Micromagnetic simulation has been used to calculate magnetization distribution and dynamics in nanoscale magnetic materials. A method reducing the calculation time is needed because it requires prolonged periods for calculation. We propose a method of calculation to reduce the calculation time with a graphics processing unit (GPU). The speed with the proposed method is fifteen times faster than that with the conventional one with a CPU at maximum. Because a GPU has very fast single precision calculation units and relatively slow double precision calculation units, it is necessary to use single precision units for fast calculation with a GPU. The difference between the results of one calculation with single and double precision units is very small. However, this difference accumulates in simulations. There is a possibility the results may change due to this difference. This effect is investigated with static and dynamic simulations with a vortex structure in a nanodot. The relative differences in static and dynamic calculations are 10^-6 and 0.04, respectively. These differences are sufficiently small for practical simulations. Therefore, a method with a GPU is useful to obtain micromagnetic simulations of vortex core motion.

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  Bit patterned media (BPM) recording has been studied as one of the possible candidates for future perpendicular magnetic recording. For the basic R/W evaluating of BPM system, small areas of media samples are suitable in most cases. A static tester is useful to measure such small areas of BPM to evaluate their read-write characteristics. We have prepared a tester, and found that measurement with write synchronization was possible. We report here the experimental results obtained from investigations into the write-margin measurements of the BPM. A position margin of 50∼71 % of the dot period was confirmed, and we found the margin loss was related to the formation of multi-domain dots.

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  A media design for thermally assisted magnetic recording was simulated, with particular consideration given to data stability during the cooling process after writing, and data stability in the adjacent tracks at each time of rewriting. The areal density was assumed to be 2 Tbpsi. The targeted media and the recording type were of the granular media and the thermal gradient, respectively. Under the conditions that (1) the thermal stability factor at 330 K should be more than 60, (2) the cooling rate after writing should be less than 10 K/nm, and (3) the information on the adjacent tracks should be preserved at temperatures up to 330 K under the head field, it was found that an anisotropy constant of about 1-2 × 10⁷ erg/cm³ is required for media with a thickness of 5-15 nm. Increasing the thickness is an effective means of decreasing the anisotropy constant, despite a decrease in the Curie temperature. By increasing the thickness, the limiting factor shifts from the thermal stability factor (1) to the information on the adjacent tracks (3). If the head field at the adjacent tracks can be decreased, the limiting factor for thick media will shift from the information on the adjacent tracks (3) to the cooling rate (2).

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  The influence of crystal orientation on the magnetic properties of 3% Si-Fe single crystals was investigated by measuring iron losses and observing magnetic domain patterns in TiN-coated and chemically polished specimens. The iron loss was reduced in a TiN-coated sample whose β angle was less than 4 degrees and increased in a sample whose β angle was larger than 4 degrees. The improvement in the iron loss was due to a decrease in eddy current loss that corresponded to a decrease in the magnetic domain width caused by the tensile stress effect of the TiN coating. In the sample whose β angle was larger than 4 degrees, the eddy current loss did not change due to the TiN coating because the magnetic domain width was almost the same under alternate magnetization. Additionally, hysteresis loss increased because of increase magnetic coercive force.

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  The magnetization and demagnetization processes in the 11 types of Dy-free Nd-Fe-B sintered magnets based on the NEOMAX-50 type magnets were investigated using the step method that is our original method of measurements and analyses of minor loops with step by step increased maximum applied fields. With the decrease in average grain sizes (D_AVE), the single-domain grains (SDG) volume fraction increased and inversely that of the saturable multi-domain grains (SMDG) decreased, the changing behavior was monotonous. The dependence of coercivities on the D_AVE, however, showed clear change around D_AVE=3.0 μm, from the increase with decreasing D_AVE to the opposite relationship less than D_AVE ‹ 3.0 μm. We consider that the changing in the relationships should come from the ratios of the domain width (W_D) to the D_AVE, and conclude that the well cutting of magnetic interactions at the grain boundaries should realize the coercivity of _μ0H_C ≈ 2 T in fine grain size Dy-free samples.

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  Bi_2.5Y_0.5Fe₅O₁₂ and Bi₃Fe₅O₁₂ thin films were prepared on Gd₃Ga₅O₁₂ substrates using a metal organic decomposition (MOD) method as magneto-optical indicators (MOI) to visualize the distribution of magnetic flux. Their structural and magneto-optical properties were studied. The Faraday rotations of Bi_2.5Y_0.5Fe₅O₁₂ and Bi₃Fe₅O₁₂ thin films were 15.5 μm/degrees at a wavelength of 524 nm for the former and 23.0 μm/degrees at 530 nm for the latter. We found that these films prepared by MOD were suitable for MOI, because of their large Faraday rotation, in-plane magnetic anisotropy, and extinct magnetic domain structures.

