May 1, 1991 - digital audio tape (RDAT) system. AC bias recording is a very important technique, but one whose physical basis is very poorly understood.
J. Phys. D Appl. Phys. 25 (1992)1-23. Printed in the UK
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1 1
REVIEW ARTICLE
Magnetic characterization of recording media
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R W Chantrellt and K O'Grady#
* t
Physics Department. University of Keeie, Keele, StaffordshireST5 5BG, UK School of Electronic Engineering Science, UCNW, Bangor, Gwynedd LL57 1UT UK Received 1 May 1991, in final form 2 September 1991
Abstract. Magnetic recording media are important technology materials whose
behaviour remains very poorly understood. However, magnetic measurements form a very important part of the practical characterization of recording media. This review outlines the current techniques of magnetic characterization and considers the effects of current research developments.
1. Introduction
Magnetic recording has been around in some form since early this century, although most of the growth has occurred during the past 2&30 years. The field now encompasses a diversity of applications such as the consumer market of audio and video recording, in addition to the storage of digital information on rigid and floppy disks. This varied range of materials and techniques is often referred to as magnetic information technology (MINT). Magnetic storage of information, in whatever form, has many innate advantages, particularly in terms of erasability coupled with long-term stability of the stored data, and is likely to remain viable for the foreseeable future. However, far from being a mature and established technology, MINT is presently facing enormous challenges, particularly as regards the increased requirements of information storage density and faster access times. These challenges are presently being met by developments in particulate recording media, although many alternative materials and techniques are under consideration. These include thin film media and magneto-optic recording, and the vertical hloch line technique which has considerable theoretical promise. It is not possible to cover the many aspects of these materials in a brief review therefore this review will concentrate on the properties and physical problems confronting the development of existing media. It is not possible, however, to consider the physical properties and characterization of the medium in isolation from the recording process. For this reason we start with an overview of the analogue and digital recording processes, which provides the necessary background 0022-37271921010001+ 23 $03.50 @ 1992 IOP Publishing Ltd
for the full consideration of the scientific problems presented by the media themselves. It should also he acknowledged that although the physics of recording media is a central problem in the field, magnetic recording spans many scientific and engineering disciplines, and covers areas outside the scope of this review. There are, however, many excellent hooks covering the whole of the field of magnetic recording, particularly those by Jorgensen (1980) and Mee and Daniel (1987). 2. The recording process
A recording system consists of a transport mechanism for the medium and one or several transducers by means of which information is transferred to and from the medium. In addition to this there is also external circuitry to process the information. In this review we shall consider only the physics involved with the storage and replay of the information, which depends crucially on the recording technique. It is useful to consider in detail the most common techniques, which are broadly categorized as saturation recording of digital information and analogue recording of audio and video signals. Each of these techniques makes many different demands on the recording medium and so it is necessary to consider the behaviour of recording media in the specific context of the intended application. 2.1. Digital recording
This is the most easily visualized form of recording. The medium, in the form of a rigid or floppy disk 1
R W Chantrell and K OGrady
LOGIC and TIMING C I R C U I l S
WRITE AMPLIFIERS
Motion O f m e d i m
! n,
Region 1 M
M 2 Region 2
.
recorded
signal
replayed signal (b)
Figure 1. (a) Schematic of the digital recording process using a ring head. (b) Schematic write and read waveforms
for the digital recording process.
