A review of the relationships between film composition, deposition process and properties of sputtered amorphous rare earth-transition metal thin films for magneto-optical recording applications is given. It is shown that to maximize Kerr rotation, anisotropy, corrosion resistance and reflectivity, the sputtering conditions should be selected to produce Zone T featureless microstructure. Although equivalent properties may be achieved with either RF diode or DC magnetron sputtering, DC magnetron sputtering is preferred for manufacturing because of higher deposition rates and lower substrate temperatures. To maximize magnetooptic effects, alloys of Fe and Co should be used as the transition metal. It is shown that perpendicular anisotropy, coercivity, compensation temperature and Curie temperature may be adjusted over a wide range through careful selection of rare earth constituents and Fe/Co ratio. Systematic data on the dependences of the magnetic properties on film composition are given.

A typical magneto-optic data storage medium consists of substrate, magneto-optic active layer, and dielectric layers. The properties of the involved materials are discussed based on their impact on the overall performance and reliability of the magneto-optic data storage media, with an emphasis placed on the key magneto-optic material requirements.

Ultra high vacuum deposited rare-earth transition metal thin film materials have been systematically investigated with respect to a high read out performance ∝ R1/2 ·θK in a magneto-optical differential detection scheme (R: reflectivity, θK: Kerr-rotation). In-situ determined XPS valence band spectra have been correlated to magneto-optical and reflectivity spectra ex-situ measured in the photon energy range 0.5 ≤ Eph ≤ 5.5 eV. At temperatures above 100 K, contributions to the Kerr effect from 4fn → 4fn−15d1 optical transitions amount to some tenth of a degree at maximum. In particular, Eu is found to be divalent in amorphous Eu-FeCo intermetallic alloys but no contribution to the Kerr-effect could be detected both at room temperature and at 100 K. We present a general argument that 4f excitations of RE ions in an RE-TM alloy can never be expected to contribute more than several tenth of a degree to θK.

A substantial improvement of the performance can be expected, however, if the optical constants n and k of a storage material with a given magneto-optical activity are lowered. A novel approach in this direction is the use of artificially layered structures where a maximum enhancement of R1/2 ·θK of about a factor of 2.5 may be expected.

The origin of magnetostriction and magnetoelastic effects in amorphous rare earthtransition element alloys is discussed in terms of local magnetoelastic coupling. Experimental determinations of saturation magnetostriction, elastic moduli and local anisotropy are presented and related to the picture of local magnetoelastic coupling.

Experimental and theoretical research into the magnetic and magnetooptic properties of rare earth-transition metal (RE/TM) multilayers Is reviewed. The occurrence of perpendicular magnetic anisotropy is understood in terms of a detailed model based on single-ion anisotropy. Magneto- optic properties of RE/TM multilayers are, in general, similar in magnitude to those of the TM. Plasma-enhancement and related effects in magnetic/metallic bilayers are observed and discussed in detail.

Both single layers (SL) of Tb-Fe and multilayer (ML) TbFe/Al2O3 films have been prepared and the magnetic and magnetooptical properties have been studied as a function of Tb-Fe layer thickness (d,) and modulation characteristics. Perfect rectangular loop is observed only for d1 > 20 na. However in ML, even for d1 -8 nm such characteristics are obtained. Hc shows a stronger dependence on d1 particularly in ML samples. The properties of ML samples are determined by d1 A12O3 layer thickness and the number of bi-layers. The Faraday rotation for Tb19Fe81 films at 633 nm is found to be 2×105 deg.cm-1. Magnetic inhomogeneities cause steps and peaks in Faraday and Kerr loops respectively.

Materials with anisotropies giving rise to perpendicular magnetization are important for optical data storage applications. However, the microscopic origin of the anisotropies is not well understood. It is now widely recognized that understanding the origin of surface and interface anisotropies is one of the most important problems in magnetism. Ultra-high vacuum deposition techniques allow the sequential deposition of layers of several elements with great regularity, and little interdiffusion and contamination at the interfaces. The microstructure of the resultant multilayer materials can vary from amorphous, or polycrystalline with only short range order, to high quality superlattices with long range structural coherence in all three dimensions. This paper gives an introduction to rare-earth/transitionmetal and transition-metal/transition-metal multilayers and superlattices. Sputtering and Molecular Beam Epitaxy (MBE) processes used to grow these materials are then described, as are the in situ and ex situ characterization techniques used to determine their electronic and physical structure, and to measure their magnetooptic properties. Some of the magnetic and magneto-optic properties of materials produced by sputtering and MBE related to optical data storage are discussed.

Exchange coupled layers are suggested as a viable approach to the problem of making a magneto-optic (MC) storage medium for high densities. The couple consists of a storage layer with the requisite square loop properties and a weak perpendicular layer with a large MO rotation at short wavelengths suitable for diffraction limited optics MO storage. The layer with good magneto-optic properties is used as a transducer to read out the information stored as magnetic domains in the storage layer. The dependence of the square loop and MO properties on the magnetic properties and thickness of the individual layers is discussed. The systems NdFeCo/TbFeCo and CoPd/TbCo are used as examples. In general, a domain wall will form at the interface between the two layers and the energy stored in this wall is an important factor determining the switching field of the couple. It has been found that when the read out layer thickness is comparable to the wall width, the spins in the wali also significantly contribute to the MO signal.

