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To realize high areal density, hard disk drives larger than 4 Tbit in⁻², ultrafine FePt grains of less than 5 nm and grain density larger than 24 T in⁻² are required. Although there have been many investigations to reduce the grain size of FePt, there are only a few reports on the control of grain density. To increase the grain density, we focused on three aspects of the surface morphology and grain density: nucleation sites on the substrate surface, surface free energy, and lattice mismatch. We achieved 14 T in⁻² by maximizing the number of nucleation sites in the FePt-C granular film and found that the surface free energy and lattice mismatch are crucial parameters for controlling the grain density.

In this study, we investigated the relationship between the temperature rise in recording dots of a bit-patterned medium and its thermal conductivity during heated dot magnetic recording (HDMR) using numerical calculation (electromagnetic field and heat conduction analyses). When the thermal conductivities of the recording and heat sink layers were anisotropic, the temperature rise of a dot's lower cell could be increased while maintaining a small temperature difference between the upper and lower cells. The HDMR process was calculated via micromagnetic simulation using the Landau–Lifshitz–Bloch equation at vertical and in-plane thermal conductivities of 24.0 and 1.0 W mK⁻¹ (12.0 and 10.0 W mK⁻¹), respectively, for the recording (heat sink) layer. Results showed a bit error rate of 0%, and thus almost no error.

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr₂O₃/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr₂O₃ layer. The magneto-optic Kerr rotation angle (θ_K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr₂O₃ layer. The temperature dependence of θ_K at 2.67 eV (λ = 465 nm), at which the largest θ_K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr₂O₃ thin films. Although the θ_K spectra measured at several temperatures are generally explained by the classical interference model, θ_K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ_K is discussed on the basis of the anomaly in the optical parameters of the Cr₂O₃ layer associated with the antiferromagnetic–paramagnetic transition.

Manipulation of magnetic moments through spin transport is determined by the effective spin mixing conductance g_eff^↑↓ in the ferromagnetic metal/heavy metal heterostructure. Magnetization dynamics detection and evaluation of the Gilbert damping constant α is usually employed to estimate g_eff^↑↓. An all-optical pump-probe method is a powerful method for investigating α with high accuracy because an extrinsic contribution on α can be suppressed at a high precession frequency with an increasing external magnetic field. A large voltage signal was recently observed in half-metallic Heusler alloy Co₂MnSi thin film due to spin current injection; however, that voltage signal may also include contributions from thermal effect and spin Hall angle in the heavy metal layer. In this study, a large enhancement of α was observed in Co₂MnSi/Pt heterostructure by using the all-optical method. g_eff^↑↓ for Co₂MnSi/Pt was 2 times larger than that for Fe/Pt. This large g_eff^↑↓ is promising for spintronic device applications.

The Gd thickness dependence of spin–orbit torque (SOT) and critical current density of SOT switching in ferrimagnetic Gd/FeCo multilayers grown on a Ta seed layer was investigated and compared with the results on GdFeCo alloy/Ta bilayers. Critical current density of SOT switching J_sw of Gd/FeCo multilayers was almost the same as those of the GdFeCo alloy films. The magnitudes of damping-like and field-like SOT effective fields, H_DL and H_FL, respectively, in Gd/FeCo multilayers were confirmed to increase near the compensation point, and the sign of H_FL in Gd/FeCo multilayers was confirmed to change across the compensation point as well as GdFeCo alloy. Damping-like and field-like SOTs, estimated by multiplying the SOT effective fields by net magnetization, exhibited slightly smaller values for TM-dominant Gd/FeCo than for TM-dominant GdFeCo, while RE-dominant Gd/FeCo shows almost the same SOTs as RE-dominant GdFeCo.

Hexagonal rare-earth manganese oxide YMnO₃ thin films were prepared on yttria-stabilized zirconia (111) substrates by metal organic decomposition method. The crystallinity and morphology of YMnO₃ thin films crystallized at various temperatures were examined by X-ray diffraction and atomic force microscopy measurements, respectively. Single phase YMnO₃ was obtained for the sample prepared by annealing temperatures of 950 °C. AFM analysis revealed that a smooth surface with a roughness of 0.15 nm was achieved for YMnO₃ thin film annealed at 950 °C and 1000 °C, while three-dimensional growth for other samples prepared at 750–900 °C, 1050 °C, and 1100 °C. A narrow band at 1.6 eV and a broad band at 5 eV due to electronic transitions in the manganese and oxygen bands were observed in an absorption spectrum.

