B. A. J. Jacobs

Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth

The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of...

Hysteresis, microstructure, and magneto‐optical recording in Co/Pt and Co/Pd multilayers

The factors that influence the coercive field Hc and the shape of the magnetic hysteresis loop for Co/Pt multilayers (MLs), applied as a magneto‐optical (MO) recording medium, have been investigated. The hysteresis loop of MLs, made by evaporation of Kr sputtering, became more rectangular when the total film thickness was reduced, i.e., the saturation field Hs decreased and the nucleation field Hn increased. Hs was calculated using a stripe domain model and Hn was modeled assuming that nucleation took place at a field that destabilized small vestigial magnetic domains. Both models assumed that domain walls can move in the ML film and the agreement with experiment was good. However, these models could not explain the hysteresis loops for the MLs deposited on a thick Pt base layer or made at high sputter gas pressure. These MLs showed a more particulate microstructure and the hysteresis loop was sheared, consistent with magnetic reversal by uniform rotation rather than domain‐wall motion. From thermomagneti...

Laser‐induced crystallization phenomena in GeTe‐based alloys. II. Composition dependence of nucleation and growth

The laser‐induced crystallization behavior of GeTe‐based amorphous alloys has been measured with a novel multipulse laser technique. This enables the composition dependence of the nucleation rate and crystal growth speed to be independently followed. Two types of crystallization are investigated. The first involves single‐phase crystallization of quaternary alloys based on Ge39Sb9Te52, in which the composition dependence of nucleation and growth is followed as Se, S, Sn, and Si are included. Both the nucleation rate and crystal‐growth speed vary exponentially with the composition, and a correlation is found between crystallization behavior and bond strengths. The second involves multiphase crystallization in the GeSbTe ternary system. It is shown that the observed variations in crystallization behavior primarily arise from the composition dependence of nucleation rather than crystal growth. The implications of this finding for the importance of long range diffusion during crystallization in the GeSbTe sys...

Scanning magnetic force microscopy on Co/Pt magneto‐optical disks

High resolution images of domains written on Co/Pt magneto‐optical disks were obtained using scanning magnetic force microscopy. The key advantage of this technique compared to electron microscopy techniques is that regular disks can be used. We studied the influence of the multilayer composition and preparation conditions as well as the effect of the pregrooves and the incorporation of a Pt underlayer, on the structure of the domains on the disk. Marks were written both with laser and magnetic field modulation, and carrier and noise levels were determined. Regularly shaped and subdomain‐free marks for both writing schemes were obtained for multilayers with about 4‐A‐thick Co layers and Pt layers thinner than 15 A. The magnitude of the coercive field had only a minor influence on the domain structure. On the other hand, the preparation conditions and the structure of the base layers have a large impact on the recording conditions and domain structures.

Laser‐induced crystallization phenomena in GeTe‐based alloys. III. GeTeSe alloys for a CD compatible erasable disk

A novel multipulse laser technique has been used to study the crystallization behavior of the GeTeSe ternary system, and the dependence of the nucleation and growth rates on composition has been related to the optical and structural properties. We show that the Se content is dominant in determining both the nucleation rate and the amorphous optical constants, while the crystal‐growth speed depends primarily on the single or multiphase nature of the resultant crystalline state. In this respect compositions on the GeTe‐GeSe line are unique in that they are single‐phase for a Se content up to 25%. This characterization of both the optical and crystallization behavior has allowed us to identify materials suitable for use in a CD‐compatible erasable optical disk based on the phase change principle, in which local amorphous regions in a crystalline film represent data bits. Readout compatibility with a CD imposes extremely stringent conditions on the reflection from the amorphous and crystalline states as well ...

Laser-induced crystallization in Ge-Sb-Te optical recording materials

Abstract Laser-induced crystallization behaviour in Ge-Sb-Te optical recording thin film materials, that are sandwiched between dielectric layers, was studied by a multipuise laser technique. The reflection changes at 780 nm laser wavelength caused by local heating by a pulsed high-power laser beam are monitored. Both the nucleation time and the complete crystallization time are found to decrease significantly with increasing phase change layer thickness up to about 25 nm, above which both tend to become constant. Furthermore, the maximum crystallization speed depends on the type of dielectric material. The strong dependence of crystallization speed on layer thickness is explained in terms of competing interface and bulk processes. This finding is of practical use because it allows the design of phase change optical recording media which can be used at high linear recording speeds.

