Introduction

In this chapter, models will be discussed that give insight into the recording process. In contrast to the playback process, the recording of magnetization patterns is a non-linear phenomenon. Thus, except for special cases, analysis must be by computer simulation. It is the long-range magnetostatic fields that cause computer simulations to be iterative and extremely time consuming. There are two philosophical approaches to computations of the record process. One is to neglect the fine details and develop reasonably approximate models that capture the main physical features of the process. Such simplified models allow for analytic solutions of magnetization patterns or solutions that require a minimum of computer simulation. This simplified approach is useful for developing guidelines in media development (e.g. the effect of coercivity or remanence changes) or in head–media interface geometry development (effect of head–medium spacing, record gap, medium thickness). Parameters can be easily changed in simplified models.

The other approach is to develop full numerical micromagnetic models. These are required in order to compute detailed, or second order effects, of the recording process, such as noise, edge-track writing, and non-linear amplitude loss and thus, for example, give fundamental input to error-rate calculations. Simulations can give detailed information about pulse asymmetry in tape recording (Bertram, et al., 1992) or the fluctuations of the transition center across the track that leads to position jitter or transition noise in thin film media (Zhu, 1992). For example, in Fig. 8.1 the vector magnetization distribution from micromagnetic simulation of a recorded transition in thin film media is shown.

Magnetic recording is a technology that has continually undergone steady and substantial advancement throughout its history. Typically, in the last two decades areal densities in computer disk recording have increased by over two orders of magnitude. This development has occurred via the simultaneous growth in new materials for heads and media, advanced signal processing schemes, and mechanical engineering of the head–medium interface. In addition, there has been substantial growth in the theoretical understanding of the magnetic behavior of heads, media and, in general, the magnetic recording process. Fundamental understanding of the physics of magnetic recording is necessary not only for system design, but so that the specific behaviour of magnetic components can be analyzed, either analytically or numerically, saving time-consuming and expensive experimentation. For example, it is difficult to produce all the media variations required to perform a thorough comparison of different modes of recording, such as longitudinal and perpendicular recording.

In recent years there have been many publications that cover the fundamentals and applications of the magnetic recording process. These books or papers have been either technically oriented discussions of specific topics or have provided an introduction at an elementary level to the basics of magnetic recording. The philosophy of this book is to provide a pedagogical introduction to the physics of magnetic recording. The level is advanced and all basic aspects of magnetic recording are included: magnetic fields of heads and media, the linear replay process, the non-linear recording process including interferences, and medium noise.

Introduction

Magnetized recording media produce fields by virtue of divergences in the magnetization pattern. Thus, (2.8) can be utilized to obtain the magnetic fields for any specified magnetization pattern. For two-dimensional geometry it is often convenient to utilize the simple form given by (2.22), or under certain conditions (2.26). Magnetized media are particularly simple to analyze since, in general, they extend infinitely far along the x axis and possess a finite thickness or magnetization depth which does not vary along the x axis. In this section expressions are given for the fields for single magnetization transitions that are either longitudinally or vertically oriented. That discussion will be followed by a general relation for the Fourier transform of the fields. This section is concluded by a discussion of the fields from sinusoidally magnetized media. Only two-dimensional geometries will be considered.

Single transitions

We begin by deriving the fields produced by a single, perfectly sharp transition as sketched in Figs. 4.1(a) and 4.2(a) for a longitudinally and vertically directed magnetization, respectively. The coordinate system (x, y) is centered at the center of the medium at the transition center. Equation (2.26) may be utilized for both cases, since the volume charge for the case of a sharp transition of longitudinal magnetization is equivalent to a surface charge of σ = 2M at the transition center (x = x0) extending from − δ/2 < y < δ/2. The fields for a longitudinal magnetization utilizing (2.26) (r1, r2, θ are noted in Fig. 4.1 (a)) yield:

These fields are plotted versus x for fixed y in Fig. 4.1(b) along the medium centerline (y = 0) and in Fig. 4.2(c) at the medium surface y = δ/2.

