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A detection limit of 200 ppt of NO2 in N2 at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10-9 cm-1W/Hz0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ

Experiments show that the laser power margin for correct MAMMOS readout of small, packed domains becomes very small. In order to study this, a model is presented for the MAMMOS readout process. This model, supported by numerical simulations, explains the experiments and shows good agreement to the experimental data.

A Magneto-Optical recording technology is described for storage densities of 50-100 Gb/in2 and data rates of 100-200 Mb/s. A sliding optical head which forms a key component for this recording technology has been developed. The head incorporates a thin-film coil for high data rate Magnetic Field Modulation recording and a dual lens objective with a Numerical Aperture of 0.85. To limit head-disc interface problems due to heating by the laser a cover layer of a few tens of μm is applied to the disc. Cover layer requirements are discussed and MFM recording results obtained with a NA=0.6 version of the sliding head and TbFeCo media with a thin cover layer are presented.

The spatial resolution of the copy process (the copy window) of a magnetic domain from the recording layer into the readout layer of a MAMMOS disk is investigated as a function of laser power. This copy window is measured experimentally and calculated with a model of the copy process. The calculations show that higher disk speeds appear to be better concerning the power margins which apply for the MAMMOS process.

In this paper tools to handle the magnetostatic aspects of thin-film tape head design are reviewed. Some illustrative results from transmission-line calculations on inductive write heads and yoke-type magnetoresistive read heads are shown. New is the application to nonlinear saturation effects in write heads. The design of an emboss head for writing servo tracks on tape is described. Finally, a new type of read head is discussed that utilizes anisotropy of the permeability instead of structuring of the fluxguides for obtaining very small track widths.

Digital tape recording systems show the same trend as hard-disk drives: a large increase of storage density with time. The use of advanced media and highly sensitive thin-film heads with magnetoresistive (MR) readout will increase the storage density dramatically. Key improvements are narrower tracks, more sensitive MR elements attained by applying the giant magnetoresistance effect, high-saturation flux density pole materials, advanced metal powder tape, intimate head-to-tape contact, and accurate tracking. By increasing the number of channels in the multitrack thin-film head, high data rates can be obtained as well. The basics of digital magnetic recording are discussed and a short historical overview is given of the Philips activities on thin-film heads for tape recording. An outlook on future improvements is given.

Images of magnetic bits written in a Pt/Co multilayer are presented. Using photosensitive semiconducting tips in a scanning tunneling microscope the surface topography as well as the polarization-dependent optical transmission are measured. Magnetic contrast is achieved by detection of the Faraday effect. Magneto-optical lateral resolution of 250 nm is demonstrated. ©1996 American Vacuum Society

We have epitaxially grown Pt/Co multilayers by sputtering and vapor-deposition directly on single crystals of ZnO, so that Pt/Co (111)¦ZnO (0001) and Pt/Co [1 1 0]¦[1 1 0] ZnO, as characterized by high resolution electron microscopy in sputtered films, and by low energy electron diffraction in evaporated films. These multilayers had very large perpendicular magnetic anisotropy energy, K eff = 2.4 × 10 7 erg/cm 3 -Co (2.4 MJ/m 3 -Co) and 3.2 × 10 7 erg/cm 3 -Co (3.2 MJ/m 3 -Co), respectively for sputtering and vapor deposition. Whereas the coercivity was only modest for the sputtered multilayers, H c = 1.1 kOe (87 kA/m), it was relatively large for vapor-deposition, H c = 3.6 kOe (290 kA/m). We attributed these large anisotropy energies to (111)fee, multilayer epitaxy on (0001) ZnO, and differences in coercivity to individual growth-induced microstructural features, which we characterized by electron probe microscopies. Epitaxy did not occur on a ZnO(1 1 00) surface, and Pt/Co multilayers vapor-deposited on this surface had inferior (111) texture and a much smaller anisotropy energy, ∼ 1 MJ/m 3 -Co.

