Magneto-optical Storage Media Research Articles

Properties of microstructure on amorphous film of rare earth–transition metal alloy for ultrahigh density recording

Controlling the microstructure of amorphous rare earth–transition metal films via the sputtering process was found to be an effective way of controlling their magnetic properties for applications as magneto-optical storage media. This paper describes how the relationship between a TbFeCo film’s magnetic properties and its microcolumnar structure depends on the sputtering conditions. An enhancement of electric resistance value was observed for the devices with a constriction columnar width in the 5–20nm range. The measured electrical resistance was over 1.0×10−5Ωm in this case. It is believed that the change of electrical resistance on the thin film is due to fluctuations in the density on the arranged microstructure or constriction of current induced by scattering because the film structure contains impurities. These same impurities are believed to be associated with the restriction of the trapped domain wall’s mobility. Furthermore, we observed a significant resistance change subsequent to the application of lower Xe pressure sputtering. This paper also serves as a feasibility study for high-density magneto-optical recording on these media materials.

Determination of the Magnetic Compensation Composition in Al-Substituted Bi-DyCoIG Nanoparticles With Enhanced Coercive-Force

In garnet particles, higher coercive-force is required to develop a practical magnetooptical storage media. Therefore, in this paper, we develop high coercive-force particles reflecting the magnetic compensation induced by the aluminum substitution, and increasing the magnetic anisotropy under the influence of cobalt substitution for applying the garnet particles to magnetooptical storage media. Nanoparticles of aluminum substituted bismuth dysprosium cobalt iron garnet (Bi-DyCoAlIG) exhibit a magnetic compensation at room temperature. The Bi-DyCoAlIG (BiDy/sub 2/Co/sub 0.2/Al/sub x/Fe/sub 4.8-x/O/sub 12/: 0/spl les/x/spl les/0.9) nanoparticles were made using coprecipitation and heat-treatment processes. The X-ray diffraction patterns of nanoparticles are clear that all the peaks were assigned to the single phase of the garnet structure. The particles size of the Bi-DyCoAlIG nanoparticles was observed to be less than 30 nm by transmitted electron microscope images which is comparables to the estimated average particle size of Scherrer's equation. The magnetic profiles of the Bi-DyCoAlIG nanoparticles show shallow minima at the composition range of 0.6<x<0.8 reflecting the magnetic compensation induced by the aluminum substitution. Moreover, the coercive-force of the nanoparticles was also peaks at x=0.7, which can be consistently explained by the magnetic compensation. The BiDy/sub 2/Co/sub 0.2/Al/sub 0.7/Fe/sub 4.1/O/sub 12/ nanoparticles demonstrate the enhanced coercivity of 1000 Oe which is factor of 3 larger than the x=0.

Enhanced coercive-force of Al-substituted Bi-GdIG nanoparticles with magnetic compensation composition

In garnet particles, higher coercive-force is required to develop a practical magneto-optical storage media. Therefore we in this article develop high coercive-force particles reflecting the magnetic compensation induced by the aluminum substitution for applying the garnet particles to magneto-optical storage media. Nanoparticles of aluminum substituted bismuth gadolinium iron garnet (Bi-GdAlIG) exhibit a magnetic compensation at room temperature. The Bi-GdAlIG nanoparticles were made using coprecipitation and heat-treatment processes. The magnetic profiles of the Bi-GdAlIG (BiGd/sub 2/Al/sub x/Fe/sub 5-x/O/sub 12/; 0 /spl les/ x /spl les/ 1.2) nanoparticles shows shallow minima at x=0.7 reflecting the magnetic compensation induced by the aluminum substitution. The coercive-force also peaks at x=0.7, which can be consistently explained by the magnetic compensation. The BiGd/sub 2/Al/sub 0.7/Fe/sub 4.3/O/sub 12/ nanoparticles demonstrate the enhanced coercivity of 320 Oe, which is factor of 2 larger than the x=0. The average size of the BiGd/sub 2/Al/sub 0.5/Fe/sub 4.5/O/sub 12/ nanoparticles was estimated 55 nm by Scherrer's equation, and observed to be about 100 nm by scanning electron microscope image.

