as yet. The approach of nanocomposites is currently the most actively chased as well as exchange-coupled with a soft magnetic phase, which has

#### **Figure 1.**

*Magnetic characteristic lengths and illustration of typical microstructures in permanent magnets [1]. Reproducibility with permission from IOP publisher.*

**65**

**Figure 2.**

*permission from [3].*

*1.1.2 Soft magnetic materials*

*Intriguing Properties and Applications of Functional Magnetic Materials*

an intrinsic upper limit of μ0Ms = 2.43 T for an Fe65Co35 alloy, where μ0 is the perme-

*Development in the energy density (BH)max at room temperature of hard magnetic materials in the twentieth century and presentation of different types of materials with comparable energy density. Reproduced with* 

permanent magnets based on rare-earth intermetallic compounds. This is led by, for instance, the rising need for energy-efficient technologies in which these magnets often play a vital role. The need for enlarged energy densities at different operating temperatures is the main motive for the development of the rare-earth permanent magnets (RPMs). Most importantly, this comprises less Dy-containing Nd2Fe14 B-type magnets with much improved temperature stability for electromotor applications at around 450 K [4], Pr2Fe14 B-type magnets for applications at 77 K together with high-Curie temperature (Tc) superconductors, [5] and a new generation of SmCo 2:17-type magnets which are applied at temperatures above 670 K [6, 7]. It also includes magnetic-power microelectromechanical systems (MEMSs) [8–11], for example, a high-speed permanent magnetic generator that requires textured, thick RPM films [12]. Currently, importance of research is on how to control the structure of grain boundary phases to understand the relevant coercivity mechanisms and the related elementary magnetization processes. The next class of permanent magnets could be rough-surfaced nanocomposites. This would include controlling the fabrication of privately mixed multiphase and well-directed

nanoscale magnets, which cannot be done by conventional techniques.

The most characterizing properties of soft magnetic materials are the easy magnetization reversal accompanied with a small area of the hysteresis loop and a low coercivity (He). Quite similar to hard magnetic materials, essential magnetic properties and microstructure are to be optimized to obtain soft magnetic materials. However, a very low magnetocrystalline anisotropy and weak to almost zero interaction between magnetic domain walls and grain boundaries are required, which is the opposite of the favorable conditions for permanent magnets. Soft magnetic materials are very significant for the subjects of power electrical applications such as generators, distribution transformers, and a broad assortment of motors as well as in electronics

Lately, there is a much-energized interest in various types of high-performance

ability of free space and Ms is the saturation magnetization.

*DOI: http://dx.doi.org/10.5772/intechopen.81386*

*Intriguing Properties and Applications of Functional Magnetic Materials DOI: http://dx.doi.org/10.5772/intechopen.81386*

#### **Figure 2.**

*Functional Materials*

*1.1.1 Hard magnetic materials*

Magnetic nanoparticles have been the focus of research because of their interesting properties, which doubtless may see use in data storage and processing, spintronics, catalysis, drug delivery, magnetic resonance imaging (MRI), environmental studies, etc. These materials show uncommon magnetic behavior compared with bulk materials, principally because of their surface/interface effects, electronic charge transfer, and magnetic interactions. The local magnetic properties with the size scale of nanometers play the key role in the microstructure-magnetic properties interplay in permanent magnets as **Figure 1** illustrates. The typical phenomena related to nanoscale structures are the increased relevance of surface effects, defects, and the existence of new phases. Therefore, these phenomena can

Several permanent magnet materials were discovered within the past century. Techniques to effectively manufacture these magnets have been shown [2]. Device designs using such magnets in different active and inactive applications have been fruitfully exploited. The energy product of permanent magnets has been improved, commencing from ≈1 MGOe for steels, increasing to ≈3 MGOe for hexagonal ferrites, and finally peaking at ≈56 MGOe for neodymium-iron boron magnets during the previous few years. With this, almost 90% of the limit for the energy density, (BH)max, (based on the Nd2Fe14 B phase) can be attained in commercially produced sintered Nd-Fe-B grades. The historic development, spanning about 100 years, of

