The effect of magnetic pole shape on the remanent magnetization state at the track edge was studied for a ring-type head and a single-layered perpendicular medium system. An MIG head and a merged MR head were used for recording. These heads have symmetric and asymmetric magnetic pole structures, respectively. It was found that the remanent magnetization state at the track edge of a single-layered perpendicular medium was greatly influenced by the shapes of the magnetic poles. Thus it is necessary to improve the shape of magnetic poles in order to realize the higher track density for a single-layered perpendicular medium.
The noise properties of perpendicular magnetic recording were investigated by using the numerical simulation for three PRML channels; PR4, EPR4, and E2PR4. One investigation was based on the measured readback signal and noise. Another was based on the arctangent pulse and the white noise, which is mathematically easy to apply. The results for the measured signal and noise were relatively different from those for the arctangent signal and the white noise. Noise correlation after PR equalization is also discussed.
In perpendicular magnetic recording, medium noise is one of the crucial issues limiting the attainable density. The purpose of the reported work was to find a guiding principle for reducing the medium noise. Although previous work suggested that the media noise is caused by reversed domains in the bit cell, our time-domain noise measurements and a novel model using Voronoi cells suggest another noise origin: a noise source of transition irregularity caused by finite magnetic cluster size.
The bit-error rate (BER) performance of 16/17(0,6/6) coded PRML systems for a single-layered perpendicular medium was studied by conducting read/write experiments using a spinstand system. The results show that the E3PR4ML system exhibits the best performance among the PRML systems evaluated in this paper, and achieves a BER of less than 10-6 at a high linear density such as 375 kbpi.
The effect of thermal agitation of magnetization on the magnetic properties of CoCrTa/Ti perpendicular media was studied by using a pulse magnetometer. The values of the “intrinsic” remanence coercivity H0, obtained by subtracting the thermal agitation of magnetization, are several times larger than those of the remanence coercivity Hr measured experimentally at a field sweep rate of ∼5 Oe/s. The value of H0 gradually approaches Hk as the CoCrTa thickness decreases, and shows values larger than 90 % of Hk at thicknesses of less than 20 nm. This indicates the potential of perpendicular media to achieve Hr values equal to Hk. The activation volume gradually decreases as the thickness decreases, and is nearly the same as the mean grain volume at thicknesses of less than 30 nm. This suggests that magnetization reversal within the grains is uniform at thicknesses of less than 30 nm, even though the grains have an elongated shape (30 nm x 9.52 nm2 at 30 nm thickness, for instance). This will be another advantage of perpendicular media in resisting thermal agitation of magnetization.
The Magnetic properties and recording performances of CoCrTa/Ti/M (M: CoZrNb, Fe, Co) double-layered perpendicular recording media were studied. It was revealed that a thin Ti film of 5 nm gave the most enhanced c-axis alignment perpendicular to the film plane in media with CoZrNb or Fe back-layers, resulting in the highest coercivity and squareness of CoCrTa. The recording performance of media with Fe back-layers, using a single-pole-type writing head, indicated that a thinner back-layer with a high flux density can be used without any reduction of the recording sensitivity and resolution, even though the Fe back-layers do not have good soft magnetic properties. However, hcp-Co back-layers degrade the recording sensitivity and resolution. The media noise of media with Fe or Co back-layers was significantly higher than that of CoZrNb back-layer media. It is suggested that these noise spectra are related to the domain structure of the back-layers.
An ordered Fe-Pt(001) perpendicular composite medium was designed with a high Bs soft-magnetic Fe-Si back-layer. When an MgO layer was introduced between the Fe-Si back-layer and the Fe-Pt layer, a square-shaped Kerr loop was obtained for the Fe-Pt recording layer, as a result of the exchange-decoupling between the two layers. Introduction of the MgO layer led to a slight improvement of the crystallinity and orientation. The medium showed a high signal output and a recording resolution D50 of 220 kFRPI for an MR reproducing head with a Gs of 0.2 μm. A high SNR, 38.9 dBp-p/rms, was achieved for a track width of 1.2 μm at a recording density of 40 kFRPI. Furthermore, an MFM image of a 500-kFRPI recorded pattern was obtained, even though a strong exchange interaction in the lateral direction of the film was implied. It is safe to conclude that the Fe-Pt composite medium will be useful for future ultra-high-density recording.
