Douglas Greg Stinson

Magnetization and anisotropy of Co/Pd multilayer thin films

Multilayered Co/Pd thin films were prepared by sequential electron‐beam evaporation of Co and Pd onto Si substrates at room temperature. The thicknesses of the Co sublayer and of the Pd sublayer were varied between 2.0–10.3 and 4.5–22.3 A, respectively. As the Pd sublayer thickness was varied at constant Co thickness, broad maxima in the saturation magnetization Ms and intrinsic perpendicular anisotropy energy Ku were observed at a Pd thickness of about 10 A. At this maximum, Ms per Co volume is larger than the saturation magnetization of bulk Co. This is believed to be caused by the polarization of the Pd atoms within about 10 A of the Co layer. Ku and Ms per Co volume both decrease with increasing Co layer thickness.

Corrosion-resistant protective overcoat for magnetooptical media

Aluminum nitride thin films were prepared by RF reactive magnetron sputtering from an aluminum target in an Ar/N 2 atmosphere. The composition, structure, and the optical, chemical, and electrochemical properties of AlN films were investigated as a function of sputtering parameters. The ability of AlN overcoats to protect the magnetooptical (MO) TbFe layer was demonstrated by monitoring the Kerr hysteresis loop during an accelerated aging test. No significant change was observed in the MO properties for 30 days of aging at 70 °C and 30% relative humidity (RH). The results indicate that AlN is superior to SiO 2 as a protective overcoat. Electrochemical corrosion tests were carried out on AlN/TDFe films to examine the corrosion resistance provided by the AlN coating.

Effect of argon sputtering pressure on the magnetic properties and morphology of TbFeCo films

The effect of argon sputtering pressure on the magnetooptic and structural properties of TbFeCo amorphous films was investigated. The films were prepared by DC magnetron sputtering from a TbFeCo homogeneous alloy target. At low argon pressures, the Tb concentration in the film was lower than in the target, while at high pressures, the Tb concentration was greater in the film than in the target. The films deposited at low sputtering pressure have high Kerr rotation and reflectivity. The Kerr-hysteresis-loop squareness decreases and slantedness increases with increasing Ar pressure. The films prepared at low Ar pressure were very smooth and without detectable columnar growth. This supports the hypothesis that the columns are not required for the film to possess perpendicular anisotropy. The study indicates that it is possible to increase the performance and stability of the film by choosing appropriate deposition conditions. >

The orientation of the magnetization of thin films as determined by the Hall effect

The Hall effect is a useful probe of the magnetization in thin metallic films since the signal is inversely proportional to the film thickness. Previously, the minimum in the coercivity, derived from the Hall hysteresis loop as a function of the angle of the applied field, has been used to determine the hard axis of magnetic thin films. A vibrating sample magnetometer can be used in this fashion if coherent rotation is the magnetization reversal mechanism. However, we show that even in the case of the Stoner–Wohlfarth model, the minimum of the Hall coercivity does not coincide with the hard axis. We propose an alternative method for determining the easy axis, which is independent of the magnetization reversal mechanism and does not require measuring the entire hysteresis loop. In addition, since for an arbitrary orientation of the applied field the measured Hall voltage will contain components due to magnetoresistance as well as the Hall effect, it is possible to extract a measure of the in‐plane and perp...

Influence of laser read and bias power levels on the performance of thermomagnetooptic recording media

Read and bias power levels are capable of noticeably altering the response of thermomagnetooptic recording materials. The heating caused by the read power can decrease signal levels due to the temperature dependence of the polar Kerr rotation. Preheating caused by bias power levels also influences media sensitivity and responses during the writing process. Increased bias levels reduce the optimum recording power and increase the severity of mark length variations caused by write power fluctuations. Simple predictive expressions were derived to describe the influence of bias and read power levels during the mark formation and readout processes. The predicted results were in good qualitative and quantitative agreement with experimental observations.

Correlation of composition and angular uniformity with performance in magneto‐optic disks prepared from alloy targets

Sputtering from an alloy target is the preferred method for the production of magneto‐optic recording media. However, when sputtering from an alloy target, the angular dependence of the deposition rate of the rare‐earth elements is different from that of the transition‐metal elements, causing film composition to vary with position. TbFeCo‐based media were deposited on 130‐mm‐diam glass disks from 2‐in.‐diam alloy targets, under varying composition and sputtering conditions. Coercivity, Kerr rotation, and reflectivity were measured as a function of position on the disk. Dynamic performance as a function of record power was determined for 1.0‐MHz recording at 7 m/s linear velocity (3.5‐μm marks). It was found that the angular (in‐track) uniformity of the magnetic properties, as well as the average composition and magnetic properties, is correlated to noise level. Systematic variations in the noise level and the carrier‐to‐noise ratio (CNR) were observed as a function of the film composition. Minimum noise a...

Improvement in the sensitivity of the magneto‐optical media

Future magneto‐optical drives will spin disks at higher speeds to reduce latency and increase data rate. Since optimum recording power increases approximately linearly with disk velocity, and due to limited available power from laser diodes, high sensitivity magneto‐optic media will be required. We have enhanced the writing sensitivity of conventional rare‐earth transition metal alloy based TbFeCo media with the addition of a fourth element, such as Zr. We studied the static and dynamic performance of TbFeCo‐Zr media as a function of Zr addition, as well as Tb, Fe, and Co concentrations. We found that alloying TbFeCo with about 5‐at. % Zr, the writing sensitivity was increased by more than 40%. At higher Zr concentrations (≳15 at. %), the media becomes sufficiently sensitive to restrict read powers to unacceptably low levels, the domain boundaries become ragged giving rise to high noise levels, and carrier levels decrease due to reduced Kerr rotation. Using an AlN antireflection layer on TbFeCo‐Zr5 media,...

Thermally induced structural relaxation in amorphous rare‐earth–transition metal thin films

In the course of practical usage, amorphous optical data storage media are subject to multiple write‐erase cycles. The laser heating that this entails can result in an appreciable change in material properties including those relevant to data recording and retention. Media lifetime predictions based on conventional Arrhenian considerations have, in the past, proven inaccurate. We simulated the thermally induced structural relaxation of rare‐earth–transition metal media assuming the existence of a spectrum of activated processes and provided a rationale for some experimental observations within this framework.

Control of exchange coupling for direct overwrite media using Pd metal interlayer

Several kinds of direct overwrite schemes have been proposed for magneto‐optic media. Schemes using weakly coupled multilayers have shown great promise. In these methods, control of exchange coupling is key to realizing overwrite. An intermediate layer between the memory and reference layers is used sometimes to control the degree of exchange coupling and interface wall energy density. For this purpose, GdFeCo layer having in‐plane anisotropy has been used previously. However, it is easily oxidized. Also, these multilayer direct overwrite media are comprised of similar materials but with different concentrations of components. The components from one layer may diffuse into the adjacent layer, rendering direct overwrite impossible. We found that easily polarizable metals such as Pd can be used as an interlayer to control the degree of exchange interaction between memory and reference layers. Pd metal has no magnetic moment by itself, but a moment is induced on it when it is in proximity to a magnetic mater...

Dependence of maximum readout power on various properties of thin-film magnetooptic media

In magnetooptic recording it is desirable that the read laser power be as large as possible without causing degradation of the recorded information. We have developed a test that efficiently simulates the repeated reading of the recorded magnetic domains. This technique is used to determine the maximum read power and minimum write power of a variety of magnetooptic materials on a variety of substrates. We report the correlation between maximum read power, coercivity of the sample, and thermal properties of the substrate.