Vistas in Current Magnetic Materials Research

K. G. Suresh
Department of Physics
IIT Bombay
Mumbai, India
Email: suresh@phy.iitb.ac.in

Introduction

Magnetism and magnetic materials have been of scientific interest for over 1,000 years. More recently, fundamental investigations have focused on exploring the various types of magnetic materials and understanding magnetic effects created by electric currents. The practical exploitation of the latter has resulted in a large number of devices which are indispensable today. The development of the field of magnetism in materials started as a curiosity to understand this new and exciting phenomenon, but soon it materialized as an enormous reservoir for many practical or applied materials. In this context, the "miracle" material was magnetite (Fe3O4) (Aldrich Prod. No. 310069), which is also known as ferrites. Ferrites represent a large class of magnetic materials with high potential in a diverse range of applications. In a way, the history of magnetic materials research is synonymous with the development of ferrites, both for fundamental studies as well as for their potential commercial applications. As with other magnetic materials, ferrites may be classified as both soft and hard materials and find extensive applications ranging from microwave devices to permanent magnets.1

Along with ferrites, ceramic and metallic materials in bulk, thin film, and nano forms constitute the backbone of magnetic materials research in industry today. Amorphous magnetic materials also offer great interest and potential by virtue of their peculiar magnetic properties. In addition, magnetism of molecular materials is very attractive because of the exciting properties of many such molecules. The projected applications of magnetic materials range from biomedical fields to quantum computing.

From the point of view of condensed matter physics or materials science, apart from ferrites, another major class of materials is intermetallic compounds containing rare-earth and/or transition metals.2 The magnetic behavior of these materials is due to transition metal (T) ions, rare-earth (R) ions, or a combination of the two. These alloys, in general, have the advantage that they are mostly ferromagnetic with large saturation magnetization, Curie temperature, and magnetic anisotropy. In fact, these alloys constitute the bulk of permanent magnets used in devices today. Moreover, these R-T compounds have tunable magnetic properties, which are advantageous for many applications, as discussed in later sections of this article. Many interesting properties also arise when rare earths or transition metals are alloyed with nonmagnetic elements such as Si, Ge, Sb, etc. Some of the best-inclass materials discovered in the immediate past belong to this class. However, due to the exorbitant cost of rare earths, systems based on transition metals are becoming increasingly popular.

Many practical applications of magnetic materials arise from the coupling of magnetism and other physical properties. These include magneto-transport, magneto-thermal, magneto-elastic, magneto-optic, magneto-electrical, and magneto-structural couplings. These properties give rise to phenomena such as magnetoresistance, the magnetocaloric effect, magnetostriction, shape-memory effects, and magneto-impedence, to name a few. Some of these properties and the way they are commercially exploited are presented in this article.

Magnetic Recording and Spintronics

Historically, apart from permanent magnets, the major application of magnetic materials has been in magnetic recording, a field that has been revolutionized every decade with new breakthroughs in magnetic materials research.3,4 Magnetic recording is one of the most widespread applications of magnetic materials today. It was reported that the annual data storage capacity in the world reached 5×1010 GB in 2002, or about 800 MB per year per person. Among the various forms of data storage, hard disc drives (HDD) are the largest contributor. Recording media can be classified as longitudinal or perpendicular, depending on the orientation of the magnetic domains. Perpendicular media can provide much larger recording densities than longitudinal media. In the 1980s, Co-Cr based alloys were regarded as the only feasible alloy system for media of HDDs. However, in the 1990s, it was extended to include Co-Cr- Pt and Co-Cr-Ta alloys. By the 2000s, Co-Cr-Z (Z=Pt, Ta, B, etc.) for longitudinal recording media and Co-Cr-Pt-SiO2 metal oxide granular films for perpendicular recording media were also included. The twophase microstructure, as well as the large magnetocrystalline anisotropy of the Co-Cr system, offers many advantages in the HDD design. Other potential systems pursued for high density recording are Co-Mo(W), FePt, CoPt, and rare-earth based compounds such as SmCo5 (Aldrich Prod. No. 339229) and NdFeB (Aldrich Prod. No. 693790). The latter two compounds have previously attracted little notice, possibly because of their much larger magnetic anisotropy as well as their poor ability to crystallize during sputtering processes. In order to achieve recording densities greater than 1 Tb/in2, techniques such as percolation perpendicular media and bit patterned media were actively investigated.5-8