Remanence measurements are very useful for estimating the characteristics of intergranular magnetic interactions in recording layers. However, it is necessary to compensate for the demagnetizing field in remanence measurements of perpendicular magnetic recording films. In this study, a new method of compensating for the demagnetizing field is proposed and an effective reverse field Heff is used to evaluate the dc demagnetization measurements, isothermal remanent magnetization measurements, and ΔM for perpendicular magnetic recording films.
We investigated switching speed effects in perpendicular media, using micromagnetic simulations. Hysteresis loop calculations show that for field sweep rates greater than 2×1013 Oe/s the measured coercivity increases sharply. The switching speed of individual grains was calculated as 18 ps to 38 ps, with a strong dependence upon the medium thickness and the magnetisation state of surrounding grains. A simulation of recording over a range of frequencies determined that writing of bits at data rates of up to 5 Gbit/s in media thicknesses of up to 180 Å is possible before loss of output occurs.
The time decay of magnetization (ΔM/ln t) and the irreversible susceptibility (Xirr) were measured for Co-Cr/Ti perpendicular and Co-Cr-Pt-Ta/Cr longitudinal recording media in various reverse magnetic fields in the temperature range from 200 to 423 K. This experiment showed that ΔM/ln t of Co-Cr/Ti perpendicular recording media gradually increased with increasing temperature in a reverse magnetic field. In the case of Co-Cr/Ti perpendicular recording media, the value of the activation volume (Vact × Ms), which is correlated with the medium noise, is independent of the reverse magnetic field, in contrast to the tendency in Co-Cr-Pt-Ta/Cr longitudinal recording media. The temperature variation of the coercivity (Δ Hc/Δ T) of these recording media increased as (Vact × Ms) decreased, as in the case of rare-earth magnets.
Perpendicular/longitudinal composite media having a Ni(-Cr) alloy as a longitudinal magnetic underlayer were prepared by dc-magnetron sputtering. All the diffraction peaks observed in XRD experiments were Ni(111) and Co(002), indicating that composite media without an intermediate layer were successfully prepared. To obtain a higher S/N value, it was revealed that it is effective to decrease the thickness and saturation magnetization of the Ni alloy longitudinal magnetic layer. However, using a Ti underlayer caused higher medium noise.
Ferromagnetic Co77Cr20Ta3 layers were deposited on paramagnetic Co65Cr35 and Pt seed layers. The crystallographic and magnetic characteristics of the Co-Cr-Ta layer were improved by independently adjusting the substrate temperature Ts of the Co65Cr35 seed layer in the range 150°C - 200°C. The Pt seed layer results in better c-axis orientation even at a low Ts of around 250°C, and a 50-nm-thick Co-Cr-Ta layer deposited on it exhibited a high perpendicular coercivity Hc⊥ of 2.5 kOe at a Ts of 250°C. Optimization of Ts for the paramagnetic Co-Cr layer and use of the Pt seed layer were effective for depositing ultra-thin Co-Cr-Ta recording layers.
Sputter-deposited Ge was applied as an underlayer of a Co-Cr-Pt-Ta single-layered perpendicular magnetic recording medium. It is important to apply a paramagnetic Co-Cr40 intermediate layer to obtain a film that has high perpendicular magnetic anisotropy without Ge diffusion into the magnetic layer. As the Ge underlayer thickness was increased, the magnetic properties and crystalline orientation improved, the media noise power and spectrum form changed, and the noise sources also seemed to change. Application of a thicker underlayer is an effective way of obtaining a high S/N value.