Magnetic and magneto-optical properties of transition metal films (Co, Ni) deposited on TbFeCo films were studied for application to magneto-optical recording. Individual films of Co or Ni showed in-plane remanent magnetization, although for Co a component of perpendicular anisotropy was found to increase for thinner films. Double layer films of Co on TbFeCo showed perpendicular remanent magnetization for Co thicknesses up to 25 Å. The effective exchange field acting on the Co layer was estimated from Kerr loops and was found to increase for thinner Co films. Double layer films of Ni on TbFeCo showed perpendicular remanent magnetization for Ni thicknesses up to 50 Å. Because of the high intrinsic magneto-optical activity of Co, the remanent Kerr rotation of Co on TbFeCo was found to be 50 % larger than for a single layer of TbFeCo with the same total thickness. No Kerr enhancement was found for Ni on TbFeCo.

Magnetic multilayers consisting of alternating layers of cobalt and either copper or palladium have been made by magnetically enhanced dc triode sputtering onto single crystal sapphire substrates. The structural, magnetic and magnetooptic properties of these multilayers have been studied. Both multilayer systems have fcc crystalline layering. We find the magnetization in the Cu/Co multilayers is due to the cobalt layers, whereas the magnetization in the Pd/Co system comes from both cobalt and palladium layers. Also, the magnetic anisotropy of the two systems is different. The Cu/Co multilayers have an inplane easy axis of magnetization due to the strong shape anisotropy. The Pd/Co multilayers exhibit a perpendicular easy axis of magnetization for thin cobalt layers due to a large interface anisotropy contribution. The Kerr rotation of the Cu/Co and Pd/Co multilayers has been studied to determine the effect of multilayer structure on magneto-optic properties. The results of the magnetooptic characterization of these multilayer systems are presented here.

In recent years there have been reports of large magneto-optic Kerr effects (MOKE) observed in structures consisting of thin magnetic layers on top of optically thick nonmagnetic reflecting materials. Feil and Haas introduced the concept that these large MOKE could be explained in terms of metallic plasma resonances.[l] Katayama et al found enhanced MOKE in Fe/Cu bilayers at wavelengths corresponding to the plasma edge of copper [2]. Schoenes and Reim discussed the Feil and Haas work [3], and Reim and Weller demonstrated enhanced MOKE in TbFeCo/Cu at wavelengths where the optical constants n and k are small [4]. Thus it appears that enhanced MOKE can result when materials have unusual optical dielectric functions.

This paper discusses the magnetic and magneto-optical properties of Pt/Co multilayers, which we propose are an attractive candidate for a magneto-optical storage medium. Thin films, prepared by electron beam evaporation, have an easy axis of magnetization perpendicular to the film plane, a square magnetic hysteresis loop, and a coercivity of ∼1 kOe, when the individual Co layers are less than 4 A thick. The polar Kerr rotation, θK, is a modest 0.1–0.2° at 820 nm, but a large optical reflectivity is responsible for a good figure of merit. Further, θK for Pt/Co multilayers increases at shorter wavelengths, where higher storage densities are possible. Initial results of thermomagnetic writing on these layers are presented.

Highly Bi-substituted iron garnet films are prepared on glass and single crystal substrates by conventional rf diode sputtering. The chemical composition of the films, especially, BI concentration strongly depends on sputtering conditions such as Ar gas pressure. Polycrystalline film of (Dy, Bi)3 (Fe, Al or Ga)5 012 on the glass substrate is promising for MO storage applications; they are good in film quality, and exhibit good squareness in magnetic hysteresis loop due to a prominent stress-induced magnetic anisotropy normal to the film plane. The polycrystalline films have a serious problem of suffering from the media noise due to grain boundary, to which we propose counter measures. We also describe the sputter epitaxial growth of the highly Bi-substituted iron garnet films with high quality, and compare the properties of the film with those of the LPE films. We also presented magnetic and MO properties of Ce-substituted YIG epitaxial films containing a large amount of Ce which were synthesized first by us using the sputtering technique. Cerium prominently enhances Faraday rotation in YIG, which surpasses the enhancement by BI. Thus the sputtered iron garnet films are promising materials for MO storage in the next generation.

In-situ crystallized Bi,Ga:DyIG films were deposited on GGG substrates by sputtering. The films had quite smooth surfaces and showed square hysteresis loops with high coercivity. Dynamic recording was carried out using an Ar laser. The figure of merit of the disk with an Al reflector was about 10 times larger than that of amorphous rare-earth transition metal magneto-optic films. A CNR as high as 54 dB was obtained for a bit length of 1.4 μm.

