Effects of intermediate layer and sputtering condition on magnetic properties were studied in Co-Cr-Pt film with high coercivity (H_C⊥), large squareness (SQ_⊥), and large nucleation field (H_N). An ultrathin Cr intermediate layer was effective in reducing the Co-Cr-Pt film thickness while maintaining high H_C⊥. H_C⊥ as high as 5000 Oe was obtained by low power sputtering at a low substrate temperature. This method is effective in reducing both Co-Cr-Pt film thickness and Ti underlayer thickness.

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Reduction of the thickness of the initial film-growth layer of CoCrPt-based perpendicular recording media was investigated using TiCr, CoCr, and Ru seed layers. Magnetic analysis revealed that the initial layer has small perpendicular magnetic anisotropy K_u, and that the thickness of the initial layer was about 10-15 nm with a TiCr seed layer, and 1-6 nm with a CoCr or a Ru seed layer. X-ray diffraction patterns and TEM images indicated that the initial layer contains an fcc-like structure, and that many stacking faults exist in CoCrPt-based perpendicular films, even with a CoCr or a Ru seed layer, The improvement in epitaxial film growth obtained by using a CoCr or a Ru seed layer is very effective in reducing the thickness of the initial layer, while is found to prevent the segregation of Cr from Co, resulting in larger intergranular exchange coupling.

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The perpendicular magnetic properties and structures of CoCrPtB/Ti, CoCrPtB/Co₆₀Cr₄₀/Ti, and CoCrPtB/Co₆₀Cr₄₀/C/Ti perpendicular media were investigated. By examining the growth forms of a Co₆₀Cr₄₀ intermediate layer and a magnetic layer, as well as the effects of inserting a very thin C layer, the following points were clarified: (1) in the case of a Co₆₀Cr₄₀ intermediate layer fabricated without a C layer, hcp phase with a high concentration of Cr is formed at T_sub.≤ 250°C, and both σ phase and hcp phase with low Cr concentrations are formed at T_sub.≥ 300°C; (2) in the case of a Co₆₀Cr₄₀ intermediate layer fabricated on a very thin C layer, hcp phase with high concentration of Cr is formed only at T_sub.≤ 150°C. At T_sub.≥ 200°C, only the initial nucleation growth of hcp phase with low Cr concentration is formed, which inhibits the formation of σ phase; (3) in the case of a CoCrPtB/Co₆₀Cr₄₀/C/Ti medium, nanocrystalline grains are formed in the initial growth region. In the upper region, c-plane-oriented grains of hcp phase with Cr segregated structure grow in a columnar shape. As a result, the grains of the magnetic layer grow epitaxially on the intermediate layer and the perpendicular magnetic properties are improved remarkably.

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The thickenss dependence of a magnetic field from which the rotational hysteresis loss has vanished was experimentally investigated for Co-CrPtB perpendicular recording media. It was clarified that (1) in Co₇₂Cr₁₆Pt₈B₄ media with film thickenss d_mag. under 200 nm and Co₆₄Cr₂₄Pt₈B₄ media with d_mag. under 100nm, a homogeneous structure was realized, except for the existence of an initial growth region with a thickness of about 2 nm; (2) for both media, the perpendicular rotational hysteresis loss W_r, the maximum field of W_r, H_p, and the field where W_r=0, H_Wr=0, increase with increasing d_mag.; (3) for both media,H_Wr=0 increase with increasing d_mag. and saturates at a field of about magnetic anisotropy H_k^gain; and (4) for a medium with a thim magnetic film, at an applied field H_Wr=0 < H < H_k^gain in a direction parallel to the film plane, a multi-domain state is realized, even though no hysteresis loss has been observed in torque measurement at this field. The last of these findings is considered to be due to the existence of thermal agitation between the multi-domains.

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Magnetic properties and recording performance are discussed for [Co/Pd (1 nm)]₉ and [(Co-27 at% B)/Pd (1 nm)]₉ superlattice perpendicular recording media as a function of the Co or Co-B layer thickness, δ_Co or δ_CoB. The perpendicular magnetic anisotropy K_u of these media shows maximum values of 5.7 x 10⁶ erg/cm³ (Co/Pd) and 3.2 x 10⁶ erg/cm³ (Co-B/Pd) at δ_Co or δ_CoB of around 0.2-0.3 nm. The K_uV/kT (V; the activation volume, k; the Boltmann constant, T; the absolute temperature) of the Co/Pd medium has large values of more than 200 (δ_Co>0.3nm), while the K_uV/kT in both media significantly decreases as the thickness decreases, which is not coincident with the thickness dependence of K_u. The value of K_u/2πM_s² (M_s; the saturation magnetization) in both media increase monotonically as the thickness decrease, and is ∼10 at δ_Co or δ_CoB of around 0.2-0.3 nm. The signal-to-medium-noise ratio SN_mR of the Co-B/Pd media is higher than that of the Co/Pd media, and increase as δ_CoB decrease. TEM images and magnetic characterization revealed that these differences in SN_mR are mainly due to the difference in the strength of intergranular exchange coupling.