However, the search for novel hard magnetic compounds with higher remnant magnetization has, to some extent, settled and no more breakthrough is noticeable. On the other side, only a modest number of ternary and quaternary systems have been explored as yet. The approach of nanocomposites is currently the most actively chased as well as exchange-coupled with a soft magnetic phase, which has

*Magnetic characteristic lengths and illustration of typical microstructures in permanent magnets [1].* 

be utilized in developing new magnetic nanoparticles.

such permanent magnets is shown in **Figure 2**.

**64**

**Figure 1.**

*Reproducibility with permission from IOP publisher.*

*Development in the energy density (BH)max at room temperature of hard magnetic materials in the twentieth century and presentation of different types of materials with comparable energy density. Reproduced with permission from [3].*

an intrinsic upper limit of μ0Ms = 2.43 T for an Fe65Co35 alloy, where μ0 is the permeability of free space and Ms is the saturation magnetization.

Lately, there is a much-energized interest in various types of high-performance permanent magnets based on rare-earth intermetallic compounds. This is led by, for instance, the rising need for energy-efficient technologies in which these magnets often play a vital role. The need for enlarged energy densities at different operating temperatures is the main motive for the development of the rare-earth permanent magnets (RPMs). Most importantly, this comprises less Dy-containing Nd2Fe14 B-type magnets with much improved temperature stability for electromotor applications at around 450 K [4], Pr2Fe14 B-type magnets for applications at 77 K together with high-Curie temperature (Tc) superconductors, [5] and a new generation of SmCo 2:17-type magnets which are applied at temperatures above 670 K [6, 7]. It also includes magnetic-power microelectromechanical systems (MEMSs) [8–11], for example, a high-speed permanent magnetic generator that requires textured, thick RPM films [12]. Currently, importance of research is on how to control the structure of grain boundary phases to understand the relevant coercivity mechanisms and the related elementary magnetization processes. The next class of permanent magnets could be rough-surfaced nanocomposites. This would include controlling the fabrication of privately mixed multiphase and well-directed nanoscale magnets, which cannot be done by conventional techniques.

#### *1.1.2 Soft magnetic materials*

The most characterizing properties of soft magnetic materials are the easy magnetization reversal accompanied with a small area of the hysteresis loop and a low coercivity (He). Quite similar to hard magnetic materials, essential magnetic properties and microstructure are to be optimized to obtain soft magnetic materials. However, a very low magnetocrystalline anisotropy and weak to almost zero interaction between magnetic domain walls and grain boundaries are required, which is the opposite of the favorable conditions for permanent magnets. Soft magnetic materials are very significant for the subjects of power electrical applications such as generators, distribution transformers, and a broad assortment of motors as well as in electronics

where a mass of inductive components is required as shown in the road map of ultra-low-loss nanocrystalline alloy as shown in **Figure 3** [13]. The widely used soft magnetic materials are low-carbon steel and non-oriented silicon iron. They account for about 80% by weight, and approximately 55% by value of all soft magnetic materials, followed by grain/oriented silicon iron (17/13%), ferrite cores (1.5/7.5%), nickel- and cobalt-iron alloys (0.5/4.5%), and special materials and offices such as metal powder cores (2/8%). Soft magnetic materials are materials easily magnetized and demagnetized. They typically have intrinsic coercivity less than 1000 A m<sup>−</sup><sup>1</sup> and they are used to enhance and/or channel the flux created by an electric current. The main parameter for soft magnetic materials is the relative permeability (μr, where μr = B/μoH), which measures the material response to the applied magnetic field. The other important parameters are the coercivity, the saturation magnetization, and the electrical conductivity. The applications for soft magnetic materials are divided into two main categories: AC and DC. In DC applications, the material is magnetized in order to carry out an operation and then demagnetized at the end of the operation, for example, an electromagnet on a lift at a scrap yard will be switched on to attract the scrap steel and then switched off to drop the steel.

