We investigated the head noise in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads through experiments and calculations. We fabricated CPP-GMR heads having a current-screen layer (a nano-oxide layer with a confined current path) whose track width and sensor height is 80 nms. A significant increase in head noise caused by spin-torque was observed when the sensing current was negative. However, spin-torque noise was not observed at a sensing voltage of ±120 mV when the heads had an orthogonal direction for the free and the reference layer. Therefore, spin-torque noise was not a fatal issue for current-screen-type CPP-GMR heads, regardless of whether the sensing current was positive or negative. Our calculations indicate that the head noise in the current-screen-type CPP-GMR heads is 1/6 less than that in the all-metallic type at a sensing voltage of ±120 mV with a peak asymmetry of ∼0%. Even when the peak asymmetry is ∼0%, in the all-metallic type, the spin-torque noise is induced by the non-uniform alignment of the magnetization from the current-induced field. Accordingly, current-screen-type CPP-GMR heads have an advantage over the all-metallic type from the viewpoint of SNR.

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Four types of perpendicular magnetic recording media were compared using simulations to determine the most suitable candidate for use in future, high density storage products. Assuming a 75 nm track pitch, the minimum K_u values for no adjacent track erasure were determined. Roll-off curves provided an estimate of the maximum linear density. Exchange-coupled composite media appeared to be the most promising candidates, due to their high resolution and SNR and, if produced under well controlled conditions, could support areal densities of 600 Gb/in².

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Mixtures of γ'- Fe₄N/Fe₃O₄ were prepared from Fe₃O₄ nano-particles in NH₃+H₂ mixed gases. By controlling the sintering temperature and time, various samples of (Fe₄N)_x(Fe₃O₄)_1-x with values of x between 0 and 1 were successfully obtained. The magnetization σ and the magnetic parameters of saturation magnetization σ_s , remanent magnetization σ_r and coercive force H_c could be explained by the non-interactive co-existence of γ'- Fe₄N and Fe₃O₄ phase in each sample. The temperature and field dependences of the resistivity showed that good tunneling magneto-resistance (TMR) junctions were formed at x = 0.34-0.37 near and below the percolation threshold of x_c = 0.4. A large magneto-resistance ratio of -8% was observed for a sample with x = 0.34 at room temperature. The effective TMR barriers in this system are assumed to beγ(orα)-Fe₂O₃ or Fe_nN (n = 2 or 3) in the grain boundaries of Fe₃O₄/Fe₃O₄, Fe₃O₄/Fe₄N and Fe₄N/Fe₄N.

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This paper describes a new application of a spin-valve-type giant magnetoresistance sensor in the biomedical field. The hyperthermia treatment, based on the hysteresis loss of magnetite under external ac fields, requires determination of the content density of magnetite injected inside the body to control the heat capacity. We propose a low-invasive methodology to estimate the density of magnetite by measuring magnetic fields inside the cavity. For this purpose, we investigated the relationship between the density of magnetite and the magnetic fields, and developed a needle-type magnetic probe with a giant magnetoresistance sensor for low-invasive measurement. The experimental results demonstrate the possibility of estimating the density of magnetite injected into the body.

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It is necessary to improve the SPT head field gradient to achieve an area recording density of 1 Tbit/inch2 in perpendicular magnetic recording. What influence the head field gradient has on medium noise is, however, not clear and we investigated this by numerical calculations using a micromagnetic simulator in this study. we especially focused on the relationships between jitter in transitions, deviation in the transition slope (πa), and the head field gradient. As a result, we found the jitter and deviation in πa decrease with a steeper head field gradient, which results in a decrease in recording media noise.

