学生論文リスト
学術論文
博士、修士学生も自分の研究成果について第一著者として学術論文を執筆しています
2022年
1:High-Field Critical Current Properties of (Bi,Pb)2Sr2Ca2Cu3Oy Filaments
D. Kobayashi; T. Okada, S. Awaji
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 32 (2022) 6400105
Abstract
To understand the temperature, magnetic field, and angular dependence of the critical current density, Jc(T, B, θ), of (Bi,Pb)2Sr2Ca2Cu3Oy (Bi2223) core composed of commercial Bi2223 tapes, we investigated Jc(T, B, θ) of a bundle of Bi2223 filaments extracted from Ag-sheathed Bi2223 tapes under broad (T, B, θ) conditions. We found that flux pinning curves for B||c (θ = 0°) exhibit a temperature-scaling relation at high T (50–77.3 K) but not at low T (30-40 K). We also found that Jc(θ) exhibits a broad peak in the B||ab (θ = 90°) direction and attributed it to the broadening of a sharp peak due to the intrinsic pin caused by a c-axis misalignment. We plotted Jc(T, B, θ) as a function of the tape normal component of the magnetic field considering the c-axis misalignment, and it agrees well with the measured Jc(T, B, 0°). This indicates that the intrinsic pinning is strong in Bi2223 and Jc(θ) of Bi2223 is dominated by the flux pinning for the B||c component. Based on these findings, we succeeded in reproducing Jc(T, B, θ) at 50-77.3 K by a model taking account of the T-scaling law, c-axis misalignment, and intrinsic pinning.
2015年
1:Nucleation control for fine nano crystallization of Fe-based amorphous alloy by high-magnetic-field annealing
R. Onodera; S. Kimura, K. Watanabe, Y. Yokoyama, A. Makino and K. Koyama
Journal of Alloys and Compounds 637 (2015) 213–218
Abstract
The magnetization measurements for an Fe83.3Si4.2B12.5 amorphous alloy were carried out to investigate the magnetic field effect on the crystallization kinetics. From the isochronal measurements, the gain of the Zeeman energy was estimated from the increase of magnetization at the crystallization temperature. By the isothermal measurements, the growth curve of bcc-Fe(Si), which is the primary crystal phase of this material, was obtained. The growth curve at 10 T shows more abrupt time evolution, compared with that at 0.5 T. However, the growth curves as a function of the scaled time converge to a universal sig- moidal curve, regardless of the applied magnetic field strength. These behaviors indicate that the rate of this crystallization reaction is increased, whereas the crystallization process is not changed by applying the high magnetic field.
2:Applied Strain Effect on Superconducting Properties for Detwinned (Y, Gd)BCO Coated Conductors
T. Suzuki, ; S. Awaji, H. Oguro and K. Watanabe
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 25 (2015) 8400704
Abstract
We succeeded in the complete detwin of (Y, Gd)BCO coated conductors using a newly developed in-situ tensile appara- tus at high temperature. The intensities of (020) and (200) peaks in a longitudinal and a transverse directions of the tape measured by XRD were similar to each other for usual (Y, Gd)BCO coated conductors before annealing. However, the (200) peak disappears for the longitudinal direction after annealing under the tensile stress. It means that the coated conductors are detwinned by annealing under the tensile stress. We found that Tc linearly decreases with increasing strain for the B-domain (b-axis). This behavior is similar with a uniaxial-pressure effect in (Y, Gd)BCO single crystals. However, the strain dependence of Tc for the A-domain (a-axis) behaves as a power-law function, whereas the uniaxial-pressure dependency of Tc for the a-axis is reported to be linear. In addition, the strain dependencies of Jc are similar with those of Tc.
2014年
1:X-Ray Diffraction Measurements of the Fe-Rh Alloy Under High Magnetic Fields and at High Temperatures
K. Ohtake; Y. Mitsui, K. Takahashi, R. Onodera, S. Kimura K. Watanabe; et al.
IEEE Transactions on Magnetics, Vol. 50, No. 1 (2014) 1001404
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.
2:Crystallization kinetics of High Iron Concentration Amorphous Alloys Under High Magnetic Fields
R. Onodera; S. Kimura, K. Watanabe, Y. Yokoyama A. Makino, and K. Koyama
Journal of Alloys and Compunds, vol.604, (2014) 8-11.
