Michael Andrew Parker

High anisotropy L1/sub 0/ thin films for longitudinal recording

Bulk L1/sub 0/ phase materials, such as CoPt and FePt, are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are also desirable for high density magnetic recording media. This work reports the preparation, microstructural characterization, magnetic properties, and magnetic recording performance of L1/sub 0/ phase thin (25 /spl Aring/ to 200 /spl Aring/) films. The samples are prepared by sputter deposition and ex-situ anneal. The microstructural characterization is by transmission electron microscopy and X-ray diffraction. High magnetocrystalline anisotropy is apparently maintained for even the thinnest films, where coercivity, H/sub c/, >10,000 Oe is observed for a 25 /spl Aring/ sample. The H/sub c/ of films prepared for magnetic recording measurements is considerably less to ensure that test patterns could be written. Samples for recording measurements were obtained by a reduction in anneal temperature or the codeposition of an oxide with the L1/sub 0/ phase material to limit grain growth during anneal.

Giant Magnetoresistance at Low Fields in Discontinuous NiFe-Ag Multilayer Thin Films

A series of sputtered multilayers of Ni80Fe20-Ag was prepared to examine the giant magnetoresistance effect before and after annealing. For a wide range of NiFe and Ag thicknesses, no giant magnetoresistance was observed in the unannealed films. After annealing, a large, negative magnetoresistance was observed of order 4 to 6% in applied fields of order 5 to 10 oersteds at room temperature. The appearance of giant managetoresistance is concurrent with the breakup of the NiFe layers, which is attributable to a magnetostatic interaction that favors local antiparallel alignment of the moments in adjacent layers. These structures may be of significant practical importance as sensors that require large changes in resistance at low fields, such as magnetoresistive heads used in magnetic recording systems.

Ba‐ferrite thin‐film media for high‐density longitudinal recording (invited)

Ba‐ferrite is an attractive candidate for future high‐density recording media because of its large coercivity, corrosion resistance, high hardness, and durability. In contrast to most recent work on Ba‐ferrite thin film media, polycrystalline films with large in‐plane remanence for longitudinal recording are emphasized. Films are prepared on a variety of substrates by on‐axis sputtering at low temperatures from stoichiometric targets followed by a postdeposition anneal in a rapid thermal processor at ∼850 °C to induce crystallization. Structural and chemical properties have been investigated by force microscopy, Rutherford backscattering, and other means. Stoichiometric films have large 5000 A grains that are unsuitable for high‐density recording. However, grain sizes as small as 200 A have been produced by doping with small amounts of Cr2O3 and other additives. Compositional changes also influence crystallite orientation. Exceptional durability has been observed on disks without overcoats, presumably due...

Preparation and magnetic properties of epitaxial barium ferrite thin films on sapphire with in‐plane, uniaxial anisotropy

For the first time, epitaxial barium‐ferrite thin films, predominantly of the magnetoplumbite structure, with an easy axis of magnetization in‐plane have been prepared on single‐crystal sapphire substrates by reactive sputtering and post‐deposition annealing. Lattice images by high resolution transmission electron microscopy reveal the probable origin of the magnetic anisotropy to be epitaxy between the Ba‐ferrite and sapphire, causing the c‐axis of the Ba‐ferrite to lie nearly in‐plane. Coercivities of more than 9 kOe have been achieved in the easy‐axis direction in Al substituted films with magnetization reduced to approximately 25% of the value for unsubstituted films.

Grain growth and ordering kinetics in CoPt thin films

Grain growth and ordering kinetics have been studied in a 10‐nm‐thick CoPt alloy film of equiatomic composition annealed in the temperature range 550–700 °C by quantifying the grain size, grain size distribution, ordered fraction and ordered domain size. The mean grain size of the as‐deposited films is 5 nm and the film is fully face‐centered cubic. Upon annealing in the temperature range 550–600 °C, the mean grain size reaches a stagnation limit of 27 nm and the grain size distribution is lognormal. Grain growth resumes beyond 600 °C and the mean grain size reaches as high as 55 nm at 700 °C. Ordering occurs by nucleation and growth of L10 ordered domains, with a mean size of 3 nm at 550 °C and 19 nm at 700 °C. The ordered fraction shows a dramatic increase from 1% to ∼28% between the two extremes of annealing temperature. The increase in the coercivity of the annealed films follows the increase in the ordered fraction more closely than the increase in grain size. The shape of the M–H loop shows evidence...