We sputtered Pt/Co and Pd/Co multilayers on single-crystal, c-axis-oriented platelets of ZnO. The Pt/Co multilayers grew epitaxially with Pt/Co(111)∥ZnO(001) and Pt/Co[1 1 0]∥ZnO[1 1 0], as characterized by high resolution electron microscopy of the film cross-section. Both multilayers had a very large perpendicular magnetic anisotropy energy compared with identical films grown on glass substrates. Keff was 2.4×107 erg cm−3-Co (2.4 MJ m−3-Co_for Pt/Co and 2.9×107 erg cm−3-Co (2.9 MJ m−3-Co) for Pd/Co multilayers grown on ZnO, comparable with multilayers grown by molecular beam epitaxy (MBE) and four and three times larger respectively than identical multilayers grown on glass substrates. However, coercivities were smaller, Hc=1.1 kOe for Pt/Co and Hc=2.5 kOe for Pd/Co, than those reported for highly textured and crystalline multilayers MBE grown on textured metal underlayers, suggesting that larger coercivities are predominantly caused by growth-induced microstructural features and not by a larger anisotropy energy.

Summary form only given. Since high data rates are critical to the success of MAMMOS (magnetically amplified magneto-optical disc system) technology, expansion and collapse speeds of magnetic domains have become a topic of recent interest. To date, the expansion speed in a MAMMOS system has not been directly measured. One way to measure the speed, is by observing the rise and fall times of the magneto-optic readback signal and inferring wall velocities from these times. To measure the rise and fall times experimentally, a MAMMOS system with a 658 nm laser and 0.6 NA lens was equipped with a high-speed photo detector recently developed at Philips. Using this system, MAMMOS readout signals were collected for various laser powers and readout fields using a LaCroy DDA-110 to sample the photo detector output at 2 GHz.

The combination of macroscopic magnetic measurements and micromagnetic observations of domain structure by Lorentz electron microscopy provides a powerful means of studying the behaviour of magnetic thin films. Here two different Co/Pt films have been studied. The same specimens were subjected to similar magnetic field cycles in an alternating gradient force magnetometer (AGFM) and then in situ in the electron microscope. Differences in the scale of the domain structures and the strength of domain wall pinning provided an explanation for the widely differing forms of the hysteresis loops and remanence curves for the two films. The remanence curves define the irreversible changes in magnetization and hence we are able to relate directly the microstructure and the hysteresis properties.

Domain structures in multilayers of Co/Pt have been imaged in a scanning transmission electron microscope using the modified differential phase contrast mode of Lorentz microscopy. Observations have been made of films in their ac-demagnetised state and, through in-situ field application, in a series of remanent states. From the former experiments characteristic lengths related to the domain wall energy have been determined whilst the latter give insight into the magnetisation processes within the multilayers. In addition to providing data on the switching fields, direct information is obtained concerning domain nucleation and the development of domain structure approaching saturation.

High resolution images of domains written on Co/Pt magneto‐optical disks were obtained using scanning magnetic force microscopy. The key advantage of this technique compared to electron microscopy techniques is that regular disks can be used. We studied the influence of the multilayer composition and preparation conditions as well as the effect of the pregrooves and the incorporation of a Pt underlayer, on the structure of the domains on the disk. Marks were written both with laser and magnetic field modulation, and carrier and noise levels were determined. Regularly shaped and subdomain‐free marks for both writing schemes were obtained for multilayers with about 4‐A‐thick Co layers and Pt layers thinner than 15 A. The magnitude of the coercive field had only a minor influence on the domain structure. On the other hand, the preparation conditions and the structure of the base layers have a large impact on the recording conditions and domain structures.

We have studied the structure and magnetic properties of ultrathin epitaxial Pd/Co films deposited by vapor deposition in ultrahigh vacuum on a Pd(111) single crystal. The film crystal structure was characterized by LEED and RHEED and the magnetic properties were studied by Kerr hysteresis loops, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) at 10 and 34GHz. Well-defined RHEED and LEED patterns were present at all thickness proving that there was always a high degree of crystal order in the films despite the large lattice mismatch of 9%. Lattice spacing measurements using RHEED revealed that the Co relaxes to the bulk in-plane spacing in the first monolayer. FMR, Kerr and VSM measurements showed that Co films thinner than ≈ 20A had an easy magnetization axis perpendicular to the film plane. We deduced that the surface anisotropy constant was (0.80 ±0.05)MJ/m 2 .

