Kohki Takahashi

Tohoku University, Miyagi, Japan

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Publications (39)50.16 Total impact

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    ABSTRACT: We have developed an apparatus that allows the observation of the transient rotational motion of fine particles under a high magnetic field in order to determine anisotropic magnetic susceptibility. The anisotropic susceptibilities of spherical nanoparticles of bismuth and commercially available carbon nanofibers were determined. The estimated Δ = 3.9 × 10-5 of spherical bismuth nanoparticles with a diameter of 370nm was fairly consistent with the value determined previously by the magnetic field dependence of diffraction peak intensity in the X-ray diffraction (XRD) pattern, but was slightly smaller than the value for the bulk crystal. In contrast, the transient behavior of carbon nanofibers did not obey the theoretical motion of a single crystal. The wide distribution of fiber lengths, the irregularity of the structure in the fiber, and the connections between the fibers are suggested for the anomalous behavior.
    No preview · Article · Jan 2016 · Japanese Journal of Applied Physics
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    ABSTRACT: A vibrating sample magnetometer for measurements at high magnetic fields and high temperatures (HT-VSM) was developed. The first-order antiferro-ferromagnetic phase (AFM-FM) transition in B2 FeRh alloy was investigated using the HT-VSM in a wide range of temperatures and magnetic field regions. At 300 K, we succeeded in observing a field-induced AFM-FM transition by magnetization measurements up to 19 T. The transition field decreases with increasing sample temperature.
    Preview · Article · Jan 2016 · Journal of the Japan Institute of Metals
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    ABSTRACT: For investigating in-field process of melting and solidification visually and quantitatively, in-situ observation system with differential thermal analysis (DTA) utilized in high temperature and in high magnetic field was developed. Decomposition processes of the bulk sample of ferromagnetic MnBi were directly observed with collecting DTA data under high magnetic field of 10 T for the 290-770K temperature range. When the temperature was over decomposition point (ferromagnetic MnBi → paramagnetic Mn1.08Bi + liquid), liquid phase appeared on the sample surface. Furthermore, when the temperature was over peritectic temperature (∼ 700K: paramagnetic Mn1.08Bi → Mn + liquid), the sample surface was broken and a large quantity of the liquid phase appeared from the sample. The in-situ observation also suggested that the decomposition temperature increased from 620K for a zero field to 638K for a magnetic field of 10 T.
    No preview · Article · Dec 2015 · Physics Procedia
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    ABSTRACT: Magnetic fields effects on liquid-phase reaction sintering on MnBi were investigated. The liquid-phase reaction was so fast even in a zero field that the fraction of in-field sintered ferromagnetic MnBi phase was independent of the external magnetic field. However, the ferromagnetic MnBi crystals in the in-field sintered sample were oriented along the external magnetic field direction. The Lotgering factor of the in-field sintered sample was 0.99. This result indicated that almost completely anisotropic MnBi phase could be obtained by in-field liquid phase reactive sintering.
    No preview · Article · Aug 2015 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: The Fe-Fe3C binary phase diagram under high magnetic fields up to 21 T was investigated by a differential thermal analysis. Applying a magnetic field of 18 T, the α–γ transformation temperature Tα–γ for pure iron increased quadratically from 1181 K (a zero field) to 1206 K. With increasing magnetic field strength, the transformation temperature Ac1 (α-Fe + cementite → α-Fe + γ-Fe) increased linearly from ∼1000 K to ∼1030 K at the rate of 1.4–1.5 K T−1. The transformation temperature Ac3 (α-Fe + γ-Fe → γ-Fe) for Fe–0.18 wt.%C and Fe–0.38 wt.%C increased quadratically by 27 K under a 18-T field. The Ac1 and Ac3 lines shifted to higher temperatures by the magnetic field, resulting in a change in the Fe-Fe3C phase diagram. The results obtained suggest that the transformation temperatures mainly arose because of the Zeeman energy of α-Fe under high magnetic fields.