Crystallographic structure of CoPtCr/CrV thin films as revealed by cross‐section TEM and x‐ray pole‐figure analysis and its relevance to magnetic properties

The crystallographic structure and coercive force of Co75Pt12Cr13/CrVx bilayer films sputter deposited on NiP/AlMg disks used in longitudinal magnetic recording were characterized as a function of varying V content from pure Cr through pure V. The coercive force of the CoPtCr peaked at an intermediate composition of the CrVx sublayer. The origin of this effect was elucidated by x‐ray diffraction, x‐ray pole‐figure analysis, cross‐section TEM, and elongated probe microdiffraction. The improving degree of epitaxy between the CoPtCr and the CrVx sublayer, the increasing fraction of Cr grains favoring the growth of (1120) Co grains with c axis in‐plane, a narrowing of the mosaic spread in the orientation of Co grains, and an increasing size of Co regions with the <1120≳ fiber axis orientation as CrVx composition approaches values between 80/20 and 50/50 Cr/V content account for the observed coercivity increase.

The development of in situ high-resolution electron microscopy

Progress in introducing high-resolution electron microscopy at controlled elevated temperatures is described. Initial work involved the study of dynamic events in materials like cadmium telluride which can be heated to a sufficient degree by the imaging beam. However, for reproducible experiments the temperature must be carefully controlled and measured, and this involves a heating specimen holder. Results achieved recently this way on a variety of substances including CdTe, GaAs, Si, GaAs-Ti, GaAs-Ni, Si- Mo and Si-Ti, are reported. To derive information pertinent to bulk behavior both the manner of an in situ reaction and its kinetics should be compared with those for specimens prepared from macroscopic materials treated ex situ. For interface reactions in semiconductor materials it is found that events typical of the bulk can be recorded under high-resolution conditions in a large proportion of cases.

Low field giant magnetoresistance in discontinuous magnetic multilayers

Giant magnetoresistance of order 4%–6% has been observed in fields of 5–10 Oe at room temperature in annealed multilayers of Ni80Fe20/Ag prepared by magnetron sputtering. For a wide range of NiFe and Ag thicknesses, no giant magnetoresistance was observed in the unannealed films. We attribute the appearance of giant magnetoresistance to a magnetostatic interlayer interaction that promotes antiparallel order of the moments in adjacent layers fostered by a breakup of the NiFe layers. We discuss the effects of variations in the underlayers, spacer thickness, and the sputtering process on the magnetoresistance. Our results suggest that maximizing magnetoresistance and minimizing hysteresis require samples with continuous Ag layers that prevent contact between the NiFe layers and NiFe layers that are discontinuous but not too severly disrupted.

Microstructural origin of giant magnetoresistance in a new sensor structure based on NiFe/Ag discontinuous multilayer thin films

The origin of giant magnetoresistance (GMR) in a new sensor structure incorporating discontinuous multilayers (DML) of NiFe thin films separated by Ag interlayers is elucidated by means of x‐ray diffraction, x‐ray reflectivity, and cross‐section transmission electron microscopy. It is shown that the observed magnetoresistance, ∼4%–6% in fields on the order of 5–10 Oe at 25 °C, is associated with the breakup upon annealing of the initially dense, columnar superlattice structure into discontinuous multilayers of NiFe due to highly mobile Ag forming Ag bridges between the Ag layers. This observation supports a micromagnetic model for these structures that is based on the concept of initially large ferromagnetic domains in the as‐deposited NiFe structure, exhibiting negligible GMR, breaking up into discontinuous layers, exhibiting an appreciable GMR, with antiferromagnetic ordering across the Ag interlayers.

Recording Performance of Thin Film Disks with Various Crystallographic Preferred Orientations on Glass Substrates

CoPtCrTa films, with CrTi underlayer and a variety of crystallographic preferred orientations, were prepared on glass substrates. AlN, NiAl or Ta seed layers, deposited prior to deposition of the underlayer, were used to control the crystal orientation and grain size of the underlayer and thereby the magnetic layer. The correlation of magnetic and structural properties was studied. The media noise was found to depend primarily on grain size and grain size dispersion; the crystal orientation of the film had a secondary role. For a given CrTi underlayer, the [1010] preferred orientation was associated with the highest coercivity.