Magneto-optical Media Research Articles

Magnetic expansion readout of 0.2 μm packed domains on a magneto-optical disk with an in-plane magnetized readout layer

We have investigated an in-plane magnetized readout-layer-based magnetic domain expansion phenomenon in a magneto-optical (MO) medium. Very high-density 0.2 μm packed domains are expanded up to three times the readout signal level in comparison with the conventional approach of applying alternative external field in the readout process. This result suggests that more than 10 Gbits/in.Reference 2 MO recording density may be achieved by implementing the domain expansion technique, using even the currently available 680 nm laser and numerical aperture of 0.55.

Characterizing the Magneto-optic Properties of Amorphous Rare Earth - Transition Metal Thin Films

AbstractThe amorphous rare earth - transition metal (RE-TM) thin film alloys and nanolayered materials exhibit numerous properties advantageous for optical recording, including perpendicular anisotropy, high coercivity and low magnetization at room temperature, low noise, and easily adjustable Curie and compensation points. As a result these materials have been employed in all commercial magneto-optic (MO) media. On the other hand, the MO effect of these materials is relatively small and tends to decrease with decreasing wavelength. It is important to understand the useful limits of these materials in MO media, and to determine if their MO figure of merit can be substantially increased through appropriate doping or nanolayering. In this paper we discuss experimental techniques for measuring MO properties, a theoretical approach for analyzing the data and designing optical thin film stacks, and results for a variety of RE-TM thin film materials.

Amorphous Rare Earth-Transition Metal Alloys for Magneto-Optical Storage

AbstractAmorphous alloys of the heavy rare earth elements with cobalt and iron have magnetic and optical properties that make them ideally suited for magneto-optical mass storage media. These materials are used in all commercial erasable (MO) optical disk systems and in optically assisted magnetic storage such as the recordable music minidisk. The important properties of these materials for this application are: 1/ ferrimagnetic exchange with a magnetic compensation point near room temperature, 2/ perpendicular easy axis anisotropy when prepared under suitable conditions as thin films, 3/low media noise as compared to polycrystalline films, 4/low processing temperatures compatible with polymer substrates and 5/thermally stable when cycled repeatedly to the Curie temperature. The magneto-optical Kerr effect that is used to read the information off the disk is large at the 800 nm wavelength used in current devices but low at 400 nm. Since the areal storage density is set by the diffraction limit, improvements in shorter wavelength device performance are needed.

Magnetic and Magneto-Optical Properties of Al Substituted Bi-RIG Particles Dispersed in a Plastic Binder

AbstractNano-size R2BiAlxFe5-xO12(R: Dy, Gd, Th) particles were prepared with a coprecipitation and annealing method. The coating films of the particles were prepared with a coating technique. The magnetic and magneto-optical properties of the particles and films were investigated. The coercive forceHcof the particles and films was increased and saturation magnetizationMs. dropped with the Al contentx. TheHcof the coating films was about 200 Oe at the compositions of dispersed particles for which theMswas almost zero. These results suggest that the increase was made with magnetic compensation. The BiTb2A10.6Fe4.4O12coating film shows Faraday rotation spectra. The figure of merit of the film is about 0.5 at 520 nm. The coating BiTb2A10.6Fe4.4O12film is one of the candidate materials for a new economical magneto-optical storage medium.

TEMPERATURE DEPENDENCE OF THE AMORPHOUS SmTbFeCo MAGNETO-OPTICAL FILMS

A mean- field theory is developed to research the temperature dependence of the amorphous SmTbFeCo thin films. It is shown that with available Sm substitution of Tb, the composition temperature Tcomp decreases, Curie temperatureTC remains unchanged, Kerr angle becomes larger, the saturation magnetization Ms at room temperature increases. Therefore, LRE-HRE-TM amorphous films can be used as the high density magneto-optical media in a wide range of temperature. However, the Sm substitution of TM does not improve the magneto-optical performance.

Triple -Layered NdFeCo Amorphous Magneto-Optical Media with High Kerr Rotation at Short Wavelengths

Triple -Layered NdFeCo Amorphous Magneto-Optical Media with High Kerr Rotation at Short Wavelengths

Ferromagnetic particle composite polymer films for glass and semiconductor substrates

A ferromagnetic particle composite (FPC) film is proposed and demonstrated as a new type of magneto-optic media for glass and semiconductor substrates. The FPC films are composed of polymers in which very fine ferromagnetic particles are homogeneously dispersed. These films can be easily fabricated on glass and semiconductor substrates by using a conventional spin-coating method. It is shown that the measured optical properties of the FPC films are similar to the theoretical ones calculated from the Maxwell-Garnett theory. And the strong Faraday effect has been confirmed in the FPC films fabricated. It is expected that the FPC films will become very useful media for various optical devices and integrated optics.