carrying a thin magnetizable film is rotated at high speed beneath a magnetizing head, as is shown in figure l(a). Another important configuration is the standard reel-to-reel transport of magnetic tape. This is a relatively inexpensive form of digital information storage, but it does not have the rapid access times offered by the rigid or floppy disk and is only useful for long-term storage and back-up facilities. In all cases the storage of a binary digit is achieved by means of a current pulse in the recording head. This consists of a gapped toroid of soft magnetic material, wound with an energizing coil to which the current pulse is applied. The result is a fringing field in the medium, which can be made larger than the coercive force, thus causing the medium to be magnetized. On readback, the recorded information gives rise to an induced voltage produced by the Hux changes in the head. The write and read 2
waveforms are of the form shown in figure l(b); electronic processing is applied afterwards to the read signal in order to retrieve the original square pulse. The digital recording process has been investigated analytically by Middleton and Davies (1984) and Potter (1970) and many others. Magnetically the process is similar to the acquisition of the remanence following the application of a nonsaturating field, the isothermal remanent magnetization (IRM) to be defined later, although importantly, the process takes place in a very short timescale, often less than a microsecond. A further complication is the spatial variation of the head field throughout the medium, which is taken into account in the vector model of the recording process (Ortenburger and Potter 1979, Pottcr and Beardsley 1980, Beardsley 1982). These models are limited by an imperfect understanding of the recording medium itself, for reasons to be discussed fully later. Similar considerations also apply in the case of analogue recording. Before considering analogue recording it is important to point out some of the purely mechanical problems of magnetic recording which arise because of the nature of the head/medium interface. Consider, for example, a Winchester disk, which may have a rotational speed in excess of 3000 RPM. The Hying height of the head over the medium is of the order of tenths of microns, so there is an obvious requirement of a clean surface for the medium. The motion actually creates an air bearing effect which reduces wear, but on the other hand introduces a spacing loss which is an important limit on the resolution of recorded data. In the case of tapes there is often contact between the head and medium, resulting in considerable problems of wear, which can be minimized by the inclusion of lubricants in the tape formulation. There is also a pressing need to keep the surface roughness as low as possible. Such tribological problems form an important area of study which can make a vital contribution to the further development of recording technology. For the most part, however, this review will concentrate on the magnetic behaviour and characterization of recording media, since in this area considerabie progress is possible on a fundamental level.
2.2.
AC
bias recording
bias recording, although somewhat complex as a magnetic process, is an extremely neat solution to the problem of maximizing the linearity and response of the medium in analogue recording. The problem is illustrated in figure 2. Curve a here shows the isothermal remanent magnetization (IRM) as a function of the applied field. This measurement will be described in more detail later, but is essentially the DC response of the system. It can be seen that the response is very non-linear. The first attempt to linearize the response involved the application of a DC bias field Hd to ensure that the medium was operating in the optimum range. AC
Magnetic characterization of recording media IQ
P 8
--
7
Z 6
-x i
I
\
r
Figure 3. Schematic hysteresis loop defining the primary magnetic quantities used for the characterization of recording media.
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3 2 1
0
2
I
6
8 IQ 12 applied field / n e
1I
1
I6
I8
20
Figure 2. An experimental compar.son 01 the tsothermal (curve a) and anhysteretic (curve b) remanent
magnetization, showing the extended linear region lor the latter case.
The technique of AC bias is a later and much more effective method. Essentially the technique involves the application of a large, high-frequency bias field which is reduced to zero in a timescale over which the signal does not vary appreciably. The closest physical measurement to this process is anhysteresis, in which the signal field remains completely static while the AC field is reduced. Anhysteretic measurements are also shown in figure 2 (curve b). In low fields the response is very linear, and characterized by a large value of the anhysteretic susceptibility xi.. In practice, as the tape leaves the region of the head the signal and AC field strengths reduce simultaneously resulting in a reduction of xi. by perhaps a factor of two. This, however, remains the most successful method of recording analogue information. The techniques of analogue recording utilize two formats. Audio recording uses a stationary head, whereas in video recording the head is rotated at an angle to the tape transport direction (helical scan recording). This makes optimal use of the whole area of the tape for recording, as is necessary in order to store the high information density required for recording video signals. A similar technique is used in the rotary digital audio tape (RDAT) system. AC bias recording is a very important technique, but one whose physical basis is very poorly understood. This is essentially because of the strong dipolar interparticle interactions which are known to dominate the process. A further complication is the lack of understanding of the noise spectrum of a recording medium which is also strongly dependent on interactions. In
fact, the interaction problem is central to the fundamental physical investigations of these materials, as will become clear later. A further difficulty is that in general tapes are characterized in terms of their static magnetic properties, whereas anhysteresis and the AC bias recording process are clearly dyriamic phenomena. The relation between the static measurements and the practical (dynamic) response of the medium during the recording process is still very much an unsolved problem. Real progress in this area will only be possible given a much better fundamental understanding of the physics of recording media. The current high level of activity in the field is very promising in this respect.
