S.E. Lambert

Magnetic force microscopy : general principles and application to longitudinal recording media

This paper discusses the principles of magnetic force microscopy (MFM) and its application to magnetic recording studies. We use the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample. Tip fabrication procedures are described for two types of magnetic tips: etched tungsten wires with a sputter‐deposited magnetic coating and etched nickel wires. The etched nickel wires are shown to have an apex radius on the order of 30 nm and a taper half‐angle of approximately 3°. Lorentz‐mode transmission electron microscopy of the nickel tips reveals that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis. Images of written bit transitions are presented for several types of magnetic media, including CoPtCr, CoSm, and CoCr thin films, as well as γ‐Fe2O3 particulate media. In general, the written magnetization patterns are seen with high contrast and with resolution better ...

The effect of Cr underlayer thickness on magnetic and structural properties of CoPtCr thin films

Thin Co‐based alloy films have shown important recording, magnetic, and structural changes when grown on Cr underlayers of different thicknesses. We have investigated these properties using several different CoPtCr compositions on Cr underlayers ranging from 0 to 200 nm in thickness. We report epitaxial growth of the hcp Co (11.0) planes on the (100) planes of bcc Cr for the first time on a disk appropriate for magnetic recording. The 〈11.0〉 Co preferred orientation occurs only when the Cr underlayer has a 〈100〉 preferred orientation. The 〈100〉 preferred orientation in the Cr layer results from the use of sputtering rates above 150 nm/min and only persists to thicknesses of about 50 nm. The thin Cr underlayers (<50 nm) with a 〈100〉 preferred orientation have fine well‐packed grains. Thicker Cr underlayers have larger grains which are uncoupled and have a more random crystal orientation. The CoPtCr film morphologies follow these trends. The noise performance of these films improves with increasing Cr under...

Reduction of media noise in thin film metal media by lamination

The properties of laminated films of longitudinal Co-alloy media have been investigated. Significant reductions in the media noise are observed compared with single-layer films of the same total thickness, while the signal amplitude is unaffected. There are at least two reasons for this improved performance. Thin media layers exhibit inherently better signal-to-media-noise ratios, which are retained when several thin films are separated by nonmagnetic interlayers. A laminated media structure also offers a media-noise advantage since noise sources in the individual magnetic layers can be independent of one another. It is concluded that lamination can be used to improve the noise performance of metal-film media by simply repeating known process steps. >

Role of atomic mobility in the transition noise of longitudinal media

The relationship between grain growth morphology and the transition noise of high-density CoPtCr media on Cr underlayers is examined. The growth morphology is shown to depend on the sputtering pressure, substrate bias, and substrate temperature. Development of isolated magnetic grains is promoted when the mobility of the sputtered atoms is reduced, i.e. at high sputtering pressure, low substrate temperature, and no substrate bias. As the magnetic grains become more isolated, the media transition noise is significantly reduced, accompanied by a simultaneous reduction in the coercive squareness. This indicates that the transition noise is primarily governed by the intergranular exchange coupling among the magnetic grains, which can be optimized by controlling the grain growth morphology. The microstructural features of the decoupled media correspond to the zone I structure in J.A. Thorntons (1986) microstructure zone diagram. >

Beyond discrete tracks : other aspects of patterned media

The surface of a thin‐film disk can be patterned using standard lithographic techniques to form discrete tracks as narrow as 0.5 μm. These studies have been extended to patterns formed when an etched track is broken into discrete segments by etching away some portions of a discrete track. Abrupt changes in the magnetization can be obtained by dc erasing the medium, giving readback signals with ∼50% of the amplitude of conventional transitions when the gap of the readback head is aligned with the edge of the media pattern. The implications of these results for servo and read‐only applications are discussed.

Dependence of magnetics, microstructures and recording properties on underlayer thickness in CoNiCr/Cr media

The effect of Cr underlayer thickness on the magnetic, microstructural, and recording properties of sputtered CoNiCr media is reported. Cr thickness was varied from 100 to 2000 AA while the magnetic layer was maintained in the range of 260-300 AA. Measurements using a vibrating sample magnetometer show a monotonic enhancement of coercivity from 600 to 1500 Oe with increasing Cr thickness. Media noise was measured as a function of transition density using a spectrum analyzer. The media noise power at 2400 flux changes per millimeter decreased by a factor of about 10 with the increase in Cr thickness. This reduction is accompanied by a reduction in the coercivity squareness and the development of granular features in CoNiCr grains growing on the columns of the Cr underlayer. These features are consistent with a reduction of exchange coupling among CoNiCr grains. >

Influence of coercivity squareness on media noise in thin-film recording media

Signal and media noise measurements have been made for a wide variety of Co‐alloy longitudinal magnetic recording media. A strong correlation between media noise and the coercivity squareness, S*, is observed at high transition density. The optimum ratio of isolated pulse amplitude to media noise is found for S*≂0.75. This behavior arises from reduced ferromagnetic exchange coupling between neighboring grains of the film which influence both the coercivity squareness and also the uniformity of the domain boundary at the recorded transitions. The low‐noise characteristics of sputtered γ‐Fe2O3 also appear to correlate with low values of coercivity squareness. This work confirms that for systems in which media noise is a limiting factor, optimum performance may require considerably lower values of coercivity squareness than commonly used, determined by the relative contribution of media noise to the overall system noise.

Separation of magnetic and topographic effects in force microscopy

Several techniques are presented which allow magnetic force microscopy to be performed while simultaneously mapping the surface topographic features of a magnetic sample. The separation of magnetic and topographic features measured simultaneously with a scanning force microscope is made possible by an instrument based on a differential interferometer that can detect cantilever deflections of 0.005 nm at a frequency as low as 1 Hz. Two different applications are presented.

Magnetic and recording characteristics of very thin metal-film media

The recording performance of very thin sputtered Co-alloy longitudinal film media has been examined. Measurements made using magnetic layers as thin as 50 AA show that the thinner films exhibit improved signal-to-noise characteristics. Similar trends have been observed in both CoNiCr and CoPtCr alloys on Cr and CrV underlayers. Improvement in signal-to-noise ratio is accompanied by a reduction in coercivity squareness as the film thickness is reduced. Large variations of coercivity with film thickness are also observed in the range 50-300 AA. It is suggested that changes in grain size and grain boundary separation during the early stages of film growth play a role in these effects. >

Composition effects in high density CoPtCr media

The optimization of the CoPtCr alloy for use in high density longitudinal magnetic recording is discussed. Low noise media can be obtained with a combination of high Cr concentration (22 at.%) and high deposition temperature. The results are consistent with a Cr segregation mechanism that provides reduction of exchange coupling without the need for a voided microstructure. The Curie temperature decreases significantly at 25 at.% Cr preventing further increase in the Cr concentration. Pt is added to the alloy to increase coercivity with little effect on other recording properties. >