- Email: [email protected]
Magnetic recording materials since 1975
Journal of Magnetism and Magnetic Materials 100 (1991) 413-424 North-Holland
Magnetic recording materials since 1975 Geoffrey Bate School of Engineering, Santa Clara University, Santa Clara, CA 95053, USA
When information is to be recorded and stored on a re-usable medium, magnetic recording, in one form or another, has been and is today the dominant technology. Magnetic particles or thin films having a coercivity of several hundred oersteds, are easily capable of retaining a magnetic pattern of recorded information (at densities of tens of thousands of bits/inch and track densities of thousands of tracks/inch) for hundreds of years and yet when desired, the pattern can be changed by simply writing the new information over the old. Since the recording process requires only a change in direction of the electron spins, the process is infinitely reversible and the new information may be read immediately with no development process being required. This paper deals with developments of the magnetic (and some important non-magnetic) properties of recording media that have occurred since 1975. It describes their current embodiment in particles of cobalt-modified iron oxides, chromium dioxide, barium ferrite and metals and in thin films of metals, alloys and oxides. The curious and currently unexplained aspects of the behavior of these important materials is noted.
1. Introduction The earliest (and only partially successful) magnetic recording materials were wires of stainless steels (12% nickel, 12% chromium) which had been annealed so that single-domain particles of the ferritic phase precipitated in an austenitic matrix. Coercivities of 200-300 Oe are easily obtained in this way. Two practical problems limited the usefulness of the wires. They twisted so that the region of the wire which contacted the head during writing was not necessarily the same region that was close to the head during the reading operation. Secondly, the wires broke easily and could only be repaired by tying knots. For these reasons the wires were replaced in the 1940s and 1950s by splice-able tapes coated with synthetic particles of ~-Fe203 in a polymeric matrix. In 1956 the particles were also used to coat the 24" diameter disks in the IBM Ramac (Random access) 305 disk machine. From then until the mid-1970s tape and disk coatings were principally of ~/-Fe203 single-domain, particles. Magnetic disks used these particles until ca. 1990. The mechanism of magnetization reversal in the acicular single-domain particles (length, typi-
cally, 0.3 Ixm, diameter = 0.06 I~m) is believed to be incoherent rotation (e.g., "curling") of the spins [1-4].
2. Magnetic particles That "y-Fe203 should have been preferred over Fe304 is, at first, surprising since o-S is higher in Fe304 particles (by 20%). Thus the coercivity is also higher (by 15%) in these acicular particles in which shape anisotropy is more important than magnetocrystalline anisotropy. Furthermore, Fe304 is the penultimate step in the preparation of -/-Fe203 particles and thus Fe304 particles should be marginally cheaper. Particles of magnetite have acquired the reputation of being prone to oxidation to a-Fe203 (non-ferrimagnetic) rather than to ~/-Fe203. The long-term stability of information stored on media made of magnetite particles is also somewhat suspect because they are believed to suffer from magnetic "accommodation" effects. Accommodation is the phenomenon in which the material does not achieve a constant hysteresis loop at the first cycling of the applied field. Only after sev-
0304-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
414
G. Bate / Magnetic recording materials since 1975
eral cycles does the hysteretic behavior become constant [1]. Clearly this would be a highly inappropriate behavior in a magnetic recording material. A related (and equally undesirable property) is "print-through". This is found in tapes and occurs because two layers of magnetic coating are separated only by the thickness of the plastic substrate. The pattern of fields emanating from the recorded signal on one layer, passes through the substrate and may be strong enough to modify the magnetic state of the particles in adjacent layers. The effect is most serious for long wavelength patterns. It thus tends to be more of a problem in audio recording at low frequencies, where one occasionally hears echoes or "preechoes" of music recorded on adjacent layers of the tape. The solution is to control the shape and size distributions of the magnetic particles. One of the principal advantages of magnetic recording is that the recorded information may be changed at any time, by simply recording the new information over the old. The old pattern of magnetization is, in principle, replaced by the new pattern, as long as the recording fields exceed those required to saturate the magnetic coatings. Usually this requires recording fields equal to or greater than four or five times the coercivity, or, in the case of particles of "y-Fe20 3, fields of 1000-1200 Oe. In flexible disks for data recording the overwrite specification is that, after writing and reading a lowest density pattern of frequency 1F and then overwriting with the highest frequency 2F (at the same current), on re-reading the disk the remaining IF signal must be _< 5% of its original level; i.e., the unerased 1F signal must be reduced by 26 dB. Achieving more complete erasure requires the use of overwriting signals at higher currents than those used for the recording of the 1F signal. It appears to be practically impossible to achieve total erasure even though the current used for overwriting is more than five times the amplitude of that used for writing the original 1F pattern. The 1F signal may be decreased by 90 dB but is still detectable [5]. This curiously non-reversible behavior of magnetic recording media has, not surprisingly, been a cause for concern among security agencies.
3. Magnetic properties and recording performance Since the recorded information is stored as a pattern of magnetization along the track followed by the writing and reading heads, we require first that the magnetic material have high remanence, M r. This, in turn, calls for high saturation magnetization, M S. When the stored information is digital data, one (or more) reversals of magnetization correspond to each recorded bit. If the bits are to be stored at high density along the track (to reduce the cost of storage and to reduce the time taken to reach the desired record), the magnetization reversals must be sharply defined. This calls for a narrow distribution of switching fields ( A H / H c) as defined in fig. 1. The recorded pattern (fig. 2) of opposed magnetic dipoles is subject to demagnetization and hence, the coercivity, H c (or more exactly, the remanence coercivity H r) must be high. The driving force of demagnetization is the remanent magnetization M r and the ability to resist demagnetization is described by H r, thus some optimal value of the ratio M r / H r must be found and used in the recording medium.
I+M
+Ms
S'H( + Mr
.Mt..t
/ S
= Mr/Ms
--H SFD= AH/Hc
_.s
/
/ _.
Fig. 1. Magnetic properties important in magnetic recording: coercivity, He, remanence coercivity,Hr, remanence, M T / M s. Switching Field Distribution (SFD)= H / H c (alternate form of switching field distribution = 1- S *).
G. Bate / Magnetic recording materials since 1975
4. High-coercivity particles Particles of unmodified iron oxide are now used only for low recording density applications such as telephone answering recorders. For high fidelity audio recording, video recording and high density data recording, particles having higher coercivity than 300 Oe are needed. 4.1. Cobalt-modified iron oxide Cobalt, when substituted in small amounts (23%) in magnetite and ~/-Fe203, has been known for many years to cause increases of hundreds of oersteds in the coercivity (by increasing the magnetocrystalline anisotropy). Unfortunately, the temperature sensitivity of H c is also increased to unacceptable levels by the substitution of cobalt. The coercivity decreases with increasing temperature. Since the magnitude of the magnetostriction constant, As, also increases with increasing cobalt content, magnetization of the cobalt-doped iron oxide particles is also sensitive to stress [6]. A solution to the problem of using cobalt to increase the coercivity of iron oxide particles
b't Mr5 Hc TRANSITION - - M _ _ _
20.40
_
M - -
CRYSTALLITES
Fig. 2. (a) The recorded transition in digital recording. The magnetic medium, moving from left to right is initially magnetized in the negative direction. The recording field is presently in the positive direction. The transition region width a is inversely related to the maximum recording density. (b) When the recording medium consists of particles or crystallites, the transition region has irregular boundaries which are an important source of media noise.
415
without also increasing their temperature dependence, was reported by Umeki et al. in 1974 [7]. It consisted of immersing the iron oxide particles in a solution of cobaltous sulphate to which sodium hydroxide was then added. Cobaltous hydroxide Co(OH) 2 precipitates on the surface of the particles. On warming to 45°C for 8 h, cobalt ions diffuse into the surface of the iron oxide particles and bring about an increase in coercivity. In particles whose coercivity has been increased by the diffusion of Co ions into a surface layer of the particle, the temperature dependence of coercivity is much less than is found when cobalt is uniformly distributed throughout the volume of the particle. This was clearly demonstrated in an experiment of Kishimoto et al. in 1984 [8]. Starting with particles of ~-Fe20 3 suspended in an alkaline solution containing cobaltous and ferrous ions in a molar ratio (Co2+/Fe 2+) of 0.5, they heated the suspension at 45 °C for 8 h "to crystallize cobalt-ferrite on the ~,-Fe20 3 particles. Fhe resultant particles (cobalt-epitaxial iron oxide particles) had the uniaxial magnetic anisotropy along the acicular axis". After measuring the magnetic properties, the particles were heated at 450 °C in air for 2 h after which "cobaltous ions were uniformly substituted into the ~-Fe20 3 particles". By changing the cobalt content from 3.6 to 7 wt%, the coercivity at 20 °C was increased from 800 to 1300 Oe and the "squareness" ( M r / M s) was increased from 0.70 to 0.72. On measuring the effect of temperatures on He, it was found that (as shown in fig. 3) the temperature coefficient of coercivity was much higher when the cobalt was uniformly distributed throughout the particles than when it was confined to the surface of the particles. The second result was that the value of coercivity at room temperature for a given percent of cobalt was approximately the same whether the cobalt was on the surface or distributed throughout the volume of the particle. The effect of cobalt on the magnetocrystalline anisotropy constant of magnetite (Fe304) was measured by Bickford et al. [9] in 1957 as a function of temperature. Slonczewski [10,11] explained the large change in magnetocrystalline anisotropy constant, Kj, that they observed in
G. Bate / Magnetic recording materials since 1975
416 2000
1500
~
3.6 wt% cobalt
\
Hc (Oe)
1000 o,.~>. ~~~..~. 500
0 - 50
/
impregnated
~ ~ ~~
dope~
~
i
1
i
L
0
50
100
150
_~ 200
T(°C)
Fig. 3. Coercivity in 3.6 wt% Co-modified iron oxide particles as a function of temperature. " D o p e d " cobalt is distributed throughout the volume of each particle. "Impregnated" cobalt is confined to a surface layer of each particle.