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  We fabricated magnetophotonic crystals (MPCs) used in electric field driven magneto-optical spatial light modulators (e-MOSLMs) with a paramagnetic garnet, which is transparent in the blue wavelength region and has a sufficient Faraday rotation angle for several applications including hologram data storage, three-dimensional displays, and optical correlators. The e-MOSLM is composed of two dielectric mirrors and one defect layer comprising magneto-optic (MO) material and electro-optic (EO) material, which controls the localized wavelength due to the change in the refractive index. The e-MOSLM utilizing MPCs allows us to use thin film paramagnetic materials, i.e., a terbium gallium garnet (TGG) and a terbium aluminum garnet (TAG), instead of a rare-earth iron garnet (RIG) because the direction of magnetization does not need to be controlled in the MO layer. We fabricated TGG thin films and MPCs with the structure of a SGGG substrate/(Ta₂O₅/SiO₂)⁶/TGG/(SiO₂/Ta₂O₅)⁶ by rf-magnetron sputtering. The sample produced localized peaks in the photonic band gap of the transmission spectra and an enhanced Faraday rotation angle. These results indicate that e-MOSLMs comprising an MPC with a TGG defect layer can be useful devices at the blue wavelength.

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  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.

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  The magnetic properties of Fe nanoparticle-based bulk material, which was fabricated with a through post-annealing process under vacuum conditions, were investigated to develop a new Fe-based soft magnetic material. Fe nanoparticles (NPs) retained uniform-size and narrow-size distributions at a post annealing temperature (Ta) of 473K. Their assembly was self organized, which is referred to as nano size periodicity. Soft magnetic properties were obtained at 1.7±0.2 Oe of coercivity at room temperature, and 179 emu/g_net of saturation magnetization was obtained at 5K by using Ta=473 K . Above Ta= 573 K, saturation magnetization decreased and coercivity was increased. High temperature post-annealing above 523 K caused NP growth and the transformation α-Fe to Fe₃C, which originated from carbon impurities in the surfactant. Thus, the low-temperature post-annealing process plays an important role in achieving high performance soft magnetic properties using Fe NP-based bulk material by desorbing of surfactants without the formation of iron carbide.

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  We investigated the reliability of a simple method for manipulating the magnetic vortex chirality using an in-plane magnetic feld. Thevortex chirality stabilized in a regular pentagonal Permalloy nanomagnet is controlled by the direction of the magnetic field. The vortex alters the chirality with a period of 36° because of symmetry. However, we found that, when the direction of the magnetic feld was closed to the boundary of the transition in chirality, this simple method could not be applied because of the less difference of nucleation energy between chiralities.

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  The response to in-plane applied field of magnetizations in synthetic antiferromagnets composed of two identical ferromagnetic layers having a uniaxial anisotropy has been investigated by analytic/numeric method using the Stoner-Wohlfarth single-domain model. Evolution of the stable magnetization configuration phase diagram with increasing exchange coupling between the ferromagnetic layers is presented, in both two-magnetization-angle-parameter space and in-plane-field space. It is revealed that the phase diagram in the angular-space is composed of magnetization-configuration phase domains classified into three groups, some of which can be further classified into sub-groups. A complicated five-step diagram evolution in the field space is shown to be explained comprehensively through the comparison to that in the angular space. It is also revealed that the non-parallel magnetization configurations can be classified into two, one existing for all exchange coupling strengths and the other starting to appear at a critical value. Toggle switching is shown to start to be available after the 4-th evolution. The derivations of the analytic formulas for the characteristic points in the phase diagrams are also presented.

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  We investigated in detail the crystallinity and transport properties of fully epitaxial Fe(001)/spinel MgAl₂O₄(001)/Fe(001) magnetic tunnel junctions (MTJs). We found that high (001) orientation (< 0.20°) and an extremely flat surface (averaged roughness ∼ 0.09 nm) were achieved in the optimized Fe/MgAl₂O₄/Fe MTJs exhibiting tunnel magnetoresistance (TMR) of more than 100% as well as excellent bias-voltage dependence. The misfit dislocation density of the MgAl₂O₄/Fe interface was determined to be precisely 5 × 10^-2 nm^-1. In addition, post-annealing in the preparation process has a significant influence on TMR and its bias-voltage dependence in Fe/MgAl₂O₄/Fe MTJs, suggesting that the crystallinity of the MgAl₂O₄ barrier interfaces was effectively improved by postannealing of each layer. The results provided useful information for understanding and developing epitaxial MTJs with coherent tunneling.