Barium ferrite films were prepared by r.f. diode sputtering on AlN underlayers, whose influence on the films’ c-axis orientation and magnetic properties was investigated. Hexagonal AlN underlayers with thicknesses of around 30-100 nm were deposited at room temperature. On the underlayers with a thickness of 30 nm, barium ferrite films with a thickness of 50 nm were deposited at room temperature. As the deposited barium ferrite films were not crystallized, they were post-annealed at temperatures between 600 °C and 900 °C for 5h in air. When the films were annealed at 800 °C, superior perpendiculaly oriented films were obtained, exhibiting a saturation magnetization Ms of 203 emu/cm3, a perpendicular coercivity Hc⊥ of 5.5 kOe, a squareness ratio Sq⊥/Sq// of 2.7, and a coercivity ratio Hc⊥/Hc// of 1.6. This excellent uniaxial c-axis orientation lying perpendicular to the film surface may be presumed to result from the effect of the AlN underlayer.
Read output decay in longitudinal magnetic recording media due to magnetization decay and broadening of the transition length “a” was investigated. The “a” broadening was analytically calculated on the assumption that the magnetization in the medium always has an arctan distribution. The parameter “a” was calculated from the slope of the Mr distribution as a function of time, energy barrier ΔE and its distribution, temperature and magnetic properties of the media. The output decay for various recording densities was calculated by using the broadened “a”, <ΔE/kT>, and its standard deviation. Consistent results are obtained when the reduction of <ΔE/kT> due to the presence of demagnetizing fields within the bit is considered.
A Co-alloy thin-film medium formed on a NiP textured substrate, has a macroscopic in-plane magnetic anisotropy in the circumference direction. In this study, the mechanism of in-plane magnetic anisotropy induced by CoCrPtTa-type thin-film media was investigated in the light of crystallography, with following results: (1) with an increase in the number of magnetic crystal grains whose magnetization easy axis was in the circumferential direction, the in-plane magnetic anisotropy increased relative to that in the radial direction. (2) The in-plane magnetic anisotropy was correlated with the change in the crystal structure of the Cr underlayer. when the Cr (110) interplanar spacing becomes smaller, the correlation becomes higher.
We studied CoPt-ZrO2 granular thin films for their potential application in high-density magnetic recording media. CoPt-ZrO2 granular thin films were fabricated by facing targets sputtering. Co75Pt25-ZrO2 granular thin film (ZrO2 : 48 vol.%, film thickness : 20 nm) had a good square hysteresis loop and a coercivity of 3.5 kOe. The δM plots of films with a ZrO2 content larger than 39 vol.% indicated a decrease in inter-granular coupling, and the magnetic torque curve of the 20 nm thin film showed an average value Ku of 3.0 x 106 erg/cm3. Thus the CoPt-ZrO2 granular films have the potential to be used in high-density recording media.
The magnetic properties and microstructure of (CoPt25)-(Al2O3)8mol% granular media with a CoCr intermediate layer were investigated. Introduction of the CoCr intermediate layer under the magnetic layer increases the coercivity Hc and coercive squareness S* drastically owing to enhancement of the in-plane orientation of the c-axis. Application of a dc bias during the deposition of the magnetic layer increased Hc further, and the highest Hc (4.2 kOe) was obtained by applying a -150 V bias. The remanence thickness product Mrδ and activation magnetic moment Va·Isb of the medium were 0.47 memu/cm2 and 0.9 femu, respectively. From the transmission electron microscopy TEM analysis, the average grain size of the medium was estimated to be 7.1 nm.
One of the most important characteristics required for high-density magnetic recording media is low noise. We approached the subject from the viewpoint of the magnetic layer grain size, using dark field imaging of results obtained with a transmission electron microscope (TEM). The relationship between the grain diameter distribution of the underlayer and magnetic layer in terms of underlayer thickness, substrate temperature, and the amount of W added to the Cr underlayer were investigated. The magnetic layer grain size decreased with a decrease in underlayer thickness, with an increase in substrate temperature, and a decrease in W amount in the CrW underlayer. The magnetic layer grain diameter indicated a comparatively good correlation with the media noise property.
