Progress in magneto-optical recording materials is reviewed from MnBi, garnets and amorphous rare earth - transition metal (RE-TM) alloys to more recent inovations of exchange coupled bilayers, multilayers, half metallic ferromagnets and amorphous uranium alloys. Materials frequently have some, but very few have all, of the properties needed for this application. The amorphous RE-TM alloys, presently in use, have all the required properties for recording systems operating at 840 nm. However, these alloys will probably have to be modified or replaced to satisfy the requirements for the next generation of higher density magneto-optical recording systems operating in the 400 nm wavelength range.

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  In this paper we discuss the atomic and magnetic structure of rare earth-transition metal compositionally modulated films with characteristic bilayer thicknesses in the nanometer range. A theoretical model based on single-ion anisotropy and anisotropic pair correlations has been developed to understand the occurrence of perpendicular magnetic anisotropy for thin individual layers. Systematics of the magneto-optic properties of this class of materials are discussed, along with recent results on magnetization reversal mechanisms as studied by time decay of the Kerr effect.

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  Calculations of the Curie temperature and magnetization in cobalt-based ultrathin films and multilayers have been performed within the framework of the Green-function theory. The effect of the long-ranged dipolar interactions and of the strong interface magnetic anisotropy is fully taken into account. The influence of the interlayer coupling across the non magnetic spacer is also investigated. The results are discussed in connection with experimental data for Co/Au, Co/Pt, and Co/Pd multilayers.

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  We have prepared Co/Pt multilayers by evaporation under UHV conditions and studied the properties as a function of the nature of the substrate and the deposition temperature. For t(Co) < 8 Å the perpendicular M-H loops are rectangular with coercivity of about 1.9 kOe. The surface anisotropy K_S is found to be 0.6 erg.cm^-2 and 0.35 erg.cm^-2 for ML deposited on glass and MgO(100) substrates respectively. The growth parameters such, as the deposition temperature and a Pt buffer layer, influence the anisotropyonly for t(Co) < 10 Å. However the loop charcteristcs are sensitive to the above parameters. The results are discussed in the context of those published in the literature.

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  Optical constants have been measured for sputtered thin films of platinum, cobalt and multilayer Pt/Co. Magneto-optical constants have also been measured for the films containing cobalt. These constants are presented as a function of the thickness of the films and the wavelength of the incident radiation. The relevance of these measured constants to the optimisation of disk structures is considered.

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  Spectra of magnetooptical Kerr rotation and Kerr ellipticity in Pt/Co multilayers were measured between 1.2 and 5.4 eV. Using the optical constants obtained by the Kramers-Kronig analyses of reflectivity spectra, the diagonal and off-diagonal elements of optical conductivity tensor were obtained for the first time in this material.

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  The magnetic properties of Fe/Pt multilayer films have been investigated using ferromagnetic resonance. In the perpendicular resonance spectrum, a family of modes have been observed in (t_Fe Fe / t_Pt Pt) × N multilayer films, where 5Å ≤ t_Fe ≤ 11Å, 11Å ≤ t_Pt ≤ 15Å and N = 35 and 50. These modes are identified as the normal modes due to exchange coupling between the Fe layers with the surface layers having a nearly pinned boundary condition. The internal uniaxial anisotropy field, H_u, as well as the g-value have been estimated. The linear relation between H_u and 1 / t_Fe suggests that a major contribution to the internal uniaxial anisotropy field is from the interface anisotropy energy, K_s between the Fe and Pt layers which is about 0.14 erg / cm². The average spin wave dispersion constants for the composite material are found to be about 60 meVŲ for t_Pt = 11 ∼ 15Å which is only 20% of the bulk Fe value.

