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Magnetic properties and microstructure of Co 80 Pt 20 -30 vol.% (Al 2 O 3 , TiO 2 , WO 3 ) and Co 82.4 Pt 17.6 -27.7 vol.% SiO 2 granular films postannealed at various temperatures in a heat-treating furnace were investigated. In this paper, it is found that among these granular films, CoPt-WO 3 granular film shows the highest coercivity (H c ) because of low intergranular exchange coupling of well-isolated magnetic grains. Furthermore, the H c is enhanced with postannealing because of the increase of the magnetic anisotropy field (H k grain ) and uniaxial magnetocrystalline anisotropy (K u grain ) of the columnar growth magnetic grains. The increase of K u grain for the postannealed granular film primarily originates from the reduction of the stacking faults as well as the lattice constant ratio.

An Fe–Co nanoparticle assembly with uniaxial nanostructure was successfully obtained as a nanocomposite with polystyrene, which was polymerized in a magnetic field, Hfix (50 kOe), from a mixture of the Fe–Co nanoparticles and styrene monomer. The easy axis of magnetization was parallel to Hfix and the uniaxial magnetic anisotropy field of the nanocomposite was estimated to be 2.5 kOe. The ferromagnetic resonance frequency of the nanocomposite reached 6.8 GHz, which was larger than that of the nanocomposite prepared without Hfix.

We developed a new one-pot synthesis method for silica coated magnetite nanoparticles by way of a cheap- modified polyol process. In this reaction, polyethylene glycol was used as a solvent media and it has been found to play a key role to act as a reducing agent, stabilizer as well as a linker for silica coating, simultaneously. The Fe3O4/SiO2 sample prepared by this new method was compared with other seed Fe3O4 and Fe3O4/SiO2 samples synthesized through the modified Stober method and characterized using different analysis techniques such as transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy. The magnetic properties of the seed Fe3O4 and silica coated magnetite nanoparticles were studied by vibrating sample magnetometer at room temperature. The produced composite sample showed excellent stability against oxidation when annealed at 600 °C in presence of hydrogen gas. Thus, the facile new polyol process adopted in our study appears to be a promising route for synthesis of highly stable, hydrophilic silica coated magnetite nanoparticles.

A new type of composite target is proposed for low Ar-gas-pressure (PAr) deposition of CoPtCr-SiO2 granular perpendicular magnetic recording media. The proposed target is a Co-Pt-Cr-CoSi-enriched CoO sintered target, consisting of CoSi and CoO powders instead of SiO2 powder. Systematic analysis of film composition, microstructure, and magnetic properties of the granular films revealed that 1) the Co74Pt16Cr10-8 mol%SiO2 granular film with O/Si ratio of 2 in the film is realized by using a CoO/CoSi ratio of 3 in the target for our sputtering system, and 2) media with S=1, Hc=5.0 kOe, and α = 2.9 was realized even under a low PAr of 0.6 Pa. This means that just the oxidization of Si into SiO2 is important for magnetic decoupling.

High magnetization hydrophilic magnetite nanoparticles have been synthesized in two different batches with mean particle sizes of 32.3 and 9.2 nm by inexpensive and surfactant-free facile one-pot modified polyol method. In the synthesis, polyethylene glycol was used as a solvent media and it has been found to play a key role to act as a reducing agent as well as a stabilizer simultaneously. It was shown that the size of the nanoparticles can be effectively controlled by modifying the reaction parameters such as reaction temperature, time and polyol/metal precursor ratio. X-ray diffraction and energy dispersive spectroscopy studies confirm the formation of a pure magnetite phase without the presence of any other phases. Transmission electron microscopy and the Fourier transform infrared spectroscopy results reveal that the particle size and surface adsorption properties are very much dependent on reaction parameters. The magnetic properties of the samples measured by physical property measurement system have shown that the as-synthesized magnetite nanoparticles possess a high magnetization of 85.87 emu/g at 300 K and 91.7 emu/g at 5 K with negligible coercivities. The structural and magnetic characterizations of these polyol coated, hydrophilic, monodisperse, superparamagnetic nanoparticles clearly indicate that they are suitable for biomedical applications.