The authors report on a detailed photoluminescence study of a series of GaAs/AlAs type II quantum well structures. In these structures the thickness of the GaAs layers was kept constant at 25 AA and the thickness of the AlAs layers was varied between 28 and 114 AA. As the AlAs layer thickness was increased they observed a large change in the relative strengths of the zero-phonon and phonon-assisted type II exciton recombinations. They have interpreted this phenomenon as being due to a change in the nature of the lowest lying AlAs electron state from Xxy (k perpendicular to (001)) to Xz (k parallel to (001)) as the AlAs thickness was decreased, where (001) is the growth direction. This changeover is due to the competing effects of electron confinement and strain produced by the small but finite lattice mismatch between GaAs and AlAs. They have estimated the strain splitting of the different X states to be 23 meV. Calculations using this figure along with an envelope function treatment of the effects of electron confinement predict that the lowest confined Xz and Xxy states should cross at an AlAs thickness of the order of 60 AA. For the samples with the thickest AlAs layers (94 and 114 AA) they were able to resolve two zero-phonon lines which they attribute to the different X states, the measured splittings are in good agreement with the calculations.

The multichannel servo writer and the data read/write heads of the new DigaMax/sup TM/ tape storage system are discussed. These heads enable a storage capacity of 13 GB (uncompressed) on 300 m of 72 kA/m tape using prerecorded buried servo information for dynamic track following during reading and writing of data.

Multitrack yoke-type MR readout heads having a single shared front fluxguide with a highly anisotropic permeability, have been designed and evaluated. The well defined microtrack response profiles of these heads show that the anisotropy of the permeability confines the flux-detection to the area on tape in between the contact leads of the MR element. This can be exploited in "full-density" tape recording heads which read directly a number of adjacent tracks.

A 10 μm free working distance recording head for blue, cover-layer incident MO recording has been developed with NA=0.95 and integrated high bandwidth MFM coil. ZF-MAMMOS recording experiments are described.

Recently, M. Sekine et al. (Techn. Digest ODS, pp 61, 2001) showed the first experimental results on a new MAMMOS magneto optical super resolution media type, which they call zero-field MAMMOS, that in contrast to the earlier developed AC MAMMOS type can be read-out without the need for any external modulating expansion field. These media were designed for use in a system with a DVD-like optical light path, employing a 650 nm laser and an NA of 0.6. Philips and Hitachi Maxell have, in joint effort, made and tested the first zero field MAMMOS media for use in a first-surface MO (magneto optical) recorder using a blue-violet laser with a wavelength of 405 nm and an NA of 0.7. In this paper we present the first recording results on these media, recorded with an actuated recording head that operates at 15 /spl mu/m from the storage layer of the disc. This distance is an order of magnitude smaller than the free working distance of the Blu-Ray Disc system.

Semiconductor tips are used as local photodetectors in a scanning tunneling microscope. We demonstrate that this configuration is sensitive to small light intensity variations, as supported by a simple model. The principle is applied to the detection of Faraday ellipticity of a Pt/Co multilayer sample. The outlook of this new technique in magneto-optical imaging is briefly discussed. >

We have discovered a process for Ar-sputtering Pt/Co multilayers, a candidate for high density magneto-optical recording medium, so that they have large anisotropy energy (K/sub eff/ /spl sim/1.2/spl times/10/sup 7/ erg/cm/sup 3/-Co) and large coercivity (H/sub c/ /spl sim/3.5 kOe) simultaneously. To achieve these improved properties the multilayers were sputtered on granular Pt underlayers, made by etching thin Pt layers, initially 20-40 /spl Aring/ thick. Plan-view transmission electron microscopy revealed that the microstructures of Pt/Co multilayers, grown on etched-Pt underlayers, was also granular with excellent (111)-fcc texture. Both K/sub eff/ and H/sub c/ were optimized, when the (111)-fcc texture was a maximum. >