For magnetic recording applications, materials in particulate, thin film, or multi-layer form are usually used. The ground-breaking discovery of spintronics has added a new dimension to the field of magnetic recording.9-12 Stored magnetic information from the disk is read with the help of giant magnetoresistance (GMR). The discovery of GMR in a magnetic multilayer system (Fe/Cr/Fe) in 1988 was a breakthrough in the history of modern magnetic materials research, both from fundamental physics as well as application perspective, and was awarded the Nobel prize in 2007. In conventional electronics, one controls the charge of the electron and its manipulations with the help of an electric field. But in spintronics there is an additional degree of freedom, namely the spin of the electron, which can be controlled by a magnetic field. Half-metallic ferromagnets and spin-polarized transport are key concepts associated with the physics of spintronics. The field of spintronics continues to grow and is supplemented with the new developments in magnetic thin films and nanomaterials research. The first commercial application of spintronics in sensors for the automotive industry appeared in 1993. This was followed by the commercialization of GMR heads (Figure 1) in magnetic memory in 1997, which led rapidly to an increase of the density of information stored on disks (from 1 Gb/in2 to 600 Gb/in2) by the end of 2007. Today, spintronics encompasses many promising new phenomena such as spin transfer, magnetic semiconductors, molecular spintronics, organic spintronics, and single-electron spintronics. A key phenomenon associated with spintronics is the tunnel magnetoresistance (TMR) exhibited by magnetic tunnel junctions (MTJs). MTJs are the basis of a new concept of magnetic memory called Magnetic Random Access Memory (MRAM), which combines short access time of semiconductor-based RAMs and the non-volatile character of magnetic memories.13 The first commercial MRAM entered the market in 2006. The next generation of MRAM, using a switching process by spin transfer, is expected to have a big impact on computer technology.

Diagram showing the magnetic read-write in a perpendicular recording medium. Adapted from Reference 33

Figure 1. Diagram showing the magnetic read-write in a perpendicular recording medium. Adapted from Reference 33.

 

Magnetic semiconductors constitute an important field today because of the possibilities of good spintronic materials and devices.14 Magnetic semiconductors are very attractive because of their ability to combine potentials of conventional semiconductors (namely, control of current by gate, coupling with optics, etc.) with those of magnetic materials (control of current by spin manipulation, nonvolatility, etc.). One of the outcomes of this integration is the Spin Field Effect Transistors (Spin FETs) based on spin transport in semiconductor lateral channels between spin-polarized source and drain with control of spin transmission by a field effect gate. Research on magnetic semiconductors is mainly concentrating on 1) hybrid structures combining ferromagnetic metals with nonmagnetic semiconductors; 2) ferromagnetic semiconductors belonging to the Ga1-xMnxAs family; and 3) the Spin Hall effect, which can create spin currents in structures composed solely of nonmagnetic conductors. Other systems being pursued with interest in spintronics include ZnXO (x=Ni, Mn, Co) and GaMnN. Search for potential spintronics materials is also concentrating on the half-Heusler family of alloys. The prototypical alloy of this series is NiMnSb, which has been found to be a good half metallic ferromagnet.

Spintronics has emerged as one of the most fascinating topics in magnetic materials research today. Its potential applications, though conceived over a wide range, are yet to be fully exploited. For example, the quantum mechanical nature of the spin and the long spin coherence time in confined geometries offer great potential in the field of quantum computing. There is no doubt that spintronics will take an important place in the technology of our century.

