Introduction

Langmuir monolayers have proven themselves to be a powerful experimental system for the study of a range of issues in soft condensed matter [1–4]. Essentially, Langmuir monolayers are a single layer of insoluble molecules at the air–water interface. As such, they form an almost ideal two-dimensional system. This offers the opportunity to study fundamental questions in the phase behavior and material properties of two-dimensional systems. Secondly, they are readily transferred from the surface to a solid substrate. This has been the motivation for studying these systems for a range of technological applications. Finally, they are a natural system for the study of biological questions given that much of biology relies on processes at interfaces, such as cellular membranes and membranes of cellular components.

The experimental techniques associated with Langmuir monolayers can loosely be divided into two classes: formation of the monolayer and characterization of the monolayer. In this chapter, we will briefly review important issues in the formation of Langmuir monolayers. The focus will be mostly on the general concepts and issues, with some specific recipes given for illustrative purposes. However, it should be recognized that many groups specializing in Langmuir monolayers have refined and developed specific methods that are optimized for their system. So, some care needs to be taken in applying any given specific recipe. In the area of characterization, we will briefly review standard surface pressure characterization and provide a survey of commercial tools, including x-ray and neutron scattering techniques.

Introduction

This chapter will review aspects of soft random solids, including particulate gels, compressed emulsions, and other materials having similar basic features. Soft random solids are appealing for a number of reasons – and not just because of their taste (as in foods) or appearance (as in cosmetics). They offer the potential for insight into much broader and quite elegant problems in nonequilibrium thermodynamics such as the dynamics of phase transitions, the origin of the glass transition, and stresses and flows in granular media. They are also central players in a host of industries in the form of paint, ink, concrete, asphalt, dairy foods, and cosmetics. From the range of examples and the techniques involved, it should be apparent that investigation of these materials is a multi-disciplinary process, combining contributions from physicists, chemists, and engineers.

Since the 1980s, studies of soft random solids have benefited enormously from advances in microscopy, computer-aided image analysis, computer simulations, and new methods to synthesize colloidal particles with controlled shape, surface chemistry, and interactions. The chapter's purpose is to summarize areas of recent investigations and point out experimental breakthroughs, remaining questions, and relevant experimental methods. Despite the recent progress, a number of fundamentally interesting and practically relevant questions remain, and it is hoped that this chapter will stimulate further work in these areas.

In the previous chapter we studied two contributions to the magnetic moment of atoms – the electron spin and orbital angular momenta. Next we are going to investigate the third (and final) contribution to the magnetic moment of a free atom. This is the change in orbital motion of the electrons when an external magnetic field is applied.

The change in orbital motion due to an applied field is known as the diamagnetic effect, and it occurs in all atoms, even those in which all the electron shells are filled. In fact diamagnetism is such a weak phenomenon that only those atoms which have no net magnetic moment as a result of their shells being filled are classified as diamagnetic. In other materials the diamagnetism is overshadowed by much stronger interactions such as ferromagnetism or paramagnetism.

Observing the diamagnetic effect

The diamagnetic effect can be observed by suspending a container of diamagnetic material, such as bismuth, in a magnetic field gradient, as shown in Fig. 4.1. Since diamagnetic materials exclude magnetic flux, their energy is increased by the presence of a field, and so the cylinder swings away from the high-field region, towards the region of lower field (the north pole in the figure).

The purpose of this chapter is to understand the origin of the magnetic dipole moment of free atoms. We will make the link between Ampère's ideas about circulating currents, and the electronic structure of atoms. We'll see that it is the angular momenta of the electrons in atoms which correspond to Ampère's circulating currents and give rise to the magnetic dipole moment.

In fact we will see that the magnetic moment of a free atom in the absence of a magnetic field consists of two contributions. First is the orbital angular momenta of the electrons circulating the nucleus. In addition each electron has an extra contribution to its magnetic moment arising from its “spin.” The spin and orbital angular momenta combine to produce the observed magnetic moment.