We present a magneto-optical recording head designed for a 400 nm laser with a 0.85 numerical aperture doublet objective lens and a thin film Magnetic Field Modulation coil integrated in a flying slider.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Curie temperature of Co/Pt multilayers could be lowered by reducing the Co layer thickness, i.e., increasing the amount of 2-dimensional character and disorder or by the addition of transition metals such as Os or Re. The second method had the advantage over the first in that the inherent decrease in the Kerr effect was lower.

We have investigated thermomagnetically written marks in Co/Pt multilayers using the modified differential phase contrast mode of Lorentz microscopy. Images thus obtained allow the domain structure in individual marks to be studied with a resolution of ⋍20 nm and can be analysed quantitatively for comparison with recording data.

In a magnetic amplifying magnetooptical system (MAMMOS) disk, the copying process of a magnetic domain from the recording layer into the readout layer is investigated by using a laser spot diameter of 0.66 /spl mu/m. Interference between neighboring bits occurs when the spatial resolution of the copy process is larger than half the domain size. It is observed that a reduction of the readout power from 2.3 to 2.0 mW corresponds to a decrease of the copy window from 0.5 to 0.06 /spl mu/m. As a result, at 2.0 mW packed magnetic domains with a width of only 0.11 /spl mu/m have been resolved without interference by using domain expansion.

Perpendicular magnetic anisotropy has recently been predicted and confirmed for a Co1/Ni2 multilayer [G. H. O. Daalderop, P. J. Kelly, and F. J. A. den Broeder, Phys. Rev. Lett. 68, 682 (1992)]. This letter reports on some magneto‐optical properties of this new multilayer. Its polar Kerr effect appeared to be considerably larger than for a Co1/Pd2 multilayer and comparable to a Co1/Pt2 multilayer. For certain vapor deposition conditions, the perpendicular magneto‐optic hysteresis loop of a 120 A Co1/Ni2 layer showed 100% remanence with a coercivity as high as 140 kA/m. Thermomagnetic writing was possible despite the high Curie temperature.

The magnetooptical recording performance of Co/Pt and GdTbFe-based disks was compared at three wavelengths. At 820 and 647 nm, the carrier-to-noise ratio of the GdTbFe disks was higher (2-3 dB) due to a lower noise level. At 458-nm wavelength, Co/Pt performed 3 dB better than the GdTbFe disk. The crystalline microstructure of Co/Pt did not cause a higher noise level than that of GdTbFe. >

Transparent ZnO with an optical index of refraction n∼2 is a suitable candidate for a dielectric enhancement layer in a magneto‐optical disk. Surprisingly, sputtering a Pt/Co multilayer on a ZnO layer not only increased the optical Kerr rotation but also increased the magnetic coercivity and improved the rectangular magnetic loop shape. Sputter etching the dielectric surface prior to the metal deposition also improved magnetic properties.

Co-Cr layers deposited on Ge underlayers were investigated with ferromagnetic resonance (FMR). The thickness of the Co-Cr layers and the substrate temperature (T/sub s/) during deposition were varied. Measurements with a vibrating sample magnetometer showed that an increase of T/sub s/ results in an increase of both the bulk saturation magnetization M/sub s/ and the perpendicular coercivity H/sub c//sup perpendicular to /. The FMR spectra, obtained with the applied field perpendicular to the sample, showed two resonances for all layers. The effective anisotropy field is positive for both resonances. The difference in effective anisotropy field does not change with thickness of the layer, but increases with increasing T/sub s/. From this it is clear that the two resonances are not due to magnetic inhomogeneities in the growth direction, but must be due to lateral magnetic inhomogeneities. Inhomogeneities in composition, resulting in different values of M/sub s/, or in geometric structure, resulting in different values of the demagnetizing factor N/sub d/, are the most probable causes of the observed effects. >

A magneto-optical near-field scanning tunneling microscope is used to image the prewritten magnetic domain structure of a Pt/Co multilayer. A semiconducting tip acts as a local photodetector to measure the magnetic circular dichroism signal coming from the magnetic sample. The resolution of the magnetic imaging is given by the photoelectrically active volume of the tip. Reduction of the laser light wavelength resulted in a factor of 4 improvement of the magnetic resolution. Based on a sound and applicable definition we estimate the resolution to be (60±35)nm for a wavelength of 532 nm. © 1998 American Institute of Physics.