Exchange Bias and Magnetostrictive Materials and Applications

A topic closely related to magnetic recording is the phenomenon of exchange bias. Exchange bias results in a shift of the hysteresis loop with respect to the magnetization axis, thereby providing a DC field bias.15 Such a phenomenon is attributed to exchange anisotropy, usually associated with ferromagnetic-antiferromagnetic interfaces. Unlike the uniaxial anisotropy seen in bulk magnetic materials, exchange bias is a result of unidirectional magnetic anisotropy. A family of alloys known as magnetic shape-memory alloys is found to show large exchange bias (Figure 2). Exchange bias has become quite important from the technological point of view because it can be used to fix the magnetization of a ferromagnetic layer, which serves as the reference layer, in a magnetic sensor. Because of the strong application potential, there is a huge demand for systems that show large exchange bias at room temperature and above.

Exchange bias in a full Huesler alloy

Figure 2. Exchange bias in a full Huesler alloy.

 

Magnetic sensors, transducers, and actuators constitute another important area of application of magnetic materials. The main underlying property that enables this application is magnetostriction,16 which is the most important magneto-elastic phenomenon exhibited by magnetic materials. This is a phenomenon where the dimensions of a magnetic material change when an external magnetic field is applied. By applying a time varying magnetic field, one can set a magnetic rod into vibration, thereby convert the electrical/magnetic energy to mechanical energy. This principle can be used for energy conversion or sensing. Socalled magnetostrictive transducers are being used extensively in sonar. Magnetostrictive transducers have many advantages over piezoelectricity- driven devices such as the PZT (Lead Zirconate Titanate) devices. Rare-earth transition-metal intermetallic (Tb,Dy)Fe2 (commercially known as Terfenol-D) is a high efficiency solid-state transducer and was one of the most attractive magnetostrictive material in the 1980s and 1990s. The magnetostriction of this material generates strains 100 times greater than traditional magnetostrictive materials, and about five times greater than traditional piezo-ceramics. Terfenol also has a high Curie temperature, making it suitable for room-temperature applications. Furthermore, by adjusting the stoichiometry of the alloy, this temperature range can be extended down to cryogenic temperatures. However, large magnetocrystalline anisotropy of many such R- intermetallic compounds limits application, in spite of their giant magnetostrictive properties. Recently, Fe1-xGax alloys have emerged as potential successors to R-T compounds because of their large magnetostriction and good mechanical properties at low fields.17 Additionally, chemical and structural heterogeneity and the resulting interaction of coexisting phases in textured Co1-xFex thin films is reported to result in large magnetostriction at very low fields. Microstructural analysis of this result has revealed the giant magnetostriction is associated with the precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. This result indicates a route to the discovery of giant, lowfield magneto-elastic materials.

Research in the field of magneto-elastic phenomena has resulted in a new class of materials known as ferromagnetic shape-memory alloys.18 The most striking example of this family is a set of compounds known as full Heusler alloys with the composition X2YZ (X and Y are transition metals, and Z is a nonmagnetic element). The magnetic field induced strain in many of these alloys is quite large compared to that of conventional magnetostrictive materials. One of the first materials to show this effect was Ni2MnGa. Subsequent to this discovery, a large number of stoichiometric and non-stoichiometric alloys of the 2:1:1 family with different elements have been tested. This has resulted in the identification of several potential ferromagnetic shape-memory alloys, in bulk and thin film form.19 These materials are usually multifunctional, with significant changes in properties such as electrical resistance and magnetic entropy as a function of the applied magnetic field. From the point of view of fundamental magnetism, shape memory materials in general, and full Heusler alloys in particular, are very important because they show a first order coupled magneto-structural transition. The strong magneto-structural coupling is responsible for many anomalous properties of this series.

