Thermomagnetic Recording Process Research Articles

Development of heat dissipation multilayer media for magnetic hologram memory

AbstractHolographic memory is a strong candidate for next‐generation optical storage with high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material, is expected to be used as a rewritable and stable storage technology. We succeeded in recording and reconstruction of two‐dimensional data in magnetic hologram without error. However, the diffraction efficiency is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes, whereas the merging of fringes due to the excess heat near the medium surface generated during the thermomagnetic recording process must be suppressed. To avoid this merge of fringes, we proposed a multilayered structure in which the magnetic layers are divided by the transparent heat dissipation layers (HDL) to control the heat diffusion. In this study, we propose a simple thermal design method for designing the HDL multilayer structure. Using this model, we designed and fabricated an HDL multilayer medium in which the recording magnetic layers are discrete in the film. The HDL multilayer medium exhibited diffraction efficiency higher than that of the single‐layer medium, and error‐free recording and reconstruction were achieved using the magnetic assist technique.

Development of Heat Dissipation Multilayered Media for Magnetic Hologram Memory

Holographic memory is a strong candidate for next-generation optical storage with high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material, is expected to be used as a rewritable and stable storage technology. We succeeded in recording and reconstruction of 2D data in magnetic hologram without error. However, the diffraction efficiency is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes whereas the merging of fringes due to the excess heat near the medium surface generated during the thermomagnetic recording process must be suppressed. To avoid this merge of fringes, we proposed a multi-layered structure in which the magnetic layers are divided by the transparent heat dissipation layers (HDL) to control the heat diffusion. In this study, we propose a simple thermal design method for designing the HDL multilayer structure. Using this model, we designed and fabricated a HDL multilayer medium in which the recording magnetic layers are discrete in the film. The HDL multilayer medium exhibited diffraction efficiency higher than that of the single layer medium, and error-free recording and reconstruction were achieved using the magnetic assist technique.

Development of Heat Dissipation Multilayer Media for Volumetric Magnetic Hologram Memory

Holographic memory is a strong candidate for next-generation optical storage, featuring high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material is expected to be used as a rewritable and stable storage technology. However, the diffraction efficiency of magnetic holography depending on the Faraday rotation angle is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes, whereas it is necessary to suppress the merging of fringes owing to heat diffusion near the medium surface. In this work, we investigated the recording process of magnetic holograms in detail with experiments and numerical simulations, and developed a multilayer media with transparent heat dissipation layers to record deep and clear magnetic holograms by controlling the heat diffusion generated during the thermomagnetic recording process. To suppress lateral heat diffusion near the medium surface, we designed and fabricated a multilayer magnetic medium in which the recording magnetic layers are discrete in a film, approximately 12-µm thick. This medium exhibited diffraction efficiency higher than that of the single-layer medium, and error-free recording and reconstruction were achieved using the magnetic assist technique.

Simulation of Thermomagnetic Recording Process for High Density Magnetic Field Modulation Method

Thermomagnetic recording process was simulated using Huth’s model for the magnetic field modulation (MFM) recording on a TbFeCo medium. This study focused on the effects of introduction of a field gradient layer, reduction of the laser spot size, and application of laser pulsed MFM method. By using GdFeCo layer, field gradient of 3.7 Oe/nm was confirmed to be applied in the recording layer. This field gradient contributes to the decrease of recorded mark elongation and the suppression of the collapse of the marks, which are two major problems to record marks smaller than 100 nm. For the reduction of the laser spot size, the mark elongation was confirmed to decrease, however the improvement of the stability of small sized marks was insufficient. By applying laser pulsed MFM method, the simulation showed the improvement of recording characteristic of small sized marks.

Simulation of Thermomagnetic Recording Process Using MFM Method: Effect of Field Gradient

Thermomagnetic recording process was simulated for Tbc(Fe72Co22)100-c media by using Huth's model modified to be applied to the magnetic field modulation (MFM) recording method. In the case of the recording with 0.4μm-diameter laser spot and a single pulsed uniform recording magnetic field and TbFeCo media, the recorded mark length became much longer than designed one calculated by a linear velocity and a field pulse width, when the designed mark length was less than 100 nm. The mark elongation was more significant for the recording on a TM-rich medium (c=17.6). When the mark length was shorter than 40 nm, marks were not formed on a RE-rich medium (c=21.7) due to the collapse of domains. By introducing Gaussian profile of recording magnetic field with a maximum gradient of 10 Oe/nm together with the laser spot profile, the elongation of recorded marks was remarkably reduced. Furthermore, the stability of the marks shorter than 40 nm on the media c=21.7 was found to be improved. From the analysis of the effective field acting on the domain wall, it was found that the effect of the field gradient |∂Hc/∂χ|+|∂Hext/∂χ| at the wall stable position was crucial to reduce the mark elongation as well as to improve stability of very short marks.

