Magneto-optical Layer Research Articles

Improvement of magneto-optical characteristics of TbFeCo/Al bilayered films at a short wavelength of 400 nm

Bilayered films composed of amorphous TbFeCo alloy and Al layers were deposited successively on glass slide substrates without plasma exposure using the facing targets sputtering system. The specimen films with the magneto-optical (MO) layer thickness tMO of 14 nm have a Kerr rotation angle θK and figure of merit fM as large as about 0.36° and 0.25, respectively, at the wavelength λ as short as about 400 nm. These values of θK and fM are considerably larger than those of the bilayered films with larger tMO in the conventional MO media. Normally, the bilayered films with tMO above 50 nm have θK of about 0.25° and fM below 0.2 at λ of 400 nm.

Control of wall coercivity and its effect on dynamic characteristics in magneto-optic materials

By observing thermomagnetically recorded domains of amorphous TbFeCo(Cr) thin films inside a polarized microscope, wall coercivities were estimated from the field which was needed to move the domain walls. The values were identical to those of the initial magnetizing field of as-deposited films having maze domain patterns. The distinct dependence of the wall coercivities on sputtering conditions observed, such as Ar gas pressure during the sputtering process and especially the Ar ion etching of silicon nitride underlayer prior to the magnetooptic layer deposition. In a dynamic recording experiment, carrier level subsequently revealed a sharp response versus externally applied magnetic field for materials with low wall coercivity, but indicated tradeoffs with relevance to read-out stability of the recorded domains. Particular attention was given to the effect of etching treatment upon the recording characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

EuSe as magneto-optical active coating for use with the high resolution Faraday effect

The high resolution Faraday effect requires a magneto-optical active thin film coating for observations of penetrated flux in superconductors. The use of EuSe for this coating offers many advantages, such as better handling during the evaporation process and a higher achievable contrast compared to the ‘classical’ mixture of EuS and EuF 2. In this paper the apparatus is described and the conditions discussed for achieving optimal thickness of the magneto-optical layer. This technique was used for observations of penetrating flux in niobium and in the high T c superconductors YBa 2Cu 3O 7−δ (single crystals, sintered specimens and epitaxial thin films) and Bi 2Sr 2CaCu 2O 8−δ single crystals.

XPS and AES studies on the interface between metal and polymer layers: The effect of Ar-plasma treatment of the polymer surface preceding the metal deposition

We carried out XPS and AES studies in order to observe the effect of Ar-plasma treatment of a photopolymer-glass substrate, on which a TbFeCo magneto-optical recording layer was deposited by RF sputtering. Sputter-etching of the polymer surface prior to deposition of TbFeCo, resulted in significant reduction in the amount of terbium oxide formed at the interface between the metal alloy layer and the polymer layer when the film was fabricated on an intact polymer surface. Stability of the recording film was improved largely through this etching process. This is considered to be due to the removal of oxygen-containing functional groups, thus hindering the oxidation of terbium in the adjacent recording layer.

Analysis of magneto-optical layers by ion scattering spectrometry

Yttrium iron garnet (YIG) films are made by liquid phase epitaxy (LPE) on a gadolinium gallium garnet (GGG) substrate. The melt contains mainly lead oxide (PbO) and therefore lead can be incorporated into the films. During the annealing process it was found that lead segregates to the surface of the YIG films. The evidence for and the temperature dependence of the segregation process is studied by low energy ion scattering (ISS).

MASS PRODUCTION OF HIGH PERFORMANCE MAGNETO-OPTICAL DISKS BY IN-LINE SPUTTERING PROCESS

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.

Magneto-Optical Enhancement of Tbfeco/Al Films at Short Wavelength

AbstractThe bilayered films composed of magneto-optical(MO) amorphous Tb-Fe-Co alloy and reflective Al layers were deposited successively on glass slide substrates without plasma exposure by using the facing targets sputtering system. The specimen films with the thickness of MO layer tMO below 5 nm showed apparent perpendicular magnetic anisotropy constant Ku of 2 to 3×106 erg/cm3 and rectangular Kerr loop. The specimen film with tMO of 14 nm took the Kerr rotation angle θk as large as about 0.36 degree, at the wavelength λ as short as about 400 nm. These values of θK is considerably larger than those of the bilayered films in the conventional MO media. Normally, the bilayered films with tMO above 50 nm took θK of about 0.25 degree at θK of 400 nm.