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To study the effects of crystal orientation in a Co/Pd multilayer film, we measured the magnetic properties of a sample deposited on a single-crystal MgO substrate. A[100]-oriented Co/Pd multilayer film with thick Co-sublayer deposited on MgO(100) has a negative perpendicular anisotropy. However, the perpendicular anisotropy increases in the [100]-oriented specimen and the difference of the anisotropy energy between [111] and [100]-oriented multilayer films decreases as the Co-sublayer thickness decreases. Reduction of Co-sublayer thickness in Co/Pd multilayer film is effective for reducing the perpendicular anisotropy distribution in random crystal orientation.

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The thermal stability and medium noise performance of Co_100-X(PrTb)_X/Co-Cr-Ta (X = 21-31.7 at%) composite perpendicular media are discussed. The perpendicular magnetic anisotropy K_u shows a maximum at X = ∼25, although the saturation magnetization decreases gradually as X increases. The ratio of “intrinsic” remanence coercivity obtained by subtracting the termal agitation to the anisotropy field, H₀/H_k, increases as X increases, indicating a reduction of the exchange coupling of magnetization. The medium noise of the composite media, which have CoZrNb soft back-layers, decreases gradually as X increases and as the Co-PrTb thickness decreases, resulting in a higher signal-to-medium-noise ratio SN_mR. The SN_mR fo a Co_76.5(PrTb)_23.5 (5 nm) / Co₇₇Cr₁₉Ta₄ (25 nm) composite medium is about 8 dB higher than that of a Co₇₇Cr₁₉Ta₄ (30 nm) medium at 300 kFRPI. The composite media show high thermal stability of read-back signals even at low recording density. However, the thermal signal-decay increases gradually as the Co-PrTb thickness decreases, and also as X increases, indicating that the thermal stability has a trade-off relation with the medium noise performance.

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Reduction of medium noise is the key in perpendicular magnetic recording. The CGC medium that has a layered continuous and granular films is a candidate for obtaining a high SNR. We investigated noise characteristics of the CGC double-layered medium by focusing on transition jitter and magnetization transition analysis with an MFM. Results show that the transition smoothing by depositing the continuous layer was confirmed by both waveform analysis and MFM image analysis. The effect is due to magnetic interaction between the continuous layer and granular layer that resolves exchange-coupled clusters existing in a sole granular layer. Less transition noise, and therefor better SNR, resulted.

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We investigated the recording characteristics and thermal decay of Tb₁₅Co₈₅/Co-Cr-Pt-B composite perpendicular media with Tb-Co amorphous layers of various thicknesses. It was found that the S/N value of composite medium at a Tb-Co thickness of 5 nm was about 7 dB higher than that of Co-Cr-Pt-B medium. It was also found that composite media with a Tb-Co thickness more than 5 nm showed no thermal decay.

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We fabricated magnetic rectangular block arrays of 130 and 260 Gblock/in² by using focused ion beam lithography. The ratios R of the total block area to the array area are 0.73 and 0.64, respectively. The magnetic domain boundaries of the arrays run along the grooves between the blocks. Each block has a single domain structure and perpendicular magnetic anisotropy. The pattening increased the squareness from 0.2 of the continuous film to 0.87 of the array with 130 Gblock/in². Application of the block arrays to high-density patterned magnetic recording media is discussed.

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To find a soft underlayer with low noise characteristics for double-layer perpendicular media, we investigated the establishment of methods to control the soft-magnetic properties of FeTaN thin film fabricated by an in-line process, and studied the correlation between the soft-magnetic and noise characteristics of FeTaN thin films. As a result, it was clarified that (1) a uniaxial amisotropy field as high as 7.9 Oe with the easy axis in the radial direction of the disk can be induced by maintaining the nitrogen gas flow ratio (F_N2/F_total) between 10% and 30%, while keeping the saturation magnetization as high as 1.4 T to 1.6 T and the film roughness R_a as low as 0.3 nm; (2) in FeTaN thin film fabricated with F_N2/F_total larger than 15%, it is possible to control the easy axis of the induced uniaxial anisotropy in the radial or circumferential direction, by the combining magnetic field application with annealing and cooling processes; (3) it is important to control the stripe domain structure in order to reduce UL noise, even in FeTaN thin film; (4) in FeTaN thin films that have no stripe domain structure, as the coercivity in the easy direction decreases, the media noise tends to decrease a little; (5) FeTaN is a material that shows very low UL noise in comparison with other soft-magnetic materials such as FeBN, FeSiAl, and NiFeMo.