It is known that the structures of a Co-Cr base alloy film consist of a Co base ferromagnetic phase in grains and a Cr base paramagnetic phase in the neighborhood of the grain boundary. It has been reported that this structure is caused by a miscibility gap on magnetic transformation. The miscibility gap on magnetic transformation of the Co-Cr binary phase diagram has already been calculated, by the CALPHAD method. However, the miscibility gap on the magnetic transformation of a Ta-added Co-Cr base alloy has not yet been calculated. In this paper, therefore, we calculate the miscibility gap on the magnetic transformation of a Ta-added Co-Cr base alloy by the CALPHAD method.
Magnetization fluctuation in the transition region is one of the important factors causing transition noise. The fluctuation is closely related to magnetic microstructures such as magnetic clusters, and it is therefore necessary to quantitatively analyze these clusters. This paper proposes an analysis method that can extract two-dimensional features of clusters. The proposed method is based on a genetic algorithm (GA), and investigates the distribution of the magnetic poles that exist at the boundaries of magnetic clusters. By considering the obtained distribution, we estimate the cluster size and confirm the validity of the proposed method.
Magnetic clusters in recorded and demagnetized states were observed by MFM with tip-sample spacings from about 100 nm to 30 nm for Co69Cr21Ta2Pt8 longitudinal recording media with coercivities of 2000 to 3000 Oe. The magnetic field distribution on the medium surface was calculated from the measured MFM images, using the integrated transformation and the tip-sample distance transformation. The magnetic cluster size on the medium surface in the demagnetized state was about 130 nm for Co69Cr21Ta2Pt8 medium with Hc of 2790 Oe. The half-amplitude pulse width of the isolated magnetization transition (PW50) decreased from about 280 nm to 130 nm when z decreased from 100 nm to 10 nm. The magnetic clusters are considered to originate in the magnetic charge distribution in the local magnetization reversal region.
A magnetic force microscope (MFM) study was carried out for two kinds of 8-mm video tape recorded sinusoidally at frequencies of 0.5 to 9 MHz. The observed MFM response decreased rapidly with increasing recording frequency except in the low-frequency range. To explain the result, two models of MFM tips were considered. One, the conventional point-dipole model, was unable to explain it. The other, which takes account of the real magnetic volume of the tip, showed a better fit. The models were also used to predict qualitatively the recording frequency dependence of the residual magnetization of the recorded tapes from the spacing-height-dependence data of the MFM response. The former showed an unreasonable frequency dependence of the residual magnetization, while the latter showed a reasonable dependence.
To study the thermal stability of a magnetic recording medium, a finite-temperature hysteresis model of the medium was made by a Monte Carlo method for an ensemble of noninteracting Stoner-Wohlfarth single-domain particles. Using this model, a finite-element numerical read/write simulation was performed by solving the Poisson equation for the two-dimensional model of a Mn-Zn ferrite ring-type head and a γ - Fe2O3 coating tape system. As a result, output voltages of 50 kBPI for a medium with KuV/kBT=50 (T = 300 K) at room temperature and 373 K were found to be reduced by thermal energy by about 19% and 28%, respectively, compared with the output voltage at T = 0 K. In addition, output voltages of 50 kBPI at room temperature were found to be reduced drastically with decreasing KuV/kBT, indicating thermal instability.
Using dc-magnetron sputtering, Fe, Co, and Cu were sputtered on a glass substrate. The samples were annealed at 500°C in a vacuum, air, or nitrogen atmosphere. It was found that the samples annealed in nitrogen showed a coercive force of over 200 kA/m. Hysteresis loop measurements and the results of ESCA and X-ray diffraction analysis seem to suggest that Co-Fe2O4 ferrite was the major magnetic component formed by the residual oxygen during annealing. On this assumption, the O2 pressure during annealing was altered, and it was found that the coercive force increases as the oxygen pressure decreases, and that it reaches 115 kA/m when the oxygen pressure is about 8 m Torr.