Magnetic recording is the central technology of information storage. Utilization of hard disk drives as well as flexible tape and disk systems provides, inexpensively and reliably, all features essential to this technology. A data record can be easily written and read with exceedingly fast transfer rates and access times. Information can be permanent or readily overwritten to store new data. Digital recording is the predominant form of magnetic storage, although frequency modulation for video recording and ac bias for analog recording may persist in consumer applications. Data storage is universally digital. Superb areal densities for disk drives and volumetric densities for tape systems are achievable with extremely low error rates. In the last decade there have been extraordinary advances in magnetic recording technology. Current densities and transfer rates for disk systems are typically 60Mbits/in2 and 10Mhz, respectively, but systems with densities of 1–2Gbits/in2 are realizable (Wood, 1990; Howell et al., 1990; Takano, et al., 1991). The ability to coat tape with extremely smooth surfaces has permitted the development of very high density helical scan products (SVHS, 8mm video) (Mallinson, 1990). The digital audio helical scan recorder (DAT) is representative of very high density tape recording with linear densities of greater than 60kbits/in, track densities of 250 tpi, and volumetric densities on the order of 50Gbits/in3 (Ohtake, et al., 1986). High data rate tape recording systems near 150MHz have been developed (Ash, et al., 1990; Coleman, et al., 1984). In general, densities and data rates of magnetic recording systems have been increasing at a rate exceeding a doubling every three years.

Introduction

This chapter is devoted to a discussion of non-linearities and overwrite in digital magnetic recording. The magnetic recording process is inherently non-linear as discussed in Chapter 8. The term ‘non-linearity’ in magnetic recording technology refers to phenomena that cause linear superposition to be invalid. These non-linearities arise from interbit magnetostatic interactions that occur during the write process. Two essential non-linear effects occur: non-linear bit shift and high-density, non-linear amplitude loss. In this chapter the example of dibit recording is discussed to illustrate these non-linearities. In high-density digital disk and tape systems new information is written over previous data. Separate erase heads to ensure complete erase are not utilized. The overwrite process is a form of erasure, which at sufficiently high record currents is dominated by non-linear bit shift effects of the ‘hard’ and ‘easy’ transitions. A simple model of overwrite is presented that agrees well with experiment. Although numerical models may be utilized to determine these nonlinearities, simplified analytic models are presented here, except for the case of non-linear amplitude loss.

Non-linear bit shift

Non-linear bit shift occurs during the writing process. The magnetostatic fields from previously written transitions cause the location of a transition currently being written to be shifted away from that determined solely by the recording head field. These shifts depend on the data pattern as well as on the location of each transition in the sequence. The term ‘non-linear bit shift’ is utilized here to refer to shifts that occur due to previously written transitions. Shifts caused by ‘hard’ transitions, or overwrite phenomena, also affect the net bit shift, as clarified in Problem 9.6.

Surface symmetry

The classification and description of symmetry properties and structures of bulk (three-dimensional) crystalline materials require a reasonable understanding of crystallography; notably of the restricted number of types of translational symmetry which crystals can possess (characterised by their associated unit cell which must be one of the 14 Bravais lattices) and the finite number of point and space groups which can define the additional symmetry properties of all possible crystals. Many properties of solids are intimately related to the special symmetry properties of these materials. While a solid surface is intrinsically an imperfection of a crystalline solid, destroying the three-dimensional periodicity of the structure, this region of the solid retains two-dimensional periodicity (parallel to the surface) and this periodicity is an important factor in determining some of the properties of the surface. In particular, it plays a dominant role in allowing electron, X-ray and atom diffraction techniques to provide information on the structure of the surface, as well as strongly influencing the electronic properties of the surface. For these reasons a proper understanding of surface crystallography is important for a general understanding of many surface effects and is critical for an understanding of the electron diffraction techniques, Low Energy Electron Diffraction (LEED) and Reflection High Energy Electron Diffraction (RHEED), of surface X-ray diffraction and of He atom diffraction (see chapter 8).