Using rf-sputtering techniques, the end member of the BixY3-xFe5O12 family, Bi3Fe5O12 (BiIG) has been grown. The Bi3Fe5O15 films exhibit a large maximum Farady rotation of −2.7 × 105 deg/cm at 550 nm and it is largerthan 105 deg/cm in the range of 520 nm to 630 nm. The Curie temperatures, the temperature dependence of the magnetization, the coercivity, the internal fields, ferromagnetic resonance linewidth and the optical absorption spectra have been measure. In the high Bi concentration films, the Hansen form of the molecular field equations in which the molecular field coefficients were modified to account for the observed increase in the Curie temperature were found to be inadequate to evaluate the temperature dependence of the magnetization. In addition to those terms, a third sublattice is required to fit the data with some of the moment at the tetrahedral iron sublattice being transferred to that of the Bi sublattice. Reasonable agreement between the molecular field model and the data are obtained if the spin on the tetrahedral iron lattice is decreased by 0. 50μB and the Bi ion moment increased to 0. 25% decrease in the absorption gap is also observed in the BiIG.

An overwritable magneto-optic disk with a double-layered medium is described. The medium consists of magnetic recording (low coercivity, Hc, and high Curie temperature. Tc) and reproducing (high Hc and low Tc) layers. After a magnetic head writes data on a recording layer. a laser beam from an optical head irradiates a written track to heat a reproducing layer above its Tc. During cooling, the reproducing layer is magnetized by the flux from the bits on the recording layer with a width equal to that of the focused laser beam. These printed bits are read with the optical head. By repeating the above operations. the overwrite process is achieved. This overwrite mechanism is confirmed by using a Gd-Tb-Fe-Co (low Hc and high Tc)/ Tb-Fe (high Hc and low Tc) double-layered medium, a Winchester type magnetic head and an optical head. A wide gap magnetic head can write 1 μ m bits on the recording layer at a 0.52 μm flying height. Such wide spacing is promising for increased operational stability. Magnetic writing is of great advantage in increasing the recording density due to its rectangular bit shape.

The dependence of the magneto-optical readout signal on readout power and external field during the readout operation was examined in order to obtain the critical readout power (the upper limit below which no change occurs in the written domains) and the magnetization reversal process above this critical power. The critical readout power became maximum when an external field was applied to suppress the force acting on the wall of the written domains. The domain length along the recording track changed when the readout operation was above the critical readout power. The domain expanded with increasing readout power and the domain finally changed to a reversed stripe when an external field was applied to enhance the expansive force on the wall, while the domain shrunk and disappeared when a field was applied to enhance the contractive force. It has been shown that external field application during the readout operation assisted in obtaining a wide readout power margin. It has also been shown that magnetization reversal during the readout operation obeyed the domain wall motion process rather than the nucleation process.

Both TbCo and TbFe thin films were dc-magnetron sputtered onto carbon-coated Cu grids as well as glass substrates from a three inch mosaic target using a variety of power levels and argon pressures (PAr). The perpendicular anisotropy (Ku) displays a maximum in the range of 2.5 to 11.5 mTorr of PAr and decreased with further increases in PAr. Films deposited at low PAr (< 11.5 mTorr) exhibit a smooth and featureless microstructure. Lorentz electron microscopy images reveal the magnetic domains in them as black and white dots which are characteristic of perpendicular magnetization. On the other hand, films deposited at higher PAr (> 11.5 mTorr) exhibit a rougher surface, a high density of microvoids and in-plane magnetic domains with ripple.

A similar study was also made of the effects of deposition power on the microstructure and domains at PAr = 5 mTorr. The Ku was found to decrease with increasing power. The microstructure at 30,40 and 50 watts did not reveal any significant difference except that there was a slight increase in surface roughness with increasing power.

A simple differential set up for measuring changes in reflectivity of a flat surface of a ferromagnetic material, the so-called transverse magneto-optic Kerr effect, is described. The differential procedure is used with the aim of minimizing mechanical and optical noise.

Hysteresis cycles have been plotted for several ferromagnetic films of sputtered amorphous R-M alloys (R - Y, La; M - Co, Ni) and reveal good proportionality between ΔR/R, the relative change of reflectivity, and the magnetization measured with a SQUID magnetometer.

The TbFeCo thin films with dense, uniform and columnless microstructure have been prepared as magneto-optical(MO) recording media by using facing targets type of sputtering(FTS) apparatus. The Kerr rotation angle θk and the reflectivity R of TbFeCo(MO) layers with and without Al reflective layer were measured for the light wavelength λ in the range of 450–850 nm and the MO layer thickness tMO in the range of 30–150 nm. θk of the double layers type of TbFeCo/Al fms took maximum of 0.54 deg.(λ =830 nm) at tMO of about 40 nm. Furthermore, θk and R of the double layers type of films for various tMO were theoretically calculated, based on the already known optical constants of TbFe, Al and /or TbOx. Though they were qualitatively in agreement with the experimental data, there was a large discrepancy between the estimated values and the experimental results. This may be attributed to the uncertainty of available optical constants such as refractive indicies and dielectric tensors quoted in this calculation.