Geoffrey Bate

Hard Magnetic Films of Iron, Cobalt, and Nickel

Thin films of Fe, Co, and Ni were prepared by vacuum deposition. The angle of incidence was varied from 0° to 80° for the explicit purpose of changing the anisotropy and producing very high coercivities. The film thicknesses, determined interferometrically, were in the range from 100 to 4000 A. X‐ray and electron diffraction examination revealed a highly faulted cubic structure for Fe and Ni, and a mixture of cubic and hexagonal phases for Co, with crystallite size ranging from 20 to 1000 A. Increasing the angle of deposition from the substrate normal had a pronounced effect on the magnetic properties of the films. The maximum effect was observed in Fe, where easy‐axis coercivity exceeded 1100 Oe at angles approaching 90°, while at normal incidence it was only 20 Oe. Next was Co with maximum coercivity of 1000 Oe and Ni with maximum coercivity of 350 Oe. This order is what would be expected on the basis of shape anisotropy of elongated particles formed on account of the oblique incidence of the vapor flux...

A critical review of magnetic recording materials

A review of the methods of preparation and the relevant properties of materials which the authors consider suitable for incorporation in conventional magnetic recording surfaces such as tapes, disks, drums, strips, and cards is provided. This field is presently dominated by one material, gamma ferric oxide in fine particle form, and so it is not surprising that this material is discussed at greater length than its potential rivals. The reasons for this dominance are considered and found to be 1) the ability of gamma ferric oxide recording surfaces to perform adequately in the recording systems which have been developed so far, i.e., the magnetic properties of the oxide have apparently not been the limiting factors in the performance of recording systems; 2) the relative cheapness of the particles; 3) the existence of suitable binder systems of proven durability; and 4) the need for compatibility with existing recording devices.

Thin metallic films for high-density digital recording

Three different methods, electrodeposition, autocatalytic deposition, and vacuum deposition, by which thin metal films may be made are described. They have a combination of magnetic characteristics which should make them well suited to the purpose of high density recording. It is concluded that the factors which are primarily responsible for the superior recording performance of these films when compared with the conventional particulate coatings are their thinness and their high coercivity.

A Brief Review of High‐Coercivity Thin Films for Digital Magnetic Recording

Three methods have been used to prepare thin films of the common ferromagnetic elements and their alloys having permanent magnet properties. The methods are (1) electrodeposition, (2) autocatalytic deposition, and (3) vacuum deposition. The effect of the deposition conditions on the magnetic properties is discussed briefly for each method. It is possible to prepare thin (∼1000 A) films with coercivities 100<Hc<1500 Oe, in which the saturation magnetization is fairly close to that of the bulk material and whose values of remanent/saturation intensity approach 1·0. The magnetic properties of the films prepared by methods (1) and (2) are isotropic and those prepared by method (3) are usually uniaxial in the film plane. Although they are highly (and specularly) reflecting and are electrically continuous, the magnetic structure of the films appears to be basically that of an assembly of strongly interacting single domain particles. The application of these films is in high‐density digital magnetic recording; b...

A survey of recent advances in magnetic recording materials

Magnetic recording coatings are still made predominantly of iron oxide particles but the newer particles are significantly better in magnetic properties, dispersibility and orientability than the particles used, say, ten years ago. Chromium dioxide particles show excellent recording performance (particularly at densities above 1000 flux changes per millimeter) but they are presently being challenged by the new cobalt-modified iron oxides. These are formed by diffusing cobalt into the surface of acicular iron oxide particles and it is claimed that the particles prepared in this way are much more stable with respect to temperature and stress than the older cobalt-substituted iron oxides. Metal particles, by virtue of their high moment density and high coercivity, would be ideal for high density recording if they could be passivated permanently. The paper reviews improvements which have been made within the last nine years in the properties of particles for magnetic recording applications and discusses how the improvements were effected.