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  To understand the effects of biaxial strain on the properties of a Heusler alloy, the structural and electrical properties of Fe₂VSi thin films in various strain states were investigated. Epitaxial Fe₂VSi films were subjected to tensile strain on MgAl₂O₄ and MgO substrates, while films on LaAlO₃ were given compressive strain. The Néel temperatures varied systematically depending on c/a in a wide range from tensile to compressive strain states. Structural characterization revealed that the films on MgO and LaAlO₃ were in almost relaxed-strain states with misfit dislocation at the substrate / Fe₂VSi interfaces. The ideal resistivity of films (∼20 nm) on MgO revealed large dependence on temperature (ρ-ρ₀ ∝T^N ≥ T²), which is related to the existence of misfit dislocations with their large lattice mismatches.

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  We have studied Peltier coefficients (Π) in Cu-Ni-based submicron-sized junctions with two different Cu-Ni layer thicknesses (t_Cu-Nis) and buffer materials. The junctions for both buffer materials, Ru and Ta, showed enhanced Π when t_Cu-Ni was fixed at 100 nm. We obtained Πs of ∼245 and ∼480 mV for Ru and Ta-buffered junctions, respectively. These were more than 10 times larger than that estimated from the bulk Cu-Ni/Ru(or Au) contact (∼ 11 mV). Concerning the t_Cu-Ni dependence of Π for Ru-buffered junctions, they did not demonstrate any enhanced Π at t_Cu-Ni = 30 nm, which was only ∼6.5 mV, while they showed large Πs at t_Cu-Ni = 100 nm as mentioned above. The results suggested that the enhanced Π acquired sufficient volume of each columnar grain in the Cu-Ni layer to induce the inner-grain inhomogeneity.

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  Demands for dc current sensors have recently been increasing. We developed a hysteresis-free current sensor based on a magnetic bridge by using magnetic fluid as a core material, which is super-paramagnetic and has no magnetic hysteresis. In the experiment, the core size was 42 mm in outer diameter, 28 mm in inner diameter, and 3.5 mm in height. The sensor became saturated at ±200 A input current, but no hysteresis was observed throughout the whole range of measurement. In a measurement range of ±20 A, the sensitivity was 0.603 mV/A, the linearity error was ±0.7 %, and the temperature error characteristics were -0.2 %/°C from 0 °C to 30 °C. This hysteresis-free current sensor would be useful in energy-efficient power systems to enable accurate current monitoring, especially in dc power-distribution systems.

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  Electromagnetic interference(EMI) is a serious problem in integrated circuits, and many studies on EMI measurement have been done. Generally, a metal probe is used to measure the electromagnetic wave, however, the metal probe disarranges the state of the electromagnetic wave. Another problem is that high resolution measurement is required. Because the operating frequency of the integrated circuit is very high, a very high frequency magnetic field measurement is required. To solve these problems, measurement using the magneto-optical effect is suitable. Magneto-optical materials have a smaller effect than that using the metal probe, and the optical measurement is advantageous in the undeformed magnetic field measurement. To achieve high resolution we used a pulsed laser with a short pulse width and propose the stroboscopic method of measurement. The stroboscopic method is advantageous in that it is possible to measure a magnetic field in an arbitrary phase because it synchronizes laser pulses with the magnetic field. In measurements of magnetic field waveforms, it was clarified that the stroboscopic method was effective for measuring magnetic field waveforms and distributions at an arbitrary phase.

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  We have developed a transmission-line thin-film sensor by using a ceramic substrate with high permittivity (ε_r=115) and amorphous CoNbZr film. We fabricated a coplanar line type sensor with CoNbZr film (25 x 25 mm and 8 μm in thickness). A phase change of 150 degrees/Oe was achieved, which was more than a 10-fold improvement compared with that for a previously reported sensor using a Teflon substrate (ε_r=5). The signal-to-noise ratio improved and was about 3 times higher than that for the previous sensor when a small AC field (270 pT, 11 Hz) was applied.