SIMULATION OF THERMOMAGNETIC RECORDING PROCESS USING MAGNETIC FIELD MODULATION METHOD

Thermomagnetic recording process was simulated for Tbχ (Fe78Co22) 100-χ media with the thickness of 40 and 20nm by using Huth's model modified for the MFM recording.In the recording of marks smaller than-200nm, the recorded mark length was found to exhibit larger value than the designed mark. From the analysis of the effective fields acting on the domain wall, the stray field was found to be important factor in the recording of the small size marks.

Micromagnetic modeling of the thermomagnetic recording process

A thermally-activated micromagnetic model has been developed to model the thermomagnetic recording process. In this model the crystal anisotropy, K/sub u/, and saturation magnetization, M/sub s/, of each grain depend on temperature. To model the thermomagnetic recording process a high-coercivity, H/sub c/, medium is heated with a thermal profile from a laser so that the medium is near its Curie point. In this state transitions are written with a Karlquist type write head. Thermomagnetically-written transitions are distinguished by curved transitions and sharp track-edges with no erase bands. Results are presented which discuss the steps required to optimize the thermomagnetic recording process.

Simulation of Thermomagnetic Recording Using Extended Huth's Equation.

Thermomagnetic recording process for various TbFeCo films on PMMA has been simulated in the case of the irradiation of the laser spot with 1.0 and 0.2 μm by using a modified Huth’s equation. In the case of TM-rich alloy, it is found that the domains smaller than 0.1 μm in diameter are formed at temperature below the Curie point because of the large demagnetizing energy, which results from the large magnetization. On the other hand, the large temperature gradient makes it difficult to stabilize the small size domain in RE-rich film.

SIMULATION OF THERMOMAGNETIC RECORDING IN RARE EARTH-TRANSITION MAGNETIC FILM USING VERY SMALL LASER SPOT

Thermomagnetic recording process has been simulated for transition metal(TM)-rich and rare earth(RE)-rich TbFeCo films assuming heam spot size of 1 and 0.1 micron, where very simple temperature dependence is assumed for the wall energy. For the TM-rich Tb18(Fe78Co22)82 film, domains smaller than 0.1 micron can be written owing to the large stray field. On the other hand, for the RE-rich Tb25.6(Fe78Co22)74.4 film, small domains cannot be written due to the large effective field which, originating from the wall energy and its spatial gradient, forces to shrink the domain.

A dynamic study of domain formation mechanism during thermomagnetic recording based on micro-Hall effect measurements

A method for analyzing the dynamics of domain formation during the thermomagnetic recording process has been developed based on the extraordinary Hall effect. A magnetic domain is written at the center of a cross-shaped magneto-optical sample having an area of 5×5 μm2, and the Hall voltage is monitored during the recording process. As far as domain nucleation is concerned, we find that the temperature gradient around the transition region (i.e., the region whose temperature is between the critical temperature for magnetization reversal and the Curie point) is very important. Under the conditions of high power and short pulse-width laser, a domain can form only during the cooling period. However, it is possible for a domain to form during the heating cycle under a low power, long pulse laser beam.

Thermomagnetically written domains in compositionally modulated DyFeCo thin films

Magnetic properties of critical importance to the thermo-magnetic recording process are shown to be structure dependent. The use of structure as an additional tool in the control of these properties is illustrated by experiments that compare the size of domains written thermo-magnetically into different structured DyFeCo films. Samples with a common average composition and all 40 nm thick and with form that varies from the conventional homogeneous amorphous alloy to a material consisting of discrete Dy and FeCo layers, have been investigated. It is found that, whilst the Curie temperature, compensation temperature and saturation magnetization reflect the consistency of the average composition and remain constant across the range, the perpendicular anisotropy and coercivity are structure dependent and increase as the films become more layered, reaching a maximum when the rare earth (RE) layers are nominally one monolayer thick. In the writing experiments this behaviour is observed as the creation of different sized stable domains under constant write conditions.

Computer simulation of thermomagnetic recording process in exchange-coupled magneto-optical bilayer films

For exchange-coupled RE-TM bilayer films, the thermomagnetic recording process was studied via computer simulation by use of an extended cell model that takes into account the interface wall energy. It is found that magnetization reversal and thus the size and the shape of written domains in the layer of a lower Curie temperature are strongly influenced by those in the layer of a higher Curie temperature.

Measurement of Domain Wall Mobility in Magneto-Optic Media

Domain wall mobility in magneto-optic (MO) media is important in determining the thermomagnetic recording and erase process. However, measurement of domain wall mobility has been difficult due to the large coercivity of MO media near room temperature and the weak MO signal at high temperature. With an improved high speed polar Kerr MO camera system, the dynamics of domain wall motion can now be imaged and measured at temperatures near Tcurie. The domain wall mobility in GdTbFe samples was measured with the bubble collapse method. The experimental results show that the domain wall motion velocity is a linear function of applied bias field and the domain wall mobility is about 0.3 cm/sOe. Samples with different preparation methods are compared and the experiment shows that whereas the dynamic coercivity varies with the preparation method, the mobility is an intrinsic property and dependent primarily upon sample composition.

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.

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.

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. >

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. >