Investigation of the domain contrast in magnetic force microscopy

Domains were imaged by magnetic force microscopy (MFM) on materials where the domain size exceeds the sample thickness by three orders of magnitude. Selected samples are a magnetooptical medium and ultrathin cobalt films, all with perpendicular magnetization. A strong domain contrast is observed in both cases. This fact is confronted to the usual theory of MFM image formation, in which the stray field from one body (tip or sample) is sensed by the other, without altering of the magnetization distributions. It is shown that the domain contrast in such extreme conditions cannot be explained with that theory. On the contrary, Abraham and McDonald’s model, which considers the response of the sample to the tip field, is quantitatively compared to experiment. It is shown to provide a good qualitative description of the results, but not a quantitative one, because of oversimplification.

New high performance magneto-optic media: Tb/Bi/FeCo and Tb/Pb/FeCo superlattices (invited)

Tb/Bi/FeCo and Tb/Pb/FeCo superlattices are demonstrated to offer Kerr rotation and figure of merit (reflectivity times Kerr rotation) substantially exceeding that offered by conventional TbFeCo recording materials, particularly at blue wavelengths. Superlattices such as 4.4 Å Tb/2.0 Å Bi/5.0 Å Fe0.6Co0.4 exhibit squarenesses suitable for magneto-optical recording and figure of merits of 0.20, 0.18, and 0.14 at wavelengths of 780, 650, and 430 nm, respectively. Dynamic testing with 780 and 490 nm light demonstrates that the high figures of merit translate to carriers several dB above those produced by TbFeCo alloys. Furthermore, very high carrier-to-noise ratios such as 54 dB at a mark length of 0.75 μm (4 MHz) and 60 dB at long mark lengths (1 MHz) are obtained using 490 nm light and a bandwidth of 30 kHz.

Vector spatial solitons influenced by magneto-optic effects in cascadable nonlinear media

The theory of vector spatial solitons in cascadable nonlinear media, bounded by magneto-optic material, is established. The main work is preceded by a discussion of the fundamental-harmonic wave phenomenon and four coupled equations for the system are established. A number of simulations are provided that illustrate both the vector nature of a quadratically nonlinear film, bounded by magneto-optic media and the crucial role of the magnetic field. It is demonstrated that the powerful polarization control exercised by the magnetic field suggests a number of interesting applications.

Thin Films and Fine Powders of Ferrites : Materials for Magneto-Optical Recording Media

In the near future, the magneto-optical (MO) recording will require materials having high MO effects at short wavelengths (400-500 mm). Ferrites, especially garnets, but also spinel ferrites have highly advantageous MO properties in this spectrum and also have very stable structural and chemical properties. The present paper evaluates their used as MO media, in comparison with the rare-earth transition metals alloys and the metallic multilayers.

Highly Field Sensitive (150 Oe) Quadri-Valued Magnetooptical Media for High-Speed Overwriting

For high-density and high-speed magnetooptical (MO) recording, the use of magnetic multi-valued (MMV) MO media has been proposed. [rf:1] A quadri-valued recording has been achieved using this media and a field modulation recording method. For practical use of this media, it is important to reduce the critical magnetic field to minimize the applied field for realizing the quadri-valued recording. We have achieved reduction of the critical magnetic field down to 150 Oe using a new coupled film, TbFeCo/GdTbFeCo.

Magnetic and magneto-optical properties of Co-containing Bi-DyIG particles dispersed in a plastic binder

Nano-size Co/sub x/:BiDy/sub 2/Al/sub 0.8/Fe/sub 4.2/O/sub 12/ (0/spl les/x/spl les/2.0) particles were prepared with a coprecipitation method. The coating films of the particles were prepared with a coating technique. The magnetic and magneto-optical properties of the particles and films were investigated. The M/sub S/ and H/sub C/ of the particles and films increased with Co content, x. The Faraday rotation dropped with x. The x=0.5 film has high H/sub C/ about 450 Oe, and the figure of merit at 520 nm is about 0.63. The particle size in the film was about 50/spl sim/100 nm which is smaller than the wavelength of the read and write laser (633 nm). These results show that the coating Co/sub x/:BiDy/sub 2/Al/sub 0.8/Fe/sub 4.2/O/sub 12/ film is a candidate material for a new economical magneto-optical storage medium.