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We analyzed the mag-noise of fabricated CIP-GMR and TMR heads to clarify the properties of mag-noise and what influence it had on the head signal-to-noise ratio (SNR). We clarified that the measured mag-noise of CIP-GMR heads is quantitatively consistent with the calculated mag-noise. We found that when the MR ratio is small, the head-amp SNR increases as the MR ratio increases. The head-amp SNR saturates at a certain value of “mag-noise SNR” as the MR ratio increases. When the read head becomes small, the mag-noise SNR decreases. This is due to a large thermal fluctuation of the magnetization “mag-noise” in heads with a small volume. The mag-noise needs to be reduced to achieve high head-amp SNR. The volume of the free layer must be maintained at the highest possible level to reduce mag-noise.

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The influence of saturation magnetization of a master disk on its perpendicular magnetic contact duplication characteristics was investigated by using the two-dimensional finite element method. An investigation of bit printing (where the duplication field is applied in a direction perpendicular to the disk plane) and edge printing (where the duplication field is applied in an in-plane direction), revealed the following points. In bit printing, the influence of saturation magnetization of a master disk on perpendicular duplication characteristics is very small when the duplication field is nearly equal to the coercivity of the slave (4 kOe), which is the optimum field. On the other hand, in edge printing, the duplication characteristics are better for high saturation magnetization of a master disk than for low saturation magnetization. The cause of this difference in the influence of saturation magnetization of a master disk is the demagnetizing field of the master. In bit printing, the influence of saturation magnetization of a master disk is small because of the high demagnetizing field coefficient, while in edge printing, the influence of saturation magnetization of a master disk is large because of the low demagnetizing field coefficient.

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We investigated enhanced in-stack longitudinal biasing, which was newly designed to strengthen longitudinal biasing by investing the antiferromagnetic exchange coupling, H_{int, LBL}, in a spacer between a free and a ferromagnetic layer of the in-stack biasing layers. Micromagnetic simulation showed that H_{int, LBL} diminishes hysteresis and improves the linearity of a transfer curve through cooperation with a magnetostatic coupling effect at the sensor edges. We found that CPP-GMR sensors with various H_{int, LBL} values and magnetostatic coupling effects were formed by adjusting the thickness of the spacer and the ferromagnetic layer. Transfer curve measurements for fabricated head devices using such CPP-GMR sensors revealed that H_{int, LBL} played an importance role in diminishing hysteresis in transfer curves, and that higher output was obtained at moderate H_{int, LBL} values. Consequently, the enhanced in-stack longitudinal biasing was demonstrated to have both higher stability and higher output.

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The relationship between the temperature coefficient of coercivity {α(H_cJ)} and the ratio S_v to H_cJ was investigated for M-type Sr-ferrite substituted by La and Co (Sr_1-xLa_xFe_12-xCo_xO₁₉) in the temperature range from 173to 423 K, where S_v and H_cJ were the magnetic viscosity constant, which essentially determined the thermal stability, and the coercivity, respectively. The value of x was varied between 0, 0.3, and 0.6. The value of α(H_cJ) for x = 0 with S_v/H_cJ = 0.038 was 0.35%/K, whereas that for x = 0.3 with S_v/H_cJ = 0.034 was 0.1%/K at 298 K. Furthermore, α(H_cJ) for x = 0.6 with S_v/H_cJ = 0.026 was almost zero at 298 K. The following relation of α(H_cJ) to S_v/H_cJ was also found to hold in these M-type Sr-ferrite magnets substituted by La and Co, as in the case of Nd-Dy-Fe-Al-B-Sn sintered magnets. α(H_cJ) was expressed as {28.5k − (δE/δT)_H}/kT ·(S_v/H_cJ), where, k, E, T, and H were the Boltzmann constant, the activation energy, the absolute temperature, and the magnetic field, respectively. The equation shows that smallerS_v/H_cJ values are necessary for ensuring small α(H_cJ) values, which are very important for permanent magnets.