The effect of replacing Co with Fe in disordered CoPtRh alloy films was investigated with the aim to increase the spin magnetic moment. The addition of Fe into a pure Co film produced a Co98Fe2 film with maximum uniaxial magnetocrystalline anisotropy (Ku), because the magnetic moment is increased with the same degree of stacking faults (SFs) induced into the hcp grains as that for a pure Co film. (Co0.98Fe0.02)PtRh films were compared with CoPtRh films with the same Pt and Rh contents, which revealed that: 1) the degree of SFs were increased; 2) the magnetic moment was increased; and 3) Ku was decreased for all compositions in the (Co0.98Fe0.02)PtRh films. In particular, the (Co0.98Fe0.02)PtRh and CoPtRh films with a larger Rh content than Pt have a greater range for increase of the Ku magnitude by enhancement of the magnetic moment with the retention of or further decrease in the degree of SFs.

A rapid magneto-optical (MO) spectrometer is introduced. In conventional MO measurements, a monochromator produces quasimonochromatic incident light. The MO spectrum is then found by measuring the hysteresis loop at each wavelength in small steps, which is time consuming. In contrast, the new system uses 1) incident white light, 2) polarization rotation by a Faraday cell and spectroscopic detection using an area sensor, and 3) subtraction of spectra for small changes of polarization. By this method, an MO spectrum can be measured in only a few minutes with a single sweep of the magnetic field, enabling rapid exploration of the magnetic properties of a wide variety of materials.

This paper shows a new effective approach to measure crystallization temperature of soft magnetic underlayer (SUL) for next generation of heat assisted perpendicular recording media. This approach uses temperature dependent reflectivity, which shows a clear jump when samples are crystallized. To achieve this measurement, an optical system is set up using hot plate and infrared laser. Reflectivity of SUL (Co 70 Fe 30 ) 92 Ta₃Zr? shows a clear jump at its amorphous-crystalline transition temperature. Experiment results show this effect is clear in infrared region, and is weak for visible light.

High magnetization FeCo nanoparticles with different Fe/Co ratios have been successfully synthesized by surfactant free simple modified polyol method. Polyethylene glycol (PEG) was used as solvent and reducing agent simultaneously in this synthesis process. All the synthesized samples of FeCo nanoparticles were annealed at 600 °C before characterizations. X-ray diffraction (XRD) data on the samples confirm formation of a body-centered-cubic single phase structure in all the compositions. Transmission Electron Microscopy (TEM) data suggest that the annealed FeCo nanoparticles are of 50–90 nm in size. The use of PEG and the annealing procedure employed ensure that the obtained nanoparticles are stable in air. This observation is well supported by both the analysis of Energy Dispersive Spectrometry (EDS) and the images of TEM which establish the formation of a thin passive oxide layer over the FeCo nanoparticles thereby resulting in the stability of the nanoparticles. The physical Property Measurement System (PPMS) reveals that the Fe 60 Co 40 composition among all the samples exhibit highest saturation magnetization of 230.14 emu/g at 5 K.

The intergranular exchange (coupling) field Hex of single/stacked granular perpendicular magnetic recording media with a switching field (SF) distribution was measured by ferromagnetic resonance (FMR) by sweeping the external field HDC through the internal effective field Heff. Heff consists of components corresponding to the anisotropy field Hk, the dipole interaction field, the self-demagnetization field, and the Hex for up and down magnetic moments of magnetic grains classified by their SF. FMR measurements were performed for one kind of weak intergranular exchange coupled medium (medium I) and two kinds of strong intergranular exchange coupled media (medium II (single magnetic layer structure) and III (stacked structure)). These measurements revealed that the increase in Hex with decreasing 4πM for media II and III was larger than that for medium I. In addition, Hex/Hk was determined to be 0.25 (for grains with low SF) and 0.37 (for grains with high SF) for medium II and 0.34 (for both low and high SF) for medium III under M=0.3. The variation in Heff for the multidomain state was smaller for the strong intergranular exchange coupled media than for the weak intergranular exchange coupled medium. The values normalized by Hk were approximately ± 0.038-0.060, ± 0.008-0.015, ± 0.012-0.030 for media I, II, and III, respectively.

