**4.2 Magnetic materials in data storage**

Magnetic materials are used in high-capacity disk drives and magnetic-semiconductor memory devices. The disk drive devices have reached the largest growth in data capacity over time, making disk drives the preeminent storage system for digital data [42]. The growth in areal density is more than 100% per year recently. The overall data capacity of a disk is nearly the areal density times the recording area depending on the disk size (the most common diameter is 2.5 and 3.5 inches, that is, 64 and 90 mm, respectively). Many technologies have aided in this speedy increase in areal density, together with enhancement of the technology of "flying" heads with shrunk space of the disk surface, data coding, error discovery and rectification, advanced servo-control systems for correct management of magnetic recording heads on data tracks, and advances in the mechanical structures comprising a disk drive, together with advances in the motors used to push the disks. Recently, there has been a significant emerging technology for fast memory devices—the magnetic random-access memory or MRAM. The MRAM device is a possible substitute for the familiar semiconductor memories used in modern computers—dynamic and static random-access memory (DRAM and SRAM). The MRAM technology combines a magnetic storage technology together with metal-oxide semiconductor (MOS) devices to result in fast and high-density data memory devices. The technology on which the magnetic part of MRAM is based is an extension of the technology used in magnetic-recording devices identified as the magnetic tunneling junction or MTJ.

**73**

*Intriguing Properties and Applications of Functional Magnetic Materials*

The technology of magnetic recording is over one century old [43]. The fundamental concept of magnetic recording is to use a magnetic structure (as the "write" head) driven by a current that represents the data to be recorded, to create a magnetic field capable of changing the state of the magnetization in a closely spaced magnetic medium, which was formerly a magnetic wire, and today it is the known tape or a magnetic layered hard drive. The data are retrieved by an output electromotive force generated in the "red head" by sensing the magnetization in the recording medium, for example, by Faraday's law. The magnetic recording system is that used to store digital data, in which instance the current supplied to the write head as pulses coded to represent the digital information (1 or 0 s) [44–47]. In the case of disk drives, the write and read heads are distinct thin-film structures deposited on the back of a mechanical slider that uses a hydrodynamic air bearing to "fly" over the surface of the disk [46]. The read and write parts are viewed together with the magnetic recording surface, which is a thin cobalt metal alloy film. The digital data are written in the magnetic film in the form of transitions among the two magnetization states (the "left" or "right") and with the width almost equal to the write head width. The transition region between the oppositely directed directions of the magnetization is similar to that between magnetic domains and has a length (*l*). The write head is formed from thin films of ferromagnetic alloys patterned in the form of a magnetic chain. The current is coupled to the chain to generate a magnetic field at the gap by a pancake coil of 10 or less turns. The coil is insulated from the metallic magnetic bondage by layers of polymer photoresist. The most frequently used alloy in the past for the magnetic films in the write head is Ni80Fe20 permalloy which can be deposited in thin films using electroplating. The ability to record on recording media with increased coercivity is not the only issue with the magnetic materials used in write heads. It is also important that the write head have high efficiency.

where Hg is the value of the magnetic field in the gap of the write head and I is the amplitude of the write current pulse. High efficiency is important to allow write-current amplitudes that are easily supplied from integrated circuits.

Functional magnetic materials are a huge source of technological applications because they can simultaneously display intriguing properties such as tunable mechanical, magnetic, electric/dielectric, thermal, and optical properties. These materials have the potential to be used in information storage and processing, refrigeration, hyperthermia, and recording technology. Though most attention is paid to the pure magnetocaloric properties and materials costs, other properties like mechanical properties, heat conductivity, electrical resistivity, and environmental impact are recently getting attention. With the refrigeration market being a multibillion dollar market, this novel technology offers great opportunities. The ideal magnetic refrigerant should contain at least 80% transition metals having large magnetic moment such as Fe or Mn. In addition, it should contain some inexpensive p-metal such as Al or Si, which can be used to tune the working point of the material. A wide range of magnetic materials is essential for the advance of magnetic

<sup>N</sup><sup>ω</sup> <sup>⋅</sup> <sup>I</sup> (2)

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

**5. Technology of magnetic recording**

Efficiency (η) in this case is defined as the ratio

**6. Summary and perspectives**

<sup>η</sup> <sup>=</sup> Hg <sup>⋅</sup> <sup>g</sup> \_\_\_\_\_

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

### **5. Technology of magnetic recording**

*Functional Materials*

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.