A short overview is given of the giant magnetoresistance (GMR) effect and its device applications. As an example of present research topics, the optimization of the ferromagnetic-layer composition of spin valves for application in recording heads is discussed. The influence on output voltage and stability has been investigated in test structures and compared with material parameters that were experimentally obtained from unpatterned films. It is found that in practical applications a small addition of cobalt is advantageous, since, although the output voltage is decreased, it leads to an improvement in reproducibility, linearity, signal-to-noise ratio and dynamic range.

Images of magnetic bits written in a Pt/Co multilayer are presented. Using photosensitive semiconducting tips in a scanning tunneling microscope, both surface topography as well as polarization-dependent optical transmission are measured. Magnetic contrast is achieved by detection of the Faraday effect. Magneto-optical lateral resolution of 250 nm is demonstrated. © 1995 American Institute of Physics.

The interlayer exchange coupling in several Co/Ru multilayers and bilayers prepared by sputtering has been investigated from magnetization and ferromagnetic resonance experiments. The hysteresis loops have been discussed in detail, particularly regarding their dependence on the number of layers and the effect of an easy axis direction perpendicular to the film plane. Ferromagnetic as well as antiferromagnetic coupling strengths have been determined in an extended range of Ru thicknesses up to 44 \AA{}. The analysis yields an oscillatory magnetic coupling as a function of the Ru thickness having a period of approximately 12 \AA{}. The peak-to-peak-distance has been found to be smaller at thinner Ru layers indicating a preasymptotic or multiperiodic behavior in the coupling. The envelope function of the coupling was found to decrease, initially as ${\mathit{t}}_{\mathrm{Ru}}^{\mathrm{\ensuremath{-}}2}$ but considerably faster above a Ru thickness of 20 \AA{}. The latter is discussed in terms of mean-free-path effects and of Fermi surface smearing effects both of which are active as a result of structural defects and of the finite measuring temperatures.

In this paper we give an overview of the magnetic and magneto-optical properties of Co/Pt and, to a less extent, Co/Pd multilayers as optimized for magneto-optical storage applications. The Co layers should be very thin, i.e. about 4 A , and the Pt layers about 10-20 A to achieve a 100 % remanent layer with perpendicular magnetic anisotropy and high coercivity (80-100 kA/m). Furthermore, the hysteresis loop becomes rectangular for total film thicknesses below about 20 nm. We measured the optical properties as a function of the wavelength of the light and calculated the figure of merit at three wavelengths (820, 633 and 410 nm) for various disk structures. Optimal figures of merit are obtained for film thicknesses below 20 nm, i.e. thicknesses that are optimum for the magnetic properties. The figure of merit at A. = 820 am for Co/Pt is comparable to that of GdTbFe and increases towards shorter wavelengths which favor higher-density recording. We discuss the recording performance using Lorentz images of the written domain patterns. Very regularly shaped domains can be written at normal writing conditions, i.e. at a laser power of 5-10 mW and fields of 25 kA/m. The highest carrier-to-noise ratio measured for Co/Pt is 51 dB (375 kHz carrier, 1.4 m/s, 10 kHz bandwidth). Aging experiments in dry oxygen-nitrogen atmosphere showed that Co/Pt multilayers are chemically stable up to 1 50 °C . Finally, no change in carrier-to-noise ratio is observed for Co/Pt multilayers with a Curie temperature below 300 °C after 2x104 write/read/erase cycles.

