Exchange-coupled Films Research Articles

Magnetically Induced Superresolution Using Domain Stability

Two types of magnetically induced superresolution (MSR) readout methods were investigated. In one MSR method, triple-layered films in which the readout layer was coupled magnetostatically with the recording layer were used. In another MSR method, exchange-coupled triple-layered magnetic films were used. We utilized the phenomena of domain nucleation and domain collapse in readout layers in these MSR readout methods, and obtained a short rise time with domain nucleation and a short fall time with domain collapse in their readout layers.

Existence conditions for interface bilayer spin waves

Conditions for the existence of in-plane propagating interface spin waves (ISW) are established analytically in the limit of a very thick exchange-coupled bilayer film. The existence of ISW is found to require (at fixed static interface conditions) an appropriate propagation direction of the spin wave in the plane of the film.

Computer simulation of thermomagnetic recording process in exchange-coupled magneto-optical bilayer films

For exchange-coupled RE-TM bilayer films, the thermomagnetic recording process was studied via computer simulation by use of an extended cell model that takes into account the interface wall energy. It is found that magnetization reversal and thus the size and the shape of written domains in the layer of a lower Curie temperature are strongly influenced by those in the layer of a higher Curie temperature.

Exchange-coupled Ni-Fe/Fe-Mn, Ni-Fe/Ni-Mn and NiO/Ni-Fe films for stabilization of magnetoresistive sensors

We have studied the unidirectional anisotropy field (H/sub UA/) and coercivity (H/sub CE/) in exchange-coupled Ni-Fe/Fe-Mn, Ni-Fe/Ni-Mn and NiO/Ni-Fe films as functions of thickness and temperature, and evaluated these films for the stabilization of magnetoresistive (MR) sensors. The as-deposited Ni-Fe/Fe-Mn and NiO/Ni-Fe films exhibit strong exchange coupling, while the Ni-Fe/Ni-Mn films require annealing to enhance exchange coupling. Both H/sub UA/ and H/sub CE/ follow an inverse dependence of the Ni-Fe film thickness in all cases, as expected from the interface nature of exchange coupling. The Fe-Mn film thickness required for the onset of exchange coupling is much smaller than the Ni-Mn and NiO film thicknesses. The thermal stability of the Ni-Fe/Ni-Mn films is superior to that of the Ni-Fe/Fe-Mn and NiO/Ni-Fe films. To explain annealing effects on exchange coupling in the Ni-Fe/Ni-Mn films, we propose an exchange-coupling model consistent with models applied to the Ni-Fe/Fe-Mn and NiO/Ni-Fe films in previous studies. Uniform-field tests of the MR sensors longitudinally biased by the three types of exchange-coupled films show quiet and stable MR responses in all cases. We evaluate the exchange-coupled films based on desired properties for the sensor stabilization, and then discuss their merits and deficiencies.

REAR APERTURE DETECTION OF EXCHANGE-COUPLED TRILAYER FILM WITHOUT INITIALIZING MAGNETIC FIELD

We propose an exchange-coupled magnetic trilayer film, which is composed of a readout layer, an intermediate layer, and a writing layer, to achieve magnetically induced super resolution (MSR) by rear aperture detection (RAD). The film doesn't require several kilo oersteds of magnetic field to align the magnetization direction of a readout layer. We obtained a carrier-to-noise ratio (CNR) of over 40 dB for a 0.4 μm mark length (0.8 μm mark pitch) and cross-talk of under -40 dB for a 0.78 μm mark length on 0.8 μm effective track pitch.

An Investigation of Exchange-Coupled Films by 0-K Monte Carlo Simulation Using the Classical Heisenberg Model

Monte Carlo simulations of antiferromagnetic spin configurations were performed in order to understand ferro/antiferro-exchange-coupled films. The validity of the simulation was confirmed by comparison with experimental data for bulk materials. The calculated antiferromagnetic spin configurations of the ordered ferromagnetic layer showed decayed oscillation in the spin directions. However, the calculated boundary exchange coupling was larger than the estimated experimental value, as is usual. A reasonable value for the effective boundary exchange coupling was obtained by applying a weak staggered field in the antiferromagnetic region and removing the ferromagnetic layer.

Microstructure of sputtered epitaxial Ni0.80Fe0.20/NixCO1−xO exchange coupled bilayers on α-Al2O3 (0001)

Epitaxial Ni0.80Fe0.20/NixCo1−xO bilayers have been grown on α-Al2O3 (0001) substrates by dc-sputtering X-ray diffraction and transmission electron microscopy have been employed to characterize these exchange-coupled films. The x-ray diffraction spectrum shows only the (111) family of peaks in both Ni0.80Fe0.20 and NixCO1−xO films. Growth orientation relationships have been determined from diffraction patterns taken in planar view and cross section. The relationships are: (111) Ni0.80Fe0.20//(111) NixCo1−xO//(0001) α−Al2O3 and [1\(\bar 1\)0]Ni0.80FE0.20//[1\(\bar 1\)0]NixCo1-xO//[1\(\bar 1\)00] α-Al2O3. The microstructure of these films as well as the interfacial structure between Ni0.80Fe0.20 and NixCo1−xO have been analyzed in high resolution electron microscopy and are described in this paper. In addition, the dependence of the exchange coupling field on interfacial roughness is discussed.

