Thermomagnetic Recording Research Articles

COMPUTER SIMULATIONS OF MAGNETIZATION REVERSAL DYNAMICS

Computer simulations of a two-dimensional lattice of magnetic dipoles are performed on the massively parallel Connection Machine Supercomputer. These lattices constitute a discrete model for thin films of amorphous rare earth-transition metal (RE-TM) alloys, used in erasable optical data storage systems. The simulated dipoles follow the dynamic equation of Landau-Lifshitz-Gilbert. Using mean-field theory, we have calculated the temperature dependence of the subnetwork magnetizations, the effective fields, the gyromagnetic coefficient, and the Gilbert damping parameter. These results are then used in the simulation of the thermomagnetic recording process, where a focused laser beam creates a hot spot and allows an external magnetic field to reverse the direction of local magnetization. Reversed domains are seen to nucleate during cooling of a heated spot with Gaussian spatial profile. In the absence of defects of the size of a domain wall, these nuclei are seen to collapse in low external fields. In contrast, samples with patches of uniform anisotropy and weakened inter-patch exchange form stable reversed domains. Color visualizations of the magnetization distribution show local pinning of the cooling reverse domain on patch boundaries.

Thermomagnetic recording in ferrimagnetic garnets

In order to optimize a new MO disk in which garnet is at the surface, optical and thermal calculations have been performed. The thickness of the chromium must be chosen so as to obtain the best writing durations. The garnet film must be sufficiently thick to ensure a large Faraday rotation. Thicknesses of 30 nm for the chromium layer and 250 nm for the garnet layer seems to be the best results we can achieve.

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 thermomagnetic recording experiments it was concluded that these latter MLs were less suitable as a MO recording medium because of higher noise in spite of the larger Hc. Co/Pt disks made by either evaporation or Kr sputtering showed lower noise and a higher carrier-to-noise ratio than those deposited on a Pt base layer or made by Ar sputtering.

Time resolved thermomagnetic recording process in GdTbFeCo films

The dynamics of domain formation in a GdTbFeCo film during a full cycle of thermomagnetic (TM) recording has been studied by a time-resolved polarized-light microscope. The microscope has 5 ns temporal resolution and sub-μm spatial resolution. Nucleation generally occurs within 15 ns after a write laser pulse. Domain growth and contractive rates are strong functions of irradiated laser power. Domain contractive rates are much higher than growth rates in all cases. The domain growth, shrinkage, and stabilization processes are very reproducible regardless of whether writing is started from nucleation or from a seed domain. A bias field affects dynamic domain size, but not growth rates.

EFFECT OF SiNx LAYER ON THE THERMAL BEHAVIOR OF MAGNETO-OPTICAL DISK WITH QUADRI-LAYER STRUCTURE

For optimizing quadri-layer magneto-optical (MO) disk, we have studied thermal behavior of MO disk. In this study, we found that thickness and composition of SiNx layer strongly influence not only thermal diffusion but also read/write characteristics in thermo-magnetic recording process.

DYNAMICS OF MAGNETIZATION REVERSAL IN AMORPHOUS FILMS OF RARE EARTH-TRANSITION METAL ALLOYS

Computer simulations of a two-dimensional lattice of magnetic dipoles are performed on the Connection Machine. The 256 x 256 hexagonal lattice is a discrete model for thin films of amorphous rare earth-transition metal (RE-TM) alloys, which have application as the storage media in erasable optical data storage systems. In these simulations the dipoles follow the dynamic equation of Landau-Lifshitz-Gilbert under the influence of an effective field arising from local anisotropy, near-neighbor exchange, classical dipole-dipole interactions, and an externally applied field. Using mean-field theory, we have calculated the temperature dependencies of the subnetwork magnetizations, the effective fields, the gyromagnetic coefficient, and the Gilbert damping parameter. These results are then used in the simulation of the thermomagnetic recording process, where a focused laser beam creates a hot spot and allows an external magnetic field to reverse the direction of local magnetization. The onset of nucleation and the dynamics of growth/contraction by domain wall motion have been studied by means of these simulations.