Magnetic-Super-Resolution Overwritable Magneto-Optical Medium

We developed a magnetic-super-resolution overwritable magneto-optical medium. It be arranged an enhanced structure, because a readout layer and overwritable layers are coupled magnetostatically. Direct overwriting is done by light intensity modulation, and readout uses the magnetic-super-resolution method. When the length of a recording mark is short, the carrier-to-noise-ratio is high and the erasability is low in comparison with those of a conventional medium.

THE DYNAMIC FARADAY EFFECT AND IT′S-MECHANISM OF LOSSES

There is evidently a difference between the dynamic and static Faraday effects in magnetooptical medium.The real part θ′t of the alternating Faraday rotation θt all is less than the real part θ′s of the static Faraday rotation θs,and the imaginary part θ″t all is bigger than the imaginary part θ″s.It is shown by the experimental result of Faraday effect in (BiTm)3(FeGa)5O12 films that the frequency dependence of θt has a direct bearing on the mechanism of magnetic lag in the high-frequency region of ω>104Hz,and is mainly depend on the magnetic hysteresis in the low-frequency region of ω≤103Hz.Even if ω→0,θt does not equal θs as a result of the magnetic hysteresis loss in the medium.It is also indicated by the theoretical analysis that it is impossible to realize completely the on off state of the switch for a modulator utilizing the magnetooptical medium with magnetic losses.

Future Perspective Magnetic Superresolution

The magnetic superresolution (MSR) method improves readout resolution of magnetooptical (MO) signals, with a mask that the readout layer creates. The MSR method is advantageous for high-density recording. However, the recording sensitivity and overwrite function of MO media must be improved in order to render it suitable for general use. To meet these requirements, we investigated the MSR method using static coupling force and its future extension.

A computer modelling approach to predicting the long term thermal stability of CoPd magnetic multilayers

A thermodynamic and kinetic model for the behaviour of the supercooled fcc phase of the CoPd system has been developed using the Workbench, an artificial intelligence based modelling system. A miscibility gap is predicted which helps preserve the layered structure of multilayered thin films. The thermodynamics have been incorporated into diffusion calculations which model composition profiles near an interface and in a layered structure. An estimate for the stability of these layers of about 20 years is obtained under projected operating conditions as a magneto-optic data storage medium. A modification to the structure to increase its stability is suggested.

Magnetic thin films for high-density recording

Magnetic and magneto-optic recording technologies are continuing to evolve at a rapid pace resulting in longer playing times and more data being stored in ever decreasing volumes. Thin-film media are playing an important role in this process. Three different type of thin-film media are discussed here, namely, a hard disk application for data recording, metal-evaporated tape for audio/video applications and CoNi/ Pt multilayer for application as a magneto-optic media for blue laser recording. The relationship between deposition parameters (evaporation and sputtering), structural parameters and magnetic and recording behaviour are discussed.

Mark observation of TbFeCo recorded on land/groove substrate by spin-polarized scanning electron microscopy

Using spin-polarized scanning electron microscopy, the laser power dependence of recorded mark shapes and sizes is studied for the TbFeCo, magneto-optical recording medium, prepared on a land/groove-shaped substrate. The bit width which increases with laser power is suppressed to the land or groove width during higher power, showing that the land/groove border acts as a barrier to bit expansion. These results confirm the advantages of a land/groove shaped medium for high density recording.

Submicron characterization of recording media using magnetic force microscopy (invited) (abstract)

Magnetic force microscopy is finding widespread use in the analysis of magnetic structure at length scales relevant to modern storage densities. We present a new technique which uses the extremely localized stray fields of MFM tips to probe submicron media characteristics. The tip is brought into contact with the magnetized media, and a uniform external field Hext is momentarily added to the tip’s stray field. If the net field exceeds the local media coercivity, the magnetization is reversed locally, and a ‘‘bit’’ is written. High-resolution MFM imaging of the results is done immediately using the same probe. Bit arrays of several Gbit/in2 can be produced with the lithography software of a commercial MFM. Using the external field Hext as a parameter, the threshold for bit writing can be determined, giving a measure of the local, or ‘‘point’’, coercivity of the media on a 100 nm scale. Results for perpendicular Co–Cr and magneto-optical media can deviate significantly from conventional bulk hysteresis measurements, in some cases reversing the bulk coercivity ranking. Intermediate values of Hext reveal spatial variations in the point coercivity due to fluctuations in composition or microstructure. In square media, sufficiently strong fields Hext cause existing bits to grow, leading to bulk reversal via front propagation, and allowing a direct measurement of wall motion coercivity. Possible extensions to longitudinal media will be discussed. In general, combining the imaging and writing capabilities of MFM probes gives a direct assessment of media response to very localized fields, and leads to a powerful method for relating microstructure to bulk hysteresis properties.