terms of "one-ion" model involving the spin-orbit coupling of Co 2+ ions placed in an electric field of trigonal symmetry in the lattice. There is at present no completely satisfactory explanation of the origin of the coercivity and its temperature dependence of cobalt surface-modified iron oxide particles and this is one of the most intriguing of the unsolved problems in the field of recording materials. The absence of understanding of the origin of coercivity in these particles has not prevented their use in high recording density applications such as video and high-performance audio tape, instrumentation tapes and flexible disks. 4.2. Other iron oxides
Even without the complications caused by the addition of cobalt the magnetic behavior of the iron oxides is difficult to understand. The final step in the preparation of particles of `/-Fe203 is the oxidation of Fe304 particles at low temperatures in the presence of water. In some commercial processes this is not the final step. The -/Fe203 particles are reduced back to Fe304 and then re-oxidized and re-reduced, etc., so that the final commercial product may have a composition somewhere between Fe304 and Fe203. Horiishi and Takedoi [12] treated particles of -/-Fe203 with an aqueous solution of either ferrous salts or ferrous hydroxide at 50-100 ° C and pH of 8.0-
14.0 in a reducing atmosphere and then reoxidized to -/-Fe203. The first cycle produced an increase in the particle coercivity from 370 to 420 Oe. The cycle could be repeated several times. The end product had an atomic ratio of Fe 3+ :Fe 2+= 1:0.23; i.e., the material was midway between Fe203 and Fe304. These are often referred to as "Berthollide oxides". Haneda and Kojima [13] investigated the effect of varying the proportion of magnetite in (Fe 3 O4)x(-/-Fe203)l-x and found that the maximum coercivity was achieved at a higher value of x = 0.75. They suggested that their particles were composed of finely divided crystallites of ~/-Fe203 and pure Fe304. Knowles [14] explained the magnetic properties of particles whose composition was intermediate between Fe304 and Fe203 as resulting from "an inner core of Fe304 with an outer layer of `/-Fe203. Owing to the large change in volume which occurs on oxidation, these ... stress each other. Over a certain range of composition, the stress field interacts with the magnetization to increase Hc, and it is also responsible for H c increasing with time after the oxidation process. A similar situation applies to partially reduced ` / - F e 2 0 3 " . 4.3. Chromium dioxide
The existence of a ferromagnetic oxide of chromium, CrO2, was reported as early as 1935, but the material was not made in single-domain form and used a magnetic recording material until 1961 [12]. The process involved the decomposition of CrO 3 under pressure and usually in the presence of water and results in highly acicular particles (aspect ratios of 20 or 30:1 are not unusual) with a very regular prismatic shape. The appearance in the electron microscope is in marked contrast to that of `/-Fe20 3 and Co--/ particles. The two principal problems were the cost of preparation (largely due to batch processing) and the low Curie temperature (O c ~ 120 ° C). Since 1961, much effort has been expended in attempts to reduce the former and increase the latter. Chromium dioxide particles are used commercially in high-quality audio tapes (an application now dominated by particles of cobalt-im-
417
G. Bate / Magnetic recording materials since 1975
pregnated iron oxide) and in the tape cartridges for the IBM 3480 computer tape drive. The low Curie temperature has been used to advantage in the thermoremanent copying of tapes. The recorded tape (made of magnetic materials with Oc considerably higher than that of CrOz) is placed in (stationary) contact with an unrecorded tape of CrO 2 particles at temperatures between 25 and 120 °C. The field pattern emanating from the master tape then records on the CrO 2 slave tape as the temperature is reduced to ambient. The advantage of thermoremanent transfer is that it can be carried out at much higher speeds than are possible by conventional copying; (i.e., by taking the amplified signal, reading from the master and using it to write conventionally on the slave tape). Chromium dioxide tapes have been suspected of suffering from short-term and long-term loss of remanence. It is well established [16] that if a field sufficient to bring a sample of CrO 2 tape to saturation remanence is applied and is followed by a field in the reverse direction slightly less than the coercivity, the magnetization legel will decrease with time even though the applied field is held constant (fig. 4). However, tapes made of particles of cobalt-treated -,/-Fe203 change their level of magnetization in much the same way and so, to a lesser extent, do tapes made of untreated ~/-Fe203 particles, as seen in fig. 4. The long-term effect is, perhaps, more serious, particularly when CrO z particles are used in a medium like 1 / 2 " computer tape which is intended for archival storage of data. The concern arises because of the possible reactions between CrO 2 particles and the polymers used to isolate the particles and to bind them to the substrate which results in a loss of magnetization over time. 4. 4. Metal particles Single-domain particles of the ferromagnetic metals, principally iron may be prepared by three methods: 1) reduction in hydrogen of the oxides or oxyhydroxides, 2) decomposition of the borohydride by heat and 3) condensation of the vaporized metal. The advantages of using the metal, rather than the oxide, in the form of particles for
03,°o°:
,o,K' 1-
• 6K
O_uO 4 -
e~
o 00
,"
-0.5 H(kOe)
J
• 300K
4
~
-1.0
15
L'
' 'II
O0o ,oo,yo,''' ~o-I
~
%
O0
-0.5
-1.0
-1.5
-2.0
HikOe) i
:
_
,
i
31
n
. oo,
II
o~,<
li
I
-i,I
"T o
\
,
.:o
/
,":,1oo
-5
H(kOe) Fig. 4. Time decay of magnetization at 300, 100 and 6 K, for particles of: (a) "y-F%O3; (b) cobalt-surface modified 3,-Fe203; (c) CrO 2. After applying a saturating field in the positive direction fields plotted along the abscissa are applied in the inverse direction and the magnetization is measured as a function of time that the plotted field is applied. M(100 s) is the magnetization after the reverse field is applied for 100 s. M(1000 s) is the magnetization after the reverse field is applied for 1000 s [16hi.
recording are the higher saturation and remanent magnetizations .and thus, in particles whose anisotropy is dominated by shape, higher coerciv-
418
G. Bate / Magnetic recording materials since 1975
ities. The disadvantages are higher cost of preparation and greater chemical reactivity with the surrounding organic binder materials and with the atmosphere. The particles currently in use in high-quality audio cassettes (analog and digital) have coercivities up to 2100 Oe [17]. This is made up of the contribution of shape anisotropy ( = 1900 Oe) and magnetocrystalline anisotropy ( ~ 200 Oe). Since the outer shell of the particles is deliberately oxidized to provide a barrier against further corrosion, the saturation magnetization of the particles is somewhat less (984 to 1338 e m u / c m 3) than the 1714 e m u / c m 3 for pure iron. Similar values for the saturation magnetization are obtained if the iron particles are converted to iron nitride, Fe4N.
4.5. Barium ferrite particles Barium ferrite B a O . 6 F % O 3 has the distinction of being the most widely used permanent magnet material. It contains no expensive ingredients and has, in fine particle form, coercivities of 2000-3000 Oe. The saturation magnetization of 60 e m u / g at room temperature not surprisingly is less than that of the iron oxides. Before the material can be used in fine particle form for magnetic recording purposes, it is necessary to reduce the coercivity to >_ 1000 Oe without reducing still further a magnetization level which is already low. A method of accomplishing both goals was achieved by Kubo et al. [17]. They melted a mixture of BaO, F e 2 0 3 and B 2 0 3 and rapidly cooled the liquid by running it between steel rollers. The boron oxide provided a glass-like matrix within which single-domain platelets of barium ferrite formed and were then crystallized by reheating. The B203 was removed with hot acetic acid from the BaO - 6Fe203 particles which had an average diameter of ---0.08 ixm and a thickness of = 0.03 Ixm; i.e., the particles had the form of hexagonal platelets. To reduce the coercivity to levels suitable for a recording medium, cobalt and titanium in the form of CoO and TiO 2 were added to the starting material and incorporated in the crystal lattice, BaFet2_2xCoxTixO~9. When x = 0.75 the coercivity is reduced to = 900
Oe, while the value of magnetic moment density o-= 60 e m u / g (almost the same as its value at x = 0). The addition of cobalt and titanium is believed to support the hexagonal axis as the easy axis of magnetization. Thus crystalline anisotropy encourages magnetization along the hexagonal axis while shape anisotropy supports magnetization in the basal plane. The coercivity is then given by the difference between the crystalline term 2K/M~ and the shape term NMS or ( H c) = 0.48(
2KM~-NMs) '
where K = + 9.45 x 105 is the magnetocrystalline anisotropy constant (erg/cm3), [0.48] is a "randomizing" factor which is applied to particle samples in which the c-axis of the platelets are not aligned. M~ is the saturation magnetization (emu/cm3); N = 2xr is the demagnetizing factor along the c-axis. For a temperature of 20 o C and x =0.8; H = 9 6 0 Oe. As the temperature increases M S becomes smaller and the first term (2K/Ms), grows larger while the second term (-NM~) becomes smaller in magnitude and remains negative in sign. Thus both terms encourage H c to grow larger with increasing temperature; an unusual behavior for a magnetic recording material. Dispersion of the particles within the solvent and binder materials is difficult since, in addition to surface tension effects, the particles experience a magnetic force of attraction to other particles, thus forming a stack which is difficult to separate into individual particles. Corradi et al. [18] and Kishimoto and Kitahata [19] showed that the increase in Hc with increasing temperature obtained by Kubo et al. [17] becomes more complicated as the range of temperatures investigated is increased. Below --- 250 K and above = 450 K the coercivity of Co, Ti doped barium ferrite particles decreases with increasing temperature. No simple explanation of these results has been found. Suzuki et al. [20] described a potentially important, new application of barium ferrite particles. They are used as the magnetic component of
419
G. Bate / Magnetic recording materials since 1975 Table 1 Table of properties of magnetic particles Property
Particle y-Fe203
Co-Fe203
CrO 2
BaO.6 Fe20 3 + C o , Ti
Fe
336-340 0.5 [863]
350-355 0.5-0.8 [863]
340-390 0.5 390-400
380 0.6-0.7 590
870-1100 0.23 + 0.52 1040
-4.64×103 -5×10 -6 20-28
- 5 to + 1 0 0 X 1 0 3 - 5 to - 1 5 × 1 0 - 6 43.8-60
+2.5>< 104 +1×10 6 36-53
+ 3.3 x 105 +10X10 -6
+ 4.4 x 10 4 +4X10 6
26-157
30-131
0.26-0.61
0.30-0.6
0.35-0.6
0.16-0.6
0.5-0.75
+3.1 xl0 -3
-0.6x
15-31 5.28
26 5.8
Saturation magnetization, M S [kA/m]
Mr/M S Curie temperature, Tc [K] Magnetocrystalline anisotropy constant, K~ [ J / m 3] Saturation magnetostriction, As Coercivity, H c [ k A / m ] Switching field distribution, AH/H c Temperature coeff, of coercivity, AHc/Hc°C(20-70°C)
-1×10
3
impreg, d-2.4×10
-3
-5×10
3
10 3
doped - 1 0 X 10 - 3 Specific surface area ( B E T ) [m2/g]
Density p Crystal structure
20-40 4.60
20-40 4.80
cubic
cubic
25-37 4.88-4.95
a 0 = 0.25 n m ,
Ferro- or ferrimagnetic Particle size [~,m]
m3m ferri needles
ferri needles
l = 0.3 d = 0.06
- / = 0.3 d = 0.06
equi ax. l = 0.2
tetragonal
hexagonal
bcc
a = 0.4218 nm c = 0.9182 nm
c = 2.32 n m a = 0.589 nm
a = 2861 nm
ferro needles
ferri hex. platelets dia = 0 . 0 8 - 0 . 1 thick = 0.001
ferro needles
l = 0.5 d = 0.05
1 = 0.3 d = 0.06
-0.025
the "slave" tapes in the contact duplication of DAT (Digital Audio Tape) masters. The coercivity of the barium ferrite tape was 600-700 Oe well below the coercivity, 2100 Oe, of the metalparticle master tape. Table 1 summarizes the properties of magnetic particles for recording applications.