For DC applications, the main regard for material selection is very likely to be the permeability. Where the material is used to produce a magnetic field or to create a force, the saturation magnetization may also be important. For AC applications, the important thought is how much energy is lost in the system as the material is cycled around its hysteresis loop. The energy loss can arise from three different sources: (1) hysteresis loss, which is related to the area contained within the hysteresis loop; (2) eddy current loss, related to the generation of electric currents in the magnetic material and the interrelated resistive losses; and (3) irregular loss, related to the movement of domain walls within the material.

Soft magnetic alloys have competed a key role in power generation and conversion for the electrical grid. The necessity for efficient generation, transmission, and distribution of electric power is ever growing; but, at the same time, the annual electric losses are overtaking annual increases in electricity consumption. In the USA, electricity is regenerated to high-voltage AC current at voltages between 138 and 765 kV and transmitted to substations close to its end-use location. The voltage is then turned down to lower values (between 13 kV and 120 V) for distribution to

#### **Figure 3.**

*Development road map of ultralow-loss nanocrystalline alloy. Reproduced with permission from [13].*

**67**

*Intriguing Properties and Applications of Functional Magnetic Materials*

different consumers. These generation, transmission, and distribution systems are aging, inept, and imperfect to meet the future energy needs of the USA without important changes in operation and infrastructure. For these reasons, advanced electric storage systems, smart controls, and power electronics for AC-DC conver-

Modern society depends on readily available refrigeration for preserving food and providing comfortable living places. Ordinary refrigerators use ozone for reducing harmful chemicals such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and ammonia (NH3) in a vapor compression cycle to supply cooling. Ordinary refrigerators tend to be unwieldy, hefty, and lack energy efficiency despite they have met the cooling needs. Recently, an alternate refrigeration method using magnetocaloric effect (MCE) has been investigated as a way to deal with the defects

Magnetic refrigeration has three outstanding advantages when compared to gas compressing refrigeration. First, it involves no harmful gasses; second, it can be compactly built as its main working material is a solid; and third, magnetic refrigerators are almost noiseless. Also, the cooling efficiency while operating with gadolinium can reach 60% of the theoretical efficiency limit [14] compared to only about 45% in the best gas-compressing refrigerators. While commercial refrigerators of this kind are still in the development stages, research efforts to develop new materials with improved MCE are targeted on maximizing the cooling capability and energy efficiency of this newborn technology. In this part, the different materials are compared, focusing on transition metal-containing compounds. When a material is subjected to an applied magnetic field, its magnetic order changes, leading to subsequent change of the entropy related to the magnetic degrees of freedom (magnetic entropy, Sm). Under adiabatic conditions, ΔSm must be covered by an equal, opposite change in the entropy associated with the lattice, resulting in a change in the temperature of the material. This temperature change, ΔTad, is usually called the MCE. It is correlated to the magnetic properties of the material through

> \_\_\_ ∂*s* <sup>∂</sup>*B*)*<sup>T</sup>*

is large and c (T,B) is small at the same temperature conditions. As effects at high temperatures are concerned, the heat capacity on the order of Dulong-Petit law is c = 3 NR, where N is the number of atoms and R is the molar gas constant. Consequently, we should focus on finding a big change in magnetization at the

= ( \_\_\_ ∂*M* <sup>∂</sup>*<sup>T</sup>* )*<sup>B</sup>*

From magnetization measurements taken at different temperature periods, ΔSm can be calculated as illustrated in Refs. [15, 16]. For materials showing a first-order phase transition with large hysteresis, these magnetization measurements should be performed cautiously so as not to overestimate values of the entropy change [17]. Otherwise, the magnetic entropy change can be acquired straight from a calorimetric measurement of the field dependence of the high temperature capacity, c, and then integrating. It has been validated that the values of ΔSm (T, B) derived from the magnetization measurement concur with the values from calorimetric measurement [18]. Numerical integration of the adiabatic temperature change, [ΔTad (T, B)], can then be done using the experimentally or theoretically predicted magnetization and heat content values. Clearly, the MCE will be large when (

(1)

\_\_\_ ∂*M* ∂*T* )*<sup>B</sup>*

sion are technologies that are being supported to reform the desired way.