Magnetic materials are used in high-capacity disk drives and magnetic-semiconductor memory devices. The disk drive devices have reached the largest growth in data capacity over time, making disk drives the preeminent storage system for digital data [42]. The growth in areal density is more than 100% per year recently. The overall data capacity of a disk is nearly the areal density times the recording area depending on the disk size (the most common diameter is 2.5 and 3.5 inches, that is, 64 and 90 mm, respectively). Many technologies have aided in this speedy increase in areal density, together with enhancement of the technology of "flying" heads with shrunk space of the disk surface, data coding, error discovery and rectification, advanced servo-control systems for correct management of magnetic recording heads on data tracks, and advances in the mechanical structures comprising a disk drive, together with advances in the motors used to push the disks. Recently, there has been a significant emerging technology for fast memory devices—the magnetic random-access memory or MRAM. The MRAM device is a possible substitute for the familiar semiconductor memories used in modern computers—dynamic and static random-access memory (DRAM and SRAM). The MRAM technology combines a magnetic storage technology together with metal-oxide semiconductor (MOS) devices to result in fast and high-density data memory devices. The technology on which the magnetic part of MRAM is based is an extension of the technology used in magnetic-recording devices identified as the

**4.2 Magnetic materials in data storage**

magnetic tunneling junction or MTJ.

**72**

The technology of magnetic recording is over one century old [43]. The fundamental concept of magnetic recording is to use a magnetic structure (as the "write" head) driven by a current that represents the data to be recorded, to create a magnetic field capable of changing the state of the magnetization in a closely spaced magnetic medium, which was formerly a magnetic wire, and today it is the known tape or a magnetic layered hard drive. The data are retrieved by an output electromotive force generated in the "red head" by sensing the magnetization in the recording medium, for example, by Faraday's law. The magnetic recording system is that used to store digital data, in which instance the current supplied to the write head as pulses coded to represent the digital information (1 or 0 s) [44–47]. In the case of disk drives, the write and read heads are distinct thin-film structures deposited on the back of a mechanical slider that uses a hydrodynamic air bearing to "fly" over the surface of the disk [46]. The read and write parts are viewed together with the magnetic recording surface, which is a thin cobalt metal alloy film. The digital data are written in the magnetic film in the form of transitions among the two magnetization states (the "left" or "right") and with the width almost equal to the write head width. The transition region between the oppositely directed directions of the magnetization is similar to that between magnetic domains and has a length (*l*). The write head is formed from thin films of ferromagnetic alloys patterned in the form of a magnetic chain. The current is coupled to the chain to generate a magnetic field at the gap by a pancake coil of 10 or less turns. The coil is insulated from the metallic magnetic bondage by layers of polymer photoresist. The most frequently used alloy in the past for the magnetic films in the write head is Ni80Fe20 permalloy which can be deposited in thin films using electroplating. The ability to record on recording media with increased coercivity is not the only issue with the magnetic materials used in write heads. It is also important that the write head have high efficiency. Efficiency (η) in this case is defined as the ratio

$$
\boldsymbol{\eta} \quad = \frac{\mathbf{H}\_{\boldsymbol{\theta}} \cdot \mathbf{g}}{\mathbf{N}\_{\boldsymbol{\alpha}} \cdot \mathbf{I}} \tag{2}
$$

where Hg is the value of the magnetic field in the gap of the write head and I is the amplitude of the write current pulse. High efficiency is important to allow write-current amplitudes that are easily supplied from integrated circuits.

#### **6. Summary and perspectives**

Functional magnetic materials are a huge source of technological applications because they can simultaneously display intriguing properties such as tunable mechanical, magnetic, electric/dielectric, thermal, and optical properties. These materials have the potential to be used in information storage and processing, refrigeration, hyperthermia, and recording technology. Though most attention is paid to the pure magnetocaloric properties and materials costs, other properties like mechanical properties, heat conductivity, electrical resistivity, and environmental impact are recently getting attention. With the refrigeration market being a multibillion dollar market, this novel technology offers great opportunities. The ideal magnetic refrigerant should contain at least 80% transition metals having large magnetic moment such as Fe or Mn. In addition, it should contain some inexpensive p-metal such as Al or Si, which can be used to tune the working point of the material. A wide range of magnetic materials is essential for the advance of magnetic

recording and the fast random access memory, MRAM, technology. Magnetic data storage has seen a linear rise in terms of storage capacity. The physics of magnetic nanostructures is at the core of magnetic hard disk drive technology; and in the future, it is very likely that areal densities will increase well beyond 1 Terabit/inch<sup>2</sup> by employing new technologies. In hyperthermia application, the target is the higher value of magnetic heat generation by a stable fluid in a lower exposure time. Nanoferrites are good candidates for hyperthermia applications since they offer a moderate magnetic moment, chemical stability, and a high specific absorption rate (SAR). Based on which heat generation mechanism is wanted, a suitable selection of magnetic core, surfactant layer, and liquid type can influence the cancer treatment.