The optimum writing field at finite temperatures was simulated by using the Langevin equation, in the conditions where the recording density was constant (= 200 kfci) and the velocity was varied from 5 m/s to 100 m/s. In the higher-frequency range over 350 MHz, the optimum writing field does not depend on the thermal effect, but is determined by the magnetic anisotropy field Hk. We also studied the dependence of the optimum writing field on the distribution of the anisotropy field (Hk) and the easy axis orientation. The optimum writing field increases with increasing variance of Hk, but does not depend on the distribution of the anisotropy axis orientation.
The magnetic domain control of an Ni50Fe50 electroplating film was investigated by conducting an experiment on a prototype Ni50Fe50 stripe film plated onto a photoresist stripe layer. The hard cured photoresist layer shrank as a result of annealing. The shrinkage of the photoresist layer produced a compressive stress on the electroplating film. A closure domain structure could be obtained on the condition that the compressive stress produced by the shrinkage of the photoresist layer was higher than the tensile stress of the electroplating film. We applied this domain control technique to a thin-film head and obtained a closure domain structure in the upper magnetic core. This head had good properties in the high frequency range.
Large-scale elements of high-frequency carrier-type magnetic sensors were fabricated by using the photolithographic technique, and the impedance variation rate of the elements was investigated experimentally and theoretically. It was found that the resistance component of the impedance varied drastically, and the impedance variation rate was measured at 87 %, using an Fe-Ta-N film. A high-frequency signal level of 5 mVpp/μm and an impedance variation rate of 94 % for a 0.5 μm track width were calculated by using theoretical formula
We investigated the writing properties of a single-pole head with a narrow trackwidth for high-track-density recording. We fabricated a single-pole head whose main pole was made from Fe-Si-N film having a high Bs of 1.9 T. The head was trimmed into a narrow trackwidth by using a focused ion beam system. The write properties of the head were evaluated by using a double-layered medium for perpendicular magnetic recording. Narrow trackwidth recording with a high resolution was demonstrated for trackwidths as narrow as 0.3 μm Furthermore, a significant reduction in the occurrence of reversed domain numbers was observed at low recording densities for the 0.3-μm-wide track, though the medium exhibited a low squareness ratio. This may be caused by the reduced demagnetizing field in the recorded track.
We propose a multi-track head for perpendicular magnetic recording, with a planar configuration. The planar multi-track (PMT) head consists of four single-pole write heads and two ring-yoke spin-valve MR read heads. It is used for multi-track recording on a hard disk medium without any writing interactions in recording performances. Magnetic-force-microscopy (MFM) observations were made to confirm the success of multi-track recording.
Spin-filter spin-valve (SFSV) is one of the key technologies for achieving high amplitude and controlling amplitude asymmetry at higher areal densities. In SFSV, the free layer is located between the Cu layer and the additional high- conductivity layer (back layer). This back layer improves the current distribution even in a thin free layer and keeps the MR ratio. It also decreases the magnetic field in the free layer from the bias current, and reduces the asymmetry sensitivity of the bias current and stripe height. In this paper, we describe a simulator that we developed by using micromagnetics and the Boltzmann equation, and discuss the design optimization of SFSV.
The magnetoresistive properties of spin-filter spin valves (SVs) with a synthetic structure were investigated. Spin-filter SV heads were fabricated and evaluated in order to verity the advantages of the spin-filter structure. Spin-filter SV films exhibit a maximum MR ratio for a back-layer thickness of 10 Å, which is larger than that of conventional SV films for a thin free layer of less than 40 Å thickness. In spin-filter SV heads, the asymmetry is adjustable even if the free layer becomes as thin as 20 Å. Therefore, high read-back amplitude and an asymmetry close to zero are simultaneously realized in spin-filter SV heads.
The estimation of error rates is becoming increasingly important with the continued growth in the density of magnetic recording. It is known that the error rate is strongly affected by the overwrite performance, while it strictly depends on the bit shift caused by hard-transition shift (HTS) or non-linear transition shift (NLTS). We designed a bit-shift model based on the mechanism of bit-shift occurrence. Our model confirms that the overwrite depends on the overwritten transition density and can be characterized by HTS and NLTS. This paper discusses simulations of the dependencies of the write gap length, the write field, and the spacing on overwrite.