IRISTER - magneto-optical disk for magnetically induced SuperResolution

IRISTER (IRIS Thermal Eclipse Reading) is a remarkable technique for achieving superresolution in optical disks. Two applications of IRISTER are Magnetically induced SuperResolution (MSR) and Premastered optical disk by SuperResolution (PSR). Magnetically induced SuperResolution (MSR) has been realized in magneto-optical disks using exchange-coupled magnetic multilayer films. Two new detection methods have been developed. In the front aperture detection (FAD) method, the heated area in the light spot works as an optical mask so that the signal is read out only from the remaining crescent-shaped area in the spot. In the rear aperture detection (RAD) method, the signal is read out only from the heated area. The cutoff spatial frequency for both types of detection is more than two times higher than that in conventional detection. A high C/N of 42 dB is obtained in the MSR disks by both methods for a mark length of 0.3 /spl mu/m, which is much shorter than the optical limit. Premastered optical disk by SuperResolution (PSR) has been realized in read-only optical disks using phase-change materials. In the front aperture detection (FAD) method for PSR, the prerecorded signal is read out only from the high reflective crescent-shaped area in the light spot, because the reflectance of the heated area is too low. A C/N of 51 dB for a mark length of 0.3 /spl mu/m has been obtained for the PSR disk.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Direct overwrite in magneto-optic recording

Progress on direct overwrite methods in magneto-optic recording is reviewed. Light-intensity-modulation overwrite methods in particular are shown to offer several advantages over the magneto-field-modulation method. Recent progress in exchange-coupled multilayered films to enhance the promising laser-intensity-modulation overwrite methods known as the “Nikon” and “Mitsubishi” methods is discussed. Three kinds of assistance layers added to the films give these methods significantly better overwrite characteristics. Attention is also given to high density data recording in light-intensity-modulation overwrite methods.

AN INTERMEDIATE LAYER FOR OVERWRITABLE FIVE-LAYER MEDIA

Direct overwrite is one of the principal ways to improve the data transfer rate of magneto-optical disks. One direct overwriting method is light intensity modulation using exchange-coupled five-layer film. The intermediate layer in this method is important for controlling the interface-domain wall energy between the memory and writing layers. We fabricated an overwritable five-layer film which uses Cu-doped DyFeCo for the intermediate layer. By changing the Cu content in this film, it is possible to control the interface-domain wall energy. For a Cu content of 45%, we obtained a wide power margin for a carrier-to-noise ratio which exceeds 45dB.

EXCHANGE-COUPLED QUADRILAYER FILMS FOR DIRECT OVERWRITE MO DISKS

The design of the direct overwrite magneto-optical disk with the exchange-coupled quadrilayer film using light intensity modulation is described. The relation between the overwrite characteristics and the magnetic properties of each layer was investigated. The writing sensitivity for the high power and the low power depended on the Curie temperature of the writing layer and the memory layer respectively. This system possesses a fairly wide composition tolerance, especially for RE content in each layer.

DIRECT OVERWRITING AND SUPER RESOLUTION READOUT BY EXCHANGE-COUPLED MULTILAYER FILM

A magneto-optical disk with the functions of direct overwriting by light intensity modulation method and of magnetically induced super resolution readout was realized by utilizing an exchange-coupled multilayer film. Magnetic hysteresis loop measurement clarified that these two functions can work independently at different temperature ranges. Write and read characteristics measurement also proved this and an equivalent CN ratio with that of a disk without direct overwrite function was obtained.

Recording field sensitivity of magneto-optical disks using very thin exchange-coupled films

Recording field sensitivity of magneto-optical disks using very thin exchange-coupled films

Stability of the interface wall in overwrite exchange-coupled multilayer films

The stability of recorded bits with an interface magnetic wall in the overwrite exchange-coupled magneto-optical films was studied. The wall energy of the recorded bits was increased by the existence of the interface magnetic wall and the recorded bits were made unstable, compared to the conventional ones. However optimization of the coercivity and the interface wall energy of the memory layer prevented the recorded bits from being unstable. It was found that the recorded bits in the overwrite exchange-coupled multilayer films were stable in the storage environment for practical use.

Ferromagnetic-bilayer spin-wave resonance: Reassignment of interface-affected lines.