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  Interior permanent magnet (IPM) motors have large amounts of torque and are much more efficient than other motors since they can utilize both magnetic torque and reluctance torque together. However, the current vector needs to be controlled to utilize both torques effectively. Hence, it is necessary to design and analyze motor drive circuits in addition to motor structures. We propose a method of calculating the characteristics of an IPM motor based on reluctance network analysis (RNA). The proposed RNA model of the IPM motor consists of multiple nonlinear reluctances and magnetomotive forces, which periodically change with the rotor position so that the rotary motion of the rotor can be simulated. The RNA model can easily be coupled with an electric circuit and mechanical system. Therefore, it is suitable for designing and analyzing the IPM motor-drive system. The validity of the proposed method is proved by comparing its results with those obtained from the finite element method (FEM).

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  This paper discusses what effects of magnetization saturation has on the cogging torques of 12-pole motors with 9-slot cores, using Nd-Fe-B ring magnets. The core flux density is changed with sintered, bonded and thermally demagnetized magnets. Cogging torques especially 36th harmonics, increase with the higher flux densities of the core at the teeth, and then magnetic saturation makes cogging torques larger. However, the harmonics of core flux densities are low although 36th cogging torques are related to the harmonics of core flux. Therefore, increasing cogging torques with magnetization saturations may be affected by the reduction in permeabilities due to magnetic saturations and the generation of core-flux density harmonics.

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  This paper discusses the stress dependence of the magnetizing properties of non-oriented (NO) electrical steel sheets. Compressive stresses deteriorate the magnetic properties at low flux density, but improve them at high flux density, where magnetization rotations are induced. Tensile stresses reduce iron losses in all types of sheets, i.e., 50A290, 50A470, 50A1300, and SPCC, at low flux density and those of high-grade 50A290 and middle-grade 50A470 at high flux density, but iron losses of low-grade 50A1300 and SPCC increase at high flux density. These stress dependences may depend on magnetization behaviors (domain wall displacement and magnetization rotation) and magnetostriction. Domain wall displacement mainly occurs at lower flux density, where tensile stresses improve magnetic properties and compressive stresses deteriorate them because magnetic domains that orient the magnetization to ‹100› and λ100 are positive. However, magnetizations rotate to exciting directions at high flux density, with the result that the ‹111› component of magnetization is induced. Here, compressive stresses improve magnetizing properties because λ111 is negative, whereas tensile stresses decrease the iron losses in 50A290 that has large and 50A470 that has middle crystalline grains and increase the iron losses in 50A1300 and SPCC that have small crystalline grains. Therefore, the stress dependences may be modeled by magnetization behaviors, magnetostriction, crystalline grain size, and analysis of the magnetic properties using this model of stress dependence may improve NO steels and their applications.

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  What influences materials, exciting direction and flux density have on the dependence of a non-oriented (NO) electrical steel sheet on stress are discussed. The dependence on stress changes depending on flux density, because the magnetic mechanisms are domain-wall displacement at lower flux density and magnetization rotation at higher flux density. The magnetic properties of NO at low flux density are improved by tension, but deteriorated by compression because of the positive magnetostriction of easy axis ‹100›. Magnetizations are easy to rotate at high flux density. Then, those of NO, especially 50A1300, are deteriorated by tension in negative magnetostriction of hard axis ‹111›. The measured loop-width changes in the shape of a hysteresis loop due to compression which is attributed to hysteresis loss, and the effect of the steel surface in the high flux density region of the loop is reduced, and then hysteresis loss decreases. However, the loop widens with higher compression in the low-flux density region, where grain size mainly affects magnetic properties and its effect can be observed.

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  This paper proposes a magnetic levitation and rotation system with the capability of adding alternating ampere force to the eddy current in an aluminum disk and describes results where torque was gained under stable levitation conditions. Specifically, ac electromagnets that generated levitation, centripetal, and ampere forces were arranged to suspend an aluminum disk using three points. Adjusting the angle of the ac electromagnets for centripetal force allowed us to apply part of the centripetal force as torque. Our evaluation of the generation of torque and its dependence on the angle of the magnets and the status of additional ampere force under stable levitation conditions for the aluminum disk is discussed in this paper.

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  We measured fetal magnetocardiograms (fMCGs) to analyze variability in the heart rate of a fetus. The heart rate variability was calculated from the interval of the R wave peak of fMCGs. However, the fMCGs had a very weak signal that was buried in noise, and noise rejection was necessary. Independent component analysis (ICA) is a useful method of distinguishing signal and noise components. However, it is difficult to separate noise from the signal, when the measured data have a low signal-to-noise ratio. We propose a method that uses a Wiener filter for post processing in ICA. The Wiener filter can reduce the amount of noise present in a signal by comparison with estimates of the required noiseless signal. The required signal in this study was created by measuring the data. The proposed method was evaluated for 15 fMCGs. As a result, the accuracy of R wave peak detection was improved by using the Wiener filter for post processing in ICA.