*DOI: http://dx.doi.org/10.5772/intechopen.81386*

**2. Magnetocaloric materials**

of vapor-compression refrigeration.

the thermodynamic Maxwell relation

(

*Intriguing Properties and Applications of Functional Magnetic Materials DOI: http://dx.doi.org/10.5772/intechopen.81386*

different consumers. These generation, transmission, and distribution systems are aging, inept, and imperfect to meet the future energy needs of the USA without important changes in operation and infrastructure. For these reasons, advanced electric storage systems, smart controls, and power electronics for AC-DC conversion are technologies that are being supported to reform the desired way.

### **2. Magnetocaloric materials**

*Functional Materials*

where a mass of inductive components is required as shown in the road map of ultra-low-loss nanocrystalline alloy as shown in **Figure 3** [13]. The widely used soft magnetic materials are low-carbon steel and non-oriented silicon iron. They account for about 80% by weight, and approximately 55% by value of all soft magnetic materials, followed by grain/oriented silicon iron (17/13%), ferrite cores (1.5/7.5%), nickel- and cobalt-iron alloys (0.5/4.5%), and special materials and offices such as metal powder cores (2/8%). Soft magnetic materials are materials easily magnetized and demagnetized. They typically have intrinsic coercivity less than 1000 A m<sup>−</sup><sup>1</sup>

they are used to enhance and/or channel the flux created by an electric current. The main parameter for soft magnetic materials is the relative permeability (μr, where μr = B/μoH), which measures the material response to the applied magnetic field. The other important parameters are the coercivity, the saturation magnetization, and the electrical conductivity. The applications for soft magnetic materials are divided into two main categories: AC and DC. In DC applications, the material is magnetized in order to carry out an operation and then demagnetized at the end of the operation, for example, an electromagnet on a lift at a scrap yard will be switched on to attract

For DC applications, the main regard for material selection is very likely to be the permeability. Where the material is used to produce a magnetic field or to create a force, the saturation magnetization may also be important. For AC applications, the important thought is how much energy is lost in the system as the material is cycled around its hysteresis loop. The energy loss can arise from three different sources: (1) hysteresis loss, which is related to the area contained within the hysteresis loop; (2) eddy current loss, related to the generation of electric currents in the magnetic material and the interrelated resistive losses; and (3) irregular loss,

Soft magnetic alloys have competed a key role in power generation and conversion for the electrical grid. The necessity for efficient generation, transmission, and distribution of electric power is ever growing; but, at the same time, the annual electric losses are overtaking annual increases in electricity consumption. In the USA, electricity is regenerated to high-voltage AC current at voltages between 138 and 765 kV and transmitted to substations close to its end-use location. The voltage is then turned down to lower values (between 13 kV and 120 V) for distribution to

*Development road map of ultralow-loss nanocrystalline alloy. Reproduced with permission from [13].*

the scrap steel and then switched off to drop the steel.

related to the movement of domain walls within the material.

and

**66**

**Figure 3.**

Modern society depends on readily available refrigeration for preserving food and providing comfortable living places. Ordinary refrigerators use ozone for reducing harmful chemicals such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and ammonia (NH3) in a vapor compression cycle to supply cooling. Ordinary refrigerators tend to be unwieldy, hefty, and lack energy efficiency despite they have met the cooling needs. Recently, an alternate refrigeration method using magnetocaloric effect (MCE) has been investigated as a way to deal with the defects of vapor-compression refrigeration.