Optically detected magnetic resonance in zero field as well as in a finite magnetic field has been used to study the excitons in type-II GaAs/AlAs quantum wells. The spectra are analyzed using the appropriate spin Hamiltonian for the quasi-two-dimensional indirect excitons. The electron-hole exchange interaction and the g factors for the electron and hole are obtained for several thicknesses of the GaAs and AlAs layers. Good agreement exists between the trend in the exchange interaction and the effective-mass theory of Rejaei Salmassi and Bauer. The anisotropy of the electron g factor is in accordance with a lifting of the threefold degeneracy of the AlAs X conduction-band minimum by the quantum-well potential giving the ${\mathit{X}}_{\mathit{z}}$ valley the lowest energy in the thin-layer quantum wells studied. The effective heavy-hole g value of \ensuremath{\sim}2.5 is much smaller than in the bulk and depends on the GaAs well thickness. This is probably a consequence of the valence-band mixing in quantum-well structures. Two classes of excitons are observed, each with a symmetry that is lower than the anticipated ${\mathit{D}}_{2\mathit{d}}$ point-group symmetry for excitons in quantum wells. The actual symmetry of the type-II excitons and the width of the exciton resonances are related to the microscopic structure of the GaAs/AlAs interface.

The combination of a high resolution and essentially no sample preparation, makes magnetic force microscopy a powerful characterization technique for magnetic storage media. This is illustrated by images of domains recorded on GdTbFe and Co/Pt magneto-optical disks. Scanning force microscopy with magnetic as well as nonmagnetic tips has been used to obtain more insight into the effects of a mild sputter etch of the dielectric layer before the deposition of a Co/Pt multilayer. The main effect of the sputter etch is a reduced roughness of the multilayer which results in a lower disk noise.

The key advantage of scanning magnetic force microscopy compared to electron microscopy techniques is that regular MO disks can be investigated. This enables one to relate the writing conditions and carrier and noise levels of the read-out signal directly to the magnetic microstructure of the domains on the disk. High resolution images are shown of domains on Co/Pt multilayer MO disks. Marks were written with both laser and magnetic field modulation and tracks were erased under different conditions. The influence of the Co and Pt layer thicknesses is briefly discussed.

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.

Experiments are presented on microwave-induced optical nuclear polarization (MIONP) using the photoexcited triplet states in a crystal of fluorene doped with phenanthrene. The effects of modulation of the magnetic field and the microwave frequency on the dynamic nuclear polarization rate were studied. The results show that MIONP must be described in this case by the solid effect theory taking into account the burning of holes in the inhomogeneously broadened ESR line.

The proton spins in a single crystal of fluorene have been polarized by dynamic nuclear polarization via the photoexcited triplet state of phenanthrene guest molecules. It is possible to reach an enhancement of the proton polarization of 90 at 1.2 K where the thermal equilibrium polarization is 0.033%.

By using time resolved luminescence with polarized light we have studied spin relaxation in type II and type I GaAs AlGaAs quantum wells (QW's). The sample is, at 1.7 K, resonantly excited at the exciton transition with circularly polarized light and detection is on one of the two circularly polarized components in a direction perpendicular to the layers. In a type II QW the electrons are confined in the AlAs layer and the holes in the GaAs well. Therefore the optical transition is indirect and a long lifetime results. During the energy relaxation of the exciton we observe that the polarization is partially lost when a longitudinal magnetic field of 100 mT is applied. After the localization of the excitons the polarization ratio stays constant during the decay of the luminescence. This means a spin relaxation time longer than the lifetime of 7 μs. At zero magnetic field no polarization is observed. The long spin relaxation time for the type II QW is in contrast with the time obtained for the type I QW. In the latter case the polarization of the luminescence disappears at times shorter or equal than the instrumental response of 60 ps which is much shorter than the lifetime of about 200 ps. From the time integrated polarization ratio combined with the lifetime we estimate a spin relaxation time of 50 ps.

A quantitative analysis of microwave induced optical nuclear polarization is given for the system fluorene doped with phenanthrene. The description is based on the solid effect theory and includes the effect of holeburning in the inhomogeneously broadened E.S.R. line.