A new method for evaluating the switching field distribution (SFD) in perpendicular magnetic recording media is proposed using the saturation magnetic field of a minor loop, Hsminor. In this method, the switching fields of focused reversal magnetic domains are measured with all magnetic moments in other domains saturated in the upward direction. Thus, the proposed SFD evaluation is not affected by variety of domain structure, and reveals tails of distribution. Using the proposed method, the SFD for various granular media was evaluated.

In this study, we have systematically investigated the effect of sputtering conditions on the microstructural properties of Ag/CoFeB thin film on MgO substrate. It was found that the crystallinity and surface roughness of the Ag film strongly depends on the Ar sputtering pressure and sputtering power. Epitaxial growth of Ag(100) film on MgO(100) substrate was achieved under the sputtering conditions of high sputtering power and elevated temperature. XRR (X-ray reflectivity) and high-resolution TEM (transmission electron microscopy) measurements also revealed the interfacial roughening in the Ag/CoFeB interface due to the island structure formation and intermixing between Ag and CoFeB.

Oblique-incidence evaporation and sputtering were performed to investigate the effect of reducing the Ru interlayer thickness on the recording characteristics of perpendicular magnetic recording media. Deep gaps were observed at grain boundaries on oblique-incidence evaporated and sputtered films when the incident angle was over 60°. Evaporated Ru films have a lower density than sputtered Ru films at the same incident angle. This indicates that the self-shadowing effect predominantly occurs due to suppression of migration of deposited atoms for evaporation films. However, degradation of the crystal orientation is observed in evaporated Ru films due to the present experimental limitations. Examination of the recording characteristics revealed that the Ru interlayer thickness can be reduced by using an oblique-incidence sputtered Ru interlayer and the writing ability can be improved by reducing the spacing loss between the head and the soft magnetic underlayer.

In order to obtain mono-dispersed Fe NPs with high saturation magnetization, quantitative analysis method to investigate the growth dynamics of the Fe NPs synthesized by a conventional thermal decomposition method has been developed. As a result, fast nucleation process promotes formation of ~4 ㎚ of initial nucleus with a non-equilibrium phase, resulting in low saturation magnetization. And slow particle growth with atomic-scaled surface precipitation mode (＜ 100 atoms/(minㆍ㎚²)) can form the growth layer on the surface of initial nucleus with high saturation magnetization (~190 emu/gFe) as an equilibrium a phase of Fe. Therefore, higher stabilization of small initial nucleus generated just after the injection of Fe(CO)? should be one of the key issues to achieve much higher M s of Fe NPs.

Fe 3 O 4 nanoparticles coated with an oleic acid as a surfactant were reduced and nitrided by hydrogen plasma and nitrogen–hydrogen mixed plasma generated by 13.56 MHz Radio-Frequency. Fe 3 O 4 nanoparticles could be reduced to α-Fe nanoparticles with small average grain size about 35 nm and with little carbonization by hydrogen plasma treatment. In the nitrogen–hydrogen mixed plasma treatment, it was clarified that the mixture of hydrogen with ratio of 10–30% to nitrogen accelerated nitridation of α-Fe nanoparticles. This effect was derived by generating exited species with higher activity such as NH ions and its radicals. Furthermore, it was also revealed that nitridation behavior strongly depended on average grain size of as-prepared α-Fe nanoparticles. In the plasma nitridation of α-Fe nanoparticles with average grain size of 35 nm, the nitridation was easily promoted with lower input power. This could be attributed to large surface energy of nanoparticles because nanoparticles had wide surface area than bulk materials.

The AMR (anisotropic magnetoresistance) and PHR (planar Hall resistance) contribution was analyzed for fabricated ring type sensor junctions in single and multiring bridge sensors, and their field sensitivity was examined. The voltage profile, i.e. the sum of AMR and PHR effects, reveal anti-symmetric behavior with the magnetic field with small offsets due to the self-balancing of ring arm resistances, but the voltage variations for the external field are additive for all junction components. The field sensitivity of the resistance for a single ring sensor is 9.5 m Ω / Oe , and its value monotonously increased to 102.6 m Ω / Oe for 17 rings with an enhanced active area.

This study reports an alternative method for measuring the magnetoresistance of unpatterned magnetic tunnel junctions similar to the current-in-plane tunneling (CIPT) method. Instead of using microprobes, a series of point contacts with different spacings are coated on the top surface of the junctions and R-H loops at various spacings are then measured by the usual four-point probe method. The values of magnetoresistance and resistance-area products can be obtained by fitting the measured data to the CIPT theoretical model. The test results of two types of junctions were highly similar to those obtained from standard CIPT tools. The proposed method may help to accelerate the process for evaluating the quality of magnetic tunnel junctions when commercial CIPT tools are not accessible.