Abstract
Crystallization kinetics of a Fe38.3Si4.2B12.5 amorphous alloy and a Fe83.3Si4B8P4Cu0.7 hetero-amorphous alloy under magnetic fields was investigated by the differential thermal analysis (DTA). In DTA for Fe83.3Si4.2B12.5, the first crystallization peak, which indicates the precipitation of bcc-Fe shifts about 10K toward the lower temperature side, whereas the second peak which indicates the precipitation of iron-compunds, shifts about 4 K toward the higher temperature side by applying a magnetic field of B = 20 T. On the other hand, in the case of Fe83.3Si4B8P4Cu0.7, the first peak at B = 20 T does not change in comparison with that at a zero magnetic field, whereas the second peak shifts toward a higher temperature side as well as Fe83.3Si4.2B12.5.
3:High-field optical spectroscopy of the chromium spinal CdCr2O4
Y. Sawada; S. Kimura, K. Watanabe, and H. Ueda
Journal of Physics: Conference Series, vol.568, (2014) 042028.
Abstract
We have performed high-field optical spectroscopy measurements of the geometrically frustrated chromium spinel oxide CdCr2O4 in the visible light region. We succeeded in observing the pure exciton excitation peak, and identified some other absorption peaks as exciton-magnon excitation peaks from the theoretical calculations of density of states of the spin wave. Furthermore, from the high-field experiment, large variations of absorption peaks were observed at around 4 T. This result indicates that some magnetic phase transition occurs around this field.
2013年
1:High-Field Optical Spectroscopy of the Spin-Crossover Complex [MnsIII(taa)]
Y. Sawada; S. Kimura, K. Watanabe ; et al.
Journal of Low Temperature Physics, 170 (2013) 424
Abstract
In this study, we have observed the optical absorption spectra of the man- ganese(III) complex [MnIII(taa)] (H3taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine), in high magnetic fields. From the measurements at zero field, we have found a vari- ation of the spectrum between 100 K and 4.2 K due to the thermally-induced spin- crossover transition, which occurs around Tc ≈ 47 K. Furthermore, from the high- field experiment, a magnetochromism, accompanying with the field-induced spin- crossover, has been observed just below Tc.
2:Magnetic Field Effects on Crystallization of Iron-Based Amorphous Alloys
R. Onodera; S. Kimura, K. Watanabe ; et al.
Materials Transaction, Vol.54, No. 2 (2013) 188
Abstract
The crystallization behavior of iron-based amorphous alloys has been investigated in high magnetic fields by differential thermal analysis (DTA) and magnetization measurements. DTA of FeSiB amorphous alloys showed that the exothermic peaks shift toward higher temperature under high magnetic fields. At 10 T, peak temperatures of the crystallizations increased approximately 3 K, compared with those at a zero magnetic field. Such a variation in the crystallization peak was not observed in the FeBNbY bulk metallic glass (BMG). In the temperature dependence of the saturation magnetization for the FeSiB amorphous alloy, the sudden increase of the magnetization was found at the crystallization temperatures. The magnetization of FeSiB increased 56 and 40 Am2 kg-1 at the first and second crystallization temperatures, respectively. This magnetization behavior indicates the magnetic transition from the paramagnetic to the ferromagnetic state, accompanying the crystallization, whereas there is only a slight increase in magnetization at the crystallization temperature in FeBNbY BMG. The effect of magnetic field on the crystallization peak as observed from DTA is related to the increase in the magnetization at the crystallization temperature.
3:Isothermal Crystallization of Iron-Based Amorphous Alloys in a High Magnetic Field
R. Onodera; S. Kimura, K. Watanabe ; et al.
Materials Transaction, Vol.54, No. 7 (2013) 1232
Abstract
Isothermal crystallization of soft magnetic Fe–Si–B amorphous alloys under high magnetic fields was investigated by magnetization measurements. The increase of magnetization due to the precipitation of ferromagnetic α-Fe(Si) from the paramagnetic amorphous matrix was observed as a function of time. The magnetic field dependence of the time evolution of the crystallization was also examined. The relaxation time increases under high applied magnetic fields indicating that the suppression of the precipitated crystal growth rate occurs. Moreover, by normalizing the relaxation time of the crystallization, the time evolutions of the crystallized fraction converge to a universal sigmoidal curve regardless of the applied magnetic field strength. These behaviors indicate that the magnetic field influences the growth rate rather than the elementary processes of crystallization.