3. Basic materials requirements
We shall shortly go on to consider the physics of the most common types of recording media. Before doing this, however, it is worthwhile stating explicitly the basic specifications for a useful recording medium in order to provide a link with the technical aspects of recording and additionally to introduce the primary magnetic quantities by which a medium is characterized. The basic physical attribute of a magnetic material which makes it usable for information storage is non-equilibrium behaviour. This can be considered as introducing a ‘memory’ which is clearly essential to any information storage technique. In the case of a magnetic material the non-equilibrium behaviour is represented by the hysteresis loop obtained by measuring the magnetization M as the applied field H is cycled. Figure 3 is a schematic hysteresis loop illustrating the primary magnetic quantities. These are the coercivity Hc, the saturation magnetization M , and remanent magnetization M,. From this we can also define the squareness of the loop S, = M,/M,. A useful recording medium requires a large enough value of M , coupled with a large value of squareness, the actual values depending on the details of the application. The requirements relating to the coercivity are somewhat more stringent. Ideally H , should he large, in order to 3
R W Chantrell and K OGrady
resist demagnetizing effects, although clearly increasing H , makes the write process more difficult. The practical value of H , depends upon the application. The smallest values of H , tend to be in low-density applications. Here, coercivities range from -300400Oe and tend to increase with increasing information density, up to values in excess of 1000 Oe as the specification for video recording. No magnetic material has a perfectly well defined field at which magnetization reversal takes place, since this would result in a square hysteresis loop which is never observed in practice. It is perhaps useful to consider a recording medium as comprised of characteristic activation volumes, each of which has its own 'switching field' at which it reverses its state of magnetization. This parameter depends on detailed considerations of the material and the reversal mechanism as will be discussed later. What is important in the present context is the fact of a distribution in these intrinsic coercivities, known as the 'switching field distribution' (SFD). The SFD naturally tends to reduce the squareness of the hysteresis loop, and a convenient measure of the width of the SFD is obtained from the Williams-Comstock (1971) construction which is shown in figure 3. The parameter (1 - S*), which characterizes the width of the SFD, is essentially a measure of the slope of the hysteresis loop at H = H,:
This parameter has been recommended (Koester 1984) as the hest single parameter for the characterization of recording media on a practical basis. Other methods for measurements of the SFD, based on remanence curves will be discussed in detail later. The SFD has a considerable effect on the analogue recording process as shown by Koester et a1 (1981) who found that the high-frequency output was drastically reduced by increasing (1 - S*) in a series of tapes prepared by mixing cobalt modified iron oxides. The optimum bias current was found to decrease at a rate of -5.6dB per unit (1 - S*). The origin of this effect is ascribed to the intrinsic spread of coercivities associated with (1 - S*) coupled with the overbiasing phenomenon to be discussed later. Generally a small (1 - S') value is required for good recording properties. In digital recording the information storage density is limited by the minimum transition width as determined by demagnetizing effects. Essentially, the maximum demagnetizing field for a thin medium with an arctangent magnetization transition is Hd = -16/2n where a is the transition width (here, and in the rest of the paper, the symbol I is used to represent the magnetization in EMU cm-) ( M = 4x0). Consequently the important parameters are the coercivity H , film thickness 6 and remanence I,. Potter (1970) has shown that the minimum transition width is (for a square hysteresis loop with 1 - S* = 0) 4
a = 61r/Hc.