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  Co/pt multilayered films were prepared on heated glass slides by electron beam evaporation. It was found that perpendicular anisotropy K_⊥ increased with elevating substrate temperature T_s. Interface anisotropy K_s and volume anisotropy K_v were evaluated from the dependence of K_⊥ on bilayer period λ In the range of T_s from 30 to 130°C, K_s increased with elevating T_s while K_v hardly depended on T_s. On the other hand, at T_s = 300°C, K_s was almost zero but K_v was positive and large, which made K_⊥ almost independent of λ. It was also found that substrate heating was quite effective to enhance coercive force H_c⊥. Though K_⊥ was hardly dependent on Pt buffer layer thickness, H_c⊥ increased with thickening Pt buffer layer. There was no correlation between low angle X-ray diffraction intensity I₁ and K_s or between I₁ and K_⊥. Though the dispersion of (111) orientation, Δθ₅₀, decreased with elevating T_s, there was no clear correlation between Δθ₅₀ and K_⊥ or between Δθ₅₀ and H_c⊥. The films deposited on room temperature substrates were annealed in vacuum. With elevating annealing temperature T_a, K_s decreased while K_v slightly increased. Though Δθ₅₀ was hardly changed by annealing, I₁ decreased with elevating T_a.

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  The magneto-optical (M-O) properties of a Co/Pt multilayered film (MF) and a Co/Pd MF were investigated. Three optical models for the Co/Pt MF were considered. Reflectivity (R), transmissivity (T), Kerr rotation angle (θ_K) and Faraday rotation angle (θ_F) of the MF were measured and those results were compared with the calculation ones of each model. From the comparison, it was found that the almost alloyed model was suitable for the M-O effect of the Co/Pt MF.

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  Co/Pt multilayers with thickness ratio d_Co/d_Pt = 1/1 have been annealed in vacuum at 300°C, 400°C and 500°C. With annealing, the saturation magnetization M_s and the saturation Kerr rotation angle increase and approach the values of alloyed films. On the other hand, the perpendicular anisotropy energy decreases monotonically owing to both the reduction of the interfacial anisotropy energy and the increase of the demagnetizing energy 2πM_s². These changes in the magnetic properties are explained by the alloying at the layer boundaries due to annealing. The diffusion process is analyzed by means of X-ray diffraction method and is suggested to be connected with the "grain boundary diffusion". The change in the composition profiles in the double layered films of 1000ÅCo/1000ÅPt has been examined by Rutherford Backscattering Spectroscopy. The RBS data also indicate that Co and Pt atoms migrate through the grain boundaries and the column boundaries.

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  For realizing a very high density magneto-optical disk using a shorter wavelength laser, magnetic properties and read/write characteristics of (Pt/Co)/TbFeCo exchange coupled double layered film were studled. It was found that exchange coupling force working on TbFeCo layer must be smaller than coercivity of TbFeCo layer to write good shaped domains. Moreover, multl-domain structure of Pt/Co film, which increases disk noise, could be removed by exchange coupling.

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  The light wavelength λ dependence of Kerr rotation angle θ_k in the multilayered films composed of Pt and MnSb layers have been investigated. The relatively flat peak in the θ_k-λ spectrum of the as-deposited Pt/MnSb films changed to apparently sharp one by post-annealing at temperature of 400°C for 150 minutes. The θ_k of the multilayered film with 13 Å-thick Pt and 50 Å-thick MnSb layers had a maximum as high as 0.83° at wavelength of 630 nm. This value is smaller than that (1.2°) of the bulk PtMnSb heusler alloy. It was found that the peak position of the spectra shifted to short wavelengths by increasing the thickness of the Pt layer.

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  The compensation temperature of each TbFe layer in a TbFe/SiO compositionally modulated film, which is composed of thin amorphous TbFe layers and interposing SiO layers between them, is determined through measurement of the temperature dependence of magnetic Kerr hysteresis loops. The difference in coercivity of TbFe layers in the film is attributed to the difference in compensation temperature. The nearer to substrate the TbFe layer is, the lower the compensation temperature it has. This is explained by a slight change in Tb concentration or effective Tb concentration. The latter could be caused by a change in microstructure of the TbFe layers from layer to layer, which results in the change of magnetic structure and/or degree of oxidation of the TbFe layers.