The formation of Langmuir-Blodgett films

The technology of the formation of LB films has much in common with the trough technology discussed in Chapter 3. In addition to the equipment described there, one needs a mechanical device to raise and lower the substrate through the air/water interface at a predetermined rate. Various devices have been employed but it is usual to provide the vertical movement by driving a large micrometer screw by an electric motor via a reduction gear train. Practical velocities are such that they are usually measured in millimetres per minute. It is essential to be able to vary the dipping rate as there is an upper effective rate which can be employed for any particular material. This rate is determined by the speed at which water drains from the film as it is withdrawn from the subphase and by the viscosity of the film and hence the rate at which material can approach the substrate. For a material of high viscosity this procedure is difficult to carry out properly and a substantial difference in pressure may occur between the pressure sensor and the region immediately in contact with the substrate. In fully automated troughs the substrate is withdrawn from the subphase and maintained in this position while the film at the air/water interface is respread and compressed to a predetermined dipping pressure. In such systems it is necessary to carry through a cyclic compression and expansion process several times to arrive at a good approximation to an equilibrium situation before the dipping process is reactivated.

In writing any scientific work it is difficult to decide what background knowledge one ought to assume in potential readers. This difficulty is particularly acute when, as in this case, the book is of an interdisciplinary nature and deals with topics which belong properly to physics, chemistry and, to a certain extent, biology. When in doubt I have decided to assume ignorance rather than knowledge. For example, the text is sprinkled with diagrams illustrating the structures of chemical compounds as it is likely that readers with a physics background will be unable to deduce these structures from the names of the compounds. On the other hand, the derivation of formulae, the origins of which are readily available in common text books, has usually been omitted. When derivations are not so easily come by, they have been given. This is true, for example, in the case of the basic expression for the refractive index of a material as experienced by neutrons. I have been unable to find a derivation of this expression in any of the various books on neutron diffraction which I have examined.

The study of thin organic films has expanded enormously in recent years and it has been necessary to be very selective in order to prevent this work degenerating into a bibliography. I have attempted to discuss material in which structure and order are dealt with and to ignore the many papers, interesting from other points of view, in which these matters have not been mentioned or are given only a minor place.

General considerations

Introduction

A large number of surface techniques involve the detection of electrons in the energy range 5–2000 eV which are emitted or scattered from the surface. A number of features are common to most of these techniques. In particular, all derive their surface sensitivity from the fact that electrons in this energy range have a high probability of inelastic scattering, so that if electrons are detected at an energy which is known to be unchanged by passage through the surface region of the solid, we know that they have passed only through a very thin surface layer; i.e. the techniques are surface specific. Secondly, because this surface specificity derives from a knowledge of the energy of the electrons, some form of electron energy analyser is required by most of these techniques. This piece of instrumentation is therefore common to many techniques.

Of course, no classification scheme is perfect. Electron energy analysers can also be used to determine the energy spectrum of other charged particles, notably ions as in ion scattering spectroscopy. Inverse Photoemission Spectroscopy (IPES) and Appearance Potential Spectroscopy (APS) are not strictly electron spectroscopies as ultraviolet and X-ray photons are detected, but IPES is very closely related to photoemission in the basic physics, and both share with electron spectroscopies a surface specificity which is governed by electron inelastic scattering.

Introduction

At low incident kinetic energies (at most a few tens of eV) the interaction of incident ions with a surface is dominated by charge transfer to neutralise the ion. This produces electron emission characteristic of the electronic structure of the surface, and therefore forms a valence level spectroscopy known as INS. This will be discussed in detail in the next section.