Sputtering-made magnetic thin films with metal–ceramic multilayer structures have been reported to achieve excellent magnetic properties and/or new functions for various magnetic materials. However, sputtering-made magnetic films tend to have 10 or fewer layers because the main applications of thin films, such as magnetic sensors, recording heads and recording media, require thin films thinner than 1 μm. This paper reports an Fe–Co/glass multilayer thick-film that has an approximate thickness of 10 μm (above 100 layers). This thick-film was prepared via pulsed laser deposition using several composite targets (two- or three-layer targets) consisting of an Fe–Co alloy (or Fe–Co sheet) and a glass plate. The fabrication of an Fe–Co single-layer film and a glass single-layer film was examined to determine the optimal defocus rate for the composite target. Then, we confirmed that the use of a [glass plate]/[Fe–Co alloy] two-layer target enabled the local formation of a [20 nm thick Fe–Co]/[10 nm thick glass) laminated structure with approximately 130 layers. An Fe–Co/glass multilayer structure was formed by using a three-layer target of a [Fe–Co sheet]/[glass plate]/[metal alloy]. Nonetheless, the results also suggest that large glass droplets should be removed in future work to create homogeneous multilayer thick-films.

A new high-sensitivity giant magneto-resistance (GMR) sensor system was developed and applied to a magnetic field microscope for which a magneto-impedance (MI) sensor was used as a high-sensitivity magnetic probe. The GMR sensor system achieved a detectivity of 13 pT/√Hz at 100 Hz. The sensing limit and spatial resolution were examined compared with the MI sensor. A 100 Hz AC magnetic field from a patterned Cu line was imaged. The GMR sensor showed better spatial resolution than that of the MI sensor owing to its small sensor unit size. The sensing limit was the same for both the GMR sensor and the MI sensor. This limit is due to ambient magnetic field noise. The GMR sensor could reduce this noise by differential detection using two sensor units. These results suggest that the GMR sensor system has advantages for application to the magnetic field microscopes compared with the MI sensor.

To investigate the magnetization process in the intermediate frequency region, the frequency dependences of coercivities in Ni and Co–Fe–B thin films were determined by anisotropic magnetoresistance measurements up to ∼160 kHz. In the low-frequency region (<5 kHz) the coercivity of Co–Fe–B was lower than that of Ni. However, the increasing rate of the coercivity in Ni was lower than that in Co–Fe–B. Consequently, above 19 kHz, the coercivity of Ni was lower than one of Co–Fe–B. Considering the basic material properties, better soft magnetic properties of Ni compared with Co–Fe–B should arise from the higher Walker breakdown field. This difference was mainly due to the Gilbert damping constant.

We observed helicity-dependent all-optical magnetization switching (HD-AOS) phenomena in GdFeCo films, which have equivalent magnetic properties but different optical properties due to optical interference layer thickness. Consequently, we found that these films have different properties of HD-AOS with the duality relation between magnetization direction and light helicity in light absorption, which generally means magnetic circular dichroism (MCD). Therefore, we also evaluated the effective absorption of the same GdFeCo samples which have similar intrinsic MCD by CW laser and intense femtosecond laser pulses. We confirmed that the contribution of this duality relation is much smaller in the absolute value of absorption. Furthermore, the magnitude and the sign of the duality relation change by the interference layer's thickness. As a result, we concluded that the duality relation in HD-AOS is derived from the duality relation in absorption. Furthermore, its duality relation can be changed effectively using multiple interference effects.

We study cobalt/platinum (Co/Pt) superlattices by means of ferromagnetic resonance spectroscopy in the frequency domain. The magnetization easy-axis of superlattices, which lies in a plane of the film with Co layers of 1.5 and 1.2 nm thickness, changes to the direction perpendicular to the film surface with Co layers of 0.9 and 0.6 nm thickness. As the Co layer thickness decreases from 0.9 to 0.6 nm, the Gilbert damping parameter significantly increases while the g-value is almost constant. As the Pt layer thickness decreases from 0.6 to 0.3 nm in the superlattices with 0.9 nm thick Co layers, the damping parameter slightly decreases. The present study demonstrates that the Co/Pt superlattices with 0.9 nm thick Co layers and 0.3 nm thick Pt layers are suitable for time-varying magnetic metamaterials realizing a larger magnetic permeability modulation.