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  The crystallographic structure and the magnetic properties of Tb/Fe sputtered multilayers have been studied as a function of the thickness of Fe-layer (d_Fe). The crystallographic structure of Fe layer changes from bcc to amorphous at the critical thickness of Fe layer (d_Fe^c ≒ 17Å) with decreasing d_Fe. In corresponding to the change in the crystallographic phase, the magnetic properties of the saturation magnetization M_s, the Kerr rotation angle θ_k, the Curie temperature T_c and the perpendicular coercivity H_c(⊥) change abruptly at the critical thickness d_Fe^c, where the Curie temperature drops to below room temperature and the magnetization curve shows the spinglass like or superparamagnetism like nature at room temperature. Below d_Fe^c, with decreasing d_Fe the Curie temperature T_c increseases owing to the exchange interaction between Fe and Tb around the layer boundaries and then the saturation magnetization increases again approaching the value of the alloys with the same composition. The perpendicular coercivity also increases steeply with decreasing d_Fe.

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  Optical response of layered lossy anisotropic media displaying magnetic order is considered. The analysis is based on classical optical terms. Planar magnetic multi layers are represented by Yeh's 4 × 4 matrices which relate the field amplitudes in the isotropic half spaces on both sides of a multilayer (ML) structure. Each layer is characterized by a complex permittivity tensor of general form while magnetic permeability assumes its vacuum value. The basic assumption is that the electromagnetic field in a layer can be represented as a linear combination of four eigenmodes following from the wave equation of the layer. Then the boundary conditions at interfaces along with a set of four field amplitudes completely determine ML electric and magnetic field distributions. The approach is the most general of those published so far. Assuming minimum previous experience in the ML optics the case of polar magnetization and normal incidence is treated in detail to provide a simple way to the solution to the problems most often encountered in practice.

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  We discuss the present status of the first-principles theory of magneto-optical properties of elemental metals and compounds. We succeeded in giving an accurate first-principles description of the optical conductivity and, for the first time, of the magneto-optical Kerr spectrum. The progress made is illustrated by our results for the transition metals Fe and Ni. For Fe we find that the ab initio calculated magneto-optical Kerr rotation is in excellent agreement with experiments, while only reasonable agreement with experiment is found for Ni. Further investigations were carried out on several compounds with including FeCo, FeRh, FeV, GdFe, and GdCo. We find that there is in general no simple connection between specific spectral peaks and the electron band structure. But by calculating the magneto-optical spectra of selected chemical compositions and crystal structures, we are gradually building up a general picture of the relationship between magneto-optical spectra and atomic properties, such as the spin-orbit coupling strength and exchange splitting, as well as of influence of local atomic environments.

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  The complex polar magneto-optic Kerr spectra at the photon energy range 1.5 to 5.2 eV in amorphous Tb_xFe_1-x films with 0.15 ≤ x ≤ 0.23 rf sputtered onto water cooled glass substrates and covered by Al₂O₃ layer, are presented. They extend the previously published data to higher photon energies where the combined effects of the enhancement by the Al₂O₃ and the oxide layer created at the metal dielectric interface and those of 4f-5d electronic transitions of terbium become important. An explanation of the spectra is proposed based on the knowledge of the Tb effect in iron garnets.

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  A large negative Faraday rotation is reported in Ir-substituted magnetic garnet in the visible wavelength region. From the electronic states calculation of Ir-O octahedral cluster in the magnetic garnet using the unrestricted SCF-XαSW method, it is assigned that this rotation originates from the charge transfer transitions from O 2p to Ir 5d levels. Furthermore, the Faraday rotation spectrum is calculated on the charge transfer model by taking account of the spin-orbit interaction and the molecular field from the iron magnetic moments.

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  A noncontinuum model was used to calculate the magnetization distribution in exchange-coupled Co/TbFeCo double-layered films from a given set of individual (single-layered) film properties. The perpendicular remanent magnetization was found to be stable in the Co layer on top of a 500 Å -layer of TbFeCo when the thickness of the Co layer was less than a critical value of 22.5 Å. The result was compatible with the experimentally observed value of 25 Å. The applied field dependence of the Co layer magnetization was also found to be consistent with the observed Kerr hysteresis loop. On the basis of this model, a multilayered structure was suggested as a means of further enhancing the Kerr rotation.