By the end of this chapter we will understand some of the quantum mechanics which explains why some isolated atoms have a permanent magnetic dipole moment and others do not. We will develop some rules for determining the magnitudes of these dipole moments. Later in the book we will look at what happens to these dipole moments when we combine the atoms into molecules and solids.

Introduction

The data storage industry is huge. Its revenue was tens of billions of U.S. dollars per year at the end of the 20th century, with hundreds of millions of disk, tape, optical, and floppy drives shipped annually. It is currently growing at an annual rate of about 25%, and the growth rate can only increase as the storing and sending of digital images and video becomes commonplace, with the phenomenal expansion of the world wide web and in ownership of personal computers and mobile computing platforms.

Magnetic data storage has been widely used over the last decades in such applications as audio tapes, video cassette recorders, computer hard disks, floppy disks, and credit cards, to name a few. Of all the magnetic storage technologies, magnetic hard-disk recording is currently the most widely used. In this chapter, our main focus will be on the technology and materials used in writing, storing, and retrieving data on magnetic hard disks. Along the way we will see how some of the phenomena that we discussed in Part II, such as magnetoresistance and single-domain magnetism in small particles, play an important role in storage technologies.

RAMAC, the first computer containing a hard-disk drive, was made by International Business Machines Corporation (IBM) in 1956.

The term “magnetic anisotropy” refers to the dependence of the magnetic properties on the direction in which they are measured. The magnitude and type of magnetic anisotropy affect properties such as magnetization and hysteresis curves in magnetic materials. As a result the nature of the magnetic anisotropy is an important factor in determining the suitability of a magnetic material for a particular application. The anisotropy can be intrinsic to the material, as a result of its crystal chemistry or its shape, or it can be induced by careful choice of processing method. In this chapter we will discuss both intrinsic and induced anisotropies in some detail.

Magnetocrystalline anisotropy

In Chapter 7 we introduced the concept of magnetocrystalline anisotropy, which is the tendency of the magnetization to align itself along a preferred crystallographic direction. We also defined the magnetocrystalline anisotropy energy to be the energy difference per unit volume between samples magnetized along easy and hard directions. The magnetocrystalline anisotropy energy can be observed by cutting a {110} disk from a single crystal of material as shown in Fig. 11.1, and measuring the M–H curves along the three high-symmetry crystallographic directions and contained within the disk.

Schematic results for single-crystal samples of ferromagnetic metals such as iron and nickel were shown in Fig. 7.4. Body-centered cubic Fe has the 〈100〉 direction as its easy axis.

In Chapter 8 we described the original 1956 experiment on Co/CoO nanoparticles in which the shift in hysteresis loop known as exchange bias or exchange anisotropy was first observed. The goal of this chapter is to describe the exchangebias phenomenon in more detail and to point out open questions in the field, which remains an active area of research. Significantly, a simple theoretical model that accounts for all experimental observations is still lacking.

Remember that exchange bias appears when a ferromagnetic/antiferromagnetic interface is cooled in the presence of a magnetic field through the Néel temperature of the antiferromagnet (Fig. 14.1). The Curie temperature of the ferromagnet should be above the Néel temperature of the antiferromagnet so that its moments are already aligned in the field direction; this is usually the case for typical FM/AFM combinations. In a simple model, the neighboring moments of the antiferromagnet then align parallel to their ferromagnetic neighbors when their Néel temperature is reached during the field cooling process. An exchange-biased system shows two characteristic features: first, a shift in the magnetic hysteresis loop of the ferromagnet below the TN of the AFM, as though an additional biasing magnetic field were present, resulting in a unidirectional magnetic anisotropy; and second, an increase in coercivity and a wider hysteresis loop, which can even occur independently of the field cooling process.

In Chapter 2 we introduced the concept of ferromagnetism, and looked at the hysteresis loop which characterizes the response of a ferromagnetic material to an applied magnetic field. This response is really quite remarkable! Look at Figs. 2.3 and 2.4 again – we see that it is possible to change the magnetization of a ferromagnetic material from an initial value of zero to a saturation value of around 1000 emu/cm3 by the application of a rather small magnetic field – around tens of oersteds.