5. Thin film recording media The particulate recording media discussed thus far have undoubtedly been conspicuously successful as storage media for audio, video and data over the past quarter century, but they are still far from ideal. Their magnetic constituent, in the form of fine particles of metals, alloys or oxides, occupies 40% of the coating volume in tapes and flexible disks and only 20% in hard disks. Since
the 1960s many attempts have been made to displace particulate recording media with metallic thin films, but it is only in the last five years that those efforts have been successful. Particulate recording materials still dominate flexible media; e.g., computer tapes and flexible disks, but hard disks now increasingly are made of thin films deposited by vacuum deposition or, more commonly, by sputtering. It is appropriate, at this point to review briefly the salient characteristics of each type of medium.
5.1. Particulate coatings The magnetic properties of the finished coating are determined almost entirely by the magnetic properties of the particles which can be measured in the "dry" state before the ink is mixed and coated on the substrate. Changes in
420
G. Bate / Magnetic recording materials since 1975
magnetic properties from the dry powder to the finished coating are relatively slight and generally quite predictable. The mechanical properties of the coating are determined overwhelmingly by the binder polymers (which occupy 60% of the coating volume), while the magnetic properties are determined by the particles. Thus, to a considerable extent it is possible to vary and control the mechanical properties independently of the magnetic properties. In the manufacture of tapes or flexible disks, high coating speeds (hundreds of f e e t / m i n u t e ) and wide rolls (_< 2 m) can be used to achieve high production rates. Uniform properties within a roll of coated material (and from roll to roll) are achievable. The materials and processes are generally well-understood and thus yields are high and coating costs are low.
react differently with the solvent and, much more importantly, with the binder polymers, dispersants, etc. affecting the durability and coefficients of friction of the coating as well as its recording performance. The saturation magnetization of thin films can be high, almost equal to that of the pure metal. The coercivity and other magnetic properties are continuously variable, within limits, by changing the deposition conditions and thus fine-tuning of the magnetic properties is possible to do in hours rather than the months required for particulate coatings. Four methods of deposition have been used to prepare thin metallic films for recording purposes. They are: 1) electroplating, 2) autocatalytic plating, 3) evaporation and 4) sputtering.
5.2. Thin-film coatings
In this process the metallic ions are reduced from an aqueous solution at the cathode using electrons from an external source of current. The process can be continuous, inexpensive and can be used to deposit two metals at the same time if, as with cobalt and nickel, their deposition potentials are similar. It was found experimentally that, to obtain high-coercivity films, a high cobalt content was required. Alloy films of C o - N i rather than C o - F e are used because the deposition potentials of Co and Ni are very similar and films of C o - N i were more likely to show good corrosion resistance than would films of C o - F e . The basic plating path was introduced by Bonn and Wendell [21] and a practical form of it was described by Koretzky [22]. The electrolyte was made up of cobalt and nickel chlorides, ammonium chloride (found to enhance the compositional uniformity of the film) and sodium hypophosphite. The latter ingredient was used to add 1 - 3 % phosphorous to the plated films where it segregated at the crystal boundaries and served to control the size of the crystallites and hence, the coercivity. The other parameters found to influence grain size and grain separation (and hence, noise in the finished films) were pH, temperature, substrate smoothness and, most importantly, plating current density.
It is relatively easy to make coatings thinner than 0.25 I~m by sputtering or vacuum evaporation but relatively difficult to make such thin coatings from particles. A thin coating is required to achieve high recording densities, particularly on disks where there is no erase head and a single head must achieve a compromise between the differing requirements of writing and reading. Thin coatings allow a greater length of tape to be packed into a reel of a given diameter and thus enable high volumetric densities to be achieved. The intensity of magnetization is much higher in thin films since the magnetic constituent is not diluted by a binder. Furthermore the coercivity and other relevant magnetic properties are continuously variable (within limits) by changing the deposition conditions; e.g., in a system in which the film is deposited by sputtering, varying the target voltage, the bias voltage and the partial pressure of argon allows fine tuning of the coercivity and remanence of the deposited film. In contrast, changing the magnetic properties of a particulate coating usually involves a major development effort. Changing the magnetic properties almost always involves changing the chemical behavior of the surface particles which then
5.3. Electroplated films
G. Bate / Magnetic recording materials since 1975
~p~//
/
Substrate
5.4. Autocatalytically deposited films It is possible to deposit from solution, films of cobalt and nickel alloys by a deposition process in which the potential required for the reduction of the metal ions is provided by a reducing agent, such as a hypophosphite or a borohydride, rather than by an external power supply. Each layer of atoms of Co and Hi deposited on the substrate serves to catalyze the deposition of a further layer. Films having a coercivity of < 1800 Oe and saturation magnetization up to 80% of that of the bulk alloy of C o - N i have been made autocatalytically. It is also possible to use the autocatalytic process to deposit films of C o - N i - P continuously on polyethylene terephthalate films on which, to start the autocatalysis, a thin film of N i - P has first been chemically deposited.
421
.. S.ie,. /
\ \
,
Crystal monitor
/
-
/
tl /
©
/ i~ l. " Shutter, ,i
5.5. Films prepared by vacuum evaporation Films of the ferromagnetic elements and their alloys deposited by vacuum evaporation are continuous, highly conducting but of coercivity < 50 Oe, much too low for recording purposes. Some way must be found to give the films a single-domain structure. A method of overcoming this problem was described by Speliotis, Bate, Alstad and Morrison [23]. They controlled the coercivity of the deposited film by varying the angle of incidence of the arriving metal atoms so that needle-shaped crystallites grow roughly along the direction of deposition. In this way coercivities < 1000 Oe could be achieved. Substrate temperatures were in the range 2 0 - 5 0 ° C and so the method could be used to make tapes on polyethylene terephthalate substrates. The metals were evaporated by electron bombardment and films of thickness in the range 0.01-0.4 ~m were deposited. This approach was used by Rossi et al. [24,25] shown in fig. 5. A magnetic film of Fe (42.5 at%), Co (42.5 at%), Cr (15 at%) was deposited at an angle of incidence of 60 ° while the substrate was rotated to ensure circumferential uniformity of the deposited film. It was found necessary to control to close tolerances the rate of deposition since the vapor pressure (and hence the rate of
(
1/6 of substrate surface
Fig. 5. Vacuum evaporation (by e-beam gun) at an angle of 60 ° to the surface of a rotating disk Substrate. The angle and the thickness distribution are controlled by the apertures in the shields [24]. evolution) of chromium is appreciably higher than that of iron or cobalt. An underlayer of Haynes 188 stainless alloy was sputtered onto the conventional rigid-disk substrate of AI-Mg. This layer served to act as a corrosion barrier between the substrate and the metal film and also could be polished to a smooth but textured underlayer for the magnetic film. The purpose of texturing (which has become an important feature of thinfilm rigid disks) is to prevent adhesive contact between disk and head. It is a critical step in the manufacturing process since if the texturing is too light the head and magnetic film may wring together, while texturing that is too heavy will intro-
G. Bate / Magnetic recording materials since 1975
422
d u c e an u n a c c e p t a b l e s e p a r a t i o n b e t w e e n h e a d a n d m a g n e t i c c o a t i n g . It s h o u l d b e r e m e m b e r e d that, at c u r r e n t r e c o r d i n g d e n s i t i e s , a h e a d - t o - r e c o r d i n g film d i s t a n c e o f 4 ixin. (1000 A ) m a y b e i n t o l e r a b l y large. In a d d i t i o n to e l i m i n a t i n g h e a d - d i s k sticking, t h e t e x t u r i n g c a n also b e u s e d to e n h a n c e t h e c i r c u m f e r e n t i a l easy axis o f m a g n e t i z a t i o n o f t h e F e C o C r layer.