The cylindrical symmetry of the angular distribution of particles in magnetic tapes

To understand the writing process in magnetic recording, it is necessary to take account of the perpendicular component of magnetization in the coating as well as the in-plane component. To do this, we need to determine the shape of the magnetization curve in the perpendicular direction. We have measured this magnetization curve for eighteen samples of tape whose thicknesses ranged from 2.5 μm to 10 μm and orientation ratios from 1.0 to 3.0 and corrected each curve for the effects of demagnetization, Comparing each of these curves with the curve measured in the transverse direction, we found that the ratio of transverse remanence to perpendicular remanence (corrected for demagnetization) was 1.092, standard deviation 0.166. Two disk-shaped samples were further measured as a function of the angle ψ where (90-ψ) is the angle between the field direction and the polar axis of the sample. Our conclusion is that for these tape coatings the angular distribution of particles was cylindrically symmetric about the direction of coating within about 10%. Consequently, it is possible to determine the magnetization curves of the coatings in the perpendicular direction from measurements made in the transverse direction.

The remanent state of recorded tapes

Measurements are reported on the in-plane and the perpendicular components of the remanence a tape acquires on passing through the steady field of a recording head. The tape coatings were oriented and unoriented particles of γ-Fe2O2, oriented CrO2, and unoriented Co-substituted γ-Fe2O2. The two writing heads used had 10µm- and 2.25µm-gaps, respectively. In each case the in-plane magnetization increases at first with increasing writing current, and eventually reaches a peak that is less than the maximum in-plane remanence produced on the same sample by an electromagnet. For higher values of writing current, in-plane magnetization in the tape actually decreases. The perpendicular remanence is not large enough to explain the difference between the in-plane remanence acquired from the head and the remanence acquired in a magnet. The perpendicular component of the field from the writing head is shown to have two adverse effects on the remanence of the tape. First, it produces a perpendicular magnetization that ranges from 2 percent to 15 percent of the in-plane component. Second, it causes a reduction in the in-plane component to occurn ear the surface of the tape closest to thew riting head. The reduction can bea s large as 15 percent of the maximum in-plane remanence and will obviously have an adverse effect on recording performance, particularly at high densities.

Magnetization in recorded tape

We have measured the in-plane magnetization and the perpendicular magnetization which 10-μm-thick tape of γ-Fe 2 O 3 particles acquires on passing through the field of a head driven with direct current. The perpendicular component of the head field has two adverse effects on the remanent state at the surface of the tape closest to the head. First, it induces a perpendicular component of magnetization which leads to asymmetry in the output pulse. Second, it leads to a reduction in the in-plane magnetization which reduces the signal output level at high recording densities. The perpendicular moment can be ≥5% of the in-plane moment in the normal writing current range of the head. In the same current range the reduction in in-plane moment represents about 5% of the tapes maximum remanent moment.

An investigation of separation losses in high-speed, high-density recording tapes

The trend in digital tape recording is toward larger storage capacity and faster accessing, which necessitate packing information at greater densities on tapes moving at higher speeds. The losses that inevitably arise from the finite separation between the head and the moving tape become much more serious as the length of the magnetized regions in the tape is reduced and as the relative velocity of head and tape is increased. The purpose of this paper was to investigate experimentally the dependence of separation losses on bit density and head gap length, and to distinguish from the reading losses those losses introduced during the writing process. Briefly, results showed that, for the heads and tapes used in the experiment, virtually all the losses could be attributed to the reading process. Furthermore, writing with a wide gap head and reading back with the four heads established that the percentage reading losses as a function of separation were apparently the same whatever reading head was used.

Static Magnetization Reversal in Thin Films of Co–Ni–P

Electrodeposited films of Co–Ni–P containing 14% nickel and 1% phosphorus by weight and having thickness 0.05 to 0.2 μ showed the following magnetic properties—high coercivity (300–550 Oe), high ratio of remanent‐to‐saturation moment (∼0.90), steep‐sided hysteresis loops and isotropy in the film plane.The process of magnetization reversal was studied by using a sensitive Hall probe to measure and map the horizontal component of the flux above the surface of the film after increasing reverse fields has been applied. This technique was used since neither the Bitter pattern method nor the Kerr effect revealed the natural pattern of magnetization reversal. However, it was possible to introduce, deliberately, a small region of reversed magnetization by means of the field from a magnetic recording head and examine this region by all three methods. This provided a method of checking the interpretations of the Hall effect data.It was found that these films employ a unique mode of magnetization reversal. Initially...