A granular-type medium with a highly oriented high-B_s Fe-Co soft underlayer (SUL) and a thin Ru intermediate layer of 4 nm was prepared. It displayed low H_ch (H_c in the hard axis direction) value of 1.1 Oe in the SUL and high H_c of 6.0 kOe in a Co-Pt-TiO₂ recording layer. The recording layer in the medium has a novel granular structure, namely, well-isolated clusters composed of 5-15 pieces of small Co-Pt grains 4-7 nm in diameter segregated by TiO₂. The clusters were oval, with an average area of 213 nm². At present, the shape and arrangement of the clusters cannot be controlled successfully.

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We evaluated the interest of exchanged-biased magnetic materials for integrated toroidal inductors with operating frequency above 1GHz. Exchange-biased magnetic materials enable designers to use closed magnetic cores up to 5GHz, which lead to very high inductance densities. We used simple considerations about DC resistance and gyromagnetic losses in order to estimate the upper limitations of such devices in terms of quality factor. These estimations were used in order to optimize the properties of toroidal magnetic core at 2GHz operating frequency. We used physical parameters based on experimental data for Fe₆₅Co₃₅ ferromagnetic layers coupled with NiMn, NiO or IrMn antiferromagnetic layers. We found that (Fe₆₅Co₃₅/IrMn) multilayers provide the best trade-off for integration and that quality factor limitations can be overcome by the use of thick (≥1μm) multilayer cores. Also, an important issue for high quality factor is to lower the damping parameter (≤0.01). Inductance densities of optimized devices were also calculated and were found to be in most cases higher than for air-core spiral inductors. These results may orientate material developments in order to meet the requirements for future high density microwave inductive devices fabrication.

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The suspension used in a hard disk drive (HDD) mainly supports the magnetic head, which flies at a height of several nanometers over a recording medium. Micro-strip transmission lines for the read and write data signals are installed on the stainless steel of the suspension. The conventional transmission line system uses stainless steel as the base metal, which determines the suspension’s mechanical characteristics. This stainless steel base metal is the dominant contributor to high-frequency signal losses. The base metal signal loss contributions are revealed by electromagnetic field simulation. At high frequencies, a significant amount of power is consumed in the base metal by induced currents and the base metal’s resistance. To reduce the base metal power, we calculated the power consumption, using a copper base metal. Electromagnetic simulation results support our findings on the contributions of the base metal to the signal losses. The results indicate that the thickness of the copper base metal should be more than 2 micrometers. We suggest a new layer structure for the suspension with reduced transmission loss. To confirm that the new layer structure of the transmission line system reduces the transmission loss, measurements were made on sample coupons using copper base metal and coupons using stainless steel base metal. The results clearly show the benefits of using a copper base metal suspension to reduce the transmission loss for suspension interconnects.

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The magnetic nanoparticle assembly is the candidate as a high-frequency magnetic material with isotropic magnetic properties. For practical applications, the dynamic properties of the superparamagnetic nanoparticle assembly, especially the upper limit of frequency for superparamagnetic properties, should be clarified. According to the phenomenological thermal relaxation theory, the upper limit of frequency has been assumed to be the blocking resonance frequency, f_b. However, considering that the superparamagnetic nanoparticle is one of the ferromagnetic materials, the influence of the ferromagnetic resonance of magnetic nanoparticles should be considered in high frequency range near the ferromagnetic resonance frequency, f_r. In this study, in order to clarify the upper limit of frequency for the superparamagnetic properties, we investigated its dynamic magnetic properties at high frequencies close to f_r using Fe₃O₄ nanoparticles. As a result, we found that magnetic nanoparticle assembly can be made to maintain its superparamagnetic properties almost up to approximately f_r by reducing the nanoparticle volume.

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This paper discusses the basic performance of a soft magnetic film as the RF electromagnetic noise suppressor and its application to a LSI chip. The 1.0μm-thick soft magnetic CoNbZr film has been integrated onto a bare LSI chip by RF sputtering method. Magnetic field above the chip has been evaluated by a planar shielded-loop probe from 1 to 2 GHz. The maximum suppression observed was 37.8 dB at 1.12 GHz.