    No preview · Article · May 2015 · Journal of Alloys and Compounds
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    ABSTRACT: In order to investigate behavior of magnetic field-induced reverse martensitic transformation for Ni-Co-Mn-Sb, magnetization experiments up to a static magnetic field of 18 T and a pulsed magnetic field of 40 T were carried out. In the thermomagnetization curves for Ni41Co9Mn39Sb11 alloy, the equilibrium transformation temperature T-0 was observed to decrease with increasing applied magnetic field, H-0, at a rate of dT(0)/d(0)H = 4.6 K/T. The estimated value of entropy change evaluated from the Clausius-Clapeyron relation was about 14.1 J/(Kkg), which was in good agreement with the value obtained by differential scanning calorimetric measurements. For the isothermal magnetization curves, metamagnetic behavior associated with the magnetic field-induced martensitic transformation was observed. The equilibrium magnetic field, H-0(0) = (H-0(Af) + H-0(Ms))/2, of the martensitic transformation tended to be saturated at lower temperature; that is, transformation arrest phenomenon was confirmed for the Ni-Co-Mn-Sb system, analogous with the Ni(Co)-Mn-Z (Z = In, Sn, Ga, Al) alloys. Temperature dependence of the magnetic field hysteresis, H-0(hys) = H-0(Af) - H-0(Ms), was analyzed based on the model for the plastic deformation introduced by the dislocations. The behavior can be explained by the model and the difference of the sweeping rate of the applied magnetic field was well reflected by the experimental results.
    No preview · Article · Dec 2014 · Metals - Open Access Metallurgy Journal
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    ABSTRACT: The effect of the high order structure of an isotactic polypropylene (PP) composite on the resistivity of composites containing magnetically aligned Ni particles was studied. Only a small amount of particles needed to be added for the composite material to become conducting after heating while in a magnetic field. The Ni columns formed on applying the field were distorted by the formation of large PP spherulites. Changes to the crystallization process due to the addition of a nucleating agent gave rise to changes in the columnar structure, resulting in large changes in the resistivity of the composite material. Controlling the high order structure of the polymer matrix including its morphology is very important in order to be able to control the magnetically aligned Ni structure.
    No preview · Article · Oct 2014 · Polymer
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    ABSTRACT: Formation of bismuth nano-particles by polyol process have been studied in terms of the effect of additives on its size and shape. Bismuth trichloride solved in ethylene glycol was reduced at high temperature with the existence of poly(vinylpyrrolidone) (PVP) and ferric chloride. Spherical particles were mostly formed with the solitary addition of PVP. Sphere size was decreased from 400 to 200 nm with increasing amount of the PVP. Addition of larger weight-average molecular weight (Mw) of the PVP decreased the sphere size slightly. On the other hand, hexagonal or rectangular plate-like particles were obtained at a high number ratio over 70% by the co-addition of ferric chloride with the PVP. Plate size and the fraction of plate-like particles tended to increase slightly with the increase of amount of the PVP. The plate size also increased with increase of molecular weight of the PVP and square plate-like particles larger than 1 micrometer were obtained with the addition of the large molecular size of PVP (Mw ~1,300,000). Adequate amount and size of PVP tended to decrease totally the growth speed of particles and to suppress coalescence with each other, and ferric ions suppressed the growth speed toward the c-axis by alternative re-oxidization of bismuth metal.
    No preview · Article · Oct 2014 · Journal- Ceramic Society Japan
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    ABSTRACT: Birefringence is an indicator of structural anisotropy of materials. We measured the birefringence of Pb(II)-doped silica hydrogels prepared under a high magnetic field of various strengths. Because the silica is diamagnetic, one does not expect the structural anisotropy induced by a magnetic field. In previous work [Mori A, Kaito T, Furukawa H 2008 Mater. Lett. 62 3459-3461], we prepared samples in cylindrical cells made of borosilicate glass and obtained a preliminary result indicating a negative birefringence for samples prepared at 5T with the direction of the magnetic field being the optic axis. We have measured the birefringence of Pn(II)-doped silica hydrogels prepared in square cross-sectional cells made of quartz and reverted the previous conclusion. Interestingly, the magnetic-influenced silica hydrogels measured have been classified into four classes: two positive birefringent ones, no birefringent one, and negative birefringent one. Proportionality between birefringence and the strength of magnetic field is seen for the former two.