Magnetic refrigeration has three outstanding advantages when compared to gas compressing refrigeration. First, it involves no harmful gasses; second, it can be compactly built as its main working material is a solid; and third, magnetic refrigerators are almost noiseless. Also, the cooling efficiency while operating with gadolinium can reach 60% of the theoretical efficiency limit [14] compared to only about 45% in the best gas-compressing refrigerators. While commercial refrigerators of this kind are still in the development stages, research efforts to develop new materials with improved MCE are targeted on maximizing the cooling capability and energy efficiency of this newborn technology. In this part, the different materials are compared, focusing on transition metal-containing compounds. When a material is subjected to an applied magnetic field, its magnetic order changes, leading to subsequent change of the entropy related to the magnetic degrees of freedom (magnetic entropy, Sm). Under adiabatic conditions, ΔSm must be covered by an equal, opposite change in the entropy associated with the lattice, resulting in a change in the temperature of the material. This temperature change, ΔTad, is usually called the MCE. It is correlated to the magnetic properties of the material through the thermodynamic Maxwell relation

$$\left(\frac{\partial \mathbf{s}}{\partial \mathbf{B}}\right)\_T = \left(\frac{\partial \mathbf{M}}{\partial T}\right)\_B \tag{1}$$

From magnetization measurements taken at different temperature periods, ΔSm can be calculated as illustrated in Refs. [15, 16]. For materials showing a first-order phase transition with large hysteresis, these magnetization measurements should be performed cautiously so as not to overestimate values of the entropy change [17]. Otherwise, the magnetic entropy change can be acquired straight from a calorimetric measurement of the field dependence of the high temperature capacity, c, and then integrating. It has been validated that the values of ΔSm (T, B) derived from the magnetization measurement concur with the values from calorimetric measurement [18]. Numerical integration of the adiabatic temperature change, [ΔTad (T, B)], can then be done using the experimentally or theoretically predicted magnetization and heat content values. Clearly, the MCE will be large when ( \_\_\_ ∂*M* ∂*T* )*<sup>B</sup>* is large and c (T,B) is small at the same temperature conditions. As effects at high temperatures are concerned, the heat capacity on the order of Dulong-Petit law is c = 3 NR, where N is the number of atoms and R is the molar gas constant. Consequently, we should focus on finding a big change in magnetization at the

#### **Figure 4.**

*Schematic representation of a magnetic refrigeration cycle that transports heat from the heat load to the ambient environment. Yellow and green boxes depict materials in low and high magnetic fields, respectively. Reproduced with permission [20]. Copyright 2005, Institute of Physics.*

appropriate temperature. A large MCE is anticipated not far from ( \_\_\_ ∂*M* <sup>∂</sup>*<sup>T</sup>* )*<sup>B</sup>* peaks at the magnetic-ordering temperature since the order parameter of the phase transition changes intensely within a narrow temperature interval. In the magnetic-refrigeration cycle, shown in **Figure 4** [19, 20], initial random-oriented magnetic moments are ordered by a magnetic field, resulting in heating of the magnetocaloric material and the heat is then transmitted from the material to the surrounding atmosphere. Upon removing the field, the magnetic moments disorder resulting in cooling of the material below ambient temperature. Heat from the system can then be withdrawn by a heat-transfer medium which may be water, air, or helium depending on the working temperature. Consequently, magnetic refrigeration is considered an ecofriendly cooling technology.

#### **3. Magnetic nanoparticles**

Over time, nanotechnology has penetrated all branches of science like physics, chemistry, and especially biomedical research and related industries. Broadly, nanoparticles are defined as materials having particle sizes in the range of 1–100 nm [21]. Bulk materials have definite physical properties, which, however, get altered when they are converted to nanoparticles, depending on their final size. One of the main changes in the properties of nanoparticles is the substantial increase in number of atoms/molecules on the surface of particles, and hence availability of effectively high surface area compared with bulk material. The high surface area of particles can be used to attach ligands and/or capping agents, which make them more suitable for effective labeling of drug/tracer molecules. The change in physicochemical properties during conversion of bulk material to nanoparticles makes them suitable for reaching the diseased site because of their better diffusion ability. A diversity of nanoparticles, including magnetic nanoparticles (MNs), has been synthesized and characterized for different industrial, biomedical, and clinical applications.