Assemblies of α-Fe/Au nanoparticles (NPs) with various volume fractions of α-Fe NPs were prepared by simultaneous agglomeration of α-Fe NPs and Au NPs in solution. Only the dipole interaction was affected by the magnetic interaction between α-Fe NPs in this system. The magnetic properties were significantly changed according to the volume fraction of the α-Fe NPs, with a high α-Fe NP volume fraction assembly exhibiting good soft magnetic properties.

The trial for quantitative evaluation of inter-granular coupling in stacked perpendicular recording media is reported. The evaluation is realized by analyzing magnetic domain in relation with microstructure of the cap layer initial growth layer. When the thickness of cap layer is increased, the change of magnetic domain from single magnetic domain to maze magnetic domain can be observed. The critical thickness of the cap layer where the change happened was around 5.1 nm. According to this analysis, when granular layer and cap layer of stacked media are assumed to be single layer, inter-granular coupling with amount of around 2.9 erg/cm2 was obtained which agrees with the simulation result qualitatively.

Fe/Au nanoparticle (NPs) assemblies with various Fe NPs densities were prepared by simultaneous agglomeration of Fe NPs and Au NPs in solution. The density of the Fe NPs was widely controlled below 43% by modifying the mixture ratio of both NP solutions. Only dipole interaction effectively induced magnetic interaction between Fe NPs in this system. With the increase in the volume fractions of Fe NPs from 0.5% to 43%, the collective behavior of magnetic moments of Fe NPs was induced by strong magnetic dipole interactions. As a result, the tendency of soft magnetic properties was found. The diameter analysis revealed that the magnetic dipole interaction energy depending on close-packed assembly consisting of only Fe NPs was 13 times that of the magnetic dipole interaction energy between two Fe NPs. The maximum magnetic dipole field was estimated to be approximately 3-5 × 105 A/m for around 8 nm of the assembly of Fe NPs. This value is much larger than that of the anisotropic field of Fe NPs. Therefore, the magnetic properties in this system were dominated by magnetic dipole interactions.

We demonstrate real time on-chip translocation of bio-functionalized superparamagnetic beads on a silicon surface in a solution using a magnetophoresis technique. The superparamagnetic beads act as biomolecule carriers. Fluorescent-labeled Atto-520 biotin was loaded to streptavidin-coated magnetic beads (Dynabead(®) M-280) by means of ligand-receptor interactions. The magnetic pathways were patterned lithographically such that semi-elliptical Ni(80)Fe(20) elements were arranged sequentially for a few hundred micrometers in length. An external rotating magnetic field was used to drive translational forces on the magnetic beads that were proportional to the product of the field strength and its gradient. The translational force at the curving edge of the pathway element of 6 μm diameter was calculated to be ∼1.2 pN for an applied field of 7.9 kA m(-1). However, the force at the flat edge was calculated to be ∼0.16 pN. The translational force was larger than the drag force and thus allowed the magnetic beads to move in a directional way along the curving edge of the pathway. However, the force was not sufficient to move the beads along the flat edge. The top and bottom curving edge semi-elliptical NiFe pathways were obliquely-arranged on the left and right sides of the converging site, respectively. This caused a central translational force that allowed the converging and diverging of the Atto-520 biotin loaded streptavidin magnetic beads at a particular site.

Monodisperse wustite (core)/spinel (shell) nanocubes with controllable size from 9 to 22 nm were synthesized by the decomposition of iron oleate complex at high temperature. The composition of the nanocubes was confirmed by X-ray diffraction and magnetic analysis, meanwhile the distributions of wustite and spinel phases within the nanocubes were directly observed by high resolution transmission electron microscopy using the dark-field image technique. The core/shell structure is quite unique, in which spinel phase is distributed not only preferentially on the surface, but also in the interior, while almost all of the wustite phase is located in the core of the nanocubes. The formation of wustite is inherent in the decomposition of the iron oleate complex, as indirectly inferred through the detection of a huge quantity of carbon monoxide generated from the reactor.