(1) More recently, Bertram (1986) gave a more general relation for a medium with a finite value of (1 - S*): a = d(l+*)+[(
nQ
d(l
- S') ) 2 + - ]
Ir6d 'I2
(2)
nQK
where Q is a parameter related to the head field and d is the head to medium separation. Generally speaking equation (2) is dominated by the second term with only a weak dependence on (1 - S*). It should be noted that if equation (2) predicts a smaller transition width than the demagnetization limited case (equation (1)) the latter should he chosen. Thus in saturation digital recording the relevant static properties are I, and H,. From the previous discussion it is is clear that the primary characterization of recording media should be in terms of the static properties, and this is certainly the case in practice. However, there are many additional important magnetic and electromagnetic properties which characterize a medium. For the remainder of the review we shall concentrate on introducing those properties and relating them to, on the one hand, the record/replay process itself, and on the other the underlying physics of the medium. Before this, however, we describe briefly the preparation of particulate and thin film recording media, which introduces many important factors relevant to their characterization.
4. Preparation of magnetic recording media 4.1. Particulate media
Elongated particles consisting of a single magnetic domain are the most common magnetic recording media in use today. A wide variety of particles are used and in this section only a brief description of the preparation techniques commonly used is possible. Of course, much of the detail of the particle preparation processes is proprietary to the major manufacturers and :hi ieadir should bc iiiidci iio ilktsion as :o :hc level of care and sophistication that is involved in the bulk preparation of the highly uniform particles which are in current use. To prepare particles which have good magnetic properties and dispersibility, direct precipitation techniques should ideally he used, which will result in particles which have smooth surfaces and are highly uniform in terms of size, anisotropy, etc. Unfortunately many of the particles which have the appropriate properties cannot be prepared directly and more elahorate techniques are required. These techniques are reviewed below. 4.1.1. Preparation of elongated y-Fe,O, particles. The
most widely used material for all recording applications consists of elongated gamma ferric oxide particles. The particles used are between 0.4 and 0.8pm long with
Magnetic characterization of recording media
D
-
lFeOlOH
Goethite
i orlhorhombir I
Feso, no 0 i 1 ,,.",4 Na OH I
Figure 4. The principal stages in the production of elongated y-Fe203 particles lor recording media.
aspect ratios between 6:l and 12:l. The basic reaction to form the particles is shown in figure 4. Needle-shaped iron oxyhydroxide FeOOH is grown on precipitated seeds from a solution containing iron salts (typically FeCI2). Usually orthorhombic aFeOOH, synthetic geothite is used. The dehydration and reduction processes usually require temperatures up to 700°C which can result in the particles sintering together. Accordingly the FeOOH particles are usually coated with chemical complexes often incorporating other elements such as zinc, nickel or tin which inhibit sintering and are also instrumental in determining the size and the aspect ratio of the particles. It is this step in the preparation which is vital to the morphology, dispersibility and magnetic properties of the final product. Thus the precise nature of the additives and the method of their utilization is proprietary to the manufacturers and is not revealed. The reduction process is usually undertaken in a hydrogen or hydrogen rich atmosphere produced using a variety of reducing oils. The final controlled oxidation of the synthetic magnetite to the defect spinel y-Fe20, is undertaken by heating to between 300 and 400°C. This stage of the process much also be carefully controlled to avoid the transformation of y-Fe203 to weakly magnetic a-Fe203.The final value of the coercivity can be enhanced at this stage by failing to complete the oxidation process leaving a final product with a composition (Fe,O,),(Fe,O,), --I (synthetic betholite). The final product is mechanically compressed between rollers to reduce the porosity of the particle although care must be exercised at this final stage to avoid damaging the particles which results in a well known reduction in the coercivity. During this final stage initial dispersants and other chemicals may be added to coat the particles thereby assisting the eventual dispersibility.