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Multilayers composed of Ni-Fe and amorphous Si interlayers were perpared in order to improve the soft magnetic properties and surface smoothness of the soft magnetic backlayer and reduce media noise originating in the domain walls. The magnetic structure, such as stripe domains, of the soft magnetic backlayer was restrained. Excellent c-axis orientation as well as high perpendicular coercivity of the Co-Cr-Ta recording layer was attained. It was revealed that [Ni-Fe/Si]_n multilayers were suitable for improving the soft magnetic properties of the backlayer.

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Activation volume and inter-particle in interaction were evaluated using superfine Ba-ferrite-particle-coated media, which were either oriented or non-oriented. It was clarified the peak value of the ΔM plot, which was used for evaluation of inter-particle interaction, changes according to the degree of media orientation. The activation volumes of superfine particulate media were found to be twice as large as their physical volumes. This suggested that more than two particles are stacked and act together in magnetization reversal. The effective value of K_uV_ac/kT was also estimated. For media composed of superfine Ba ferrite particles with diameters of about 22 nm, the K_uV_ac/kT values were close to the critical value in thermal stability.

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The post-oxidation effects of reactive-ECR-sputtered Co-containing ferrite thin films are described, focusing on their morphology and their magnetic and recording properties. A combination of reactive-ECR-sputter deposition and post-oxidation with ECR oxygen plasma was proved to be the best method for preparing Co-containing ferrite thin film media with good magnetic properties, smooth surface, low medium-noise characteristics, excellent roll-off curve, and small thermal fluctuation.

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To study the origin of high coercivity in Co-Pt film with an Ru underlayer, we measured the magnetic properties and performed transmission electron microscopy (TEM) observation of Co-Pt films deposited on single-crystal MgO. The structure of the Ru layer is twin. MgO [100] and the c-axes of Ru are arranged at an angle of about 60 degrees. Although most of the Co-Pt grains on the Ru have an hcp structure, an fcc structure can be observed at the upper surface of the grain-boundary region. The MgO [100] and [110] directions are normally magnetically easy and hard, respectively. The anisotropy field increases when an Ru underlayer is used in the Pt-rich region. Coercivity is enhanced by the increase in magnetic anisotropy originating from the hcp Co-Pt alloy, which is stabillized even in a Pt-rich composition by using the hcp-Ru underlayer.

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The effect of interparticle interactions on recording characteristics was investigated by computer simulation on an oblique metal-evaporated tape. It was shown that the magnetic anisotropy derived from the interparticle interactions plays an important role in the minor loop magnetization process and the recording mechanism of the medium. In the present study, the interparticle interactions in metal-evaporated media are considered to come mostly from magneto-static interactions. Computer simulation results based on a medium model that takes account of the magnetic anisotropy derived from the interparticle interactions are quite consistent with the data obtained from experimental measurement. As a result, it is suggested that the magnetic anisotropy originated in magneto-static interactions should be considered in order to understand the recording characteristics of the medium.

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We studied CoPt-(C₄F₈)_n granular thin films for use in high-density magnetic recording media. The (C₄F₈)_n matrix in granular film is very hard and flat, and these properties are advantageous for recording media. CoPt-(C₄F₈)_n granular films were fabricated by the facing targets sputtering method with plasma polymerizing reaction. The CoPt alloy was deposited on substrate and the (C₄F₈)_n matrix was polymerized at the same time. After annealing at 300°C, the CoPt-(C₄F₈)_n thin film had a high coercivity of 4.1 kOe. AFM images of the CoPt-(C₄F₈)_n granular thin films indicated that the grain size was decreased by addition of (C₄F₈)_n matrix. It is considered that the CoPt-(C₄F₈)_n granular films has the potential to be used in high-density recording media.

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Magnetic clusters in recording media were observed using a magnetic force microscope with applied magnetic fields up to 6 kOe. Magnetic clusters are annihilated by an increase of magnetic field in three steps: convergent structure → vortex structure → “⊂” structure. MFM image observed at saturation magnetization seem to reflect inhomogeneity and the surface roughness of films.