A microscopic theory of standing spin-wave resonance in exchange-coupled bilayer films is presented. The interface-inhomogeneity model (an effective interface exchange coupling and intrinsic uniaxial interface anisotropy parameters are introduced) and the Heisenberg model with spin (exchange and Zeeman) Hamiltonian are used to derive an energy dispersion relation and spin-wave-mode functions for exchange-coupled bilayer ferromagnetic films. The theory is valid for Bravais lattices with arbitrary film-surface or interface orientations in a configuration where the static external field is perpendicular to the film surface and the values of the ferromagnetic and antiferromagnetic interface exchange couplings are arbitrary. The analysis is based on the exact solution of the eigenvalue problem of a bilayer film achieved with the interface rescaling approach. It is shown that the pattern of the resonance spectrum is determined by the nature of the interface exchange-coupling integral ${\mathit{J}}^{\mathit{A}\mathit{B}}$, with the first two (high-field side) lines exhibiting pronounced intensities when the interface coupling is antiferromagnetic. We show moreover that the highest-field line arises by excitation of the mode that is localized on the bilayer interface. We also show that this mode represents a complex bilayer vibration formed by the in-phase combination of sublayer vibrations, and this result corrects the general belief of experimenters that the mode under consideration is due to an out-of-phase combination of sublayer vibrations.

High density overwriting in exchange-coupled magneto-optical multilayer films

The possibility of high-density overwrite recording using the light modulation method in magnetooptical disks has been investigated. It was found that the area around a mark written by irradiation of high power was erased spontaneously, regardless of the prerecorded states. It is effective for high-linear-density recording. On a disk with a track pitch of 1.4 mu m, signal deterioration by thermal interference was not observed after recording at an adjacent track with an off-track of 0.2 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Super Resolution in Magneto-Optical Disks

Super-resolution in magnetooptic disks is reviewed. Magnetically induced super-resolution (MSR) has been achieved using exchange-coupled magnetic multilayer films. Two types of detection method, Front Aperture Detection and Rear Aperture Detection, have been developed. The principle of MSR is discussed in this paper based on the exchange coupling between magnetic layers. The cutoff spatial frequency at which a signal approaches zero is more than twice as high for the MSR disk as for a conventional disk. A high C/N of more than 42 dB was obtained in both types of disk for a mark length of 0.3 μm, which is much shorter than the cutoff in conventional detection.

Exchange Coupled Films for Magnetooptic Recording Applications

The magnetic properties of exchange-coupled films are discussed from the viewpoint of applications in magnetooptic recording. Various magnetic properties, such as the magnetization process, coercive force, spin orientation and Curie temperature, can be controlled through the exchange coupling. The most important and useful application involves control of the temperature dependence of the coercive force in rare earth-transition metal amorphous films. Highly controlled thermomagnetic responses bring high performance as well as new functions to magnetooptic recording media. High recording sensitivity, overwriting capability and super-high magnetic resolution all become possible through the use of exchange-coupled multilayer films. Exchange-coupled films are also expected to be prime candidates for use in future magnetooptic recording systems employing blue-light lasers.

Exchange-Coupled Multilayer Films for Direct Overwrite in Magneto-Optical Disk

Direct overwriting techniques are essential to improve the data transfer rates of magnetooptic disk systems. Overwrite methods employing light-intensity modulation using exchange-coupled multilayer film media in particular have been intensively investigated in the last few years because of the many advantages they offer. This paper reviews exchange-coupled multilayer films enabling direct overwriting ofbstract magnetooptic disks. The principle mechanism employed, and conditions required to achieve direct overwriting of exchange-coupled multilayer films, including the effects of intermediate film layers, are described. Some experimental results on overwrite characteristics are then presented, and the possibility of high-density recording is discussed. Finally, the stability of bits recorded in exchange-coupled multilayer films is also considered.

Anomalous torque curves in PtCo/TbFeCo exchange-coupled bilayer films

The torque curves, T(α), of PtCo/TbFeCo (RE-dominated) bilayers behave unexpectedly when the applied field H is directed close to the perpendicular ( α=0). The Pt/Co and TbFeCo films separately are characterized by positive effective magnetic uniaxial anisotropy. Consequently we expect T(α)/α to be negative near α = 0. However, we find T(α)/α to be large, positive and approximately constant near α = 0. In this angular region the relaxation of the wall associated with the bilayer exchange coupling drives the Pt/Co and TbFeCo layer magnetizations in opposite directions. On the thin wall model basis, the wall energy, σ w, inferred from the ensuing torque slope is about two and a half times the σ w computed from Kerr loop shifts. The bilayer magnetic energy density, E(α), must be even [ E( α) = E(− α)] and in the form of an inverted parabola near α = 0. Physically more realistic distributed wall models must conform to the required symmetry as well as to fit experimental data.