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  This paper describes the usefulness of 3D biomagnetic field measurements for discriminating multiple sources closely located and overlapped in time such as in magnetocardiogram(MCG) and magnetoencephalogram (MEG), using frequency and phase analysis. 3D MCG measurements with 48 positions on the chest wall were carried out on normal subjects and on a patient with Wolff-Parkinson-White (WPW) syndrome using a 3D vector second-order gradiometer connected to 39-channel SQUIDs, which can simultaneously detect magnetic fields perpendicular (Bz) and tangential (Bx, By) to the chest wall. To discriminate multiple sources activating in the heart we developed a signal processing method consisting of singular value decomposition(SVD) to estimate the number of equivalent sources, time-frequency analysis to calculate the frequency spectrum of 3D MCG wave-forms, and coherency analysis to calculate phase coherency at each measurement position. We were able to discriminate multiple sources consisting of the source of an accessory pathway and the source of a normal pathway hidden by an intense signal such as the delta wave of the WPW patient, using coherency analysis with the tangential component (Bx, By) added to the normal component(Bz).

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  In this study, we aimed to explore the possibility of evaluating the attitudes of subjects as to whether they liked or disliked visually presented pictures. We measured magnetoencephalgraphic (MEG) responses and conducted a phase analysis of the specific alpha band frequency component. We gave ten subjects a questionnaire with images of animals prepared beforehand. We selected 30 pictures, from which 15 pictures were used as positive impression images (Good Images) and 15 pictures as negative impression images (Bad Images). The MEG responses were measured with a 122-channel magnetometer system. We obtained the power spectrum of MEG responses and carried out a phase analysis of the peak frequency of the alpha band. As a result, significant phase synchronicity was confirmed for the negative impression images.

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  Hyperthermia is a cancer therapy that necrotizes cancerous tumors with heat. Functional Hyperthermia (soft-heating hyperthermia) is one type that implants small elongated elements inside the body. The element generates heat induced by a high-frequency magnetic field produced by exciting coils and is therefore a less-invasive method.
  The most heat is generated when the element's major axis is parallel to the direction of the incoming magnetic field. However, a single coil can be excited in a constant direction. Therefore, an exciting system to generate multidirectional magnetic fields is needed. We developed this exciting system using two coils and two frequencies. The combination of two frequencies and an easy way of estimating the heating value were examined. The profiles of various combinations of these two frequencies were measured and as a result, we found the heating value was much the same, if the sum of the two frequencies was the same. Using a single solenoid coil and the magnetic field of strain waveform enabled us to successfully of and easily estimate the heating value.

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  The induction heating ability of Ti needles (φ 0.8 mm × 4.5 mm) was measured by applying an alternating magnetic field of 800∼990 kHz up to 50 Oe-rms. The needles were cold dummies of small radiation seeds (titanium capsule: φ 0.8 mm × φ 0.64 mm × 4.5 mm) used in prostate cancer brachytherapy; the needles and seeds have roughly equal potential to heat up because of eddy current losses. Eddy current losses are concentrated at the outer circumference of heating generators. Under a 990 kHz magnetic field of 40 Oe-rms, 1 ml of starch paste inserted with a Ti needle (either perpendicular or parallel to the magnetic field) was heated above 42.5°C, a threshold temperature required to kill cancer cells. Furthermore, 40 Ti needles were inserted into an artificial prostate (φ 36 mm × 20 mm) made of polyacrylamide gel. The artificial prostate was then immersed in 36°C water, and a 990 kHz-40 Oe-rms magnetic field was applied for 20 minutes, which increased the temperature of the inner part of the artificial prostate to above 42.5°C. However, the peripheral part was kept at a lower temperature. Therefore, when the radiation seeds are jointly used as heating mediators for hyperthermia, thermal damage to organs surrounding the prostate may be suppressed. Prostate cancer tissue was implanted on a mouse and grown to 3 ml in volume. Three Ti needles were then inserted into the tumor. After exposure to a 990-kHz-50-Oe-rms magnetic field for 20 minutes, the model tumor was successfully heated above 44.5°C, while the surface temperature of the mouse's body was kept below 38°C. All of these results indicate a promising combination of prostate brachytherapy and magnetic hyperthermia, using the Ti radiation seeds as heating mediators.

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