Kerr effect of two-medium layered systems

Detailed and practical expressions are given for the magneto-optical Kerr effect (MOKE) for various configurations of two media. One is a magneto-optic (MO) one, and the other is a nonmagnetic (NM) medium. For a system of two thick media with a single interface, with a first-order approximation in MOKE term Q, the Kerr function is determined by the product of a MOKE term Q and an optical term η. A second type of system includes a thin MO (or NM) layer deposited on a thick NM (or MO) substrate. For a MO/(NM-substrate) configuration, the Kerr function is related to the Kerr effects from the air/MO and MO/NM interfaces, and to the Faraday effects of the MO layer, as well as to interference effects. The enhancement factor can be expected to be large by proper choice of materials. For a NM/(MO-substrate) configuration, the total Kerr function is related to the Kerr effect from the NM/MO interface and can be enhanced by interference. The enhancement factor is expected to be less than one if the NM layer is strongly absorbing. Calculations of Kerr effects for examples of the PtMnSb/AuAl2 and Fe/Cu configurations are given. These indicate that the peaks shown in the onset region of the interband transitions of Cu can be attributed to a dramatic change of the refractive index in that region. The merits of a MO/(NM-metallic) structure have been evaluated, and indicate that a better Kerr enhancement effect can be achieved if the refractive index of the MO layer is larger than one and is much larger than that of the metallic material. A drawback to this configuration comes from the fact that a MO material with a large refractive index value usually is not expected to have a large intrinsic Kerr effect.

Spectroscopic study of potential magneto-optic storage layers

We present an experimental investigation of magneto-optic (MO) thin film materials of interest in optical storage. XPS valence band spectra of ultrahigh vacuum deposited rare earth — Fe50Co50 alloys are correlated to magneto-optic and reflectivity spectra measured ex-situ in the photon energy range 0.5 ≦E ph ≦ 5.5 eV. It is found that 4f n →4f n−15d 1 optical transitions contribute to room temperature Kerr spectra in Pr-FeCo and Nd-FeCo, but only in the UV spectral region (λ≦300 nm). Eu is found to enter Eu-FeCo intermetallic alloys with a 4f 7 ground state at −2 eV binding energy. However, no substantial contribution to the Kerr effect from excitations of this electronic state has been observed n either at room temperaturen or at 100 K. We present a general argument that 4f excitations of RE ions in typical intermetallic alloys can never be expected to contribute more than some tenths of a degree toθ k in contrast to the well known huge rotations in RE-chalcogenides. For Yb-FeCo we have observed in XPS a mixed valence behavior of Yb showing a discontinuous transition from divalent Yb2+ in the pure metal to mixed valent Yb2.55+ in amorphous Yb-FeCo films. Nof→d contributions to the room temperature MO spectra have been detected. Strong optical enhancement effects are presented for metallic bilayer and multilayer structures of the type MO-layer/reflector. The total thickness of an optimized structure corresponds to about two times the penetration depth of light. Enhancement occurs predominantly at photon energies below the plasma frequency of the reflector material and is caused by low values of the optical constantsn andk of the reflector. In storage materials like Tb-FeCo the read-out performance can be improved by about a factor of 2 with a bilayer of 30 nm Tb-FeCo on Cu. Quantitative modelling of the optical and magneto-optic properties of these films is discussed.

Dynamic writing of magnetic domains in GdTbFe: Observations made by Lorentz microscopy

In this paper the sizes and shapes of thermally written magnetic domains in a GdTbFe layer are discussed. The magneto-optical layer is deposited on a specially prepared silicon wafer disk, provided with Si3N4 windows. This allows dynamic writing of domains under realistic recording conditions on a rotating substrate and direct observation of the magnetization patterns in the transmission electron microscope, using Lorentz microscopy. Domain patterns were written in two different ways: With the first method, laser modulation (LM), the laser beam was modulated while applying a fixed field. In the second method, magnetic field modulation (MFM), the laser was on continuously and the applied magnetic field was modulated. With LM it was observed that both size and shape are closely dependent on the magnitude of the applied field. This behavior is in good agreement with observations made with optical readout of similar magneto-optical disks. Domains written with MFM have somewhat irregular contours. The degree of irregularity depends on the magnitude of the applied magnetic field and its switching time. Reduction of these irregularities can be achieved by simultaneously modulating the laser beam.

Design Concept of Magneto-Optical Disk

We have developed a 4-layer magneto-optical disk (SiN/TbFeCo/SiN/Al) with erase/write tolerance for laser power (energy window) at temperatures from 10°C to 60°C. We found that the thickness of the third SiN layer and of the second magneto-optical layer has a critical influence on optical and thermal properties. The optimized thickness is about 500 Å for the third SiN layer and 200 Å for the magneto-optical layer. The energy window is 80% of the write power in writing but only 20% of the erase power in erasing. Since laser power can fluctuate by 30% of the nominal power in practical use and disk sensitivity can vary by 10%, we think temperature compensation is needed, which would make it possible to have an energy window of 50%, even in erasing.