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  By means of a coupling of spin and obital moment under action of crystal electrical field, we deduced the theoretical expression of the interface energy K_s supporting the perpendicular anisotropy for one monolayer Co/19ÅPt multilayer. As compared with the experimental value, K_s=-0.25erg/cm², the permission combination of obital quantum number L and the ratio of the charge number Z_Pt/Z_Co, carried by Pt and Co atoms respectively, is given for fitting the experimental K_s value.

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  Magnetic domain structures of Co/Pt multilayers sputter-deposited with the substrate bias V_B were observed. The application of V_B decreases the coercivity of Co/Pt drastically. The stripe domains were observed for the film deposited at V_B = -30 V and small coercivity made the wall motion easy, on the contrary to films deposited without V_B, which have relatively large coercivity of about 1 kOe and almost square hysteresis. The flatness and sharpness of interface in multilayers become better by the application of V_B, and the well-defined columnar structure observed in 0 V biased film dissappears with V_B, These changes in the structure cause easy wall motion.

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  Optical and magneto-optical properties of Fe_xO_1-x (x>0.5) oxide-metal composite films were measured in photon energy range from 1.5eV to 4eV by ellipsometry and polar Kerr effect. The observed complex reflectivity ratio Rp/Rs were compared with those calculated from anisotropic effective dielectric constant by a model that fine Fe metallic ellipsoids are precipitated in dielectric matrix (FeO). Volume fractions of metallic phase thus obtained are in good agreement with those from magnetization measurement, but depolarization factor of metallic precipitates does not explain the perpendicular magnetic anisotropy of the films. Kerr rotation angle, θ_k, increases linealy with average magnetization of the composite films up to 20 min for σ_g=100 emu/g at photon energy 2eV. The ratio θ_k/ σ_g is twice as large as that for pure Fe.

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  Magnetic, magneto-optical properties and Auger depth profiling performed in Ni_xY_1-x amorphous films are reported. Kerr effect measurements were carried out at both film/air (f/a) and glass/film (f/g) interfaces. Anomalous magnetic surface effects on amorphous Ni_0.91Y_0.09 have been detected. We discuss these results and contrast them with those of Co_xY_1-x thin films.

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  The polar Kerr rotation (1.5∼5.5eV) and the reflectivity (1.4∼6.2eV) in for Fe-Ni alloys of different composition were measured and the optical constants and the Kerr ellipticity were estimated by Kramers-Kronig analysis. The Kerr effect of the alloys was briefly discussed in connection with the off-diagonal element of the dielectric tensor.

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  The refractive index (n), extinction coefficient (k), Kerr rotation angle (θ_k), ellipticity (η_k) and saturation magnetization (M_s) were investigated for binary Fe-Ni, Ni-Co and Fe-Co and ternary Fe-Ni-Co alloy films. Both θ_k and η_k at λ=500 nm can be expressed as a power series of M_s, θ_k(rad.) = 3.7×10^-7M_s + 1.1×10^-13M_s³ and η_k(rad.) = 1.1×10^-6M_s - 1.9×10^-13M_s³ , respectively.

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  Applications of time domain testing techniques which permit characterization of the media contributions to the performance of an MO channel are discussed; including both small (μm) scale phenomena appropriate to recording physics studies, and, larger (mm) scale phenomena appropriate to disk mapping and uniformity studies. Recording physics studies are described wherein the components of timing shifts in the channel due to properties of the media are characterized in terms of thermal, optical, and, magnetic crosstalk contributions. In this context laser power precompensation of edge positions is characterized. Timing jitters in the channel due to the properties of the media are characterized in terms of the noise contributions which limit performance, such as media and writing noises which scale linearly in the read power. Disk mapping studies are described wherein a software bit-error rate mapping technique is used to measure and characterize media defects.

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  Recently there has been renewed interest in phase transformation based optical storage technology. Included among the many reasons for this interest has been the achievement of : (1) fast erase in conjunction with data stability, (2) demonstration of direct overwrite with good erasability, and in the last couple of years, (3) good progress in increasing the cyclability of the media. Direct overwrite has been demonstrated at media speeds from 1.25 to 22m/s. More than 2×10⁶ cycles have been achieved with low BER. In addition, phase change media exhibits strong signal response at wavelengths shorter than 830nm and requires a much simpler optical head. In this paper we review past progress and assess the current status.