Thermomagnetic recording mechanism on TbFeCo disks

In order to make clear a domain formation mechanism in magneto-optical disk with TbFeCo amorphous film, a numerical simulation based on a heat flow analysis is reviewed and discussed. In a conventional laser-power-modulation method, diameters of circular domains are calculated by using forces acting on the domain wall. Calculated change of the diameter with laser power agrees with optically observed results. Moreover, the shape of chevron type domains for a magnetic-field-modulation method can be explained by using the same simulation methods.

The thermal conductivity of rare-earth–transition-metal films as determined by the Wiedemann–Franz law

Measurements have been made of the electrical conductivity of sputtered rare-earth–transition-metal films. The Wiedemann–Franz law has been used to relate these values to the electronic component of the thermal conductivity of the films. A small lattice contribution to the thermal conductivity was estimated and added to the electronic component to obtain the total thermal conductivity of the materials. This total thermal conductivity is about one order of magnitude smaller than that of the pure bulk materials composing the films. Knowledge of the thermal conductivity of such rare-earth–transition-metal films has proved to be useful in the modeling of the thermomagnetic recording processes. These processes are used to record information on magneto-optic data storage disks.

Critical assessment of the physics underlying direct overwrite in magneto-optic recording (invited)

The basic requirements for achieving direct overwrite in magneto-optic recording are discussed from the basic physics of localized magnetization reversal during thermomagnetic recording. The operating principles and the key issues are reviewed and critically assessed for several major direct overwrite approaches including beam modulation using single-layer media, beam modulation using exchange-coupled multilayers, and magnetic field modulation.

ChemInform Abstract: Magneto‐Optical Recording Materials

Abstract High density data storage can be obtained by thermomagnetic recording techniques by means of which remanent magnetization is produced on small spots of a disc surface when cooling from a critical temperature. The digital information is read out using the magneto-optical Kerr effect. In this report the physical principles of the various magnetic phenomena associated with thermomagnetic writing and magneto-optical reading are briefly discussed. A review is given of various materials that may be used for this purpose, including the degree to which these materials meet the stringent requirements for thermomagnetic writing and magneto-optical reading.

Writing temperature estimation in thermomagnetic recording

A thermal conduction model calibrated with the aid of the measured temperature dependence of the magnetooptic (MO) Kerr effect and MO recording readback data taken with variable reading laser power and disk velocity has been used to provide an estimate of the absolute writing temperature in thermomagnetic recording on MO media. This procedure assumes that such writing is essentially a thermal thresholding phenomenon and that magnetic effects in writing are secondary. The key elements of this method include the following: (a) availability of a flexible thermal model properly calibrated for the MO media under study; (b) measurements of the Kerr rotation of the MO material versus temperature; (c) an accurate experimental method for measuring the differential Kerr readout signal with a dynamic MO disk tester; and (d) experimental data of thermomagnetic writing that is well fitted by a model illustrating the validity of the simple threshold writing temperature concept. The simultaneous regression fitting of a thermal model of MO readback to experimental data results in a fully self-consistent procedure. >

Domain edge formation and jitter characteristics in thermomagnetic recording with magnetic field modulation

The role of the dynamics of the magnetic bias field in field modulation thermomagnetic recording has been studied experimentally and through computer simulation. Using a continuous-wave, focused laser to heat a magnetooptic recording medium, the magnitude and the spatial gradient of the modulated bias field were varied, and the behavior of medium noise and jitter in magnetooptic recording was measured. The experimental measurements yielded a universal curve for the relation between signal pulse jitter and the spatial gradient of the recording bias field. Micromagnetics simulations provided insight into the connection between bias field magnitude and its spatial gradient and the degree of domain wall irregularity in recorded marks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magneto-optical recording on evaporated Co/Pt multilayer films

Co/Pt multilayers composed of 0.3-nm Co and 1-nm Pt layers were prepared by e-beam evaporation from Co and Pt sources. These Co/Pt multilayers with a total film thickness of 20-30 nm have a large perpendicular magnetic anisotropy, a perpendicular hysteresis loop with a saturated remanence, a coercivity of about 1.8 kOe, and a Curie temperature of about 300 degrees C. They are structurally stable enough for thermomagnetic magnetooptical recording. Moreover, these Co/Pt multilayers are corrosion resistant and have enhanced Kerr rotation at short wavelengths. On a disk of 15*(Co(0.32 nm)/Pt(1.15 nm)) multilayer enhanced with 80-nm silicon nitride, a CNR (carrier-to-noise ratio) of 59 dB was achieved with a reading laser ( lambda =820 nm) power of 1 mW for a 2-MHz carrier at 10 m/s, and 64 dB with 3-mW read power for a 2.5-MHz carrier at 20 m/s. It is concluded that these Co/Pt multilayers are very promising as magnetooptical recording materials. >