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It is possible for servo signals to be written on hard and floppy disks at high speed and low cost by using a magnetic contact duplication technique with lithographically patterned master media. The duplicated magnetization patterns of the slave media are elucidated. In this duplication technique, a dc magnetic field is applied at the time of duplication. When the degree of duplication is nearly maximum at the optimum magnetic field, the duplicated magnetization is reversed at transitions and the magnitude of the magnetization is close to saturation level. Therefore, when this technique is applied to hard and floppy disks, it is expected that a high signal-to-noise ratio can be attained.

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A tunneling magnetoresistive (TMR) element with a magnetic tunneling junction has the advantage of large MR ratio compared with a GMR element, and has high potential for applications such as magnetic random access memory and magnetic read heads for high-density recording. This paper describes the design and performance of yoke-type read heads with the TMR element. Magnetic field calculations were carried out for a front-yoke stacked-type TMR head. The head has the advantage of a high signal-to-noise ratio, and produces Lorentzian waveforms without any distortion in combination with perpendicular magnetic recording media.

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Electroplated CoNiFe films with bcc crystal structure and large saturation flux density (B_S) were investigated. CoNiFe films deposited in conventional conditions have a high B_S of 2.0 T, but show large coercivity (H_Ch > 30 Oe) because of their large grain size. Through several investigations of the electrodeposition conditions, it was found that the grain size is decreased by applying a higher deposition current density, which leads to in lower coercivity while preserving high B_S.

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An ultrathin CoFe free layer is very important for future high density recording head such as those using specular spin-valve films, because a CoFe free layer gives a higher MR ratio than a conventional NiFe/CoFe free layer. In order to realize a single CoFe free layer, we investigated the magnetostriction (λ_s) of an ultrathin CoFe free layer in spin-valve films. We compared the magnetostriction dependence on the underlayer structure among Cu, Ru, and Ru/Cu underlayers on a Ta buffer layer. For an Ru/Cu double-layered underlayer, λ_s becomes positively large when the Ru underlayer thickness is increased; on the other hand, it becomes negatively large when the Cu underlayer thickness is increased. This tendency can be explained by a model in which the λ_s is changed by the lattice elastic strain, where the volume ratio of Ru/Cu changes the lattice strain of an ultrathin CoFe free layer. The sensitivity of λ_s to the Cu underlayer thickness becomes large with increasing Co concentration in the CoFe free layer. This indicates that λ_s of a CoFe free layer becomes sensitive to the elastic strain with increasing Co concentration.

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The magnetostatic interaction between the GMR elements and the shields of shielded GMR heads was investigated by micromagnetic modeling of the GMR elements and the shields. The magnetization patterns in the shields were determined in order to minimize the total energy of the shields. The biasing field of the GMR element was defined by the sum of the permanent magnet field and its images in the shields. It was confirmed that the magnetization curves of shields with permanent magnets show hysteric behavior, and double- or single-flux closure appeared in the domain structure. The transition from double-flux closures to a single-flux closure causes sudden permeability change in the shields. A permeability change will produce changes in the imaging field of the permanent magnets in the shields. As a result, a Barkhausen jump corresponding to a permeability change appears in the easy-axis transfer curves (i.e., the GMR response vs. the easy-axis field).

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In order to study the thermal stability of magnetic recording media, a hysteresis model of a medium at finite temperatures is proposed, based on an ensemble of non-interacting Stoner-Wohlfarth single-domain particles. The model takes into consideration the switching rate of each particle by using the Neel-Arrhenius law and the master equation. Using this model, a finite-element numerical read/write (R/W) simulation is performed by solving the Poisson equation for the two-dimensional model of a thin-film head and longitudinal media system. As a result, the output voltages at 77 kBPI and 141 kBPI at room temperature are found to be reduced linearly with the logarithmic time, on account of the thermal energy. In addition, the slope of the output voltage reduction is much larger for media with a lower K_uV/k_BT, a finding consistent with experimental results. R/W simulation using the proposed model may be useful for evaluating the thermal stability of the recording medium.

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A read/write simulation based on the Poisson equation by a finite element method was performed on antiferromagnetically coupled (AFC) magnetic media composed of two laminated recording layers and a thin intervening Ru layer, and a thin-film head system. To estimate the thermal stability of the AFC media, a new medium model that introduces interlayer exchange coupling as proposed. This model explains well the ovserved minor loop shifts to the first quadrant in M-H loops due to the interlayer exchange coupling. However, the recorded magnetizations calculated by using this model are found to be in antiparallel magnetization configurations that are independent of the interlayer exchange coupling, and the thermal stability of the AFC media is found to be much improved in comparison with that of the conventional medium. This suggests that within the parameters used in the present simulation, the thermal stability of the AFC media is mainly enhanced by the magnetostatic interlayer coupling.