A magnetic recording head with a Co-Ni-Fe/Ni-Fe composite write pole was designed. The saturation induction Bs of Co-Ni-Fe is 2 T. In a computer simulation of a 3D static magnetic field, a recording head with Co-Ni-Fe films on both sides of a write gap generated a longitudinal write field of 9.9 kOe at a magnetic spacing of 40 nm from the write gap. The write characteristics of the recording head with the dual high-Bs structure designed above was tested for media with high coercivities of 4.5 to 7 kOe. The overwrite performance is over 30 dB for a medium with high coercivity of 7 kOe. Straight patterns of magnetic transitions written on the medium with a coercivity of 7 kOe were observed by using a magnetic force microscope (MFM). On the other hand, bends were found in the track-edges of magnetic transitions written on a medium with a coercivity of 4.5 kOe. This indicates that the recording head generated too large a write field for the medium with a coercivity of 4.5 kOe.
Two-dimensional read/write simulation based on the Maxwell equations was performed by a finite element method on a system consisting of a keepered medium (KM) with a thin soft magnetic layer (keeper layer) over a recording layer and a thin-film head. In the write process, the KM is given a weaker head field than the UKM. But the KM’s magnetization is larger than that of the UKM because of an image effect in the write and after-write processes. In the read process, the keeper layer reduces the practical spacing by means of the image effect, and improves the read resolution by introducing a virtual gap (VG). Then, the keeper layer with the optimum parameters (e.g. μ = 100, t = 15 nm, Bs = 1.5 T) decreases the demagnetizing field in the recording layer (e.g. Mr = 560 kA/m, t = 36 nm, Hc = 167 kA/m) by 1/2, and increases the recorded magnetization by 1/4. As a result, the thermal stability of the recording magnetization can be doubled. Therefore, the KM will be an attractive medium with better thermal stability if the optimum parameters are chosen.
To increase the recording density of thin-film media, it is necessary to improve the recording performance, by, for instance, reducing the media noise. It is also important to ensure thermal stability. In order to obtain higher thermal stability of recorded magnetization, media with soft magnetic layers have recentry been suggested. In this study, we made media with a very thin soft magnetic film under the magnetic layer, and investigated their recording and reproducing characteristics and thermal stability. The findings were as follows: · When FeSiAl soft magnetic film is formed under the magnetic layer, it is possible to make media with a high coercive force. · Making a thin FeSiAl film (a few nm thick) is effective for improving the recording and reproducing characteristics, such as the recording and reproducing resolution and the media noise. · The thin FeSiAl film formations contribute to a significant improvement in the thermal stability.
GMR heads with lead overlaid structure were designed and fabricated, and their read-write performance was tested in comparison with that of conventional GMR heads with abutted junctions. Crosstalk was independent of the difference in the head structure, and decreased with a decrease in the magnetic read track width, defined by the half-width of the microtrack profile. Lead overlaid heads were 1.4 times as sensitive as abutted-junction-type heads when the magnetic read track width was 0.4 μm, because they were free of dead zones at both ends of the track. They also showed better stability in repeated read-write testing. This may be due to overlapping of the sensor edges by the leads that were deemed to be the most unstable, and sensing of only the stable central area. GMR heads with lead overlaid structure are promising for high-density recording.
The intrinsic magnetic non-uniformity of magnetic recording media, which disappears at room temperature on account of thermal effects, can be detected by low-temperature measurement. The temperature dependence of the coercivity squareness ‘S*’ was investigated and could be explained by the distributions of the dynamic coercivity and the activation volume. In considering the high-speed writing process on magnetic disks, where the dynamic magnetic properties should correspond to the write performance, it is informative to investigate the magnetic properties of the media in order to avoid thermal effects at low temperature.