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The dependence of the coercive squareness S* on the recording characteristics of CoPtCr-SiO₂ perpendicular magnetic recording media was investigated. Experimental results showed that the magnetic cluster size d_cl, which is evaluated by observing the ac-erased magnetization pattern of media, decreased with a decrease in S*. Ac-erased noise increased with an increase in S*. At a recording density of 1000 kfci, the S/N of a medium with S* of 0.39 was 5 dB higher than that of a medium with S* of 0.44. The recording magnetization pattern was obtained for a medium with S* of 0.39 at a high recording density of 1300 kfci. Thermal decay of the medium became very small for S* of less than 0.37. It was concluded that S* was effective parameter on the recording characteristics of CoPtCr-SiO₂ perpendicular magnetic recording media.

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The initial permeability of a composite assembly consisting of Fe and Fe₃O₄ particles whose average diameters were 1 μm and 10 nm, respectively, was investigated. It was found that the initial permeability is sensitively dependent on the volume ratio of Fe and Fe₃O₄. The effect is due to a specific structure of the composite assembly whereby Fe₃O₄ particles congregate in air gaps created by Fe particles, reducing the demagnetizing field of Fe particles. This result suggests that ferromagnetic particle assembly consisting of particles with large size difference may be a good high-frequency core material.

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This paper describes the integration of a three-phase current sensor and a zero-phase current sensor via a coaxial bar structure. A coaxial bar structure was therefore used, as it improves the accuracy due to the exclusive use of the zero current transformer (ZCT) and the current sensor. The ZCT’s high-sensitivity magnetic sensor is an MI element, while the current sensor employs a Hall-effect IC. As a result, a sensitivity ratio (the ratio of leakage to residual output) of over 8:1 is obtained for the ZCT, demonstrating the structure’s ability to isolate leakage current. Meanwhile, the linearity error of the three-phase current remains at ±7.9%, well under the ±10% specification. Thus it can be seen that this structure was many advantages, while its sole disadvantage with respect to the conventional ZCT is increased manufacturing cost.

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The high-frequency carrier-type (HFC-type) magnetic field sensor realizes very high sensitivity without any cooling apparatus for the sensor element. An important technique for obtaining high sensitivity is the use of a high-frequency current applied to the sensor element. The maximum sensitivity is obtained at frequencies up to the ferromagnetic resonance of the sensor magnetic thin film; for example, hundreds of megahertz for amorphous CoNbZr films. It is becoming important to study high-frequency circuits for driving HFC-type magnetic field sensors in order to achieve any practical applications such as nondestructive testing or biomedical applications. In this paper, a method of reflection signal measurement is studied in order to measure alternating magnetic fields with very small magnitude. A theoretical formula based on an equivalent circuit model was obtained for the relation between the sensor property and measurement sensitivity. An experimental confirmation was carried out by comparing this formula with experimental results. Finally, the measurement of a small alternating magnetic fields was effected by combining reflection signal measurement with the carrier-suppressing method.

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Magnetic materials suitable for hyperthermia are discussed taking their magnetic properties, practical limitations of treatment conditions and instrumentation into consideration. The experimental results suggest that either ferromagnetic particle with very low anisotropy constant or superparamagnetic particles with a moderate anisotropy constant are suitable. Considering the magnetic- and biocompatibility of the particles, superparamagnetic Fe₃O₄ with a diameter of 11-13 nm is considered most appropriate. Fe₃O₄ particles of various diameters were successfully synthesized using coprecipitation and thermal decomposition and their heating rates were experimentally verified. The heat generated depends on the particle diameter and solid concentration. The temperature of 3 ml Fe₃O₄ suspension with an average particle diameter of 14 nm and solid concentration of 4 wt.% rose from room temperature to 100 °C, when it was exposed to an ac magnetic field strength and frequency of 3.2 kA/m and 600 kHz respectively for 10 minutes.

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