Kohki Takahashi

Tohoku University, Miyagi, Japan

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Publications (32)47.41 Total impact

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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.
    Journal of Magnetism and Magnetic Materials 08/2015; DOI:10.1016/j.jmmm.2015.07.114 · 1.97 Impact Factor
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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.
    Journal of Alloys and Compounds 05/2015; 632. DOI:10.1016/j.jallcom.2015.01.129 · 3.00 Impact Factor
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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.
    Metals - Open Access Metallurgy Journal 12/2014; 4(4):609-622. DOI:10.3390/met4040609 · 0.88 Impact Factor
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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.
    Polymer 10/2014; 55(25). DOI:10.1016/j.polymer.2014.10.031 · 3.56 Impact Factor
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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.
    09/2014; 1(4). DOI:10.1088/2053-1591/1/4/045202
  • Naoyuki Kitamura · Kohki Takahashi · Iwao Mogi · Satoshi Awaji · Kazuo Watanabe ·
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    ABSTRACT: This chapter contains sections titled: Introduction Experimental Procedure Results and Discussion Conclusion Acknowledgements
    Innovative Processing and Manufacturing of Advanced Ceramics and Composites II, 03/2014: pages 109-116; , ISBN: 9781118771501
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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.
    IEEE Transactions on Magnetics 01/2014; 50(1):1-4. DOI:10.1109/TMAG.2013.2273363 · 1.39 Impact Factor
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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.
    Journal of Alloys and Compounds 11/2013; 577:S503-S506. DOI:10.1016/j.jallcom.2012.03.072 · 3.00 Impact Factor
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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.
    Journal- Korean Physical Society 08/2013; 63(3):747-750. DOI:10.3938/jkps.63.747 · 0.42 Impact Factor
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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.
    Polymer 01/2013; 54(2):784–790. DOI:10.1016/j.polymer.2012.12.016 · 3.56 Impact Factor
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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.
    MATERIALS TRANSACTIONS 01/2013; 54(6):969-973. DOI:10.2320/matertrans.M2012415 · 0.68 Impact Factor
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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.
    Journal of Magnetism and Magnetic Materials 11/2012; 324(22):3863–3869. DOI:10.1016/j.jmmm.2012.06.036 · 1.97 Impact Factor
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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.
    IEEE Transactions on Applied Superconductivity 06/2012; 22(3):6000204-6000204. DOI:10.1109/TASC.2011.2174553 · 1.24 Impact Factor
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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.
    Journal of the Japan Institute of Metals 04/2012; 76(4):246-250. DOI:10.2320/jinstmet.76.246 · 0.31 Impact Factor
  • 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.
    Journal of the Japan Institute of Metals 01/2012; 76(1):72-80. DOI:10.2320/jinstmet.76.72 · 0.31 Impact Factor
  • Yoshifuru Mitsui · Keiichi Koyama · Kohki Takahashi · Kazuo Watanabe ·
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    ABSTRACT: A high-field x-ray diffraction (HF-XRD) camera was developed to observe structural changes of magnetic materials in magnetic fields up to 10 T. The instrument mainly consists of a Debye-Scherrer-type camera with a diameter of 80.1 mm, a 10-T cryocooled superconducting magnet with a 100-mm room-temperature bore, an x-ray source, a power supply, and a chiller for the x-ray source. An x-ray detector (image plate) in the HF-XRD camera can be taken out and inserted into the magnet without changing the sample position. The performance of the instrument was tested by measuring the HF-XRD for silicon and ferromagnetic MnBi powders. A change of x-ray diffraction pattern was observed due to the magnetic orientation of MnBi, showing that the instrument is useful for studying field-induced orientation processes and structural properties of field-controlled materials.
    The Review of scientific instruments 12/2011; 82(12):125104. DOI:10.1063/1.3663839 · 1.61 Impact Factor
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    ABSTRACT: Martensitic transformation in a highly ordered Fe3Pt has been investigated by magnetization and X-ray diffraction measurements. We confirmed that a new type of face-centered tetragonal (FCT) martensite phase appears below 60 K in Fe3Pt with a degree of order S = 0.88. The tetragonality c/a gradually increases with decreasing temperature, and is approximately 1.005 at 10 K. This is in contrast to the tetragonality c/a < 1 generally observed in the FCT martensite with a degree of order less than 0.8. The spontaneous magnetization increases in association with the transformation.
    Journal of Alloys and Compounds 08/2011; 509(34):8530-8533. DOI:10.1016/j.jallcom.2011.06.035 · 3.00 Impact Factor
  • Masafumi Yamato · Yuki Kudo · Kohki Takahashi · Kazuo Watanabe · Naoshi Kawamoto ·
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    ABSTRACT: The magnetic alignment of poly(L-lactic acid) (PLA) containing octamethylenedicarboxylic dibenzoylhydrazide (OMBH) as a nucleating agent was studied. OMBH also affected PLA as a nucleating agent when about 5 wt % of d-type material was included. The magnetic alignment of PLA including OMBH was achieved. The c axis of the PLA was parallel to the magnetic field.
    Chemistry Letters 07/2011; 40(7):765-767. DOI:10.1246/cl.2011.765 · 1.23 Impact Factor
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    ABSTRACT: Carbon/silica composite films were prepared from colloidal silica aqueous solution in which vapour grown carbon nano-fibers were dispersed. The orientation of nano-fibers was attempted by dip-coating substrates under high magnetic field up to 10 T. The fibers started to align along the direction of magnetic field below 1 T. The degree of orientation was saturated at about 6 T. The anisotropic susceptibility, Delta chi = chi(parallel to) - chi(perpendicular to), was estimated to be (3.05 +/- 1.10) x 10(-7) cm(3)/mol from the fitting of numerical simulation. Optical anisotropy, the ratio between transmittances of light polarized perpendicular to and parallel to the direction of fiber orientation, increased immediately in the films prepared under applied fields below 1 T and almost saturated above 1 T. The optical anisotropy was in good agreement with the behaviour of magnetic orientation of fibers.
    Journal- Ceramic Society Japan 06/2011; 119(1390):430-433. DOI:10.2109/jcersj2.119.430 · 0.70 Impact Factor
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    ABSTRACT: Effect of magnetic-field direction applied during annealing at 800°C on the demagnetization process was investigated systematically in sintered Nd-Fe-B magnets. We observed a stepwise decrease in the demagnetization curve when the perpendicular field was applied during the 800°C annealing, which suggests an existence of soft magnetic grains. We then annealed this sample in zero field successively from Ta = 400°C to 720°C with the temperature increment of 10°C, and traced a change in demagnetization curves. The volume fraction Vsoft of the soft magnetic grains was estimated from the step width of the demagnetization curves. The Vsoft value showed a maximum around Ta=500°C with Vsoft = 25%, and decreased rapidly with increasing Ta. The coercivity versus T a plot exhibited a similar tendency to that of Vsoft versus Ta plot. Origin of such soft magnetic grains is considered to be a precipitation of perpendicularly-aligned Nd2Fe14B small particles in the grain boundary, which would act as a nucleation site on the large Nd2Fe14B host matrix. These results suggest that the magnetic coupling between Nd2Fe14B grains is one of the important factors to understand the coercivity mechanism in sintered Nd-Fe-B magnets.
    Journal of the Japan Institute of Metals 04/2011; 75(4):193-197. DOI:10.2320/jinstmet.75.193 · 0.31 Impact Factor