**4. Applications of functional magnetic nanoparticles**

The unique chance to control coercivity in magnetic nanomaterials has led to a number of significant technological applications, particularly in the field of information storage. Small magnetic particles are promising candidates for a further increase of the density of magnetic storage devices toward 100 Gbit/inch2 up to a few Tbit/inch2 [31]. Other than data storage, many applications of magnetic nanoparticles are known; examples are: ferrofluids, high-frequency electronics, high-performance permanent magnets, and magnetic refrigeration. Magnetic particles are also employed in many biological and medical applications such as drug-targeting, cancer therapy, lymph node imaging, or hyperthermia [32–34]. Lately, researchers have succeeded to produce multifunctional MN. There are mainly two approaches: (i) molecular functionalization, which comprises attaching the magnetic nanoparticles to antibodies, proteins, and dyes, and so on and (ii) blending of MNs with other functional nanoparticles, such as quantum dots or metallic nanoparticles [35]. As an example, magnetic nanoparticles could be used as seeds for growing semiconducting chalcogenides. In this case, the final product is core-shell or hetero nanostructures having both magnetic and fluorescent properties. This results in the display of intracellular control of nanoparticles for promising dual-functional molecular imaging (i.e., combined MRI and fluorescence imaging). MNs can be used as MRI contrast improvement agents, as the signal resulting from proton magnetic moments around magnetic nanoparticles can be recorded by resonant absorption [24]. These multifunctional MNs could be used in many biological applications such as protein purification, bacteria detection, and therapeutic removal of toxins [32]. **Figure 6** illustrates these two approaches for making multifunctional MNs and their various biological applications.

In the last three decades, magnetic data storage has seen a linear rise in terms of storage capacity. The physics of magnetic nanostructures is at the heart of magnetic hard disk drive technology. In the future, it is very probable that areal densities will increase well beyond 1 Terabit/inch2 by employing new technologies like bitpatterned media (BPM) or heat-assisted magnetic recording [31, 36].

**71**

*Intriguing Properties and Applications of Functional Magnetic Materials*

Patterned magnetic nanostructures, such as two-dimensional dot-arrays have attracted the interest of researchers due to their potential applications such as magnetic information storage [37] or nonvolatile magnetic random access memory (MRAM) [38]. The demand for ultrahigh-density magnetic storage devices drives the bit size into the nanometer scale. As the volume = 2/4 (where and are the diameter and thickness, respectively) of the grains is reduced in the scaling process, the magnetization of the grains may become unstable due to thermal fluctuations, and data loss may occur [33]. As the physical size of the nanostructures in the patterned array decreases, loss of data due to the thermal instability [also known as "superparamagnetic (SPM) effect"] would become a very crucial issue [39]. Therefore, future data storage technology has to overcome the SPM effect. In this regard, the L10-FePt alloy is one of the most promising materials for future ultrahighdensity magnetic storage devices because it possesses a huge uniaxial magneto-

*Various potential applications of multifunctional magnetic nanoparticles in biology. Reproduced with* 

*permission from [24]. Copyright 2009, American Chemical Society.*

magnetization. Also, the present longitudinal data storage media may be considered as a collection of independent particles because of their weak intergranular exchange coupling. However, as we have discussed in the super-ferromagnetic section, strong intergranular interactions can drive the system to form long-range ordered superferromagnetic (SFM) domains, which are clearly unsuitable for applications in data storage. Also, the SFM alignment counteracts large tunneling magnetoresistance (TMR) values, so magnetic random access memory applications are not promising for SFM systems. However, super-ferromagnetic materials are soft magnetics, which make them nearly ideal materials for high permeability, low-loss materials for microelectronics, power management, and sensing devices designed for high frequencies.

Recently, thermotherapy for cancer using MN has emerged as a potential mode of hyperthermia [23–26]. Hyperthermia is a type of medical treatment in which body tissue is exposed to a temperature (42–44°C) higher than physiological temperature (37°C) to kill the cancer cells. This approach is one of the modalities of cancer treatment used in combination with radiation and certain chemotherapeutic drugs. There could be two ways to heat the cancer cells: (i) applying external sources (e.g., using a water bath, microwave, ultrasound, infrared sauna), which is also

erg/cc), which leads to a high thermal stability of

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

crystalline anisotropy ( = 7 × 107

**Figure 6.**

**4.1 Magnetic materials in hyperthermia**

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

**Figure 6.**

*Functional Materials*

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.

**4. Applications of functional magnetic nanoparticles**

The unique chance to control coercivity in magnetic nanomaterials has led to a number of significant technological applications, particularly in the field of information storage. Small magnetic particles are promising candidates for a further increase of the density of magnetic storage devices toward 100 Gbit/inch2

are known; examples are: ferrofluids, high-frequency electronics, high-performance

employed in many biological and medical applications such as drug-targeting, cancer therapy, lymph node imaging, or hyperthermia [32–34]. Lately, researchers have succeeded to produce multifunctional MN. There are mainly two approaches: (i) molecular functionalization, which comprises attaching the magnetic nanoparticles to antibodies, proteins, and dyes, and so on and (ii) blending of MNs with other functional nanoparticles, such as quantum dots or metallic nanoparticles [35]. As an example, magnetic nanoparticles could be used as seeds for growing semiconducting chalcogenides. In this case, the final product is core-shell or hetero nanostructures having both magnetic and fluorescent properties. This results in the display of intracellular control of nanoparticles for promising dual-functional molecular imaging (i.e., combined MRI and fluorescence imaging). MNs can be used as MRI contrast improvement agents, as the signal resulting from proton magnetic moments around magnetic nanoparticles can be recorded by resonant absorption [24]. These multifunctional MNs could be used in many biological applications such as protein purification, bacteria detection, and therapeutic removal of toxins [32]. **Figure 6** illustrates these two approaches for making multifunctional MNs and their various