volts is a p p l i e d b e t w e e n t w o p a r a l l e l plates. T h e s e are the (positive) substrates (the surface on which t h e film is to b e d e p o s i t e d ) a n d t h e ( n e g a t i v e ) t a r g e t ( m a d e o f t h e m a t e r i a l to b e d e p o s i t e d ) . A r g o n i o n s a r e a t t r a c t e d to t h e t a r g e t a n d k n o c k o u t a t o m s o f t h e t a r g e t m a t e r i a l . By v i r t u e o f their acquired momentum the dislodged atoms migrate toward and some are deposited on the s u b s t r a t e . O n e o f t h e key a d v a n t a g e s to s p u t t e r ing o v e r v a c u u m e v a p o r a t i o n is t h a t t h e c o m p o s i t i o n o f t h e d e p o s i t e d f i l m is t h e s a m e as t h a t o f the target and does not depend on the relative vapor pressure of the target's component elem e n t s . By c h a n g i n g t h e t a r g e t c o m p o s i t i o n , t h e composition of the deposited film can be changed in a p r e d i c t a b l e w a y a n d t h u s t h e t a s k o f e x a m i n -
5.6. Sputtered films S p u t t e r i n g is d o n e in a v a c u u m c h a m b e r p u m p e d d o w n to a p r e s s u r e o f 0.75 × 10 - 6 to 0.75 x 10 - 7 T o r r a n d filled w i t h a gas, usually a r g o n to a p r e s s u r e o f 10 - 3 to 7.5 × 10 - 2 T o r r [2]. A p o t e n t i a l d i f f e r e n c e o f s e v e r a l t h o u s a n d
Table 2 Typical properties of magnetic film media Material
Co Fe Ni Co-Ni Co-Fe Co-Sm Co-P Co-Re Co-Pt Co-NiP Co-(30 at%) Ni:N 2 Co-Ni: 0 2 Co-Ni-Pt Co-Ni-W Fe30 4 ~/-Fe20 3 : Co ~,-Fe20 3 : Os Co-(18 at%) Cr Co-(20 at%) Cr Co-(22 at%) Cr
Deposition process a)
Orientation b)
Dominant crystal structure c)
M s
H c
(kA/m)
(kA/m)
OIE NIE, SP OIE SP OIE SP OIE OIE N1E (e) EL, EP SP SP El, EP SP
IPA IPI IPA IPI IPA IPI IPA IPA IPA IP1 IPI IPI IPI IPI
hcp hcp bcc bcc fcc fcc hcp/fcc bcc amorphous hcp hcp/fcc hcp/fcc hcp/fcc hcp/fcc
1100-1400 1100-1400 1600 1600 400 400 800-1200 1400-1600 500-1000 800-1100 500- 750 800-1400 600-1000 650
OIE SP SP NIE, SP SP SP SP SP SP
IPA IP* I IPI IPI IPI IPI ± ± ±
hcp/fcc hcp/fcc hcp/fcc is is is hcp hcp hcp
300800450 400 200240 300400 300-
400 900
250 550 340
S, S *
K u
(105 m -3)
61-120 30- 60 d) 60- 90 10 20- 28 30- 70 60-120 33- 55 36- 96 18- 58 --o 140 40-120 80 80 60- 70 30- 50 17- 32 40-100 160 80-100 ( ± ) 65- 95 ( ± ) 80-105 ( ± )
4 (bulk) 0.3-3.0
0.9, 0.9 1.1 0.9, 0.9 0.9, 0.9 0.8, 0.8 0.96 0.7-0.8 0.9, 0.97 0.8, 0.8 0.8, 0.8 0.8, 0.8 -
1.0 0.15 0.4
a) OIE = Oblique Incidence Evaporation; NIE = Normal Incidence Evaporation; SP = SPuttered; EL = ElectroLess plated; EP = ElectroPlated. b~ 1PA = In-Plane Anisotropic; IPI = In-Plane Isotropic; ± = perpendicular. c) hcp= hexagonal close-packed; fcc = face-centered cubic; bcc = body-centered cubic; is = inverse spinel. d) Coercivities for Co films deposited on Cr or W underlayers. ¢) Co-Sm values for films deposited in the presence of an orienting field.
G. Bate / Magnetic recording materials since 1975
ing the effect of varying the composition of binary, ternary or quaternary alloys is greatly simplified and accelerated. The principal independent variables (other than target composition) are the argon pressure and the target and substrate voltages. Belk, George and Mowry [26] studied experimentally and theoretically the noise in thin-film longitudinally oriented magnetic films of CoP, CoNiP, CoRe, CoSm and found in each case that the disk noise in n V e / H z increased markedly with increasing bit density. Only with particulate disks of "y-Fe203 and a perpendicularly magnetized film of CoCr was there no increase (~/Fe203) or decreasing noise (CoCr). They showed experimentally and theoretically that the physical mechanism for the noise was the fluctuation that occurs from bit to bit in the geometry of the zigzag transitions separating the bit cells. The effect of changes in argon pressure, substrate bias and substrate temperature on noise level in similar, sputtered films of CoPtCr on a chromium underlayer, were measured by Yogi et al. [27]. Noise decreased as argon pressure increased and as substrate temperature and bias voltage decreased. These changes in the condition under which the films were made are ones that might be expected to produce grains that are increasingly more isolated from neighboring grains. Thus the current explanation of the resuits of Belk et al. [26] and Yogi et al. [27] appears to be that the noise arises principally because, in the case of Belk's films, the grains are sufficiently close together to enable exchange forces to correlate the spin directions of crystallites on opposite side of a recorded transition. By increasing the argon pressure and reducing substrate temperature and bias; as in Yogi's films, non-magnetic layers form at the surface of the grains, thus separating the grains and strongly reducing the. extent of the exchange coupling. The modulation noise is thus reduced. The effects of argon pressure on magnetization, S* (coercivity squareness), coercivity and noise level in Yogi's films are shown in fig. 6. The structure, orientation and magnetic properties of thin films for recording media are summarized in table 2 (Arnoldussen [30]).
423
600
u 400 " ~ 200 . . . . . . . .
O0 o
L
.......
,
i0 ~
•
. . . . .
.......
1o2
1.0 I 0.9
e
~
e
~
t
~
0.8 ~ e
~
e
0.7 0.6 . . . . . . . .
0.5 0o
J
.......
to ~
i
; ', ; ' , ' , ; i o 2
1800 1600 o "--" 1400
1200 1000 0°
PAr (mtorr) '
'
i
' ' ''I
,
CoPtCr/Cr
O
(/1
E E C3 0 0
Oq v
E Z
10-2 9 i
i
I
i
i
I
I I
i
I
i
I
I
PAr (mtorr) Fig. 6. The magnetic properties and normalized media noise of C o P t C r / C r media as a function of argon sputtering pressure [27].
424
G. Bate / Magnetic recording materials since 1975
6. Conclusion
During the past 15 years some very difficult problems in magnetic recording materials and drives have been tackled and solved. As a result the performance (i.e., bit density, track density, access time to the data, data rate, reliability, etc.) of recording systems have been greatly improved. The information storage density (the product of bits/inch and tracks/inch) allows us to quantify one aspect of recording performance. In 1975 the IBM 3350 disk drives stored 3 × 106 bits/in. 2 by flying the head 0.4 txm over the disk. In 1990 IBM published the results of successful laboratory demonstrations of disks operating at 1 × 109 bits/in. 2 [27,28]. This involved "flying" the head 0.04 Ixm (i.e., 0.4 mean free paths in air) above the surface Of the disk in order to record and read at a linear density of 158000 bits/in. The track density was 7470 tracks/in. In 1991 the areal density was increased in the Hitachi laborators to 2 × 109 bits/in. 2 [29]. The IBM recording medium was built of five layers in which the sputtered, CoPrCr magnetic layer (12-40 nm thick) was isolated from corrosive interaction with the disk substrate by a layer of chromium 50-200 nm thick. A protective layer of hard carbon (12.5 nm thick) was deposited and was itself covered by a thin film of lubricant. It is significant that the magnetic part of the disk consisted of only one layer out of the five. This strongly suggests that, as we look forward to further improvements in magnetic recording media, over the next 15 years, the most challenging problems will increasingly be the non-magnetic ones.
References [1] G. Bate, Recording Materials, in: Ferromagnetic Materials, vol. 2, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980) chap. 7, p. 381. [2] E. Koster and T.C. Arnoldussen, Recording Media, in: Magnetic Recording, vol. 1, Technology, eds. C.D. Mee and E.D. Daniel (McGraw-Hill, New York, 1987) chap. 3. [3] W. Steck, J. de Phys. 46 (1985) 33. [4] M.P. Sharroek, IEEE Trans. Magn. MAG-25 (1989) 4374.
[5] W.A. Manly, Jr., IEEE Trans. Magn. MAG-12 (1976) 758. [6] P.J. Flanders, 1EEE Trans. Magn. MAG-12 (1976) 770. [7] S. Umeki, T. Uebori and Y. Imaoka, IEEE Trans. Magn. MAG-10 (1974) 655. [8] M. Kishimoto, M. Amemiya and F. Hayama, J. Appl. Phys. 55 (1984) 2272. [9] LR. Bickford, J.M. Brownlow and R.F. Penoyer, Elec. Eng. 104B suppl. 5 (1957) 238. [10] J.C. Slonczewski, J. Appl. Phys. 29 (1958) 448. [11] J.C. Slonczewski, J. Appl. Phys. 32 (1961) 253S. [12] N. Horiishi and A. Takedoi, Japan. Patent 75 101 299 (1975). [13] K. Haneda and H. Kojima, J. de Phys. 40 (1979) C2583. [14] J.E. Knowles, J. Magn. Magn. Mater. 22 (1975) 263. [15] T.J. Swoboda, P. Arthur, N.L. Cox, J.N. Ingraham, A.L Oppegard and M.S. Sadler, J. Appl. Phys. 32 (1961) 3745. [16] (a) R.M. Kloepper, B. Finkelstein and D. Braunstein, IEEE Trans. Magn. MAG-20 (1984) 757. (b) S.B. Oseroff, D. Clark, S. Schultz and S. Shtrikman, IEEE Trans. Magn. MAG-21 (1985) 1495. [17] O. Kubo, T. Ido and H. Yokoyama, IEEE Trans. Magn. MAG-18 (1982) 1122. [18] A.R. Corradi, D.E. Speliotis, G. Bottoni, D. Candolfo, A. Cechetti and F. Masoli, IEEE Trans. Magn. MAG-25 (1989) 4066. [19] M. Kishimoto and S.-I. Kitahata, IEEE Trans. Magn. MAG-25 (1989) 4063. [20] T. Suzuki, T. Ito, M. Isshiki and H. Saito, IEEE Trans. Magn. MAG-25 (1989) 4060. [21] T.H. Bonn and D.C. Wendell, Jr., US Patent 2 644 787 (1953). [22] H. Koretzky, Proc. 1st Austr. Conf. on Electrochemistry (1963) 417. [23] D.E. Speliotis, G. Bate, J.K. Alstad and J.R. Morrison, J. Appl. Phys. 36 (1965) 972. [24] E.M. Rossi, G. McDonnough, A. Tietze, T.C. Arnoldussen, A. Brunsch, S. Doss, M. Henneberg, F. Lin, R. Lyn, A. Ting and G. Trippel, J. Appl. Phys. 15 (1984) 2254. [25] T.C. Arnoldussen, E.M. Rossi, A. Ting, A. Brunsch, J. Scheider and G. Trippel, IEEE Trans. Magn. MAG-20 (1984) 821. [26] H.R. Belk, P.K. George and C.S. Mowry, IEEE Trans. Magn. MAG-21 (1985) 1350. [27] T. Yogi, T.A. Hguyen, S.E. Lambert, G.L Gorman and G. Castillo, IEEE Trans. Magn. MAG-26 (1990) 1578. [28] T. Yogi, C. Tsang, T.A. Hguyen, K. Ju, G.L Gorman and G. Castillo, IEEE Trans Magn. MAG-26 (1990) 2271. [29] M. Futamoto, F. Kugiya, M. Suzuki, N. Takano, H. Fukuoka, Y. Matsuda, Y. Inaba, T. Takagaki, Y. Miyamura, K. Akagi, T. Nakao, H. Sawaguchi and T. Munemoto, 5th Joint MMM-Intermag Conf., Pittsburgh (1991) submitted. [30] T.C. Arnoldussen, Proc. IEEE 74 (1986) 1526.