MNs are the nanoparticles synthesized from magnetic elements like iron, nickel, and cobalt or their chemical derivatives [21–26]. Each particle of bulk magnetic materials has many domains separated by walls, and each domain represents a region with a specific direction of magnetization. When bulk material is converted to MN, each particle can approach a single domain [22–24]. In larger particles (micrometer

**69**

**Figure 5.**

*Physics.*

*Intriguing Properties and Applications of Functional Magnetic Materials*

size), surrounding thermal energy [kT, where k is the Boltzmann constant and T is the temperature (K)] is much less [when T = 300 K (room temperature), kT = 0.026 eV] than particle energy (Kv, where K is the anisotropic constant and v is the particle volume) and thus the direction of magnetic moment does not change with time. When particle size decreases (sub-micro-meter size), particle energy decreases and thus direction of magnetic moment also changes with respect to original direction, that is, with angle (θ). However, with further decrease of particle size (nanosize), the direction of magnetic moment changes to the opposite direction (θ = 180), which is known as superparamagnetic behavior of magnetic nanoparticles. Super paramagnetism is due to particle size, whereas paramagnetism is an intrinsic property of the material caused by its atomic nature (e.g., Na). Superparamagnetic

is prefixed to "paramagnetic" because particles show paramagnetic behavior in the absence of a magnetic field and no magnetization is retained after removal of the magnetic field. Decreasing particle size below the critical size, ferromagnetic particles can be changed to superparamagnetic particles. Paramagnetic materials (e.g., Na and K) [22] do not have magnetic interactions between the atoms; hence, the net magnetic moment is equivalent to the number of atoms in the particle. However, the interatomic magnetic interaction in ferromagnetic or superparamagnetic materials gives the net magnetic moment of the particle. On either decreasing temperature or increasing magnetic field, there is a possibility of transition from superparamagnetic to ferromagnetic (**Figure 5**) [29, 30] because of increasing extent of the arrangement

*(A) Paramagnetic particles under a magnetic field. No variation of magnetization is shown and (B) superparamagnetic particles under a magnetic field or at low temperature [30]. Copyright 2003, Institute of* 

–104 μB [27, 28] and thus the term "super"

*DOI: http://dx.doi.org/10.5772/intechopen.81386*

particles have high magnetic moment of 103

of spins of MN.

#### *Intriguing Properties and Applications of Functional Magnetic Materials DOI: http://dx.doi.org/10.5772/intechopen.81386*

*Functional Materials*

**Figure 4.**

appropriate temperature. A large MCE is anticipated not far from (

*Reproduced with permission [20]. Copyright 2005, Institute of Physics.*

ecofriendly cooling technology.

**3. Magnetic nanoparticles**

magnetic-ordering temperature since the order parameter of the phase transition changes intensely within a narrow temperature interval. In the magnetic-refrigeration cycle, shown in **Figure 4** [19, 20], initial random-oriented magnetic moments are ordered by a magnetic field, resulting in heating of the magnetocaloric material and the heat is then transmitted from the material to the surrounding atmosphere. Upon removing the field, the magnetic moments disorder resulting in cooling of the material below ambient temperature. Heat from the system can then be withdrawn by a heat-transfer medium which may be water, air, or helium depending on the working temperature. Consequently, magnetic refrigeration is considered an

*Schematic representation of a magnetic refrigeration cycle that transports heat from the heat load to the ambient environment. Yellow and green boxes depict materials in low and high magnetic fields, respectively.* 