Monodisperse Fe nanoparticles with a high saturation magnetization were synthesized by thermal decomposition of a newly developed Fe(CO)x−oleylamine reacted precursor. The coordinate bond of the Fe(CO)x−oleylamine reacted precursor is different from conventional Fe(CO)5, and the CO ligands are partially replaced by oleylamine. The coordinate bond of the Fe(CO)x−oleylamine reacted precursor had a large influence on the size of the Fe nanoparticles, and as a result, the nanoparticle diameter could be controlled in the range from 2.3 to 10 nm by changing the reaction temperature and time for the precursor. The saturation magnetizations for 10 and 2.3 nm diameter NPs were 192 and 160 emu/gFe at 5 K. The high MS of the Fe NPs was attributed to lesser amounts of impurities, such as C and O.

MgZnO (magnesium–zinc-oxide) films were grown on (11–20) sapphire substrates and Zn-polar ZnO substrates by plasma enhanced metal-organic chemical vapor deposition (PE-MOCVD) employing microwave-excited plasma. Structural, electrical and optical properties were investigated by X-ray diffraction, atomic force microscope, Hall, transmittance and photoluminescence measurement. The c-axis lattice constant decreases proportionally to an increase in the Mg content of MgxZn1 − xO films. Therefore, this indicates that Mg atoms can be substituted in the Zn sites. Mg contents in films on ZnO substrates increase up to 0.11. In addition, Ga doped ZnO films were grown on (11–20) sapphire substrates. The resistivity of the films on (11–20) sapphire is controlled between 1.2 × 10− 3 Ω cm to 1 Ω cm by changing the process conditions. The overall results indicate the promising potential of this PE-MOCVD method for related (Zn, Mg)O films formation because of the reactivity of the radicals, such as oxygen radicals (O⁎).

Concept of ultra clean (UC) dry-process proposed by present authors in 1990s has been widely used in various thin film fabrication fields from the view point of research as well as volume production stages of electronic devices. Original basic concept for UC-process sometimes are lead to another wrong concept direction, usually caused by a compromise for temporal trial, space limitation, facility, and so on. In this paper, UC technological items originally proposed is briefly reviewed. Influence of vacuum atmosphere by chamber baking, moisture content in process gas lines, outgasing of co-sputtering cathode equipped with magnetic fluid, and particle sources in fore-pump. are re-checked again in research process for perpendicular magnetic recording media.

The uniaxial magnetic anisotropy field, Hk, and g-factor of a c-plane oriented CoPtCr alloy film was quantitatively evaluated by using Q-band (∼34 GHz) ferromagnetic resonance (FMR). An FMR signal with a high signal-to-noise ratio was successfully obtained by using a 2mmφ-Macor holder, which achieves phase matching, a decrease in the background of the signal, and accurate alignment of the external field parallel to the film normal or in-plane. An external field was applied perpendicularly or horizontally to the film, and a single magnetic domain was maintained during the measurements. The measured FMR signals demonstrated that 1) the Hk and g-factor were simultaneously derived for CoPtCr films with Co 75-90 at.%, 2) the magnitude of Hk was supported by that obtained with a Torque magnetometer, and 3) the g-factor increased from 2.11 to 2.19 when Co content was decreased from 85 to 65 at.% and the Cr content was increased from 5 to 20 at.%.

A Faraday-effect system to measure small patterned areas with about a 6-μm sized laser spot was constructed. We found that highly accurate Faraday measurements were possible for a patterned array of nanodots and also obtained the dependence of the Faraday effect on the angle of the sample. Two lasers with wavelengths of 633 nm and 690 nm were close to the local surface plasmon resonance (LSPR) wavelengths of the noble material of Au. A nanodot array, consisting of magnetic materials made up of a Co/Pt multilayer, sandwiched by Au, that was not more than 100 nm in diameter, was fabricated, and the optical and magneto-optical properties were investigated.