Magnetic Refrigeration

Magnetic cooling is a relatively new application of magnetic materials.20 Magnetic cooling is based on the principle of adiabatic demagnetization of a magnetic material wherein magnetic entropy of the solid is manipulated by changing the applied field. This gives rise to desired changes in temperature, thereby enabling cooling (refrigeration) or heating (heat pumps) applications. The main advantages of magnetic refrigerators are based on their eco-friendly nature and anticipated superior performance compared to existing conventional gas-based refrigerators. The efficiency, compactness, and adaptability of magnetic refrigerators have given the field of magnetic cooling a separate and recognized identity. In view of these advantages, magnetic refrigeration is termed as "green and clean" technology.

Though chronologically speaking, magnetic cooling/heating is not a new idea, exploitation of ferromagnetic materials for this application has revolutionized the concept in a significant manner. The central component of a magnetic refrigerator (Figure 3) is a magnetic material which possesses a very high (giant) magnetocaloric effect (MCE). An MCE manifests as isothermal magnetic entropy change or adiabatic temperature change when the material is subjected to a magnetic field. The other components are the field generating magnet assembly and the design that result in cooling of the desired volume. The higher the MCE, the smaller the field required; thus, the practical design of a working refrigerator demands materials with giant MCE. From the commercial point of view, it is desirable that the refrigerator works in fields that can be generated by permanent magnets. Therefore, giant magnetocaloric materials are of great importance. The terminology of giant magnetocaloric effect (GMCE) came into existence with the ground-breaking discovery of large MCE in Gd5Ge2Si2 (Aldrich Prod. No. 693510) by Pecharsky and Gschneidner.21 Since then, various materials systems have been studied to identify such materials. Usually, materials that undergo first-order magnetic transitions/magneto-structural transitions possess GMCE,22 and most current work tries to address this issue. Many systems in this family have been developed in the recent past, mainly from the rare-earth intermetallics family. As all potential permanent magnets are also made of rare earths, the development of low field (efficient, viable) magnetic refrigeration technology implies a huge demand/market for rare earths. In fact, this has led to vigorous exploration and extraction/recovery of various rare earths throughout the world.

Schematic of vital components of a magnetic refrigerator

Figure 3. Schematic of vital components of a magnetic refrigerator.24

 

While GMCE materials are highly important, there are several challenges associated with them. Main challenges are the large thermal hysteresis associated with the first order transition and the long delay in achieving the maximum adiabatic temperature change. Another problem is to get the refrigerants in the required form, usually spheres or ribbons, to constitute the regenerator bed.

In the history of magnetocaloric materials, the greatest achievement has been the discovery of the Gd5(Ge,Si)4 alloy series (Aldrich Prod. Nos. 693510 and 693502). In addition to GMCE, this system shows a very interesting phenomenon known as spontaneous generation of voltage (SGV).23 SGV occurs in the vicinity of first-order magnetostructural phase transition, which is also responsible for the giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance. Materials which show SGV are promising multifunctional miniature sensors capable of sensing changes in temperature, magnetic field, and pressure. Increasing research in this field is expected to yield many similar systems.

Intermetallic alloys without rare earths also can show giant MCE.24 In addition, there are a few ceramic systems found to be promising for magnetic refrigeration. Work is ongoing to understand the magnetocaloric efficiency of several materials in amorphous, nanocrystalline, single crystalline, thin film and molecular forms, though most do not possess a giant magnetocaloric effect. There is intense activity in the field of molecular magnets in general and molecular magnetic refrigerants in particular and research on magnetocaloric effect now encompasses materials belonging to a wide range of sizes, geometries, and crystalline forms.