Over time, nanotechnology has penetrated all branches of science like physics, chemistry, and especially biomedical research and related industries. Broadly, nanoparticles are defined as materials having particle sizes in the range of 1–100 nm [21]. Bulk materials have definite physical properties, which, however, get altered when they are converted to nanoparticles, depending on their final size. One of the main changes in the properties of nanoparticles is the substantial increase in number of atoms/molecules on the surface of particles, and hence availability of effectively high surface area compared with bulk material. The high surface area of particles can be used to attach ligands and/or capping agents, which make them more suitable for effective labeling of drug/tracer molecules. The change in physicochemical properties during conversion of bulk material to nanoparticles makes them suitable for reaching the diseased site because of their better diffusion ability. A diversity of nanoparticles, including magnetic nanoparticles (MNs), has been synthesized and characterized for different industrial, biomedical, and clinical

MNs are the nanoparticles synthesized from magnetic elements like iron, nickel,

and cobalt or their chemical derivatives [21–26]. Each particle of bulk magnetic materials has many domains separated by walls, and each domain represents a region with a specific direction of magnetization. When bulk material is converted to MN, each particle can approach a single domain [22–24]. In larger particles (micrometer

\_\_\_ ∂*M* <sup>∂</sup>*<sup>T</sup>* )*<sup>B</sup>*

peaks at the

**68**

applications.

size), surrounding thermal energy [kT, where k is the Boltzmann constant and T is the temperature (K)] is much less [when T = 300 K (room temperature), kT = 0.026 eV] than particle energy (Kv, where K is the anisotropic constant and v is the particle volume) and thus the direction of magnetic moment does not change with time. When particle size decreases (sub-micro-meter size), particle energy decreases and thus direction of magnetic moment also changes with respect to original direction, that is, with angle (θ). However, with further decrease of particle size (nanosize), the direction of magnetic moment changes to the opposite direction (θ = 180), which is known as superparamagnetic behavior of magnetic nanoparticles. Super paramagnetism is due to particle size, whereas paramagnetism is an intrinsic property of the material caused by its atomic nature (e.g., Na). Superparamagnetic particles have high magnetic moment of 103 –104 μB [27, 28] and thus the term "super" is prefixed to "paramagnetic" because particles show paramagnetic behavior in the absence of a magnetic field and no magnetization is retained after removal of the magnetic field. Decreasing particle size below the critical size, ferromagnetic particles can be changed to superparamagnetic particles. Paramagnetic materials (e.g., Na and K) [22] do not have magnetic interactions between the atoms; hence, the net magnetic moment is equivalent to the number of atoms in the particle. However, the interatomic magnetic interaction in ferromagnetic or superparamagnetic materials gives the net magnetic moment of the particle. On either decreasing temperature or increasing magnetic field, there is a possibility of transition from superparamagnetic to ferromagnetic (**Figure 5**) [29, 30] because of increasing extent of the arrangement of spins of MN.

#### **Figure 5.**

*(A) Paramagnetic particles under a magnetic field. No variation of magnetization is shown and (B) superparamagnetic particles under a magnetic field or at low temperature [30]. Copyright 2003, Institute of Physics.*

Owing to their unique feature of attraction and interaction under magnetic field conditions, these MNs have been applied for separation of cells/biological materials and drug delivery. MNs have attracted the researchers' attention because of their ability to act as contrast agents in magnetic resonance imaging (MRI) for diagnostic applications. It may be apposite to observe here that lower toxicity, biocompatibility, and significant accumulations of MNs at the diseased site make them suited for remedial applications. When these MNs are placed under magnetic field effects, a phase interval between the applied magnetic field and the direction of magnetic moments results in thermal losses. The orientation of magnetic moment fluctuates thermally, involving two main mechanisms: (i) Neel's fluctuations of the magnetic moment relative to the crystal lattice (internal dynamics) and (ii) Brownian fluctuations of the particle itself relative to the medium in which the particle is placed (external dynamics). These are affected by viscosity of the medium and other processes, which can affect the movement of particle. These external and internal frictions generated on MN under external magnetic field conditions result in "foci" of heat generation, which may be sufficient enough to kill the cell. Thus, selective heat generation by MN at the tumor site can provide the significant advantage of killing tumor cells without affecting the normal tissues much.