    Full-text · Article · Sep 2014
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    ABSTRACT: This chapter contains sections titled: Introduction Experimental Procedure Results and Discussion Conclusion Acknowledgements
    No preview · Chapter · Mar 2014
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    ABSTRACT: Practical superconducting wires are designed with a composite structure to meet the desired engineering characteristics by expert selection of materials and design of the architecture. In practice, the local strain exerted on the superconducting component influences the electromagnetic properties. Here, recent progress in methods used to measure the local strain in practical superconducting wires and conductors using quantum beam techniques is introduced. Recent topics on the strain dependence of critical current are reviewed for three major practical wires: ITER-Nb3Sn strand, DI-BSCCO wires and REBCO tapes.
    No preview · Article · Jan 2014 · TEION KOGAKU (Journal of the Cryogenic Society of Japan)
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    ABSTRACT: The first-order antiferromagnetic-ferromagnetic phase transition in B2 FeRh was studied by magnetization and X-ray diffraction measurements in magnetic fields up to 5 T at high temperatures, in order to investigate the relationship between magnetic and structural properties affected by the field. It turned out that the compound shows the field-induced isostructural transformation by applying a magnetic field, accompanied by the first-order magnetic transition from the antiferromagnetic to the ferromagnetic state.
    No preview · Article · Jan 2014 · IEEE Transactions on Magnetics
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    ABSTRACT: The tetragonality c/a of FCT martensite phase in an Fe3Pt depends on the degree of order S of the L12-type parent phase. As the degree of order increases, the tetragonality c/a changes from being less than 1 (S = 0.75) to being larger than 1 (S = 0.88). In this study, we have investigated a martensitic transformation behavior in a single-crystalline Fe3Pt with a degree of order which is in between 0.75 and 0.88, and have found the existence of orthorohombic martensite phase in the Fe3Pt.
    No preview · Article · Nov 2013 · Journal of Alloys and Compounds
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    ABSTRACT: Magnetization and electrical resistivity measurements were carried out for polycrystalline Mn2Sb1- x Ge x (0.05 ≤ x ≤ 0.2) in magnetic fields up to 16 T in the 4.2 - 600 K temperature range in order to investigate the magnetic and the electrical properties under high magnetic fields. Mn2Sb0.92Ge0.08 showed a Curie temperature, T C , of 532 K and a first order magnetic transition from a ferrimagnetic (FRI) to an antiferromagnetic (AFM) phase at T t = 230 K with decreasing temperature in a zero magnetic field. With increasing x, T C decreased and T t increased. For Mn2Sb0.92Ge0.08, a matamagnetic transition from an AFM to a FRI phase was observed at 215 K. The magnetic phase diagram of Mn2Sb1- x Ge x is presented.
    No preview · Article · Aug 2013 · Journal- Korean Physical Society
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    ABSTRACT: Magnetization, electrical resistivity and X-ray diffraction measurements were carried out for Mn1.8Co0.2Sb with a tetragonal structure in 4.2 ≤ T ≤ 280K and in field B up to 16 T. For B = 0 T, anisotropic structural deformation occurred at Tt ∼ 140K for zero-field cooling (ZFC), accompanied by a first-order magnetic transition from the ferrimagnetic (FRI) to antiferromagnetic (AFM) phases. In this deformation, the lattice parameters a and c changed by δ a/a=+0.15% and by δ c/c = -0.44%, respectively. By applying magnetic fields of 5 T, Tt decreased to 60K with thermal hysteresis of 35 K. The two-phase coexistence of the AFM and residual FRI phases was observed even at 10K for field cooling of 5 T, while a single phase of AFM was confirmed for ZFC. At B = 16 T, the transition did not occurs. In addition, a field-induced structural deformation was observed, accompanied by the metamagnetic transition just below Tt. The obtained results are discussed from a viewpoint on entropy change for the transition.