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  Neary 40 years have passed since magnetic storage devices first began being used for computers. The most important of these devices is the magnetic disk. This article covers magnetic disk evolution, advances in magnetic recording, disk system prospects.

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  The future developments needed in magneto-optical recording are discussed. It is projected that by the year 2000, personal computers will have processing speeds of 100 million instructions per second and as much as 1 gigabyte of semiconductor memory. Magnetic disk drives could range in size from 1 inch to 3.5 inches. A 3.5 inch drive is expected to store as much as 60 gigabytes while even a 1 inch drive could store 600 MBytes. The 1 inch drives could be used in arrays on boards with a 4 X 4 array providing 10 gigabytes of storage, faster access time and much greater reliability than a single drive. To work with this memory and storage hierarchy, relatively high-performance removable media will be required for off-line storage and interchange between computers. It is expected that data will consist of a lot of images, requiring vast amounts of storage. Two inch magneto-optical disks could provide 4 gigabytes of storage and the required removability. Magneto-optical drives are expected to provide faster access times and higher data rates than today, but to still lag rigid magnetic drives in performance. These advances will come about through use of shorter wavelength laser sources, improved servoing techniques, faster rotation rate spindles, improved signal processing, improved modulation and error correction codes, direct overwrite and lighter weight and smaller optical heads.

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  Pt/Co and Pd/Co multilayers are attractive candidates for a magneto-optical recording medium, particularly because of their larger Kerr rotation at short wavelength where future ultra-high density recording is possible. In this paper we discuss the attributes of multilayers and point out some of their deficiencies which need improvement in order to meet the overall requirements for a high density, high performance recording medium.

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  Using an in-line sputtering system composed of seven chambers and one outgassing module, the effects of sputtering conditions on the performance and reproducibility of Magneto-Optical disks based on TbFeCo have been studied. A dc reactive sputtering superimposed with an ac has remarkedly reduced arcings in the fabrication of silicon nitride films without excessive substrate heating. The influences of total pressures, Ar/N₂ ratios and input powers on the refractive index of a silicon nitride film were extensively investigated. The protective ability of a silicon nitride film was found to be mainly dependent on the refractive index, but an amount of the warp of polycarbonate substrates was dependent on the process parameters. The influences of sputtering conditions on the corrosion resistance and magnetic properties were also studied. Good uniformities of the thickness and composition of a silicon nitride layer and a magneto-optical layer could be achieved by sputtering at a low pressure. An excellent reproducibility in fabricating the disks having a quadri layered structure was achieved and BER was at the low level of 10^-6.

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  The practical use of Rare-Earth (RE) Transition-Metal (TM) MO media for erasable optical recording applications advanced through major steps during the last several years. Fundamental understanding of alloy thin film properties, high performance multilayer MO media designs and large-scale manufacturing methods were the keys to establishing MO media in the market place. The professional use of MO media and their potential for higher performance in future generations of optical drives will lead to increasing commercial use of RETM alloys in the next decade. This paper covers the main steps of MO media research, development and manufacturing which lead to production of a commercial product.

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  Magneto-optical and magnetic properties of single and exchange-coupled double-layered TbNdFeCo films were examined. Very thin TbNdFeCo films with small Tb content were found to be suitable as the 1st layer of double-layered TbNdFeCo films. Large coercivity and large Kerr rotation were concurrently realized in exchange-coupled double-layered TbNdFeCo films. It was also shown that the exchange-coupled double layered TbNdFeCo film has a good recording characteristics at the wavelength of 830nm and the potentiality as a recording medium for shorter wavelength is suggested.

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  The structure, magnetic and magneto-optical properties of the MnBiRe (Re = Ce and Nd) thin films are discussed. It has been found for those films that the magnetization is in the range from 20 to 50 emu/g, Hc = 1 - 4 kOe. The dependence of the Kerr rotation angle θ_K and reflectivity R on the wave-length (400-900 nm) was measured at room temperature. The θ_K was in the range from 1 to 2.5 and R = 0.3 to 0.45. X-ray diffraction shows that the crystal to be mainly MnBi hcp and similar for MnBiNd films.