We propose a technique for low-gas-pressure (0.6 Pa) deposition of a granular film consisting of Co-based metallic grains and stoichiometric SiO2 grain boundaries by using reactive sputtering. As a typical example, we experimented in forming Co-SiO2 granular films by using dc magnetron sputtering with Ar+O2 mixed gas with a Co92.3Si7.7 (at%) alloy target. Detailed analyses of the composition, microstructure, and magnetic properties of the granular films revealed that: (1) a high deposition rate of 3.4 nm/s was realized under a Si-selective oxidization condition of the surface of the CoSi alloy target. (2) The granular film with an O/Si composition ratio of 2 was successfully achieved under the selective oxidization conditions. (3) c-plane-oriented Co grains with SiO2 grain boundaries were formed on a Ru underlayer. (4) The low-gas-pressure process made it possible to enhance the columnar grown of Co grains without tapering off. These results suggest that the low-gas-pressure process will be effective in improving corrosion and impact resistance even for the current mass-produced perpendicular magnetic recording media with CoPtCr (-SiO2, -TiO2) granular materials.

A relationship between boron (B) diffusion into the MgO barrier and pinhole creation in CoFeB/MgO/CoFeB-magnetic tunnel junctions (MTJs) was investigated. The diffused B in the MgO layer was identified by secondary ion mass spectrometry for the MTJs annealed at 350degC , which provide the giant magnetoresistance (TMR) ratio. The pinhole density, estimated from the statistic distribution of breakdown voltage of the TMR properties, increased as either the thickness or the B content of the CoFeB layer became thicker or higher. These experimental findings imply that the diffused B into the MgO barrier creates pinholes to short-circuit the tunnel conduction, since the amount of diffused B into the MgO barrier might be related to the total amount of the B content in the CoFeB layer. Three different techniques were found to be useful for the reduction of diffused B into the MgO barrier layer; usage of materials having boron affinity for capping layer, decrease of the total amount of B-content in CoFeB layer, and reduction of grain boundaries in the MgO barrier layer.

Nanostructured magnetic tunnel junction rings (MTJ) based on CoFeB free layer were fabricated to investigate the corresponding magnetization reversal processes. The dimensions of the MTJ rings were with outer diameter/linewidth of 1 mum/100 nm and 0.7 mu m/100 nm , respectively, in which the fabrication was carried out by a top-down technique in combination with electron beam lithography, ion-milling and etch-back processes. Double transition processes in magnetoresistance (MR) responses are observed in these nanostructured MTJ rings. With considering the surface roughness, an OOMMF simulation was employed to explore the magnetization evolution. The simulated hysteresis loops are consistent with the MR behaviors, unfolding that the magnetic switching process initiates from the forward to reversed onion states through an incomplete vortex state. This two-step switching behavior is reasonably assumed to be due to a pinning site resulting from the surface roughness in a region of the ring. In addition, a series of in situ magnetic force microscopy images extracted from nanostructured MTJ rings reveals the similar magnetization evolution with the simulated results.

The unidirectional anisotropy constant, JK, of the Mn76Ir24/FM (FM =Ni-Co, Co-Fe, Fe-Ni) bilayers was investigated, systematically changing the FM layer composition. On the (111)-oriented face centered cubic (fcc) Mn-Ir layer, the FM layers were grown in the respective crystal structure, corresponding to their composition. The phase boundaries between fcc and body centered cubic (bcc) structures existed around Co80Fe20 and Fe70Ni30, roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, Ms, of the FM layer showed a broad peak around Fe60Co40 and a shallow dip at Fe70Ni30 , corresponding well with the Slater-Pauling curve of bulk alloys. On the other hand, the JK reached a maximum around Co70Fe30 and showed different variation from that of the Ms against the FM layer composition. Therefore, the correlation plot between JK and Ms did not provide any clear relationship. From the experimental results, we found a general feature that bcc structured FM layers are more likely to induce strong exchange anisotropy than fcc structured FM layers.

In order to open a way to realize ultra high density hard disk (HD) drives with fast operation speed utilizing a spin nano-technology, precisely controlled fabrication technology with nano-scale for head and media according to the required properties is essential from material, processes, and physical view point. In the present paper, correlation between process parameter of ultra clean (UC) dry-process proposed by us and structure and physical property for ferro/antiferro interface, magnetic tunnel junctions and perpendicular magnetic recording media will be widely discussed.