Though several potential materials have been developed in recent past, elemental gadolinium (Aldrich Prod. No. 263087) remains the standard reference material for comparing MCE. Advantages of gadolinium are: 1) its moment is quite high, thereby giving rise to a reasonably large entropy change; 2) the S-state nature of the 4f orbital enables it to be free from any large crystal field effects, which would be undesirable for a good refrigerant; and 3) its magnetic ordering temperature is relatively high (close to room temperature) at least among various 4f metals. In many materials, though the magnetic entropy change is quite substantial, their adiabatic temperature change is too low to be considered for these applications. Another drawback of certain materials such as FeRh is that their MCE is irreversible and as such they cannot find a place in a practical refrigerator. Applications of magnetic materials are not just restricted to cooling. Certain materials are known to show an inverse magnetocaloric effect, which implies that the application of the field increases the magnetic entropy and are expected to be useful in devices such as heat pumps. Apart from Gd5(Si,Ge)4 compounds, other important systems identified so far include: La(Fe,M)13 (M=Si, Al); MnFe(P1-xAsx); MnAs; full Heusler alloys, namely Ni2MnX (X=Sn, In, Sb); and molecular magnets.25

The projected applications of magnetic refrigeration are quite wide ranging from air conditioning to food preservation, liquefaction of gases, and cryogenic detectors. Thin film-based magnetic refrigerators are expected to be valuable for miniature devices. Another field where biocompatible magnetocaloric materials can play a role is in medical applications, i.e., as hyperthermia. Growth of the field of MCE has resulted in the exploration of other novel cooling techniques such as the electrocaloric,26 elastocaloric,27 and barocaloric28 effects.

Table 1. Magnetocaloric properties (isothermal entropy change and adiabatic temperature change) of certain potential magnetic refrigerant materials for a field change of 50 kOe.

Material Magnetic Transition
Temperature (K)
Isothermal Magnetic
Entropy Change
(ΔSm)max [J/kg K]
Adiabatic
Temperature Change
(DTad)max [K]
Gd5Si2Ge2 276 20 15
ErCo2 35 36 9.5
LaFe11.57Si1.43 188 24 7.5
MnAs0.75Sb0.25 235 25 10
NiCoMnSb 250–300 30
La0.87Sr0.13MnO3 197 5.8
Gd 292 11 11.2

In addition to the role as active (working) materials in a refrigerator, certain magnetic materials serve as passive magnetic regenerators. The difference is that while active materials take part directly in cooling through the field cycle, the latter use their large heat capacities to absorb or desorb heat throughout the thermal cycle. These materials have appreciable magnetic heat capacity associated with magnetic transition at temperatures where phononic and electronic contributions are very small. Research in the field of magnetic refrigerants has also led to identification of several such passive materials that are quite useful in extending the temperature span of a refrigerator, especially to low temperatures. Examples of such materials include the rare-earth intermetallic compounds Gd-Er-Rh, R-Co, and R-Ni.

Future Application Areas for Magnetic Materials

In addition to the exploitation of change in specific heat of a material under the influence of a magnetic field, one can also search for a change in electrical resistivity. This change can be huge in some systems, resulting in transition from a parent insulating magnetic material to a metallic ferromagnet. In this case, change in resistivity is called colossal magnetoresistance (CMR). Several transition metal oxides are known to undergo this kind of field-induced insulator to metal transition, resulting in CMR. There are a large number of metallic and ceramic (both in bulk and thin film form) systems that show large magnetoresistance suitable for applications like field sensing.29

A large number of magnetic materials are being prepared in amorphous form for applications related to soft materials. They can be important in the synthesis of nano-structured materials by giving proper heat treatments. Magnetic impedance and magneto-optics are two other fields of great importance today. Magneto-optics deals with change in the polarization of light by a magnetic field, either internal or external. This is of great potential in magnetic recording. Nano/Bio/Molecular magnetism is another field that has recently attracted much attention, as have ferrofluids, colloidal suspensions of ferro/ferri magnetic particles.30

In summary, the scope of magnetic materials research is constantly expanding. Discovery of new phenomena, increasing understanding of magnetism, the availability of novel and sophisticated experimental probes, utilization of advanced theoretical and computational tools, and identification of new materials have all contributed to the growth of this field. It is quite certain that many of the present challenges for commercialization will soon be overcome.

Materials

     

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