Magnetization reversal process and domain dynamics in thermomagnetic recording of TbFeCo amorphous films

The domain wall dynamics in thermomagnetic recording of TbFeCo amorphous films is interpreted in terms of a model comprising magnetization reversal nucleation followed by a wall displacement process. The nucleation of the reversed domain from the saturated magnetization state occurs with a field usually observed as a nominal coercivity in Kerr or magnetization hysteresis loops. The concept of wall coercivity, which is often smaller than the nucleation field, is explicitly incorporated in the stability condition of the cylindrical domain during the wall displacement process. The model predicts both the recorded domain size and its dependence on recording power and applied field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magneto-optical recording materials

High density data storage can be obtained by thermomagnetic recording techniques by means of which remanent magnetization is produced on small spots of a disc surface when cooling from a critical temperature. The digital information is read out using the magneto-optical Kerr effect. In this report the physical principles of the various magnetic phenomena associated with thermomagnetic writing and magneto-optical reading are briefly discussed. A review is given of various materials that may be used for this purpose, including the degree to which these materials meet the stringent requirements for thermomagnetic writing and magneto-optical reading.

Modeling magneto-optical domain erasure without cylindrical symmetry

We describe a technique for modeling domain dynamics in the context of thermomagnetic magneto-optical recording. The new feature of our model is that no assumption of cylindrical symmetry is required. We illustrate the potential of our technique by applying it to domain erasure on a moving disk.

Thermal mark characterization on static magneto-optical media

The authors explore the thermal behavior of multilayer magnetooptic (MO) storage media in thermomagnetic recording. An attempt has been made to develop a variety of useful models for heat transfer in these disks that can be readily interpreted for ongoing media design activity. Experimental techniques have been devised that can probe the spatial and temporal distribution of the thermal field in the MO storage film. An important feature of these experimental methods is that they use conventional recording hardware for testing recording performance. The experimental work focuses on static, zero-velocity recording experiments in order to clarify the methodology of using thermomagnetic recording process as a high-speed, high-resolution thermometer for the MO film. It is found that each of the models (finite element and method of images) provides an accurate description of experimental measurements of the thermal field in typical MO disk structures. >

Thermomagnetically Induced Nucleation and Growth of Domains in GdTbFeCo

The nucleation and growth kinetics of magnetic domains below Curie temperature during thermomagnetic recording in a typical compensation point writing material, GdTbFeCo, were investigated with a dynamic tester using both pulsed and cw laser beams. Nucleation of domain is shown to be more difficult than growth. Furthermore, both nucleation and growth of domains are rather nonuniform. The implications of these observations are discussed.

Domain formation characteristics during thermomagnetic recording for amorphous TbFe and TbFeCo alloy thin films

The configurational characteristics of thermomagnetically written domains under a static laser irradiation condition were observed by using a polarizing microscope. The shapes of the domains were categorized into three distinctly different types: (1) circular domains with size almost independent of the applied field, (2) domains growing in size and assuming smoother boundaries with increasing applied field, and (3) domains unrecordable at any condition. It was confirmed that the domain configurations were determined mainly by which one was the dominant controlling factor between demagnetizing and domain wall energy at elevated temperatures in the thermomagnetic recording process. The bit shape irregularity found in some alloys of TbFe was attributed to nonuniformity in magnetic properties inherent in the amorphous structure. >

Mean-field analysis of ternary and quaternary rare earth-transition metal alloys for thermomagnetic recording

A mean-field model developed to analyze ternary (TbFeCo) and quaternary (TbGdFeCo, TbDyFeCo) alloys of interest in thermomagnetic recording is based on the localized mean-field theory and emphasizes the reduction of adjustable parameters. The experimental magnetization versus temperature curves are compared with the model predictions, and good agreement is obtained in all instances. The exchange parameters and atomic moment values obtained are consistent with the current understanding of these alloys. The possibility of including an itinerant model to account for the 3d transition-metal magnetic electrons is considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>