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To attain high-density magnetic recording, the parameter a of an isolated transition, which defines the sharpness of the recorded magnetization pattern in the transition region, has to be made small as possible. Here, the values of parameter a are estimated by fitting an arctangent or a hyperbolic tangent function to the recorded magnetization patterns calculated by a finite element R/W simulation solving the Poisson equation for the two-dimensional model of a thin-film head and various media, and compared with those obtained by the analytical formula derived by using the Willams-Comstock and Hokkyou (W-C-H) model. As a result, the calculated recorded magnetization patterns are found to be fitted quite well by a hyperbolic tangent function using the parameter a estimated by using the W-C-H model, and the calculated half-amplitude pulse width PW₅₀’s are found to coincide well with those estimated from an analytical formula obtained by using the reciprocal theorem assuming the Karlqvist head field and an arctangent magnetization pattern.

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(PLZT)_x(BiFeO₃)_1-x solid solutions exhibiting both ferroelectricity and ferromagnetism were prepared by a solid-state reaction. X-ray diffraction patterns showed that the crystal structure of the solid solutions for x = 0.1−1.0 was a cubic perovskite structure, but Raman Spectra suggested the existence of a rhombohedra-type local polar order. The temperature dependence of ferroelectricity indicates that the materials for the whole range of x are ferroelectric relaxors. The materials for x = 0.10-0.45 simultaneously showed ferroelectric and magnetic hysteresis loops at room temperature. The Néel temperatures of the materials decreased with increasing x, e.g., 640 K (x = 0), 585 K (x = 0.10), and 570 K (x = 0.15). The possible origins of the weak spontaneous magnetization in the present system are also discussed.

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Nonthermodynamical Bi₃Fe₅O₁₂ films were synthesized by the reactive ion beam sputtering technique, and the relation of the compositions of the starting materials 3(1+x)×Bi₂O₃ + 5 ×Fe₂O₃ powder, targets, and sputtered films was investigated. The compositions of the target and the film were different from that of the starting materials, which can be understood by taking the high volatility and high sputtering yield of Bi into consideration. By using a starting material with x = 0.15, BIG film with the composition Bi:Fe:O = 3.0:5.0:12.0 was successfully obtained. However, a film with x = 0.1 showed the largest θ_F value. From the point of view of the magneto-optical properties, the optimized compositions of the film may deviate form the stoichiometric compositions.

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To investigate the potential of magnetophotonic crystal for use in optical isolators, theoretical and experimental studies were carried out. The calculations revealed that the required optical isolator performance (reflectance R 95% and Kerr rotation θ_K 45°) can be achieved with a reflection-type one-dimensional mgnetophotonic crystal (1D-MPC) having only one thin magnetic layer. Provided that a dielectric multilayer film and an Al film are used in combination as the reflection layer, the structure of 1D-MPC satisfying the isolator requirements becomes much simpler than that of the transmission-mode 1D-MPC. This is indeed very attractive for fabrication of actual isolators.

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We developed a prototype magnetic field modulation (MFM) coil with a magnetic core on a silicon substrate. The magnetic core, consisting of a CoFeSiB/SiO₂ composite multi-layered film, has a high quality factor of more than 70 at high frequency of 100 MHz. For a micro-MFM coil, high field strength of more than 200 G(16 kA/m) must be generated in the center of the coil at a high frequency and a low driving current. Whereas an air-core coil requires a high driving current, a magnetic core coil can be driven at a lower driving current. We simulated the magnetic field strength of the vertical section of this MFM coil using 3D-Maxwell simulation.

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The magnetic properties and crystallization behavior of low-rare-earth-content (Nd = 6 at.%) α-Fe/Nd₂Fe₁₄B-type Nd-Fe-Co-V-B system nanocomposite magnet ribbons prepared by rapidly quenching with a single roller were investigated. The roller surface velocity V_S of the rapid quenching directly affected on the amorphous volume and the magnetic properties after annealing. The formation of an Fe₃B-like phase at an early stage of the crystallization process resulted in the reduction of the soft α-Fe phase grain size, and this brought about good magnetic properties.

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NdFeB magnet powder rapidly quenched (MQ powder) and Fe₃B/NdFeB spring magnet powder were densified by using the electrically heated powder rolling method (EPR). Although MQ powder was uniaxially pressed under high temperature by EPR, magnetization recordering did not occur. Fe₃B/NdFeB spring magnet powder was also consolidated by EPR. The resultant spring magnet retained the amorphous state.