4:Improvement of Jc for GdBCO Coated Conductors by Annealing Under Strain
T. Suzuki; S. Awaji, H. Oguro, K. Watanabe ; et al.
IEEE Transaction on Applied Superconductivity, Vol. 23, No. 3 (2013) 8000104
Abstract
GdBa2Cu3Oy coated conductors were annealed un- der external strain (strain-annealing). The residual strain after an- nealing was evaluated using synchrotron radiation. We found that the residual strain is changed by the strain-annealing. The volume fraction of the a-axis domains along the longitudinal direction is increased by compression-annealing and decreased by tension- annealing. In addition, the Jc value of the tension-annealed sample increases drastically below 70 K, although the Jc value is smaller than that of the as-received sample at 77.3 K for B//c. The peak of the angular dependent Jc at θ = 0◦, which originates from the c-axis correlated pinning, for the strain-annealed sample is much smaller than that of the as-received sample. It is considered that the shrinking of the c-axis correlated pinning because of the reduction of the twin structure and the increase of carrier doping may be related to the change in Jc properties.
2011年
1:Development of an x-ray diffraction camera used in magnetic fields up to 10 T
Y. Mitsui; K. Koyama, K. Takahashi and K. Watanabe
Review of Scientific Instruments 82, 125104 (2011)
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 80.1mm, a 10-T cryocooled superconductng 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, showig that the instrument is useful for studying field-induced orientation processes and strucutural properties of field-controlled materials.
2010年
1:Observation of reverse transformation in metamagnetic shape memory alloy Ni40Co10Mn34Al16 by high-field X-ray diffraction measurements
Y. Mitsui; K. Koyama, K. Watanabe; et al.
Materials Transactions, Vol. 51 No. 9 (2010) 1648-1650
Abstract
The structural properties of metamagnetic shape memory alloy Ni40Co10Mn34Al10 were investigated by X-ray powder diffraction measurements in the temperature range from 283 to 473 K and magnetic fields up to 5 T. With increasing temperature from 283 K, the alloy undergoes the reverse martensitic transformation from the tetragonal L10-type structure with the lattice parameter at = 0.3853nm and ct = 0.3299 nm into the B2-type cubic structure with the lattice parameter ac = 0.2924nm at As = 383K. At 408 K (>As), we directly observed the field-induced reverse martensitic transformation.
2009年
1:Difference between BaSnO3 and BaZrO3 nano-rods for c-axis-correlated pinning properties in REBa2Cu3Oy
M. Namba; S. Awaji, K. Watanabe; et al.
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 19 (2009) 3495-3498
Abstract
In order to understand the vortex pinning properties by the BaZrO3(BZO) and BaSnO3(BSO) nano-rods in Er123 films, the microstructure and the angular dependence of Jc and resistivity were measured in detail for the BZO and the BSO added laser deposition. We found that the peak at B//c in the angular dependence of Jc for the Er123film with BSO nano-rods was higher in a wide magnetic field region up to the irreversible field than that for the Er123 film with BZO nano-rods, although the minimum values of the angular dependencer of Jc are similar each other. Hence, the BSO nano-rods work more effectively as the c-axis-correlated pinning center than the BZO nan-rods because of the different elementary pinning energy of nano-rods.
2:Correlated pinning behavior in ErBa2Cu3Oy films with BaZrO3 nano-rods
M. Namba; S. Awaji; K. Watanabe; et al.
Physica C 469 (2009) 1404-1409
Abstract
In REBa2Cu3Oy(RE123) films with BaZrO3(BZO), the columnar shaped BZO precipitates, called nano-rods, are formed as the c-axis-correlated pinning centers. In order tu understand the vortex pinning properties of RE123 films with BZO nano-rods, we measured the resistivity ρ and critical current density Jc of three different films, i.e. the Er123 films with 0, 1.5 and 3.5 wt% BZO prepared by pulsed lase deposition. We fund that the peak related to the c-axis-correlated pinning in the angular dependence of Jc became large in a low fields region but small in a high magnetic field with increasing BZO concentration. In addition, the irreversibility field at B//c-axis increased also with increasing BZO concentration, although the dip in the angular dependence of ρ and the peak of the Jc at B//c-axis decreased in the high magnetic field near the irreversible field. It is suggested that the BZO addition is effective to the enhancement of Jc in al low field region as well as the improvement of the irreversibility field. However, we have to consider the c-axis-correlated pinning by the edge dislocations as well as the nano-rods in order to understand it in a high field region.