The fact that the initial magnetization of a ferromagnet is zero is explained by the domain theory of ferromagnetism. The domain theory was postulated in 1907 by Weiss and has been very successful. We will discuss the details of the domain theory, and the experimental evidence for the existence of domains, in the next chapter.

The subject of this chapter is: How can such a small external field cause such a large magnetization? In Exercise 6.2(b), you'll see that a field of 50 Oe has almost no effect on a system of weakly interacting elementary magnetic moments. Thermal agitations act to oppose the ordering influence of the applied field, and, when the atomic magnetic moments are independent, the thermal agitation wins.

In the previous chapter we mentioned the current interest in combining magnetic behavior with additional desirable properties, and looked at the examples of semiconducting transport and robust insulation. In this chapter we continue this philosophy with a discussion of the so-called multiferroics, which combine magnetic ordering with other kinds of ferroic ordering – ferroelectricity, ferroelasticity, and ferrotoroidicity. We will focus in particular on the combination of magnetism and ferroelectricity, which is appealing because of its potential for magnetoelectric response, that is, the control and tuning of magnetism using electric fields, and vice versa.

The formal definition of a multiferroic is a material that displays two or more primary ferroic orderings simultaneously. The well-established primary ferroics are: the ferromagnets, which have a spontaneous magnetization that is switchable using an applied magnetic field and which we have focused on in this text so far; the ferroelectrics, with their spontaneous electric polarization that is switchable by an applied electric field; and the ferroelastics, which have a spontaneous strain that is switchable by an applied mechanical stress. Recently the ferrotoroidics have been proposed, using symmetry arguments, to complete the classes of primary ferroics. Let's begin by comparing the properties of the ferroelectrics, ferroelastics, and ferrotoroidics with the ferromagnets that we have already discussed in detail.

In this chapter we continue our survey of magnetic phenomena with a look at magnetism in magnetic semiconductors and insulators. A large practical motivation for the study of magnetic semiconductors is their potential for combining semiconducting and magnetic behavior in a single material system. Such a combination will facilitate the integration of magnetic components into existing semiconducting processing methods, and also provide compatible semiconductor–ferromagnet interfaces. As a result, diluted magnetic semiconductors are viewed as enabling materials for the emerging field of magnetoelectronic devices and technology. Because such devices exploit the fact that the electron has spin as well as charge, they have become known as spintronic devices, and their study is known as spintronics. In addition to their potential technological interest, the study of magnetic semiconductors is revealing a wealth of new and fascinating physical phenomena, including persistent spin coherence, novel ferromagnetism, and spin-polarized photoluminescence.

We will focus on the so-called diluted magnetic semiconductors (DMSs), in which some of the cations, which are non-magnetic in conventional semiconductors (Fig. 17.1 left panel), are replaced by magnetic transition-metal ions (Fig. 17.1 center panel). We will survey three classes of DMSs. First are the II–VI diluted magnetic semiconductors, of which the prototype is (Zn, Mn)Se, which have been studied quite extensively over the last decade or so.

In the previous chapter we discussed the diamagnetic effect, which is observed in all materials, even those in which the constituent atoms or molecules have no permanent magnetic moment. Next we are going to discuss the phenomenon of paramagnetism, which occurs in materials that have net magnetic moments. In paramagnetic materials these magnetic moments are only weakly coupled to each other, and so thermal energy causes random alignment of the magnetic moments, as shown in Fig. 5.1(a). When a magnetic field is applied, the moments start to align, but only a small fraction of them are deflected into the field direction for all practical field strengths. This is illustrated in Fig. 5.1(b).

Many salts of transition elements are paramagnetic. In transition-metal salts, each transition-metal cation has a magnetic moment resulting from its partially filled d shell, and the anions ensure spatial separation between cations. Therefore the interactions between the magnetic moments on neighboring cations are weak. The rare-earth salts also tend to be paramagnetic. In this case the magnetic moment is caused by highly localized f electrons, which do not overlap with f electrons on adjacent ions. There are also some paramagnetic metals, such as aluminum, and some paramagnetic gases, such as oxygen, O2.