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An experiment was carried out to investigate the effects of an Al organic compound as a new reducing agent, and SiO₂ and CaO additives, on the magnetic and physical properties of SrFe₂-hexagonal ferrite (W-type). It was found that the magnetic properties of SrO·8.5Fe₂O₃ were consideraly improved by adding 0.4 wt% of Al organic compound, 0.3 wt% of SiO₂, and 0.7 wt% of CaO. The optimum conditions for making magnets and the prime properties of typical specimens, according to chemical analysis, are as follows. Composition : Sr²⁺_0.950Si⁴⁺_0.084Ca²⁺_0.170Al³⁺_0.053Fe²⁺_0.813Fe³⁺_16.508O₂₇ ; semisintering condition (S. S. C.) : 1350°C × 5.0 h in nitrogen gas atmosphere; drying condition (D. C.): 200°C × 3.0 h in air; sintering condition (S. C.) : 1180°C × 1.5 h in nitrogen gas atmosphere. The magnetic and physical properties of typical specimens are as follows : J_m = 0.479 T, J_r = 0.434 T, H_cJ = 187.48 kA/m, H_cB = 173.40 kA/m, (BH)_max = 32.71 kJ/m³, T_c = 488.7°C, H_A = 1433 kA/m, K_A = 3.20 × 10⁵ J/m³, and η_B = 30.7 μ_B, a = 5.886 × 10^-10m, c = 32.785 × 10^-10m, and c/a = 5.570.

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Raman scattering was investigated for a single crystal of LaMnO₃. In the orbitally ordered ground state, a clear Raman structure between 120 meV and 170 meV appears, which is relevant to the collective excitation of the orbital degree of freedom called “orbital wave.” The intensity of the Raman bands rapidly decreases above the Neel temperature. This indicates that the coherence of the orbital wave is suppressed in the absence of the static spin order. The peak energy of the Raman bands is decreased with increasing temperature toward the orbital order-disorder temperature T_∞. The suppression of the Jahn-Teller distortion as well as the energy gap of the orbital excitation near T_∞ may be responsible for the observed energy shift.

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We studied the electronic properties of Zn doping on the Mn site in the colossal magnetoresistance (CMR) material La_0.7Pb_0.3(Mn, Zn)O₃ (x ≤ 0.3). Small nonmagnetic Zn²⁺ (d¹⁰) doping or magnetic dilution tends to separate the system into ferromagnetic clusters, weakening the long-range ferromagnetic order and reducing the Curie temperature. The spin polarizability of the x = 0 − 0.3 samples is estimated to be 0.97 − 1.00, indicating that the x = 0 − 0.3 samples are spin-polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x ≥ 0.1) induces the metal-insulator transition and destoroys spin-polarized matellic state with long-range ferromagnetic order.

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We synthesized and investigated the magnetic and electronic nature of layered manganese oxysulfide Sr₂CuMnO₃S, which crystallizes in an unusual intergrowth structure with Cu₂S₂ and pyramidal MnO₅ layers separated by Sr ions. The Cu 2p XPS spectrum for Sr₂CuMnO₃S indicates that Cu ions are in the monovalent state, or “Cu⁺”. The features of the Mn 2p XPS spectrum for Sr₂CuMnO₃S are similar to these of the parent compound La³⁺Mn³⁺O₃ of CMR materials. MnO₅ cluster model analysis suggests that the electronic nature of the pyramidal MnO₅ layers belongs to a strongly correlated electron system, and that the system is a so-called Mott insulator as a result of its strongly ( χ ) exhibits low-dimensional antiferromagnetic ordering of Mn³⁺ in the pyramidal MnO₅ layers. Fitting the χ data to the Curie-Weiss law gives evidence of the strong second-neighbor Mn-Mn interaction.

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New layered oxysulfides, Sr₂CuFe_1-xGa_xO₃S (x = 0, 0.05) and Sr_4-xLa_xCu₂Mn₃O_7.5S₂ (x = 0, 0.10, 0.20, 0.40) with alternating Cu₂S₂ and perovskite oxide layers were synthesized and characterized. The perovskite oxide layers in Sr₂CuMO₃S (M = Fe and Ga) and A₄Cu₂Mn₃O_7.5S₂ (A = Sr and La) have stacking sequence with -MO₂-SrO-SrO-MO₂- and -MnO₂-AO-MnO_1.5-AO-MnO₂-, respectively. Sr₂CuFe_1-xGa_xO₃S (x = 0, 0.05) is semiconducting. Theelectrical resistivity of the x = 0.05 sample is ten times lower than that of the x = 0 sample, indicating that the conduction path is in the Ga-introduced Fe oxide layer. Sr₄Cu₂Mn₃O_7.5S₂ exihibits a phase transition from paramagnetic to antiferromagnetic states below 100 K. In Sr_4-xLa_xCu₂Mn₃O_7.5S₂ (x = 0, 0.10, 0.20, 0.40), the lattice parameters a and c decrease with increasing x. All samples are semiconducting. The electrical resistivity dramatically falls with increasing x; that of the x = 0.40 sample is one hundred times lower than that of the x = 0 sample.