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Melt-spun ribbons made from low-rare-earth Pr-Fe-Co-Ti-Nb-Y-B system alloys were prepared by thesingle-roller rapid-quenching method, and the effects of composition, wheel velocity, and heat-treatment on their magnetic properties were investigated. It was found that the coercivity in Pr_3.5Fe_79-xCo₈Ti_1.5Nb₁Y_xB₇ ribbon increases with the amount of Y. In Pr_3.5Fe_78.5Co₈Ti_1.5Nb₁Y_0.5B₇ alloy ribbons, the magnetic properties were obtained (BH)_max = 109.6 kJ/m³, J_r = 1.29 T, H_cJ = 249.6 kA/m, and H_cB = 209.6 kA/m. The temperature coefficients of J_r and H_cJ of Pr_3.5Fe₇₈Co₈Ti_1.5Nb₁Y₁B₇ alloy ribbons were α(J_r)_ave = − 0.038%/°C and α(H_cJ) = − 0.390%/°C, respectively. The irreversible loss of an isotropic compression molding Pr_3.5Fe_78.5Co₈Ti_1.5Nb₁Y_0.5B₇ bonded magnet prepared by usingribbons annealed at 675°C for 5 min is −2.0% at 75°C, and this value was almost the same as that for an alnico bonded magnet.

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Magnetization reversal in nano-structured Py/Cu/Py rings was investigated in terms of the giant magnetoresistance effect (GMR). The magnetization reversal was well regulated when a magnetic field was applied parallel to the arm axis, and the GMR effect reached about 1.5%. When the external magnetic field was tilted from the arm-plane, pinning (or depinning) of the domain wall was observed in both wire and rings. This demonstrates that the GMR of a tri-layered structure is sufficiently sensitive to detect pinning (or deppining) of domain-wall motion in a nano-scaled ring, indicating that the present ring system is promising as a means for measuring the interference of electron motion affected by inhomogeneous magnetization distribution.

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Ni/Pt quadrilayers were deposited by excimer laser ablation in a sequence of Ni/Pt/Ni/Pt and a reversed sequence of Pt/Ni/Pt/Ni. Perpendicular magnetization was observed in Ni/Pt/Ni/Pt films, while in-plane magnetization was observed in Pt/Ni/Pt/Ni films. The perpendicular magnetic anisotropy observed in Ni/Pt/Ni/Pt films is ascribed to Ni₃Pt generated by an interlayer mixing.

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We report evidence of interlayer exchange coupling (IEC) through a thin MgO layer between Fe₃O₄ and Fe layers. Both interfaces of Fe₃O₄ /MgO and MgO /Fe were atomically flat according to RHEED analysis. The IEC of antiferromagnetically favorable interaction was observed in the trilayers with the MgO thickness from 10 to 2 Å. The coupling strength J₁ became larger with decreasing the MgO thickness, and the maximum value was estimated to be −2.7 erg/cm² at t_MgO=4 Å which was comparable with some of the strongest IEC’s for transition-metal spacers such as Ru, Rh for Co and so on. In addition, this J₁ was much larger than that observed in Fe/MgO/Fe system, suggesting that the IEC by tunneling mechanism are strongly dependent on electronic structure of magnetic materials.

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According to Julliere’s model, magnetic tunnel junctions (MTJs) with half-metallic electrodes lead to a large tunnel magnetoresistance (TMR) ratio. A Co₂MnSi Heusler alloy is theoretically expected to exhibit half-metallicity. We fabricated (110)-oriented epitaxial Co₂MnSi electrodes on sapphire substrates using W and Ta/W/Cr buffer layers. With the W buffer layer, we found that the W and Co₂MnSi layers formed a twin structure. However, with the Ta/W/Cr multi-buffer layers, we succeeded in fabricating a high-quality Co₂MnSi(110) epitaxial electrode. We fabricated a MTJ with the high-quality Co₂MnSi(110) electrode and investigated TMR effects in the MTJ. As a result, we observed a TMR ratio of about 40% at room temperature and 120% at 2 K.