Magnetic recording materials since 1975 Geoffrey Bate School of Engineering, Santa Clara University, Santa Clara, CA 95053, USA
When information is to be recorded and stored on a re-usable medium, magnetic recording, in one form or another, has been and is today the dominant technology. Magnetic particles or thin films having a coercivity of several hundred oersteds, are easily capable of retaining a magnetic pattern of recorded information (at densities of tens of thousands of bits/inch and track densities of thousands of tracks/inch) for hundreds of years and yet when desired, the pattern can be changed by simply writing the new information over the old. Since the recording process requires only a change in direction of the electron spins, the process is infinitely reversible and the new information may be read immediately with no development process being required. This paper deals with developments of the magnetic (and some important non-magnetic) properties of recording media that have occurred since 1975. It describes their current embodiment in particles of cobalt-modified iron oxides, chromium dioxide, barium ferrite and metals and in thin films of metals, alloys and oxides. The curious and currently unexplained aspects of the behavior of these important materials is noted.
1. Introduction The earliest (and only partially successful) magnetic recording materials were wires of stainless steels (12% nickel, 12% chromium) which had been annealed so that single-domain particles of the ferritic phase precipitated in an austenitic matrix. Coercivities of 200-300 Oe are easily obtained in this way. Two practical problems limited the usefulness of the wires. They twisted so that the region of the wire which contacted the head during writing was not necessarily the same region that was close to the head during the reading operation. Secondly, the wires broke easily and could only be repaired by tying knots. For these reasons the wires were replaced in the 1940s and 1950s by splice-able tapes coated with synthetic particles of ~-Fe203 in a polymeric matrix. In 1956 the particles were also used to coat the 24" diameter disks in the IBM Ramac (Random access) 305 disk machine. From then until the mid-1970s tape and disk coatings were principally of ~/-Fe203 single-domain, particles. Magnetic disks used these particles until ca. 1990. The mechanism of magnetization reversal in the acicular single-domain particles (length, typi-
cally, 0.3 Ixm, diameter = 0.06 I~m) is believed to be incoherent rotation (e.g., "curling") of the spins [1-4].
2. Magnetic particles That "y-Fe203 should have been preferred over Fe304 is, at first, surprising since o-S is higher in Fe304 particles (by 20%). Thus the coercivity is also higher (by 15%) in these acicular particles in which shape anisotropy is more important than magnetocrystalline anisotropy. Furthermore, Fe304 is the penultimate step in the preparation of -/-Fe203 particles and thus Fe304 particles should be marginally cheaper. Particles of magnetite have acquired the reputation of being prone to oxidation to a-Fe203 (non-ferrimagnetic) rather than to ~/-Fe203. The long-term stability of information stored on media made of magnetite particles is also somewhat suspect because they are believed to suffer from magnetic "accommodation" effects. Accommodation is the phenomenon in which the material does not achieve a constant hysteresis loop at the first cycling of the applied field. Only after sev-
0304-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
414
G. Bate / Magnetic recording materials since 1975
eral cycles does the hysteretic behavior become constant [1]. Clearly this would be a highly inappropriate behavior in a magnetic recording material. A related (and equally undesirable property) is "print-through". This is found in tapes and occurs because two layers of magnetic coating are separated only by the thickness of the plastic substrate. The pattern of fields emanating from the recorded signal on one layer, passes through the substrate and may be strong enough to modify the magnetic state of the particles in adjacent layers. The effect is most serious for long wavelength patterns. It thus tends to be more of a problem in audio recording at low frequencies, where one occasionally hears echoes or "preechoes" of music recorded on adjacent layers of the tape. The solution is to control the shape and size distributions of the magnetic particles. One of the principal advantages of magnetic recording is that the recorded information may be changed at any time, by simply recording the new information over the old. The old pattern of magnetization is, in principle, replaced by the new pattern, as long as the recording fields exceed those required to saturate the magnetic coatings. Usually this requires recording fields equal to or greater than four or five times the coercivity, or, in the case of particles of "y-Fe20 3, fields of 1000-1200 Oe. In flexible disks for data recording the overwrite specification is that, after writing and reading a lowest density pattern of frequency 1F and then overwriting with the highest frequency 2F (at the same current), on re-reading the disk the remaining IF signal must be _< 5% of its original level; i.e., the unerased 1F signal must be reduced by 26 dB. Achieving more complete erasure requires the use of overwriting signals at higher currents than those used for the recording of the 1F signal. It appears to be practically impossible to achieve total erasure even though the current used for overwriting is more than five times the amplitude of that used for writing the original 1F pattern. The 1F signal may be decreased by 90 dB but is still detectable [5]. This curiously non-reversible behavior of magnetic recording media has, not surprisingly, been a cause for concern among security agencies.
3. Magnetic properties and recording performance Since the recorded information is stored as a pattern of magnetization along the track followed by the writing and reading heads, we require first that the magnetic material have high remanence, M r. This, in turn, calls for high saturation magnetization, M S. When the stored information is digital data, one (or more) reversals of magnetization correspond to each recorded bit. If the bits are to be stored at high density along the track (to reduce the cost of storage and to reduce the time taken to reach the desired record), the magnetization reversals must be sharply defined. This calls for a narrow distribution of switching fields ( A H / H c) as defined in fig. 1. The recorded pattern (fig. 2) of opposed magnetic dipoles is subject to demagnetization and hence, the coercivity, H c (or more exactly, the remanence coercivity H r) must be high. The driving force of demagnetization is the remanent magnetization M r and the ability to resist demagnetization is described by H r, thus some optimal value of the ratio M r / H r must be found and used in the recording medium.
I+M
+Ms
S'H( + Mr
.Mt..t
/ S
= Mr/Ms
--H SFD= AH/Hc
_.s
/
/ _.
Fig. 1. Magnetic properties important in magnetic recording: coercivity, He, remanence coercivity,Hr, remanence, M T / M s. Switching Field Distribution (SFD)= H / H c (alternate form of switching field distribution = 1- S *).
G. Bate / Magnetic recording materials since 1975
4. High-coercivity particles Particles of unmodified iron oxide are now used only for low recording density applications such as telephone answering recorders. For high fidelity audio recording, video recording and high density data recording, particles having higher coercivity than 300 Oe are needed. 4.1. Cobalt-modified iron oxide Cobalt, when substituted in small amounts (23%) in magnetite and ~/-Fe203, has been known for many years to cause increases of hundreds of oersteds in the coercivity (by increasing the magnetocrystalline anisotropy). Unfortunately, the temperature sensitivity of H c is also increased to unacceptable levels by the substitution of cobalt. The coercivity decreases with increasing temperature. Since the magnitude of the magnetostriction constant, As, also increases with increasing cobalt content, magnetization of the cobalt-doped iron oxide particles is also sensitive to stress [6]. A solution to the problem of using cobalt to increase the coercivity of iron oxide particles
b't Mr5 Hc TRANSITION - - M _ _ _
20.40
_
M - -
CRYSTALLITES
Fig. 2. (a) The recorded transition in digital recording. The magnetic medium, moving from left to right is initially magnetized in the negative direction. The recording field is presently in the positive direction. The transition region width a is inversely related to the maximum recording density. (b) When the recording medium consists of particles or crystallites, the transition region has irregular boundaries which are an important source of media noise.
415
without also increasing their temperature dependence, was reported by Umeki et al. in 1974 [7]. It consisted of immersing the iron oxide particles in a solution of cobaltous sulphate to which sodium hydroxide was then added. Cobaltous hydroxide Co(OH) 2 precipitates on the surface of the particles. On warming to 45°C for 8 h, cobalt ions diffuse into the surface of the iron oxide particles and bring about an increase in coercivity. In particles whose coercivity has been increased by the diffusion of Co ions into a surface layer of the particle, the temperature dependence of coercivity is much less than is found when cobalt is uniformly distributed throughout the volume of the particle. This was clearly demonstrated in an experiment of Kishimoto et al. in 1984 [8]. Starting with particles of ~-Fe20 3 suspended in an alkaline solution containing cobaltous and ferrous ions in a molar ratio (Co2+/Fe 2+) of 0.5, they heated the suspension at 45 °C for 8 h "to crystallize cobalt-ferrite on the ~,-Fe20 3 particles. Fhe resultant particles (cobalt-epitaxial iron oxide particles) had the uniaxial magnetic anisotropy along the acicular axis". After measuring the magnetic properties, the particles were heated at 450 °C in air for 2 h after which "cobaltous ions were uniformly substituted into the ~-Fe20 3 particles". By changing the cobalt content from 3.6 to 7 wt%, the coercivity at 20 °C was increased from 800 to 1300 Oe and the "squareness" ( M r / M s) was increased from 0.70 to 0.72. On measuring the effect of temperatures on He, it was found that (as shown in fig. 3) the temperature coefficient of coercivity was much higher when the cobalt was uniformly distributed throughout the particles than when it was confined to the surface of the particles. The second result was that the value of coercivity at room temperature for a given percent of cobalt was approximately the same whether the cobalt was on the surface or distributed throughout the volume of the particle. The effect of cobalt on the magnetocrystalline anisotropy constant of magnetite (Fe304) was measured by Bickford et al. [9] in 1957 as a function of temperature. Slonczewski [10,11] explained the large change in magnetocrystalline anisotropy constant, Kj, that they observed in
G. Bate / Magnetic recording materials since 1975
416 2000
1500
~
3.6 wt% cobalt
\
Hc (Oe)
1000 o,.~>. ~~~..~. 500
0 - 50
/
impregnated
~ ~ ~~
dope~
~
i
1
i
L
0
50
100
150
_~ 200
T(°C)
Fig. 3. Coercivity in 3.6 wt% Co-modified iron oxide particles as a function of temperature. " D o p e d " cobalt is distributed throughout the volume of each particle. "Impregnated" cobalt is confined to a surface layer of each particle.