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The effect of a magnetic field on the unidirectional solidification process of the high-T_c superconductor Bi₂Sr₂CaCu₂O_y (Bi-2212) was studied. The apparatus used in this study was a high-frequency heating furnace located in the room-temperature bore of a superconducting magnet. A sinterad rod of Bi-2212 was put into an alumina tube, then moved parallel to the applied magnetic field direction through the high-frequency coil. The magnetic field intensity was about 7 T in the molten zone. After solidification, the initial and later parts of the samples thus obtained ware cut into blocks, and X-ray diffraction and magnetization measurements were performed to evaluate their orientations. When a conventional unidirectional solidification process such as the floating-zone method is used, Bi-2212 crystals tend to grow with their c-axes perpendicular to the direction of motion. On the other hand, when a high magnetic field is applied during the solidification process, samples with their c-axes parallel to the direction of motion were obtained. This phenomenon was due to the paramagnetic anisotropy of Bi-2212 crystals. It can be said therefore that a magnetic field can be used to control the grain orientations in melt-solidification processes of even feeble magnetic substances.

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The conductance quantization of Ni Nano-wire was investigated by using break-junction technique. When the width of the nano-wire decreased rapidly, the conductance in remanent magnetization states was observed to be quantized in units of 2e²/h. The unit of quantization switched to e²/h when a magnetic field was applied. When the width of the nano-wire varied slowly, however, quantized conductance plateaus in units of e²/h appeared in the remanent state. Even in this case, the plateaus with ne²/h (n = 2,4,6,...) were observed to be more stable than those with ne²/h (n = 1,3,5,...). The results imply the relevance of the local magnetic domain structrure around the contact, as well as its reconfiguration by the magnetic field, to conductance quantization in ferromagnets.

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The distributions of the switching field (H_SW) for single magnetic wires (150 nm width) were investigated as a function of temperature between 5 and 300 K. One end of each wire was connected to a square pad (large area), and magnetization reversal phenomena were very sensitively detected by using the giant magnetoresistance effect. While the distribution width of the H_SW for the Co layer in NiFe/Cu/Co drastically broadened below 100 K, that of the H_SW for the NiFe layer in NiFe/Cu/NiFe had three narrow peaks at each temperaure down to 5 K. The origin of these three peaks can be attributed to the existence of three different kind of magnetic domain structures in the pad area, which was suggested by MFM observation.

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Micromagnetic simulation based on the Landau-Lifshitz-Gilbert equation was performed to clarify the effect of self-induced anisotropy on domain wall behavior such as sall broadening and wall pinning. In this simulation, the descretization region was in the cross-section normal to the film plane. Self-induced anisotropy was modeled as follows : first, with the uniform easy axis set normal to the calculation region, the domain wall profile is calculated. Next, after relaxation, with the easy axis direction set to be the same as the magnetization direction, the domain wall profile is re-calculated. A flux-closure asymmetric vortex domain wall (C-shaped) is obtained and the domain wall width increases after the easy axis direction has been set to be the same as the magnetization direction. The increase in the wall width becomes larger with increasing film thickness, which agrees with the experimental results. Positive and negative magnetic fields are applied to estimate the depinning field of the wall. Simulation results show different depinning fields in the direction of the wall movement. The dependences of the depinning field on the film thickness and various wall configurations, such as a two-vortex state (S-shaped), are also presented.

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Current-perpendicular-to-plane magnetoresistance (CPP-MR) was measured for ferrimagnetic Co/Gd multilayers, which have twisted spin structure accompanied by spin-flop transition. The influence of the twisted spin structure on the electronic transport was exclusively obtained, eliminating the effect of anisotropic magnetoresistance. MR curves showed a downturn around the spin-flop field, suggesting that the formation of the twisted spin structure leads to a negative resistance change. This is possibly caused by the GMR-like mechanism whereby the resistance decreases in the process of the spin structure change from antiparallel to parallel alignment between Co and Gd moments.

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The asymmetric bias voltage dependence of the tunneling magnetoresistance (TMR) effect in Mn₈₀Ir₂₀/Ni₈₀Fe₂₀/Co₉₀Fe₁₀/SrTiO₃/La_0.7Sr_0.3MnO₃ (LSMO) junctions was investigated. The bias voltage dependence of the TMR effect depended on a chemical composition of SrTiO₃ (STO) barrier. In our previous report, the origin of the asymmetric bias voltage dependence of the TMR effect in such junctions was accounted for by the density of states of CoFe. However, it is necessary to consider the influences of the electronic structure in STO/LSMO, CoFe/STO interfaces, and oxygen deficiency in the STO barrier.