In the last three decades, magnetic data storage has seen a linear rise in terms of storage capacity. The physics of magnetic nanostructures is at the heart of magnetic hard disk drive technology. In the future, it is very probable that areal densities

patterned media (BPM) or heat-assisted magnetic recording [31, 36].

by employing new technologies like bit-

permanent magnets, and magnetic refrigeration. Magnetic particles are also

[31]. Other than data storage, many applications of magnetic nanoparticles

up to a few

**70**

biological applications.

will increase well beyond 1 Terabit/inch2

Tbit/inch2

*Various potential applications of multifunctional magnetic nanoparticles in biology. Reproduced with permission from [24]. Copyright 2009, American Chemical Society.*

Patterned magnetic nanostructures, such as two-dimensional dot-arrays have attracted the interest of researchers due to their potential applications such as magnetic information storage [37] or nonvolatile magnetic random access memory (MRAM) [38]. The demand for ultrahigh-density magnetic storage devices drives the bit size into the nanometer scale. As the volume = 2/4 (where and are the diameter and thickness, respectively) of the grains is reduced in the scaling process, the magnetization of the grains may become unstable due to thermal fluctuations, and data loss may occur [33]. As the physical size of the nanostructures in the patterned array decreases, loss of data due to the thermal instability [also known as "superparamagnetic (SPM) effect"] would become a very crucial issue [39]. Therefore, future data storage technology has to overcome the SPM effect. In this regard, the L10-FePt alloy is one of the most promising materials for future ultrahighdensity magnetic storage devices because it possesses a huge uniaxial magnetocrystalline anisotropy ( = 7 × 107 erg/cc), which leads to a high thermal stability of magnetization. Also, the present longitudinal data storage media may be considered as a collection of independent particles because of their weak intergranular exchange coupling. However, as we have discussed in the super-ferromagnetic section, strong intergranular interactions can drive the system to form long-range ordered superferromagnetic (SFM) domains, which are clearly unsuitable for applications in data storage. Also, the SFM alignment counteracts large tunneling magnetoresistance (TMR) values, so magnetic random access memory applications are not promising for SFM systems. However, super-ferromagnetic materials are soft magnetics, which make them nearly ideal materials for high permeability, low-loss materials for microelectronics, power management, and sensing devices designed for high frequencies.

#### **4.1 Magnetic materials in hyperthermia**

Recently, thermotherapy for cancer using MN has emerged as a potential mode of hyperthermia [23–26]. Hyperthermia is a type of medical treatment in which body tissue is exposed to a temperature (42–44°C) higher than physiological temperature (37°C) to kill the cancer cells. This approach is one of the modalities of cancer treatment used in combination with radiation and certain chemotherapeutic drugs. There could be two ways to heat the cancer cells: (i) applying external sources (e.g., using a water bath, microwave, ultrasound, infrared sauna), which is also

called "external or extracellular hyperthermia," and (ii) delivering MN inside the cancer cells [under alternating current (AC) field], which is known as intracellular hyperthermia. Because cell membrane composed of lipids is thermally insulating, tumor cells heated from external sources do not achieve hyperthermic temperature. Consequently, extra heat from an external source has to be provided to achieve the therapeutic temperature. However, this causes blisters, burns, swelling, blood clots, and bleeding in clinical conditions. Therefore, application of hyperthermia using this approach has faced practical limitations. On the other hand, intracellular heating using internalized MN at the tumor site provides an efficient and safe approach for hyperthermia application. The therapeutic efficacy and clinical advantages of intracellular hyperthermia over extracellular hyperthermia is a matter of further investigation. In addition, development of surface-functionalized nanoparticles using advanced technologies may present a better therapeutic modality for future clinical applications. Could all MNs be used in hyperthermia? Common MNs are Fe3O4; γ-Fe2O3; and Mn-, Co-, and Ni-doped ferrites because they have high magnetic moment (50–60 emu/g) under external magnetic field, which can give hysteresis loss and result in significant rise in temperature sufficient for hyperthermia therapy. However, some materials (e.g., ZnO and TiO2) become ferromagnetic when particle size decreases to the nanometer range (510 nm) [40, 41]. Owing to their very low magnetic moment (1 emu/g or less), such types of material may not be useful for hyperthermia treatment. It may be important to mention that Fe and Co nanoparticles are prone to oxidation in acidic and alkaline conditions, which are likely to be different in tissue compartments in body. In contrast, oxide nanoparticles (e.g., Fe3O4) are highly stable in slightly acidic and alkaline conditions and are biocompatible. Very small Fe3O4 (cubic phase) nanoparticles (5 nm) are not useful for hyperthermic applications because of low magnetic moment [29, 30]. However, FePd, FePt, CoPt, and CoPd (tetragonal phase) nanoparticles would result in significant heat generation, even with a particle size of 35 nm [27], but their stabilities in acidic and alkaline mediums are less than their oxide counterparts.