    No preview · Article · Jun 2013 · MATERIALS TRANSACTIONS
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    ABSTRACT: Magnetic-field induced alignment of polyethylene is firstly reported. Isothermal crystallization of low molecular weight polyethylene samples, high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), from melting state was conducted under the magnetic-field of 2 or 10 T. Isothermal crystallization at the temperatures which induced effective self-seeding achieved alignment of polymer chains. The b axis of polyethylene unit cell was aligned parallel to the magnetic-field. Low viscosity and low crystallization rate of the polyethylene samples were favorable for high degree of the magnetic-field alignment.
    No preview · Article · Jan 2013 · Polymer
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    ABSTRACT: We have demonstrated the effect of pressure on the steplike metamagnetic transition and its associated magnetostriction in (Eu1−xGdx)0.6Sr0.4MnO3 (x=0 and 0.1). The critical field initiating the field induced ferromagnetic transition in both samples is lowered by the applied pressure. The further application of external pressure up to 1.2 GPa on the x=0 parent sample causes a spontaneous ferromagnetic transition with a second-order like character, leading to collapses of the steplike transition and its concomitant lattice striction. These findings indicate a crucial role of the low-temperature phase separated state characterized by a suppressed magnetization upon decreasing temperature.
    No preview · Article · Nov 2012 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: Internal strains under a tensile load for ${\rm CuNb/Nb}_{3}{\rm Sn}$ wires with and without the prebending treatment were measured directly by neutron diffraction at room temperature and 7 K. The axial residual strains changed by 0.30% at room temperature and by 0.38% at 7 K to the tensile side due to the prebending treatment. The effective Poisson's ratio evaluated from the relationship between axial and lateral strains under an axial stress was about 0.36 at 7K with or without the prebending treatment. The applied tensile strain dependences of the deviatoric and hydrostatic strains were estimated, and the tensile strain dependence of the superconducting properties were discussed on the basis of the three-dimensional strain model. A control of the residual strains such as the prebending treatment plays an important role in the superconducting properties of ${\rm Nb}_{3}{\rm Sn}$ wires under axial stress/strain.
    No preview · Article · Jun 2012 · IEEE Transactions on Applied Superconductivity
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    ABSTRACT: Measurements of magnetization, electrical resistivity and high field X-ray powder diffraction (XRD) were carried out for polycrystalline Mn 1.8Co 0.2Sb in magnetic fields up to 5 T in 4.2-300 K temperature range, in order to investigate the structural properties affected by magnetic fields. In a zero field, the compound shows the first-order magnetic transition from the ferrimagnetic (FRI) to antiferromagnetic (AFM) states at T t = 145 K with decreasing temperature. By applying magnetic fields of 5 T, T tdecreased to 60 K with thermal hysteresis of 35 K. From the XRD measurements, not only the AFM phase but also the residual FRI phase was confirmed even at 10 K in cooling process under 5 T. Results obtained indicate that the residual FRI phase is kinetically arrested.
    No preview · Article · Apr 2012 · Journal of the Japan Institute of Metals
  • Hiroaki Kato · Takahiro Akiya · Kohki Takahashi
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    ABSTRACT: Results of a systematic study concerned with the high-magnetic-field effects on the coercivity in processing various series of sintered Nd-Fe-B magnets are given. Coercivity enhancement phenomenon was observed in several series of sintered Nd-Fe-B magnets with Cu additive after annealing under magnetic fields of 10~14 T. A strong correlation was suggested between the field annealing temperature at which maximum coercivity increase was observed and the anomaly temperature in differential scanning calorimetry experiments. We found a linear relation between the room temperature coercivity H c of the sample annealed at T a=500°C under magnetic field H a for a series of samples with different Dy contents, which can be expressed as H c = H c(0) + αH a, with H c(0) being the value for the zero-field-annealed sample and α, the coercivity enhancement factor. Although very small, α has a positive finite value in samples with 0 mass% and 2 mass% Dy. Significant increase in α value was confirmed for samples with Dy content larger than 6 mass%\ This suggests the effects of higher anisotropy in paramagnetic susceptibility of Nd-rich intergranular phase containing Dy. Also introduced is the preliminary results of the grain boundary diffusion process of sinterd Nd- Fe-B samples with surface Dy layer under strong gradient magnetic fields produced by the 18 T superconducting magnets.
    No preview · Article · Jan 2012 · Journal of the Japan Institute of Metals