Side-fringe-field effects on track edge phenomena for heads with different pole shapes were evaluated. Written tracks were partially dc-erased from both track edges, and the changes in the residual track output were measured. The erased track widths were also observed by the Bitter method. It was found that the fringe field width of recording heads strongly depends on the pole shape and that the trimming of recording head poles is very effective for reducing head-fringe-field effects. Writing misregistration capability was also evaluated through off-track overwrite measurement.
The magneto-optical Kerr spectra in PtMnSb, MnBi, and MnPt3 were calculated by the first-principles LMTO-ASA method. These three materials have large Kerr rotation angles of about 1-2 degrees in a low-energy region around 1-2eV. The large Kerr rotation peaks of the three materials are compared and discussed in detail.
The magnetic second-harmonic generation (MSHG), one of nonlinear magneto-optical effects, was measured by using a Ti-sapphire laser (λ = 810 nm) combined with filters, a polarizer, an analyzer, a detector and a photon-counting apparatus. The polar plot of the azimuthal dependence of the MSHG from a [Fe(3.75ML)/Au(3.75ML)] superlattice showed a four fold symmetry pattern. A reversal of the magnetic field caused reflection of the azimuthal patterns with respect to a certain axis. Nonlinear magneto-optical Kerr rotation as large as 31.1° was observed in [Fe(1.75ML)/Au(1.75ML) for the S-polarization incidence. The magnetic contribution to the azimuthal pattern was explained by symmetry analysis, taking into account the surface non-magnetic (electric dipole), bulk non-magnetic (electric quadrupole) and surface magnetic (electric dipole) contributions.
Faraday rotation (FR) in a Ru4+-substituted magnetic garnet was measured in the visible to near ultraviolet wavelength region. An FR contribution due to Ru4+ ions was observed at around 2.8 eV and increased with increasing temperature. On the basis of the energies of charge transfer (CT) transitions in a (Ru4+O2-6)8- cluster calculated by an unrestricted SCF-SW-Xα method and the analysis of magnetic circular dichroism spectra measured in a Ru4+-substituted gadolinium gallium garnet, the transition at around 2.8 eV is assigned to the allowed CT transition of the lowest energy from O2- 2p to Ru4+ 4d orbitals, that is, the t1u (π) → t2g* transition. Furthermore, the temperature dependences of FR spectra are explained by a thermally induced FR from the first excited state of Ru4+.
One-dimensional magnetophotonic crystals with bismuth-substituted yttrium iron garnet films were fabricated by a method that combines rf magnetron sputtering and pulsed-light annealing. The media exhibited significantly high transmittance and Faraday rotation at designated localized wavelengths of light in the photonic bandgap. These results coincide quantitatively with the theoretical prediction.
MAMMOS (Magnetic Amplifying Magneto-Optical System) is a technology for reproducing a large signal during MO readout from a small magnetic domain. It has hitherto been considered that the domain recorded on the recording layer is copied to and expanded on the readout layer by irradiating the readout layer with a laser beam and applying a perpendicular magnetic field. However, we discovered that it is very effective for readout of small magnetic domains to apply an in-plane magnetic field. Under conditions where λ = 635 nm and NA = 0.55, the bit error rate (BER) of 0.15μm packed domains was improved to 2×10-4, while the BER of 0.2 μm packed domains was 2 × 10-4 when an in-plane magnetic field was not applied.
This paper discusses double-mask RAD media for short wavelength recording. NdGdFeCo is used as a readout layer instead of GdFeCo, which is currently used in 1.3-GB, 3.5-inch MSR disk media. The Kerr rotation of NdGdFeCo is larger than that of GdFeCo at 410 nm, and the ellipticity of NdGdFeCo is smaller, which is effective for decreasing the cross-talk. By optimizing the magnetic and optical properties of the NdGdFeCo readout layer, a CNR as high as 45 dB was obtained for a mark length of 0.31 μm when a readout magnetic field of 400 Oe was applied, using a 680 nm laser.