Recording experiments on rare-earth transition-metal thin films are studied with Lorentz microscopy

Domains written thermomagnetically under recording conditions in rare-earth transition-metal thin films are studied with Lorentz microscopy. A novel technique in which the magnetooptical layer is deposited on specially prepared silicon wafer disks provided with Si/sub 3/N/sub 4/ windows is described. This technique allows direct observation of the written domain patterns in the electron microscope. It is demonstrated that the nucleation process plays a crucial role when writing in TbFeCo. Different compositions having different temperature dependences of the magnetic properties lead to markedly different domain-formation behavior. With the detailed insight into the shape and structure of domains obtained by Lorentz microscopy, carrier and noise levels obtained from recording experiments can be understood. This is demonstrated with data on GdTbFe. It is shown that magnetic-field modulation, apart from the advantage of direct overwrite, offers the possibility of very-high-density recording.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magneto-Optical Properties of TbFeCo Amorphous Films with Al Reflector

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.

THERMOMAGNETIC SWITCHING EXPERIMENTS ON GdTbFe FILMS

The switching behavior of amorphous GdTbFe magneto-optic storage layers has been investigated as a function of compensation temperature. Films with Curie temperature ranging from 420 K to 495 K and compensation temperatures ranging from 200 K to 420 K exhibit writing behavior with carrier-to-noise ratio up to 61 dB. Outside this range poor quality films are obtained.

Design and performance of magnetic heads for magneto-optic recording with magnetic field modulation

In magneto-optic recording, the most direct way to overwrite stored information is to switch the magnetization in a laser-heated region of the magneto-optic layer by means of an alternating magnetic field generated by a magnetic head. The heat dissipation in the head and the necessary voltage and current amplitude associated with fast switching, the desired high field, and a large head-to-medium distance make high demands upon the head design and the current source, respectively. Design criteria and numerical and experimental results for some circularly symmetric head configurations are presented.

Influence of position of magnetooptic layer on differential phase shift of slab waveguide

This paper examines how the position of a magnetooptic layer inside or outside of the core affects the nonreciprocal phase shift of TM modes in a magnetooptic slab waveguide. We find that, with symmetric slabs, the nonreciprocal phase shift is maximized when the magnetic material fills half the slab core. For asymmetrical slabs, the magnetic material should be located on the side of the core facing the cladding layer with the lower refractive index. For best results, the magnetic material should extend from the core boundary to the point inside the core where the longitudinal component of the electric field changes its sign. Effective nonreciprocal phase shifters can also be built by letting one of the cladding regions outside of the slab core be a magnetic material. For best results, the magnetic material should have a higher index than the nonmagnetic cladding layer on the opposite side of the core. Subject to certain qualifying restrictions, the magnetooptic effect increases with increasing penetration depth of the field into the magnetic cladding layer.

The detection of oxygen in magneto-optical layers with SIMS

The detection of oxygen in magneto-optical layers is of fundamental importance for the characterization of the stability of RE-TM material. The magnetic properties are directly influenced by oxide formation. Oxygen depth profiles are carried out by using the SIMS technique. A comparison with magnetic measurements showed a clear conformity. We were able to study the oxidation behaviour of various layers at room temperature and at higher values up to 250° C for several hours. It could be shown that aluminum is a successful material for the protection of RE-films against oxidation. The difficulties of translating SIMS counting rates into concentration values were overcome by using EPMA. Specially prepared reference samples were measured by this technique and could then be used as standard samples for SIMS.

Photoelastic modulated ellipsometry on magnetooptic multilayer films

A method is described to obtain the permittivity tensor of a magnetooptic layer in a multilayer film by phase modulated ellipsometry. It is shown that the phase difference caused by oblique incidence reflection can be separated from the phase difference caused by the magnetooptic effect by selecting the angle of the analyzer, i.e., a Wollaston prism and a dual detector. A numeric method is described to extract the optical constants from the measured quantities.

Nonreciprocal TM-mode thin-film phase shifters

TM modes guided by thin-film waveguides with one or more magnetooptic and anisotropic layers (i.e., substrate, film, and cover layers) have been studied with emphasis on its nonreciprocal characteristics. Our analysis shows that the nonreciprocity of TM modes is the result of nonreciprocal Goos-Haenchen phase shifts at the cover–film and/or film–substrate interfaces. Contributions to the nonreciprocity defined as (N+ − N−) from various regions are identified. Here N± are the effective indices of refraction of two counterpropagating modes. An accurate algebraic approximation for (N+ − N−) has been obtained to provide physical insight into the problem and facilitate numerical calculations. Guidelines for designing nonreciprocal TM mode phase shifters are also listed.

Corrosion-resistant protective overcoat for magnetooptical media

Aluminum nitride thin films were prepared by RF reactive magnetron sputtering from an aluminum target in an Ar/N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> atmosphere. The composition, structure, and the optical, chemical, and electrochemical properties of AlN films were investigated as a function of sputtering parameters. The ability of AlN overcoats to protect the magnetooptical (MO) TbFe layer was demonstrated by monitoring the Kerr hysteresis loop during an accelerated aging test. No significant change was observed in the MO properties for 30 days of aging at 70 °C and 30% relative humidity (RH). The results indicate that AlN is superior to SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> as a protective overcoat. Electrochemical corrosion tests were carried out on AlN/TDFe films to examine the corrosion resistance provided by the AlN coating.