Magnetic properties and nanostructure for FePt-30vol%TiN deposited while flowing N₂ gas are investigated. When N₂ gas addition of 3%–5% are introduced, compared with that of without N₂ gas, reduction in the lattice constant ratio and the unit cell volume, the increase in the degree of order of the granular film are observed. These indicate the segregation of Ti into grain boundaries as the result of the N₂ gas addition through nitridization of Ti dissolved in the FePt grains which lead to the enhancement of saturation magnetization, perpendicular magnetic anisotropy, and magnetic anisotropy field of the granular film. Therefore, magnetic decoupling in the FePt-TiN similar to the FePt-BN granular films is promoted by the segregation of nitrides formed by nitridization of metal element solidly soluble in FePt magnetic grains due to nitrogen deficiency.

The magnetization switching field is efficiently reduced by exciting precession with a microwave field of GHz frequency. Analytical calculations based on the Landau–Lifshitz–Gilbert equation have revealed that the effect of the thermal activation process plays an important role in magnetization switching behavior under a microwave field. In this study, we experimentally investigated the microwave-assisted magnetization switching (MAS) behavior of Co/Pt nanodot arrays under various microwave field conditions. Experimental results were compared with the calculated effective energy barrier height of MAS. Consequently, all the experimental MAS behaviors can be explained by the effect of thermal activation, but quantitative discussion will require accurate experimental studies.

In this study, post-annealing under NH₃ gas atmosphere was conducted in order to promote the two-phase separation of FePt-BN granular films. Magnetic properties and structural analysis for the Fe₅₀Pt₅₀–30 vol% BN granular films revealed that (1) saturation magnetization (M_s) of the granular film increased from 500 to 650 emu cm⁻³ with increasing post-annealing time from 0 to 10 min, (2) the orientation of FePt grains changes from (002) to (111) and fully ordered with increasing post-annealing time up to 30 min, (3) the unit cell volume of FePt grains in the granular film decreased from 55.91 to 55.55 ų with increasing post-annealing time from 0 to 10 min, which suggests dissolve and eject of solid solution element. From the above, M_s reduction for the FePt-BN granular films is considered to be caused by the solid solution of B in the FePt grains.

We experimentally demonstrated electrical detection of all-optical magnetization switching (AOS) induced by a single femtosecond laser pulse irradiation by measuring alternate rapid changes in anomalous Hall voltage and magneto-optic image pulse by pulse in a Hall-cross shape ferrimagnetic GdFeCo alloy thin film. We also demonstrated that the amplitude of the change in anomalous Hall voltage depended on the position of the AOS-created magnetic domain on the Hall cross. Furthermore, the AOS-created magnetic domains were stable against subsequent current applications in the Hall cross circuit, whereas reversed magnetic domains were not created when the laser pulse was irradiated with a high current. We found that the cooperative effect among magnetism, light, and electric current was assumed to have effects on the absence of the AOS. Combining the AOS phenomenon and electrical measurement/control techniques can realize ultrafast, deterministic, and distinguishable applications.

Optimizing switching behavior under a microwave field is a key issue for microwave-assisted magnetic recording. We study the effect of inter-grain exchange coupling induced by a continuous magnetic layer on microwave-assisted switching of CoCrPt-SiO₂ granular media. The microwave assistance effect is improved in the low-frequency region by introducing the inter-grain exchange coupling, but the coupling reduces the critical frequency. The maximum assistance effect is almost independent of the film structure since the dual effect cancels each other. Micromagnetic simulations also show similar results on exchange coupling between grains. These results indicate that the inter-grain exchange coupling can be used to optimize the critical frequency of the media in practical applications.

We developed a three-dimensional (3D) holographic display using the magneto-optical effect. The system uses a microlens array to reduce the area size of light reflected from pixels of a digital mirror device to approximately 1 μm and write in parallel to a magnetic recording medium as hologram data. This method makes it possible to write over a wide area even with a piezo stage that has high resolution but a narrow range of movement. The holograms, written at a pixel pitch of 1.1 um, enabled the reconstruction of 3D images with a wide viewing angle of 26.7 deg. By avoiding abrupt directional changes of the piezo stage, the image quality was improved by eliminating line defects in the written holograms; image distortion was reduced by setting the incident angle of the reference light in the program of a computer-generated hologram to match the model and the reconstructing optical system. Image distortion was reduced by improving the program of the computer-generated hologram.

The core-level resonant magneto-optical Kerr effect of a ferrimagnetic metal alloy, Gd₂₃Fe₆₇Co₁₀, was measured at the Fe M-shell and Gd N-shell absorption edges using rotating analyzer ellipsometry. A large Kerr rotation angle of several degrees was detected at room temperature. The signal was found to be strong enough for element-selective magneto-optical experiments to trace various magnetic events, such as all-optical magnetization switching.