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Investigation of current induced magnetization switching properties in nanosecond regime in ferromagnetic tunnel junctions is increasing its necessity for realization of fast operating magnetic random access memory (MRAM). In this work, appropriate electrode structures for such a measurement were investigated. The electrode structure was designed as a typical coplanar waveguide with 50Ω characteristic impedance (structure A). Although the buffer layer thickness and the distance between the signal line and the ground line were altered, the large frequency dependenceof the insertion loss was not improved. The revised structure (structure B) with small cross section area between the signal line and the ground line showed very flat transmission properties over 20 GHz. Consequently, submicron sized Ta/FeNi/IrMn/CoFe/Ru/CoFeB/MgO/CoFeB/Ta/Ru ferromagnetic tunnel junctions with the electrodes of structure Bwere fabricated. The tunnel magnetoresistance ratio was 108 % after annealing at 250 °C.

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The dependence of critical current density (J_c) for current-induced magnetization switching (CIMS) on external magnetic fields applied along the hard axis of a free layer (H_hard) was investigated in CoFeB/MgO/CoFeB magnetic tunnel junctions. The J_c and the intrinsic current density (J_c0), which is derived from the dependence of J_c on pulse duration, decreased as |H_hard| increased. As the reduction ratios of J_c0 while applying H_hard depend on the anisotropy field of junctions (H_k), the initial angle of magnetization, affected by H_hard and H_k, is related to the reduction in J_c0. These results are discussed in terms of the energy barrier for CIMS and the spin transfer efficiency.

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A magnetoplated wire (MPW) is a copper wire whose circumference is plated with a magnetic thin film. The effects of using an MPW are a decrease in the proximity effect, an increase in inductance, and generation of more flux than when copper wire is used. The ratio of the linearity range L_o to the outer coil diameter D of the eddy current displacement sensor is extended by using MPW for the coil. However, the relationship between the Fe film thickness of MPWs and the linearity range characteristic has not been clarified. We measured the output voltage characteristics of MPW coils with Fe film thicknesses of 0.7 μm, 1 μm, and 1.1 μm, and calculated L_o/D ratios. As a result, we found that the L_o/D ratio of an MPW coil with a 1-μm-thick Fe film had a maximum value of 0.66.

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It was found that Li-Zn-Cu ferrite has remarkable temperature dependence of complex permeability which conformed to an exponential function. In order to apply the Li-Zn-Cu ferrite to the temperature sensor devices, the fundamental investigation was executed with a LC resonant circuit using Li-Zn-Cu ferrite and capacitor. Its impedance spectra greatly depended on atmosphere temperature, and such dependence was revolved by a simple approximate line of an exponential or a linear function. Consequently, the authors suggested that a micro-chip temperature sensor device will be obtained by using Li-Zn-Cu ferrite which has both superior temperature sensitivity and high-frequency performance.

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Efficiency improvement and cost reduction are important requirements for the design of DC-DC converters. Zero voltage switching is one good solution for increasing efficiency, and an active clamp circuit is useful for implementing it effectively. A cost reduction could be achieved by devising a new transformer that functions not only as a conventional transformer but also as a DC reactor that reduces the switching device peak current. However, no design methodology for such a transformer has yet been established. This paper proposes a novel method that simulates the operations of a DC-DC converter and estimates the magnetic flux distribution in the new transformer. The magnetic circuit of the transformer is transformed into its equivalent electric circuit with two ideal transformers. Circuit simulation results agree fairly well with experimental ones.

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Sasaki et al. observed an iron ball suspended in midair below five iron balls, in a plastic box attracted by a permanent magnet [J. Appl. Phys. 95, No.4, 2090 (2004)]. The magnetic field exerted by the permanent magnet and five iron balls is analyzed theoretically and experimentally. The position of the suspended iron ball is also calculated from the field, and the results agree well with experimental findings. Without a plastic box, however, an iron ball is found to move away along the magnet ring axis, which is consistent with Earnshaw’s theorem.