terms of "one-ion" model involving the spin-orbit coupling of Co 2+ ions placed in an electric field of trigonal symmetry in the lattice. There is at present no completely satisfactory explanation of the origin of the coercivity and its temperature dependence of cobalt surface-modified iron oxide particles and this is one of the most intriguing of the unsolved problems in the field of recording materials. The absence of understanding of the origin of coercivity in these particles has not prevented their use in high recording density applications such as video and high-performance audio tape, instrumentation tapes and flexible disks. 4.2. Other iron oxides
Even without the complications caused by the addition of cobalt the magnetic behavior of the iron oxides is difficult to understand. The final step in the preparation of particles of `/-Fe203 is the oxidation of Fe304 particles at low temperatures in the presence of water. In some commercial processes this is not the final step. The -/Fe203 particles are reduced back to Fe304 and then re-oxidized and re-reduced, etc., so that the final commercial product may have a composition somewhere between Fe304 and Fe203. Horiishi and Takedoi [12] treated particles of -/-Fe203 with an aqueous solution of either ferrous salts or ferrous hydroxide at 50-100 ° C and pH of 8.0-
14.0 in a reducing atmosphere and then reoxidized to -/-Fe203. The first cycle produced an increase in the particle coercivity from 370 to 420 Oe. The cycle could be repeated several times. The end product had an atomic ratio of Fe 3+ :Fe 2+= 1:0.23; i.e., the material was midway between Fe203 and Fe304. These are often referred to as "Berthollide oxides". Haneda and Kojima [13] investigated the effect of varying the proportion of magnetite in (Fe 3 O4)x(-/-Fe203)l-x and found that the maximum coercivity was achieved at a higher value of x = 0.75. They suggested that their particles were composed of finely divided crystallites of ~/-Fe203 and pure Fe304. Knowles [14] explained the magnetic properties of particles whose composition was intermediate between Fe304 and Fe203 as resulting from "an inner core of Fe304 with an outer layer of `/-Fe203. Owing to the large change in volume which occurs on oxidation, these ... stress each other. Over a certain range of composition, the stress field interacts with the magnetization to increase Hc, and it is also responsible for H c increasing with time after the oxidation process. A similar situation applies to partially reduced ` / - F e 2 0 3 " . 4.3. Chromium dioxide
The existence of a ferromagnetic oxide of chromium, CrO2, was reported as early as 1935, but the material was not made in single-domain form and used a magnetic recording material until 1961 [12]. The process involved the decomposition of CrO 3 under pressure and usually in the presence of water and results in highly acicular particles (aspect ratios of 20 or 30:1 are not unusual) with a very regular prismatic shape. The appearance in the electron microscope is in marked contrast to that of `/-Fe20 3 and Co--/ particles. The two principal problems were the cost of preparation (largely due to batch processing) and the low Curie temperature (O c ~ 120 ° C). Since 1961, much effort has been expended in attempts to reduce the former and increase the latter. Chromium dioxide particles are used commercially in high-quality audio tapes (an application now dominated by particles of cobalt-im-
417
G. Bate / Magnetic recording materials since 1975
pregnated iron oxide) and in the tape cartridges for the IBM 3480 computer tape drive. The low Curie temperature has been used to advantage in the thermoremanent copying of tapes. The recorded tape (made of magnetic materials with Oc considerably higher than that of CrOz) is placed in (stationary) contact with an unrecorded tape of CrO 2 particles at temperatures between 25 and 120 °C. The field pattern emanating from the master tape then records on the CrO 2 slave tape as the temperature is reduced to ambient. The advantage of thermoremanent transfer is that it can be carried out at much higher speeds than are possible by conventional copying; (i.e., by taking the amplified signal, reading from the master and using it to write conventionally on the slave tape). Chromium dioxide tapes have been suspected of suffering from short-term and long-term loss of remanence. It is well established [16] that if a field sufficient to bring a sample of CrO 2 tape to saturation remanence is applied and is followed by a field in the reverse direction slightly less than the coercivity, the magnetization legel will decrease with time even though the applied field is held constant (fig. 4). However, tapes made of particles of cobalt-treated -,/-Fe203 change their level of magnetization in much the same way and so, to a lesser extent, do tapes made of untreated ~/-Fe203 particles, as seen in fig. 4. The long-term effect is, perhaps, more serious, particularly when CrO z particles are used in a medium like 1 / 2 " computer tape which is intended for archival storage of data. The concern arises because of the possible reactions between CrO 2 particles and the polymers used to isolate the particles and to bind them to the substrate which results in a loss of magnetization over time. 4. 4. Metal particles Single-domain particles of the ferromagnetic metals, principally iron may be prepared by three methods: 1) reduction in hydrogen of the oxides or oxyhydroxides, 2) decomposition of the borohydride by heat and 3) condensation of the vaporized metal. The advantages of using the metal, rather than the oxide, in the form of particles for
03,°o°:
,o,K' 1-
• 6K
O_uO 4 -
e~
o 00
,"
-0.5 H(kOe)
J
• 300K
4
~
-1.0
15
L'
' 'II
O0o ,oo,yo,''' ~o-I
~
%
O0
-0.5
-1.0
-1.5
-2.0
HikOe) i
:
_
,
i
31
n
. oo,
II
o~,<
li
I
-i,I
"T o
\
,
.:o
/
,":,1oo
-5
H(kOe) Fig. 4. Time decay of magnetization at 300, 100 and 6 K, for particles of: (a) "y-F%O3; (b) cobalt-surface modified 3,-Fe203; (c) CrO 2. After applying a saturating field in the positive direction fields plotted along the abscissa are applied in the inverse direction and the magnetization is measured as a function of time that the plotted field is applied. M(100 s) is the magnetization after the reverse field is applied for 100 s. M(1000 s) is the magnetization after the reverse field is applied for 1000 s [16hi.
recording are the higher saturation and remanent magnetizations .and thus, in particles whose anisotropy is dominated by shape, higher coerciv-
418
G. Bate / Magnetic recording materials since 1975
ities. The disadvantages are higher cost of preparation and greater chemical reactivity with the surrounding organic binder materials and with the atmosphere. The particles currently in use in high-quality audio cassettes (analog and digital) have coercivities up to 2100 Oe [17]. This is made up of the contribution of shape anisotropy ( = 1900 Oe) and magnetocrystalline anisotropy ( ~ 200 Oe). Since the outer shell of the particles is deliberately oxidized to provide a barrier against further corrosion, the saturation magnetization of the particles is somewhat less (984 to 1338 e m u / c m 3) than the 1714 e m u / c m 3 for pure iron. Similar values for the saturation magnetization are obtained if the iron particles are converted to iron nitride, Fe4N.
4.5. Barium ferrite particles Barium ferrite B a O . 6 F % O 3 has the distinction of being the most widely used permanent magnet material. It contains no expensive ingredients and has, in fine particle form, coercivities of 2000-3000 Oe. The saturation magnetization of 60 e m u / g at room temperature not surprisingly is less than that of the iron oxides. Before the material can be used in fine particle form for magnetic recording purposes, it is necessary to reduce the coercivity to >_ 1000 Oe without reducing still further a magnetization level which is already low. A method of accomplishing both goals was achieved by Kubo et al. [17]. They melted a mixture of BaO, F e 2 0 3 and B 2 0 3 and rapidly cooled the liquid by running it between steel rollers. The boron oxide provided a glass-like matrix within which single-domain platelets of barium ferrite formed and were then crystallized by reheating. The B203 was removed with hot acetic acid from the BaO - 6Fe203 particles which had an average diameter of ---0.08 ixm and a thickness of = 0.03 Ixm; i.e., the particles had the form of hexagonal platelets. To reduce the coercivity to levels suitable for a recording medium, cobalt and titanium in the form of CoO and TiO 2 were added to the starting material and incorporated in the crystal lattice, BaFet2_2xCoxTixO~9. When x = 0.75 the coercivity is reduced to = 900
Oe, while the value of magnetic moment density o-= 60 e m u / g (almost the same as its value at x = 0). The addition of cobalt and titanium is believed to support the hexagonal axis as the easy axis of magnetization. Thus crystalline anisotropy encourages magnetization along the hexagonal axis while shape anisotropy supports magnetization in the basal plane. The coercivity is then given by the difference between the crystalline term 2K/M~ and the shape term NMS or ( H c) = 0.48(
2KM~-NMs) '
where K = + 9.45 x 105 is the magnetocrystalline anisotropy constant (erg/cm3), [0.48] is a "randomizing" factor which is applied to particle samples in which the c-axis of the platelets are not aligned. M~ is the saturation magnetization (emu/cm3); N = 2xr is the demagnetizing factor along the c-axis. For a temperature of 20 o C and x =0.8; H = 9 6 0 Oe. As the temperature increases M S becomes smaller and the first term (2K/Ms), grows larger while the second term (-NM~) becomes smaller in magnitude and remains negative in sign. Thus both terms encourage H c to grow larger with increasing temperature; an unusual behavior for a magnetic recording material. Dispersion of the particles within the solvent and binder materials is difficult since, in addition to surface tension effects, the particles experience a magnetic force of attraction to other particles, thus forming a stack which is difficult to separate into individual particles. Corradi et al. [18] and Kishimoto and Kitahata [19] showed that the increase in Hc with increasing temperature obtained by Kubo et al. [17] becomes more complicated as the range of temperatures investigated is increased. Below --- 250 K and above = 450 K the coercivity of Co, Ti doped barium ferrite particles decreases with increasing temperature. No simple explanation of these results has been found. Suzuki et al. [20] described a potentially important, new application of barium ferrite particles. They are used as the magnetic component of
419
G. Bate / Magnetic recording materials since 1975 Table 1 Table of properties of magnetic particles Property
Particle y-Fe203
Co-Fe203
CrO 2
BaO.6 Fe20 3 + C o , Ti
Fe
336-340 0.5 [863]
350-355 0.5-0.8 [863]
340-390 0.5 390-400
380 0.6-0.7 590
870-1100 0.23 + 0.52 1040
-4.64×103 -5×10 -6 20-28
- 5 to + 1 0 0 X 1 0 3 - 5 to - 1 5 × 1 0 - 6 43.8-60
+2.5>< 104 +1×10 6 36-53
+ 3.3 x 105 +10X10 -6
+ 4.4 x 10 4 +4X10 6
26-157
30-131
0.26-0.61
0.30-0.6
0.35-0.6
0.16-0.6
0.5-0.75
+3.1 xl0 -3
-0.6x
15-31 5.28
26 5.8
Saturation magnetization, M S [kA/m]
Mr/M S Curie temperature, Tc [K] Magnetocrystalline anisotropy constant, K~ [ J / m 3] Saturation magnetostriction, As Coercivity, H c [ k A / m ] Switching field distribution, AH/H c Temperature coeff, of coercivity, AHc/Hc°C(20-70°C)
-1×10
3
impreg, d-2.4×10
-3
-5×10
3
10 3
doped - 1 0 X 10 - 3 Specific surface area ( B E T ) [m2/g]
Density p Crystal structure
20-40 4.60
20-40 4.80
cubic
cubic
25-37 4.88-4.95
a 0 = 0.25 n m ,
Ferro- or ferrimagnetic Particle size [~,m]
m3m ferri needles
ferri needles
l = 0.3 d = 0.06
- / = 0.3 d = 0.06
equi ax. l = 0.2
tetragonal
hexagonal
bcc
a = 0.4218 nm c = 0.9182 nm
c = 2.32 n m a = 0.589 nm
a = 2861 nm
ferro needles
ferri hex. platelets dia = 0 . 0 8 - 0 . 1 thick = 0.001
ferro needles
l = 0.5 d = 0.05
1 = 0.3 d = 0.06
-0.025
the "slave" tapes in the contact duplication of DAT (Digital Audio Tape) masters. The coercivity of the barium ferrite tape was 600-700 Oe well below the coercivity, 2100 Oe, of the metalparticle master tape. Table 1 summarizes the properties of magnetic particles for recording applications.