3:X-ray diffraction measurements in high magnetic fields and at high temperatures
Y. Mitsui; K. Koyama; K. Watanabe
Science and Technology Advanced Materials 10 (2009) 014612-014616
Abstract
A system was developed measuring x-ray powder diffraction in high magnetic fields up to 5 T and at temperatures from 283 to 473 K. The stability of the temperature is within 1 K over 6 h. In order to examine the ability of the system, the high-field x-ray diffraction measurements were carried out for Si and a Ni-based ferromagnetic shape-memory alloy. The results show that the x-ray powder diffraction measurements in high magnetic fields and at high temperature are useful for materials research.
4:Effects off BaMO3 (M = Zr, Sn) nanorods on critical temperature of ErBa2Cu3Oy films
M. Namba; S. Awaji; K. Watanabe; et al.
Applied Physics Express 2 (2009) 073001
Abstract
In order to study the degradation of critical temperature Tc for a coated conducor using nanorods, the relationship between Tc and nanorod shape was investigated by tansmission electron microscopy for ErBa2Cu3Oy(Er123) films with BaZrO3(BZO) and BaSnO3(BSO) nanorods. The dependence of Tc on the BZO or BSO content was different, however, the nanorods interface density dependence of Tc was the same for both. Thus, the interface density of the nanorods controls Tc of the nanorod-added films. The different degradation in Tc between the BZO and BSO nanorod-added films originated in differences in the nanorod dimeter and density.
5:Correlated pinning behavior in ErBa2Cu3Oy films with columnar defects
M. Namba; S. Awaji; K. Watanabe; et al.
Applied Physics Express 2 (2009) 113002-113004
Abstract
In this study, c-axis-correlated pinning centers, which are known to increase the critical current density (Jc) for B//c-axis, were produced by heavy-ion-irradiation in REBa2Cu3Oy (RE: rare earth element, RE123) coated conductors, and the subsequent changes in Jc were measured. A peak in the angular dependence of Jc induced by the defects was observed for applied magnetic fields up to 13 T for Er123 films with matching fields of Bφ = 0.1 and 1 T. We found that the columnar defects led to improvements in Jc even under applied magnetic fields more than 130 times higher than Bφ.
6:強磁場下における強磁性体MnBiの相転移
Y. Mitsui; K. Koyama; K. Watanabe; et al.
Japan Inst. Metals 73 (2009) 40-45
Abstract
X-ray powder diffraction, magnetization measurements, and differential thermal analysis (DTA) of polycrystalline MnBi were carried out in zero magnetic field as will as in magnetic fields of up to 14 T and in the temperaturees range of 300-773 K, in order to investigate magnetic phase transition. In zero magnetic field, a magnetic phase transition from ferromagnetic to paramagnetic state is exhibited at temperature (Tt) of 628 K, accompanied by decomposition. With increasing magnetic field up tp 14 T, Tt increases linearly with the rate of 2 KT-1. A metamagnetic transition between the paramagnetic and field-induced ferromagnetic states was observed just above Tt. The exothermic and endothermic peaks were detected in the magnetic field dependence of DTA signals in the interval of 626-623 K, which relates to the metamagnetic transition. The obtained results are desscussed on the basis of a mean field theory.
7:強磁場・高温X線回折測定用試料電気炉の性能評価
Y. Mitsui; K. Koyama; K. Watanabe
Journal of the Magnetics Society of Japan 33 (2009) 60-63
Abstract
An X-ray powder diffraction system in high temperature and in high magnetic fields was developed, which was performed at temperature ranging from room temperature to 473 K in the magnetic fields u to 5 T. The temperature stability was within ±1 K for 8 hours. To examine the ability of the system, high field and high temperature X-ray diffraction measurements were carried out for a novel ferromagnetic shape alloy, Ni40Co10Mn34Al16. We observed a field-induced reverse transformation in the alloy.