The effect of Ga addition on the enhancement of coercivity in HDDR-treated Nd-Fe-B powders, was investigated. Microstructural observation revealed that Ga addition suppresses grain growth of recombined Nd2Fe14B during the DR process. Analysis using the microstructural parameters in Kronmmarüllers equation shows that the α K value of Ga-added samples (0.48) is larger than that of ternary samples (0.12), which suggests that Ga leads to the development of reduced surface anisotropy in the recombined Nd2Fe14B grains.
The magnetic properties of Nd-Fe-B alloy magnet powders produced by the HDDR process were found to depend strongly on the hydrogen pressure. Anisotropic magnet powder is obtained by treatment at low hydrogen pressure, whereas isotropic magnet powder is obtained at high hydrogen pressure. Since the reaction rate is determined by the hydrogen pressure and the composition of Nd-Fe-B alloys, it is considered that the appropreate reaction rate of Nd-Fe-B and hydrogen in the disproportion step is the essential factor to induce anisotropy in the magnet powder. On the basis of the result, we established an improved HDDR process (d-HDDR) consisting of four processing step. Nd-Fe-B magnet powder treated at a hydrogen pressure of 0.03 MPa in the d-HDDR process has Br of 1.38T, iHc of 1122kA/m, and (BH)max of 342kJ/m3.
Amorphous melt-spun ribbons of Nd-Pr-Fe-Co-Cu-Nb-B system alloys were prepared by the single-roller rapid-quenching method The effects of composition, wheel velocity, and heat treatment on their magnetic properties were investigated. Amorphous Nd2.7Pr6.3Fe75.5Co8Cu0.5Nb1B6 ribbons prepared at a wheel velocity of 12.5 m/s were crystallized by heat treatment, and the optimum heat-treatment condition was found to be 675 °C for 0 min,then the value of (BH)max was 156.16 kJ/m3. The temperature coefficients of Jr and HcJ for the ribbons crystallized from the amorphous Nd2.7Pr6.3Fe75.5Co8Cu0.5Nb1B6 alloy are α (Jr) = −0.036 % /°C (reversible) and α (HcJ) = −0.46 %/°C (irreversible), respectively.
Amorphous melt-spun Nd9Fe74-xCo9V1NbxB7 (x = 0-3) alloy ribbons were prepared by the single-roller melt-spinning method, and the effects of their composition and annealing condition on their magnetic properties were investigated. The optimum condition for preparation of Nd9Fe74-xCo9V1NbxB7 alloy ribbons and some properties are as follows : composition, Nd9Fe73Co9V1Nb1B7; wheel velocity, 15 m/s; annealing condition, 700 °C × 0 min; magnetic properties, (BH)max = 166.4 kJ/m3 (20.9 MGOe). TEM observation showed that the average particle sizes of Nd9Fe73Co9V1Nb1B7 and Nd9Fe71Co9V1Nb3B7 ribbons were about 30 nm and 20 nm, respectively. Fine grains were randomly distributed from spotty rings of electron diffraction. The value of (BH)max for an isotropic compressed bonded magnet prepared by using Nd9Fe73Co9V1Nb1B7 alloy ribbons annealed at 700 °C × 0 min was 96.7 kJ/m3 (12.1 MGOe), and the density was 6.38 Mg/m3.
The microstructure of remaining amorphous-phase a Fe/NdFeB nanocomposite NdvFebalCowNbxCuyBz(v = 6-8.5, w = 0-10, x = 0-2.5, y = 0-0.5, z = 6-7 at%) magnet alloys was investigated by means of transmission electron microscopy (TEM), three-dimensional atom probing (3DAP), and Mössbauer spectroscopy. It was found by TEM that a small amount of amorphous phase remains in the intergranular region between crystal grains of α Fe and Nd2Fe14B even after heat treatment. The results of 3DAP showed that Nb and B atoms are significantly enriched in the remaining amorphous phase. Mössbauer spectroscopy revealed that the volume fractions of both the amorphous phase and the Nd2Fe14B phase increase with increasing Nb content. The coercivity-enhancing effect of Nb addition is discussed on the basis of the above information.