5. Thin film recording media The particulate recording media discussed thus far have undoubtedly been conspicuously successful as storage media for audio, video and data over the past quarter century, but they are still far from ideal. Their magnetic constituent, in the form of fine particles of metals, alloys or oxides, occupies 40% of the coating volume in tapes and flexible disks and only 20% in hard disks. Since
the 1960s many attempts have been made to displace particulate recording media with metallic thin films, but it is only in the last five years that those efforts have been successful. Particulate recording materials still dominate flexible media; e.g., computer tapes and flexible disks, but hard disks now increasingly are made of thin films deposited by vacuum deposition or, more commonly, by sputtering. It is appropriate, at this point to review briefly the salient characteristics of each type of medium.
5.1. Particulate coatings The magnetic properties of the finished coating are determined almost entirely by the magnetic properties of the particles which can be measured in the "dry" state before the ink is mixed and coated on the substrate. Changes in
420
G. Bate / Magnetic recording materials since 1975
magnetic properties from the dry powder to the finished coating are relatively slight and generally quite predictable. The mechanical properties of the coating are determined overwhelmingly by the binder polymers (which occupy 60% of the coating volume), while the magnetic properties are determined by the particles. Thus, to a considerable extent it is possible to vary and control the mechanical properties independently of the magnetic properties. In the manufacture of tapes or flexible disks, high coating speeds (hundreds of f e e t / m i n u t e ) and wide rolls (_< 2 m) can be used to achieve high production rates. Uniform properties within a roll of coated material (and from roll to roll) are achievable. The materials and processes are generally well-understood and thus yields are high and coating costs are low.
react differently with the solvent and, much more importantly, with the binder polymers, dispersants, etc. affecting the durability and coefficients of friction of the coating as well as its recording performance. The saturation magnetization of thin films can be high, almost equal to that of the pure metal. The coercivity and other magnetic properties are continuously variable, within limits, by changing the deposition conditions and thus fine-tuning of the magnetic properties is possible to do in hours rather than the months required for particulate coatings. Four methods of deposition have been used to prepare thin metallic films for recording purposes. They are: 1) electroplating, 2) autocatalytic plating, 3) evaporation and 4) sputtering.
5.2. Thin-film coatings
In this process the metallic ions are reduced from an aqueous solution at the cathode using electrons from an external source of current. The process can be continuous, inexpensive and can be used to deposit two metals at the same time if, as with cobalt and nickel, their deposition potentials are similar. It was found experimentally that, to obtain high-coercivity films, a high cobalt content was required. Alloy films of C o - N i rather than C o - F e are used because the deposition potentials of Co and Ni are very similar and films of C o - N i were more likely to show good corrosion resistance than would films of C o - F e . The basic plating path was introduced by Bonn and Wendell [21] and a practical form of it was described by Koretzky [22]. The electrolyte was made up of cobalt and nickel chlorides, ammonium chloride (found to enhance the compositional uniformity of the film) and sodium hypophosphite. The latter ingredient was used to add 1 - 3 % phosphorous to the plated films where it segregated at the crystal boundaries and served to control the size of the crystallites and hence, the coercivity. The other parameters found to influence grain size and grain separation (and hence, noise in the finished films) were pH, temperature, substrate smoothness and, most importantly, plating current density.
It is relatively easy to make coatings thinner than 0.25 I~m by sputtering or vacuum evaporation but relatively difficult to make such thin coatings from particles. A thin coating is required to achieve high recording densities, particularly on disks where there is no erase head and a single head must achieve a compromise between the differing requirements of writing and reading. Thin coatings allow a greater length of tape to be packed into a reel of a given diameter and thus enable high volumetric densities to be achieved. The intensity of magnetization is much higher in thin films since the magnetic constituent is not diluted by a binder. Furthermore the coercivity and other relevant magnetic properties are continuously variable (within limits) by changing the deposition conditions; e.g., in a system in which the film is deposited by sputtering, varying the target voltage, the bias voltage and the partial pressure of argon allows fine tuning of the coercivity and remanence of the deposited film. In contrast, changing the magnetic properties of a particulate coating usually involves a major development effort. Changing the magnetic properties almost always involves changing the chemical behavior of the surface particles which then
5.3. Electroplated films
G. Bate / Magnetic recording materials since 1975
~p~//
/
Substrate
5.4. Autocatalytically deposited films It is possible to deposit from solution, films of cobalt and nickel alloys by a deposition process in which the potential required for the reduction of the metal ions is provided by a reducing agent, such as a hypophosphite or a borohydride, rather than by an external power supply. Each layer of atoms of Co and Hi deposited on the substrate serves to catalyze the deposition of a further layer. Films having a coercivity of < 1800 Oe and saturation magnetization up to 80% of that of the bulk alloy of C o - N i have been made autocatalytically. It is also possible to use the autocatalytic process to deposit films of C o - N i - P continuously on polyethylene terephthalate films on which, to start the autocatalysis, a thin film of N i - P has first been chemically deposited.
421
.. S.ie,. /
\ \
,
Crystal monitor
/
-
/
tl /
©
/ i~ l. " Shutter, ,i
5.5. Films prepared by vacuum evaporation Films of the ferromagnetic elements and their alloys deposited by vacuum evaporation are continuous, highly conducting but of coercivity < 50 Oe, much too low for recording purposes. Some way must be found to give the films a single-domain structure. A method of overcoming this problem was described by Speliotis, Bate, Alstad and Morrison [23]. They controlled the coercivity of the deposited film by varying the angle of incidence of the arriving metal atoms so that needle-shaped crystallites grow roughly along the direction of deposition. In this way coercivities < 1000 Oe could be achieved. Substrate temperatures were in the range 2 0 - 5 0 ° C and so the method could be used to make tapes on polyethylene terephthalate substrates. The metals were evaporated by electron bombardment and films of thickness in the range 0.01-0.4 ~m were deposited. This approach was used by Rossi et al. [24,25] shown in fig. 5. A magnetic film of Fe (42.5 at%), Co (42.5 at%), Cr (15 at%) was deposited at an angle of incidence of 60 ° while the substrate was rotated to ensure circumferential uniformity of the deposited film. It was found necessary to control to close tolerances the rate of deposition since the vapor pressure (and hence the rate of
(
1/6 of substrate surface
Fig. 5. Vacuum evaporation (by e-beam gun) at an angle of 60 ° to the surface of a rotating disk Substrate. The angle and the thickness distribution are controlled by the apertures in the shields [24]. evolution) of chromium is appreciably higher than that of iron or cobalt. An underlayer of Haynes 188 stainless alloy was sputtered onto the conventional rigid-disk substrate of AI-Mg. This layer served to act as a corrosion barrier between the substrate and the metal film and also could be polished to a smooth but textured underlayer for the magnetic film. The purpose of texturing (which has become an important feature of thinfilm rigid disks) is to prevent adhesive contact between disk and head. It is a critical step in the manufacturing process since if the texturing is too light the head and magnetic film may wring together, while texturing that is too heavy will intro-
G. Bate / Magnetic recording materials since 1975
422
d u c e an u n a c c e p t a b l e s e p a r a t i o n b e t w e e n h e a d a n d m a g n e t i c c o a t i n g . It s h o u l d b e r e m e m b e r e d that, at c u r r e n t r e c o r d i n g d e n s i t i e s , a h e a d - t o - r e c o r d i n g film d i s t a n c e o f 4 ixin. (1000 A ) m a y b e i n t o l e r a b l y large. In a d d i t i o n to e l i m i n a t i n g h e a d - d i s k sticking, t h e t e x t u r i n g c a n also b e u s e d to e n h a n c e t h e c i r c u m f e r e n t i a l easy axis o f m a g n e t i z a t i o n o f t h e F e C o C r layer.