THE DIRECT OBSERVATION OF MAGNETISATION REVERSAL IN FILMS OF INTEREST FOR HIGH DENSITY INFORMATION STORAGE

Lorentz electron microscopy in a highly modified transmission electron microscope has been used to study the magnetisation reversal process in films of interest as magneto-optic storage media. Domain structures, mainly in CoNi/Pt multilayers, have been observed under a range of applied field conditions. Whilst domain propagation under increasing fields and the approach to saturation have both been investigated, the focus has been on the early stages of domain growth where time-dependent effects are prominent. Close agreement between experimental observations and micromagnetic modelling has been achieved allowing values of the domain wall energy to be extracted.

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.

Macroscopic ferrimagnets as magneto-optic media (abstract)

We have discovered a class of phase separated materials in which the two phases couple antiferromagnetically across the phase boundary. A prototypical example is cobalt containing ≂100-Å-diam precipitate particles of EuS with the NaCl structure. These materials show many of the properties of ferrimagnets such as compensation points so we have adopted the term ‘‘macroscopic ferrimagnet’’ to distinguish them from ferrimagnets in which the antiferromagnetic exchange couples individual atoms. As a result of the ferrimagnetic exchange between the Co and the EuS the net magnetization of the composite is low in the vicinity of compensation so the anisotropy field is high. Electron beam evaporated thin films have a perpendicular easy axis as a result of growth induced anisotropy. Another effect of the exchange between the Co and EuS is to order the EuS well above its normal Curie point of 16 K. A film with 80 mole % Co and 20 mole % EuS has a compensation point of 55 K showing that the Eu2+ has essentially its full moment of 7μB at this temperature in order to compensate the 1.65μB of Co. The EuS even shows evidence of magnetic order at room temperature. The wavelength dependence of the sign of the magneto-optic Kerr effect can only be explained if the EuS has sufficient rotation at high photon energies to be larger than the rotation of the Co. We have prepared more complex alloys that show square loop properties at room temperature, for example, Tb-Co-EuS and Tb-FeCo-EuO. X-ray diffraction analysis shows that these films consist of an amorphous matrix (Tb-Co or Tb-FeCo) with precipitates of a NaCl structure phase (e.g., defect solid solutions like Eu1−xTbxS1−x⧠x). Both atomic and macroscopic ferrimagnetism are operating in these compositions. Films with high remanence and coercivities over 4 kOe at room temperature have been obtained. Some of these compositions may be useful as magneto-optic storage media.

Microstructure and magneto-optic properties of unannealed and annealed Fe/Pt and Co/Pt multilayers

Two requirements for magneto-optical storage media are remanent perpendicular magnetization and large magneto-optic Kerr rotations. Several film structures and material systems have been proposed for this purpose, including amorphous rare earth-transition metal alloys, Co/Pt multilayer films and Co/Pt alloys. Another structure that has potential for magneto-optic applications are thin films of ordered alloys based on the Llo structure, with two possible materials systems being Fe/Pt and Co/Pt. Ordered CoPt and FePt both adopt the Llo crystal structure, are ferromagnetic, and are magnetically easy along the unique axis. Thin films oriented such that the c axis is out of the film plane are expected to have large perpendicular anisotropy, and both FePt and CoPt are known to have large Ken-rotations, so both of the requirements mentioned above can be met with these materials.In order to produce the ordered structures oriented with the c axis out of the plane epitaxial multilayers of the two constituents were produced by magnetron sputtering onto epitaxial Pt [001] seed layers deposited onto MgO [001], resulting in multilayers stacked along Pt [001].

Jones matrix analysis of incident angle effects in magnetooptic storage media

A ray-tracing technique employing Jones matrices was used to model the polarization state of the reflected wavefront as a function of numerical aperture for the various types of magnetooptic media. The average reflectance and MO signal for differential readout were calculated for different numerical apertures by integrating over the wavefront. Quadrilayer structures which are designed to maximize the readout SNR are most affected by incident angle effects.

Effects of incident angle on readout in magnetooptic storage media

The effects of incident angle on the readout SNR of a magnetooptic storage system employing the differential detection system were theoretically investigated. The most important parameters which depend on incident angle were found to be the Kerr reflectance and the phase difference between the two components of the reflected wave. In particular, the phase difference was found to depend strongly on incident angle for structures which are strongly antireflective.