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Ferromagnetic tunnel junctions (MTJs) in which a thin MgO seed layer was inserted between Fe bottom electrode and MgF₂ insulating barrier were fabricated through the use of molecular beam epitaxy. The structure and tunnel magnetoresistance (TMR) were investigated for Fe (200Å)/MgO (5Å)/MgF₂ (20Å)/Co (200Å) junctions. RHEED observation suggested that an MgF₂ layer deposited at room temperature was in amorphous or nanocrystalline state. The TMR ratio observed for a typical MTJ with barrier height of 1.6 eV was 1.8 % at 4.2 K and decreased with increasing temperature, down to 0.7 % at 293 K. It was found that the bias dependence of TMR shows a strong asymmetric feature, in contrast to conventional MTJs with amorphous alumina barriers.

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We studied the tunnel spectra of MTJs with a single-crystal ultrathin Fe (100) electrode. As a result, an enhancement of the MR ratio compared to that for the MTJ with thick Fe (100) electrode was observed. In addition, the tunnel spectra of MTJs with ultrathin Fe electrodes were found to have oscillatory components in the positive bias region, and the differential magnetoresistance also showed oscillations. This is the first observation of the quantum-well effect in the bias dependence of TMR. This new effect provides us with the possibility of creations new voltage-controlled spin-functional devices.

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The crystal structure of spin-valve films with a ZnO underlayer was investigated. ZnO film has a strongly (002)-oriented structure, whereas spin-valve films deposited directly on the ZnO underlayer have a weak oriented structure. TEM observation shows the possibility of using a reaction layer at the ZnO/MnPt interface. We found that the insulation of very thin Cu layer in the ZnO/MnPt interface causes a large improvement of orientation in spin-valve films, and that the MR ratio is better than spin-valve films using Ta/NiFe underlayers because of both the high crystal orientation and the reduction in current shunting.

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The effect of the interlayer coupling field (H_in) on the magnetization process of a free layer in spin valves (SVs) is discussed. SVs of the form Sub./Ta/Ni-Fe/Mn-Ir/Co-Fe/Cu/Ni-Fe/Cu/Ta, were used. The H_in was varied by changing the thicknesses of the free Ni-Fe and Cu spacer layers. The dynamic differential susceptibility (DDS) was measured to investigate the magnetic properties of the free layer, such as a uniaxial magnetic anisotropy field H_K and a structure constant S. As a result, it was found that even when H_in is compensated by an external applied field, S increases with the strength of the interlayer coupling, causing a deterioration of the magnetic softness of the free layer.

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Magnetization reversal of patterned magnetic structures consisting of single and antiferromagnetically (AF) coupled magnetic elements was simulated by Landau-Lifshitz-Gilbert equations. It is considered that AF coupled magnetic elements can make the demagnetization field small. Simulation was carried out for changing saturation magnetization, uniaxial anisotropy, magnetic exchange stiffness constant, interlayer exchange coupling, magnetic layer thickness, nonmagnetic layer thickness and element size. As a result, we found that the switching field of AF coupled magnetic elements became smaller than that of single magnetic elements in submicron elements.

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We prepared the ordered phase of FePt alloy at a low annealing temperature by adding Cu to FePt. The ordering of FePtCu film starts at an annealing temperature of 300°C or lower, whereas FePt and FePtAg films do not form the ordered phase at 300°C. By annealing at 300°C, the coercivity of the FePtCu film is increased to around 4.5 kOe. In the FePtCu film, the added Cu and FePt form a ternary alloy before and after annealing, whereas the FePtAg is decomposed into FePt and Ag phases by annealing at temperatures higher than 400°C. Comparison of the FePtCu film with the FePtAg film shows that the formation of a thermodynamically stable FePtCu ternary alloy is important for reducing the ordering temperature.

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We investigated the L1₀ ordering process of FePt films deposited on a Bi underlayer. When annealed at relatively low temperatures, Bi tends to move toward the surface of an FePt layer. It was found that Bi travels along the grain boundary of FePt. This accelerates coalescence of FePt grains with intensive atomic diffusion, which leads to faster transformation from the disordered phase to the ordered. Subsequent rapid increase of coercivity is observed after annealing at 350°C. This appreciable reduction of ordering temperature may remove the difficulty of fabrication of L1₀-type, high-density recording media.

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