By contrast, a number of techniques in surface studies utilise the kinetic energy transfer of more energetic incident ions to provide information on the surface. Most of these techniques use incident inert gas ions He+, Ne+ or Ar+ in the energy range from a few hundred eV to a few keV although some use is also made of similarly low energy alkali metal ions (Li+, Na+, K+) and oxygen ions, and there are also techniques based on the use of far more energetic (up to 1 MeV or more) incident ions of He+ and H+. While these incident ions may also suffer charge transfer at the surface, and can produce electronic excitations both in the form of core level ionisation and plasmon excitation, most techniques concentrate on the kinetic energy transfers between the incident ion and the atoms which comprise the surface.

Trough technology

Amphiphilic materials spread at the air/water interface have been the subject of intensive study over a long period of time. The type of apparatus usually used for this purpose has much in common with the apparatus needed to form Langmuir–Blodgett films and, indeed, it is usually possible to adapt the same apparatus for both purposes. In this section the problems which must be overcome if these processes are to be carried out are discussed and the most effective solutions to these problems described.

It has become traditional to use the word ‘trough’ to denote such apparatus and this usage will be adhered to here though the word trough tends to suggest such things as hogwash rather than the ultra-cleanliness needed for effective studies of monolayers. It is indeed this cleanliness which must be our first concern. Many materials which would otherwise be suitable for the fabrication of troughs tend to leach out surface active material into the water based subphase contained in the trough and thus can not be used. Most modern troughs are made in one of the two following ways.

1. Teflon (polytetrafluoroethylene) does not leach out plasticisers and can be purchased in substantial blocks, sheets and rods of various thicknesses. The preferred method is to machine a trough from a solid block of this material. This is a practicable procedure if a good milling machine is available but one is limited to a rather shallow trough. […]

Introduction

Many surface techniques involve some damage, or destruction of the surface being investigated but, with the exception of the SIMS technique described in chapter 4, this is an incidental side-effect rather than a primary feature of the technique. In the case of SIMS, the destruction of the surface is by the rather brutal method of sputtering, and the analysis of the sputtered, charged fragments is carried out primarily with the object of determining the surface composition. In this chapter, we discuss two very different types of desorption spectroscopy, in which adsorbed species specifically, are desorbed from the surface in an attempt to learn about the nature of the adsorbate—substrate bonding. Information on surface composition (or more often surface coverage of an adsorbed species) may also be obtained, but this is usually incidental.

The two general methods of desorption are by thermal and by electronic stimulation. Any species adsorbed on a suface must be bound to the surface with some specific amount of energy and will desorb at a rate determined by a Boltzmann factor. Heating the surface will increase this desorption rate, and the desorbing species may be detected in the gas phase by conventional mass spectrometers. Evidently, a study of the temperature dependence of the desorption rate can lead to information on the binding energy states of the adsorbate (or, more strictly, on the desorption energies).

Introduction

As was pointed out in Chapter 1 liquid crystals (or mesophases as they are often called) were first discovered by Reinitzer in 1888 and the first proper classification of liquid crystals was made by Friedel in 1922. Since that time various new categories of liquid crystals have been discovered and named. It would be impossible to give an extensive treatment of this important and wide ranging subject here but, as so many of the systems discussed in this book have a liquid crystalline structure, at least a brief treatment of the topic is essential. Furthermore, several methods of forming ordered thin organic films not treated in other chapters depend on the initial formation of a mesophase. It has been suggested that something like 10% of fine organic materials listed in a typical catalogue of such products are capable of existing in a mesophase within some appropriate temperature range or, in the case of lyotropic liquid crystals, when dissolved at an appropriate concentration in some solvent. It is thus obvious that the subject has immense ramifications and could not be pursued in any great breadth here.

Liquid crystals can initially be divided into thermotropic and lyotropic materials. The first category involves a single molecular species and exists in a temperature range which lies between the melting point of the solid phase and the temperature at which a true liquid is arrived at.