Co/Pt and Co/Pd multilayers as a new class of magneto-optical recording materials

In this paper we give an overview of the magnetic and magneto-optical properties of Co/Pt and, to a less extent, Co/Pd multilayers as optimized for magneto-optical storage applications. The Co layers should be very thin, i.e. about 4 A , and the Pt layers about 10-20 A to achieve a 100 % remanent layer with perpendicular magnetic anisotropy and high coercivity (80-100 kA/m). Furthermore, the hysteresis loop becomes rectangular for total film thicknesses below about 20 nm. We measured the optical properties as a function of the wavelength of the light and calculated the figure of merit at three wavelengths (820, 633 and 410 nm) for various disk structures. Optimal figures of merit are obtained for film thicknesses below 20 nm, i.e. thicknesses that are optimum for the magnetic properties. The figure of merit at A. = 820 am for Co/Pt is comparable to that of GdTbFe and increases towards shorter wavelengths which favor higher-density recording. We discuss the recording performance using Lorentz images of the written domain patterns. Very regularly shaped domains can be written at normal writing conditions, i.e. at a laser power of 5-10 mW and fields of 25 kA/m. The highest carrier-to-noise ratio measured for Co/Pt is 51 dB (375 kHz carrier, 1.4 m/s, 10 kHz bandwidth). Aging experiments in dry oxygen-nitrogen atmosphere showed that Co/Pt multilayers are chemically stable up to 1 50 °C . Finally, no change in carrier-to-noise ratio is observed for Co/Pt multilayers with a Curie temperature below 300 °C after 2x104 write/read/erase cycles.

CD-erasable (CD-E) disk technology

CD-ROM has established itself as the preferred medium for the cost-effective distribution of software and various types of multimedia information. The ongoing price reductions of write- once CD-R media and drives are broadening the application range of CD-R from CD- prototyping and small-scale data distribution towards data archiving and bck-up at the desktop. The addition of erasability would increase the attractiveness of recordable CD technology and enable its wide-spread use for personal recording of all types of data files with convenient data interchange. A CD-erasable system (CD-E) with the following characteristics would be very desirable: 1) compatible with existing computer systems and software, 2) erasable with direct overwrite (DOW), 3) recordable using CD-R recorders with minimum modification, 4) readable on CD-ROM players with minimum modification, and 5) reliable. The potential of Ag-In-Sb-Te phase-change materials for CD-E discs and some basic studies have been previously reported by Ricoh. In this paper, we present detailed results of DOW recording on Ricohs CD-E discs at 2.4-2.8 m/s (CD double speed). We conclude that reliable writing and reading as outlined in the concept above can be achieved with this CD-E system.

Co/Pt and Co/Pd multilayers as new magneto‐optical recording media (abstract)

In this paper we will discuss Co/Pt and, to a lesser extent, Co/Pd multilayers optimized for magneto‐optical storage applications. Perpendicular magnetic anisotropy and square hysteresis loops are achieved for multilayers with a total thickness below about 200 A if they consist of very thin (≂4 A) Co layers alternated with 10–20‐A Pt layers. Multilayer made by evaporation show a high coercivity (≳100 kA/m) and have proven to be suitable as magneto‐optical recording medium. Recently we showed that Co/Pt layers with high coercivity can be made by sputtering as well, if not Ar but Kr is used as sputter gas.1 We will discuss the properties of Co/Pt multilayers in relation to the preparation technique and will present magneto‐optical recording results for both evaporated and sputtered Co/Pt multilayer films. The written domain patterns can be very regular, as observed by scanning magnetic force microscopy, and a CNR as high as 55 dB (1 MHz carrier, 5 m/s, 30 kHz BW) is observed. The good oxidation and corrosio...

CD-compatible erasable disk

We have experimentally demonstrated the feasibility of a fully backward compatible erasable Compact Disc. Such discs require new phase change materials; we have identified such materials by means of novel selection criteria, finding that a range of GeTeSe alloys possess both the required recording and optical properties.