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  Thin films consisting of two-phase PtMnSb and Mn₂Sb were prepared by thermal annealing of ion-beam sputtered Pt/Mn/Sb multilayers having four different types of stacking patterns. The resulting phase morphologies such as preferred orientations and relative amounts of the constituting phases were greatly affected by the initial stacking patterns of the multilayers and annealing conditions. The magnetic measurements revealed that the perpendicular magnetic anisotropy was developed in the samples with the c-axis preferred orientation of the Mn₂Sb phase.

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  In this study, bilayered films composed of amorphous TbFeCo alloy and Al layers have been prepared by using the facing targets sputtering system. The specimen films with TbFeCo layer thickness below 20 nm showed Kerr rotation enhancement at short wavelengths near 400 nm. In addition, the films with very thin MO layers(below 5 nm) revealed apparent perpendicular magnetic anisotropy.

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  Magneto-optical recording materials with high remanent Kerr rotation at short wavelengths were successfully obtained in a trilayer form by sandwiching NdCo amorphous films with an in-plane magnetization between TbFeCo amorphous films with strong perpendicular anisotropy and high coercivity. The effect of the NdCo thickness on the magnetic and magneto-optical properties of the trilayer medium was investigated systematically. The distribution of the magnetization direction in a trilayer NdCo amorphous alloy film was calculated to manifest the reorientation of the magnetization inside the NdCo layer through magnetic exchange coupling.

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  A comprehensive phenomenological study of galvanomagnetic and magneto-optic properties of In doped amorphous TbFeCo films are presented. From the coercivity, Hc, determined by Kerr and Hall loop measurements, we find that the films prepared at Ar pressure less than 8x10^-3 mbar and thickness greater than 100 nm have good reproducible and suitable magnetic and magneto-optic properties. On time aging, Hc changes little for films prepared at lower Ar pressure.

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  TbGdFeCo and TbFeCo thin films were prepared by dc magnetron sputtering. Deposition conditions were background pressure of below 2 × 10^-6 Torr and Ar pressure of about 5∼20 mTorr. The bias voltage was about -150∼0 volt. Torque and hysteresis loop of the samples were measured by torque magnetometer and VSM with an applied field of 7.4 kOe and 10 kOe, respectively. The magnetic anisotropy constants were calculated by using Miyajima method. The single layer thin films with a thickness of about 2000 Å were prepared with replacing Gd with Tb. Here, it could be known K_u decreased with increasing Gd instead of Tb and took a sudden turn for the decrease when bias voltage was zero. All of the single layer thin films showed in-plane anisotropy. In case of multilayer thin films, the samples were prepared with varying thickness, and the composition of the samples were Tb₂₄(Fe-Co)₇₆. Most multilayer thin films represented perpendicular anisotropy.

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  Amorphous Tb-Gd-Fe thin films were prepared at various compositions and various substrate temperatures (T_sub), and their magnetic anisotropies have been investigated. It is found that the films prepared at T_sub=77 K show only the in-plane anisotropy, and that those prepared above about T_sub=300 K show the perpendicular anisotropy. Since the absolute value of the uniaxial anisotropy constant (K_u) decreases linearly with the fraction of Gd substitution for Tb, the single ion anisotropy of Tb is considered to be closely connected with the anisotropy of these films. The composition dependence of K_u obtained suggests that the perpendicular anisotropy is related to the creation of small microcrystals with the composition of 15-30% rare earth in the films.

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  Some of R₂Fe₁₄B alloys show fairly large Kerr rotation, and exhibit the spin reorientation phenomenon. It might be possible to use this phenomenon for magneto-optical recording by using the spin reorientation temperature. In Nd₂Fe₁₄B, the low temperature phase of the spin orientation which has the conical plane as the easy axis changes to the high temperature phase of the one which has c-axis as the easy axis above the spin reorientation temperature. In Er₂Fe₁₄B, the low temperature phase has the easy axis in c-plane, but the high temperature phase has the easy axis along c-axis. Corresponding to such spin reorientation, many magneto-optical Kerr spectra are reported by changing temperature and applied magnetic field to elucidate the behavior of the sublattice magnetic moments.

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