volts is a p p l i e d b e t w e e n t w o p a r a l l e l plates. T h e s e are the (positive) substrates (the surface on which t h e film is to b e d e p o s i t e d ) a n d t h e ( n e g a t i v e ) t a r g e t ( m a d e o f t h e m a t e r i a l to b e d e p o s i t e d ) . A r g o n i o n s a r e a t t r a c t e d to t h e t a r g e t a n d k n o c k o u t a t o m s o f t h e t a r g e t m a t e r i a l . By v i r t u e o f their acquired momentum the dislodged atoms migrate toward and some are deposited on the s u b s t r a t e . O n e o f t h e key a d v a n t a g e s to s p u t t e r ing o v e r v a c u u m e v a p o r a t i o n is t h a t t h e c o m p o s i t i o n o f t h e d e p o s i t e d f i l m is t h e s a m e as t h a t o f the target and does not depend on the relative vapor pressure of the target's component elem e n t s . By c h a n g i n g t h e t a r g e t c o m p o s i t i o n , t h e composition of the deposited film can be changed in a p r e d i c t a b l e w a y a n d t h u s t h e t a s k o f e x a m i n -
5.6. Sputtered films S p u t t e r i n g is d o n e in a v a c u u m c h a m b e r p u m p e d d o w n to a p r e s s u r e o f 0.75 × 10 - 6 to 0.75 x 10 - 7 T o r r a n d filled w i t h a gas, usually a r g o n to a p r e s s u r e o f 10 - 3 to 7.5 × 10 - 2 T o r r [2]. A p o t e n t i a l d i f f e r e n c e o f s e v e r a l t h o u s a n d
Table 2 Typical properties of magnetic film media Material
Co Fe Ni Co-Ni Co-Fe Co-Sm Co-P Co-Re Co-Pt Co-NiP Co-(30 at%) Ni:N 2 Co-Ni: 0 2 Co-Ni-Pt Co-Ni-W Fe30 4 ~/-Fe20 3 : Co ~,-Fe20 3 : Os Co-(18 at%) Cr Co-(20 at%) Cr Co-(22 at%) Cr
Deposition process a)
Orientation b)
Dominant crystal structure c)
M s
H c
(kA/m)
(kA/m)
OIE NIE, SP OIE SP OIE SP OIE OIE N1E (e) EL, EP SP SP El, EP SP
IPA IPI IPA IPI IPA IPI IPA IPA IPA IP1 IPI IPI IPI IPI
hcp hcp bcc bcc fcc fcc hcp/fcc bcc amorphous hcp hcp/fcc hcp/fcc hcp/fcc hcp/fcc
1100-1400 1100-1400 1600 1600 400 400 800-1200 1400-1600 500-1000 800-1100 500- 750 800-1400 600-1000 650
OIE SP SP NIE, SP SP SP SP SP SP
IPA IP* I IPI IPI IPI IPI ± ± ±
hcp/fcc hcp/fcc hcp/fcc is is is hcp hcp hcp
300800450 400 200240 300400 300-
400 900
250 550 340
S, S *
K u
(105 m -3)
61-120 30- 60 d) 60- 90 10 20- 28 30- 70 60-120 33- 55 36- 96 18- 58 --o 140 40-120 80 80 60- 70 30- 50 17- 32 40-100 160 80-100 ( ± ) 65- 95 ( ± ) 80-105 ( ± )
4 (bulk) 0.3-3.0
0.9, 0.9 1.1 0.9, 0.9 0.9, 0.9 0.8, 0.8 0.96 0.7-0.8 0.9, 0.97 0.8, 0.8 0.8, 0.8 0.8, 0.8 -
1.0 0.15 0.4
a) OIE = Oblique Incidence Evaporation; NIE = Normal Incidence Evaporation; SP = SPuttered; EL = ElectroLess plated; EP = ElectroPlated. b~ 1PA = In-Plane Anisotropic; IPI = In-Plane Isotropic; ± = perpendicular. c) hcp= hexagonal close-packed; fcc = face-centered cubic; bcc = body-centered cubic; is = inverse spinel. d) Coercivities for Co films deposited on Cr or W underlayers. ¢) Co-Sm values for films deposited in the presence of an orienting field.
G. Bate / Magnetic recording materials since 1975
ing the effect of varying the composition of binary, ternary or quaternary alloys is greatly simplified and accelerated. The principal independent variables (other than target composition) are the argon pressure and the target and substrate voltages. Belk, George and Mowry [26] studied experimentally and theoretically the noise in thin-film longitudinally oriented magnetic films of CoP, CoNiP, CoRe, CoSm and found in each case that the disk noise in n V e / H z increased markedly with increasing bit density. Only with particulate disks of "y-Fe203 and a perpendicularly magnetized film of CoCr was there no increase (~/Fe203) or decreasing noise (CoCr). They showed experimentally and theoretically that the physical mechanism for the noise was the fluctuation that occurs from bit to bit in the geometry of the zigzag transitions separating the bit cells. The effect of changes in argon pressure, substrate bias and substrate temperature on noise level in similar, sputtered films of CoPtCr on a chromium underlayer, were measured by Yogi et al. [27]. Noise decreased as argon pressure increased and as substrate temperature and bias voltage decreased. These changes in the condition under which the films were made are ones that might be expected to produce grains that are increasingly more isolated from neighboring grains. Thus the current explanation of the resuits of Belk et al. [26] and Yogi et al. [27] appears to be that the noise arises principally because, in the case of Belk's films, the grains are sufficiently close together to enable exchange forces to correlate the spin directions of crystallites on opposite side of a recorded transition. By increasing the argon pressure and reducing substrate temperature and bias; as in Yogi's films, non-magnetic layers form at the surface of the grains, thus separating the grains and strongly reducing the. extent of the exchange coupling. The modulation noise is thus reduced. The effects of argon pressure on magnetization, S* (coercivity squareness), coercivity and noise level in Yogi's films are shown in fig. 6. The structure, orientation and magnetic properties of thin films for recording media are summarized in table 2 (Arnoldussen [30]).
423
600
u 400 " ~ 200 . . . . . . . .
O0 o
L
.......
,
i0 ~
•
. . . . .
.......
1o2
1.0 I 0.9
e
~
e
~
t
~
0.8 ~ e
~
e
0.7 0.6 . . . . . . . .
0.5 0o
J
.......
to ~
i
; ', ; ' , ' , ; i o 2
1800 1600 o "--" 1400
1200 1000 0°
PAr (mtorr) '
'
i
' ' ''I
,
CoPtCr/Cr
O
(/1
E E C3 0 0
Oq v
E Z
10-2 9 i
i
I
i
i
I
I I
i
I
i
I
I
PAr (mtorr) Fig. 6. The magnetic properties and normalized media noise of C o P t C r / C r media as a function of argon sputtering pressure [27].
424
G. Bate / Magnetic recording materials since 1975
6. Conclusion
During the past 15 years some very difficult problems in magnetic recording materials and drives have been tackled and solved. As a result the performance (i.e., bit density, track density, access time to the data, data rate, reliability, etc.) of recording systems have been greatly improved. The information storage density (the product of bits/inch and tracks/inch) allows us to quantify one aspect of recording performance. In 1975 the IBM 3350 disk drives stored 3 × 106 bits/in. 2 by flying the head 0.4 txm over the disk. In 1990 IBM published the results of successful laboratory demonstrations of disks operating at 1 × 109 bits/in. 2 [27,28]. This involved "flying" the head 0.04 Ixm (i.e., 0.4 mean free paths in air) above the surface Of the disk in order to record and read at a linear density of 158000 bits/in. The track density was 7470 tracks/in. In 1991 the areal density was increased in the Hitachi laborators to 2 × 109 bits/in. 2 [29]. The IBM recording medium was built of five layers in which the sputtered, CoPrCr magnetic layer (12-40 nm thick) was isolated from corrosive interaction with the disk substrate by a layer of chromium 50-200 nm thick. A protective layer of hard carbon (12.5 nm thick) was deposited and was itself covered by a thin film of lubricant. It is significant that the magnetic part of the disk consisted of only one layer out of the five. This strongly suggests that, as we look forward to further improvements in magnetic recording media, over the next 15 years, the most challenging problems will increasingly be the non-magnetic ones.
References [1] G. Bate, Recording Materials, in: Ferromagnetic Materials, vol. 2, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980) chap. 7, p. 381. [2] E. Koster and T.C. Arnoldussen, Recording Media, in: Magnetic Recording, vol. 1, Technology, eds. C.D. Mee and E.D. Daniel (McGraw-Hill, New York, 1987) chap. 3. [3] W. Steck, J. de Phys. 46 (1985) 33. [4] M.P. Sharroek, IEEE Trans. Magn. MAG-25 (1989) 4374.
[5] W.A. Manly, Jr., IEEE Trans. Magn. MAG-12 (1976) 758. [6] P.J. Flanders, 1EEE Trans. Magn. MAG-12 (1976) 770. [7] S. Umeki, T. Uebori and Y. Imaoka, IEEE Trans. Magn. MAG-10 (1974) 655. [8] M. Kishimoto, M. Amemiya and F. Hayama, J. Appl. Phys. 55 (1984) 2272. [9] LR. Bickford, J.M. Brownlow and R.F. Penoyer, Elec. Eng. 104B suppl. 5 (1957) 238. [10] J.C. Slonczewski, J. Appl. Phys. 29 (1958) 448. [11] J.C. Slonczewski, J. Appl. Phys. 32 (1961) 253S. [12] N. Horiishi and A. Takedoi, Japan. Patent 75 101 299 (1975). [13] K. Haneda and H. Kojima, J. de Phys. 40 (1979) C2583. [14] J.E. Knowles, J. Magn. Magn. Mater. 22 (1975) 263. [15] T.J. Swoboda, P. Arthur, N.L. Cox, J.N. Ingraham, A.L Oppegard and M.S. Sadler, J. Appl. Phys. 32 (1961) 3745. [16] (a) R.M. Kloepper, B. Finkelstein and D. Braunstein, IEEE Trans. Magn. MAG-20 (1984) 757. (b) S.B. Oseroff, D. Clark, S. Schultz and S. Shtrikman, IEEE Trans. Magn. MAG-21 (1985) 1495. [17] O. Kubo, T. Ido and H. Yokoyama, IEEE Trans. Magn. MAG-18 (1982) 1122. [18] A.R. Corradi, D.E. Speliotis, G. Bottoni, D. Candolfo, A. Cechetti and F. Masoli, IEEE Trans. Magn. MAG-25 (1989) 4066. [19] M. Kishimoto and S.-I. Kitahata, IEEE Trans. Magn. MAG-25 (1989) 4063. [20] T. Suzuki, T. Ito, M. Isshiki and H. Saito, IEEE Trans. Magn. MAG-25 (1989) 4060. [21] T.H. Bonn and D.C. Wendell, Jr., US Patent 2 644 787 (1953). [22] H. Koretzky, Proc. 1st Austr. Conf. on Electrochemistry (1963) 417. [23] D.E. Speliotis, G. Bate, J.K. Alstad and J.R. Morrison, J. Appl. Phys. 36 (1965) 972. [24] E.M. Rossi, G. McDonnough, A. Tietze, T.C. Arnoldussen, A. Brunsch, S. Doss, M. Henneberg, F. Lin, R. Lyn, A. Ting and G. Trippel, J. Appl. Phys. 15 (1984) 2254. [25] T.C. Arnoldussen, E.M. Rossi, A. Ting, A. Brunsch, J. Scheider and G. Trippel, IEEE Trans. Magn. MAG-20 (1984) 821. [26] H.R. Belk, P.K. George and C.S. Mowry, IEEE Trans. Magn. MAG-21 (1985) 1350. [27] T. Yogi, T.A. Hguyen, S.E. Lambert, G.L Gorman and G. Castillo, IEEE Trans. Magn. MAG-26 (1990) 1578. [28] T. Yogi, C. Tsang, T.A. Hguyen, K. Ju, G.L Gorman and G. Castillo, IEEE Trans Magn. MAG-26 (1990) 2271. [29] M. Futamoto, F. Kugiya, M. Suzuki, N. Takano, H. Fukuoka, Y. Matsuda, Y. Inaba, T. Takagaki, Y. Miyamura, K. Akagi, T. Nakao, H. Sawaguchi and T. Munemoto, 5th Joint MMM-Intermag Conf., Pittsburgh (1991) submitted. [30] T.C. Arnoldussen, Proc. IEEE 74 (1986) 1526.