Permeability and its upper limitation frequency of superparamagnetic nanoparticle type magneto-dielectric hybrid material were theoretically and experimentally investigated. The Landau-Lifschitz-Gilbert equation without any interaction between nanoparticles revealed that the blocking resonance frequency was able to exceed the ferromagnetic resonance frequency originating from the intrinsic magnetocrystalline anisotropy field by decreasing particle size, resulting in ultra fast switching of superparamagnetic moment in GHz range. In the case of Fe nanoparticles, the blocking resonance frequency can be increased to 130 GHz by reducing particle size to 1 nm. The experiment results for Fe3O4 and Fe nanoparticle assemblies supported the validity of our calculation results. Thus, superparamagnetic nanoparticle assembly could be promising material for high frequency use over 10 GHz range.

Tailor-made nano-structured spin materials obtained by precisely controlled nano-scale fabrication technologies for use in ultra-high density hard disk drives (HDDs), as well as an understanding of their nanomagnetics, are essential from the view point of materials, processes, and physics. Artificial control of the exchange coupling among ferromagnetic layers through the RKKY interaction (indirect) and direct exchange coupling represented as the exchange bias at the ferromagnetic (FM)/antiferromagnetic (AFM) interface are of great interest and have received significant attention to induce new modulated spin structures in conventional simple FM materials. In particular, soft magnetic under layer (SUL) with strong synthetic antiferromagnetic (SAF) coupling between two adjacent soft magnetic layers, exchange coupled stacked media introducing exchange coupling between FM layers and giant exchange anisotropy at the FM/AFM interface have attracted significant attention from the view point of applications. Within the framework of the present paper, we discuss future technical trends for SUL, granular media and the spin-valve head from the viewpoint of direct and/or indirect exchange coupling based on our recent results.

The saturation magnetization factor, Ms, was investigated for a Co-based alloy-oxide granular film through the quantitative analysis of film composition and phase formation. Using a carbon cap layer deposited directly onto the Co-SiO2 film, both oxidation and carbonization of cobalt caused a reduction in Ms. The composition of the granular film strongly affects Ms, because the Ar gas pressure and discharge power during Co-SiO2 sputtering alters the Co and oxygen content in the film. In particular, for a film fabricated under a high Ar gas pressure such as 8.0 Pa, Ms was significantly decreased, due to the formation of the antiferromagnetic CoO phase.

Monodisperse Fe nanoparticles, with a size ranging from 4 to 10 nm, have been prepared by the thermal decomposition of Fe carbonyl. The influence of oxidation on the magnetization of Fe nanoparticles has been investigated with changing the concentration of oxygen (O2) gas in the glove box, the oxygen atom (O) in ether and water (H2O) in the reaction solution. When the concentration of oxygen in the glove box was more than 5000 ppm, a large decrease of the saturation magnetization (MS) was observed. There was a small decrease of MS when the O/Fe ratio was more than 10. However, in the present experimental condition, the H2O/Fe ratio from 10-2 to 10-5 had little influence on the decrease of MS. MS of Fe nanoparticles was decreased with a smaller particle size. However, MS of the Fe nanoparticles with the size from 7 to 10 nm showed the size-independent behavior. Such results strongly suggested that the decrease of MS cannot be explained by these two factors of oxidization and size effect.

We introduce our recent experimental results for three blocked layers for currently used perpendicular recording media; a recording layer (RL: for recording), a soft magnetic underlayer (SUL: magnetic flux path in writing), and a nonmagnetic intermediate layer (NMIL: underlayer of RL and separation layer between RL and SUL). For the NMIL, uniaxial crystallographic symmetry is an essential requirement for suppression of variant growth of magnetic grains in granular-type RL. From this view point, AlN with wurtzite structure and materials with pseudo-hcp structure, which means fcc structure with stacking faults, were found to be effective. For the SUL, disordered hcp CoIr with negative K u were found to well suppress both spike noise and track erasure due to a wide distribution of magnetic flux under the return yoke in writing and formation of a Neel wall instead of a Bloch wall in the SUL. For the RL, positive-/negative- K u stacked media with incoherent switching mode was found to be effective in order to solve the recent write-ability problem for high K u RL material with high thermal stability. Applying all these items, an advanced medium concept with the stacking structure of “CoPtCr-oxide/CoIr-oxide/CoIr/pseudo-hcp nonmagnetic layer/substrate” is very promising from the view point of (1) switching field reduction of a RL with high K u material, (2) conventional amorphous SUL free, and (3) conventional NMIL free.