In this process it is essential to recall that the properties of the final product are essentially determined by the initial seed formation process. In recent years synthetic lepidocrocite (y-FeOOH) has also been used commercially as the precursor for y-Fe203. The eventual product is a brown (tan) free-running powder with coercivities which range from 260 to 385 Oe. For particles which include a limited amount of Fe30a the coercivity can be as high as 425 Oe. In general the final value of the coercivity for the particles when dispersed and coated onto the substrate can differ from the original powder by up to 20 Oe. For a full review of the preparation of y-Fe203 particles the reader is referred to Bate (1980). Table 1 summarizes the typical properties and applications of y-Fe203particles. 4.1.2. Metal particles. In a purely historical context it
is interesting to note that the first magnetic recording tape was coated with elemental iron particles produced by the thermal decomposition of iron pentacarbonyl. However, small particles of the ferromagnetic elements are highly reactive and in fact may be pyrophoric when exposed to air. They may also be attacked by the dispersants or binders used in the tape coating process. Accordingly they were not used until the recent increase in demand for media with high moment and coercivity for applications such as digital audio tape and 8 mm video. Elongated metal particles can be prepared by two basic routes: either the reduction of oxides or salts of the metals or the decomposition of organometallic compounds or complexes. The former method usually involves the reduction of any of the precursors of yFe,O, particles using hydrogen or other organic reducing agents. Alternatively iron, cobalt or iron-cobalt alloy particles can be prepared by the borohydride 5
R W Chantrell and K OGrady Table 1. Typical properties and applications of y-Fe203particles. Source: Bayer UK technical data
sheet. Application Audio and computer tape, floppy discs Computer tape, floppy discs Low noise audio and video tape Professional audio tape
Particle length (pm)
Aspect ratio
M, (=4nls)
Coercivity
(Gauss)
(Oe)
0.7 0.6
6:l 8:l 10:l 7:l
4000 4000 41 00 4100
285 305 385 300-380
0.5 0.4
reduction of solutions containing the metal salts (Oppegard et a1 1961). The latter process includes carbonyl decomposition, formate decomposition, etc, but these processes are expensive due to the cost of the precursors and have not found wide application. The key to the preparation and use of metal (usually iron) particles lies in the passivation of the particle surface. Due to the reactivity of the metal, organic compounds are in general unsuitable and other metal or oxide coatings are preferred. For example Aonuma (1975) incorporated chromium and potassium sulphate in the borohydride reduction of metal salts to inhibit oxidation and produced a tape with H , = 1000 Oe and a loop squareness of 0.81. The preferred and currently the only utilized technique for the passivation of iron particles for recording is the controlled oxidation of the particle surface, leaving a core of metal which comprises approximately 50% of the particle volume. The oxidation reduces the effective value of the specific magnetization of the bulk material to around 150 EMU g-' but still results in a remanence of double that of a fully oxidized particle. As is the case for y-Fe203the coercivity is entirely controlled by the particle elongation. However, due to the high moment, particle alignment during the coating process is greater resulting in a loop squareness which is generally higher than that achieved for y-Fe203and comparable to the values obtained with C r 0 2 (section 4.1.4). 4.1.3. Cobalt-modified iron oxides. The substitution of
cobalt ions for iron gives rise to a large increase in the magnetocrystalline anisotropy of iron oxides. The cobalt saturated oxide COO Fe203, cobalt ferrite, is unique amongst the inverse spinel ferrites since it has cubic anisotropy with three easy axes along the cube edges. Cobalt ferrite has a large intrinsic anisotropy constant, K , , of 2 x 106ergcm-3 so it is hardly surprising that cobalt inclusions in y-Fe20, can raise the coercivity to the region of 1OOOOe or greater if required. The inclusion of cobalt into iron oxides can be achieved by a variety of techniques. y-Fe203 can be doped uniformly (body doped) with cobalt by adding cobalt salts to the solution before the precipitation of FeOOH or by precipitation of CoOOH onto the FeOOH particles after formation. The remainder of the process is as described in section 4.1.1. The resulting particles exhibit multi-axial or isotropic magnetic behaviour and are in general spherical or have 6
axial ratios