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A switched reluctance (SR) motor has a doubly salient pole structure. It has some merits in comparison with other motors: simple structure, robustness, low cost, and capability for high-speed rotation. However, there are problems of torque ripple and acoustic noise. In addition, the power-to-weight ratio and efficiency need to be improved. Recent advances in power electronics technology allow meticulous current control to reduce the ripple and noise. On the other hand, the core material and motor structure must be further examined to improve the power-to-weight ratio and efficiency. This paper presents a novel SR motor with permanent magnets between the stator poles. The characteristics of the proposed motor are calculated by using a finite element method (FEM) and compared with those of conventional SR motors. The availability of the proposed SR motor is then clarified experimentally.

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Stepping motors are widely used in office automation (OA) and facility automation (FA) because they are simple and easy to control. However, it is difficult to calculate their dynamic characteristics because their rotational motion is discontinuous. This paper presents electromagnetic and motion coupled analysis of a variable reluctance (VR) stepping motor. First, a reluctance network analysis (RNA) model of the VR stepping motor is presented. Next, measured Coulomb and viscous friction of the motor are discussed to take the influence of friction in the coupled analysis into account. The RNA model is combined with the motor’s drive circuit and motion-calculation circuits in SPICE. The dynamic characteristics of the VR stepping motor could be accurately calculated using the proposed coupled analysis model.

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This paper describes a cantilevered magnetic actuator driven by magnetostriction in a low magnetic field. The dimensions of the two-layered actuator were 1 mm × 5 mm. Amorphous FeSiB was used as the magnetostrictive material, and polymide film with a thickness of 30 μm was used as the substrate for the experiment. The theoretical formulas for the amount of the displacement and for the force of the actuator were obtained. The calculated values of the displacement and the force were compared with experimental values, and were found to be in close agreement with them. It was also found that the obtained formulas were suitable for the design of the cantilevered actuator.

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Magnetic gearing has many advantages. For example, it has noncontact meshing, is noiseless, oil-free and maintenance-free. We tried various types of magnetic gearing and investigated their performance characteristics. All magnetic gearing has some typical characteristics. A spur gear has internal and external meshing by changing the center distance of the gear. Recently, we tested the magnetic planetary gearing for use in wind power generators.
This report describes performance characteristics, specifically, transmitted power efficiency and load-carrying capacity, of planetary gearing with full depth using the rare earth permanent magnet NdFeB. The main test parameters were the number of planet gears, input speed, and differential speed.
Our results clarified the performance characteristics of magnetic planetary and differential gearing. The gear system was able to maintain a constant output speed for the sun gears by a differential operating outer ring gear despite a random input speed for the planetary gear. This system would be effective for wind power generators.

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Recently, magnetic planetary and differential gearing has been developed for wind power generators. This report describes magnetic planetary and differential gearing having two sun gears, a configuration referred to as twin sun gears. This magnetic gearing decreased the thrust force and increased the transmitted torque with a magnetic closed circuit between the sun and planet gears. However, this gearing slipped down at low transmitted torque when four planets were used in the magnetic planetary gear. This was due to a meshing zone between the sun and planet gears or the sun and outer ring gears. The main contact zone is narrow and not an excited effective force.
As a result, by only moving the phase shift angle of two of the four planets by 5°, the transmitted torque is increased by more than 50%.

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This paper proposes a new cable-less magnetic micro-actuator that operates on the inertia force of a one-degree-of-freedom model by using electromagnetic force. It has optimal design for the inclination angle of the one-degree-of-freedom model and an appropriate selection of suction force at the actuator support. The mechanical dc-ac inverter incorporates a cantilever beam and the one-degree-of-freedom model that switches under electromagnetic force. The volume of the prototype cable-less micro-actuator with three dry cells was about 1.5 cm³. Our experimental results revealed that the actuator could move vertically at 18.8 mm/s when the duty ratio of the produced voltage was 46.2%. The operating time for a duty ratio of 26.5% was 36 min, providing a total operating range of 26 m for horizontal travel or 15.5 m for vertical travel. The actuator has many possible applications in small pipe inspections, repairs, and machining because it can move on magnetic substances.

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