[Show abstract][Hide abstract] ABSTRACT: We report comprehensive studies of the crystallographic, magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system, Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x<0.6. A multicritical point was apparent at T=3.4K and x=0.12, where the antiferromagnetic, spin-glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x=0.6. At x∼0.6, magnetic susceptibility and specific heat were described by temperature power laws, χ∼C/T∼T-δ, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an x-temperature phase diagram in the range 0≤x≤1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.
Physical Review B 06/2015; 91(22). DOI:10.1103/PhysRevB.91.224412 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report comprehensive studies of the crystallographic, magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system, Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x < 0.6. A multicritical point was apparent at T = 3.4 K and x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x = 0.6. At x 0.6, magnetic susceptibility
[Show abstract][Hide abstract] ABSTRACT: This work investigates the spectroscopic properties of (Co1−xZnx)Al2O4 with a range of x of 0 ≤ x ≤ 1. Spectroscopic and crystallographic evaluations using XRD, Raman, FT-IR and UV-Vis spectroscopy reveal that Zn2+ substitution systematically changes the lattice constant, which mainly depends on the Co–O bonds, and the related optical characteristics of this material. The x dependence of these properties shows two trends, and the mutation point seems to be at x ≈ 0.5. This implies that the electronic structure of (Co1−xZnx)Al2O4 is not changed monotonically by Zn2+ substitution. Interestingly, some of the optical phenomena observed in this study become prominent for samples with x ≥ 0.5. That is, we observed sideband peaks near the main peaks in the Raman spectra, and their relative intensities systematically and significantly increased with increasing Zn2+ substitution. The rates of increase are not constant, and are fast for samples with x ≥ 0.5. The sideband peaks are considered to reflect the unique changes in the local electronic structure of (Co1−xZnx)Al2O4, and they are useful for evaluating the substitution level without the influence of the site change phenomenon. Thus, clarifying them is expected to be important for understanding and controlling the electronic structure of the spinel oxide. On the other hand, investigation of the visible light absorption due to the d–d transition of Co2+ reveals that the efficiency is also high for samples with high Zn2+ substitution (x ≥ 0.5). This is also considered to be valuable information for investigation of the optical properties and/or the catalytic function of the spinel oxide. Moreover, the fluorescence of the (Co1−xZnx)Al2O4 samples is also identified as a novel functional property of this material. The intensity of the fluorescence peak also dramatically increases for samples with x ≥ 0.7. The effect of Zn2+ substitution on the local electronic structure of (Co1−xZnx)Al2O4 has not been clarified yet. However, some of the interesting characteristics reviewed in this study are worth investigating from the viewpoint of materials science and applications.
[Show abstract][Hide abstract] ABSTRACT: We report on the crystallographic structure and magnetism of 5-nm Co–Al–O spinel nanocrystals synthesized under supercritical hydrothermal conditions. Structural examination using powder X-ray diffraction and chemical analysis showed the composition of the sample to be Co0.47Al2.36O4 rather than the stoichiometric composition of CoAl2O4 . The site occupancy of Co on the A-site forming the diamond lattice was 0.47, which is slightly larger than the site percolation limit. Magnetization measurements showed that magnetic clusters emerged below 40 K. At temperatures below 40 K, a Griffiths-phase-like inhomogeneous state appeared in the sample in which magnetic clusters and paramagnetic spins coexisted. The dc-paramagnetic and ac-susceptibilities exhibited an anomaly below 7 K.
Journal of Magnetism and Magnetic Materials 01/2014; 350:161-166. DOI:10.1016/j.jmmm.2013.09.012 · 1.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report comprehensive measurements of the magnetic, transport, and thermal properties of the Heusler-type compound Fe2+xV1-xAl, with x values near the ferromagnetic quantum critical point, xc ∼ 0.05. At T ∼ 60 K, a prominent Schottky-like anomaly appeared in the specific heat; this anomaly was correlated with a smaller pseudo-gap formation in magnetic susceptibility, magnetoresistance, and thermoelectric power. Furthermore, a magnetic anomaly observed in the magnetic susceptibility and resistivity at T ∼ 4 K was suppressed significantly by applying a magnetic field. A magnetically inhomogeneous phase arose below T ∼ 60 K, which appeared to consist of ferromagnetic and paramagnetic clusters.
[Show abstract][Hide abstract] ABSTRACT: In this study, we report a rapid and simple technique for obtaining cobalt aluminate having a spinel structure. The products were prepared from a hydroxide precursor synthesized by coprecipitation of cobalt (Co(2+)) and aluminum (Al(3+)) nitrates with an alkaline solution. The chosen precursor enabled low temperature fabrication of cobalt aluminate with a spinel structure by sintering it for 2 hours at low temperatures (>400 °C). Crystallographic and thermal analyses suggest that the low-temperature-sintered products contain Co(3+) ions stabilized by chemisorbed water and/or hydroxide groups, which was not observed for products sintered at temperatures higher than 1000 °C. The color of the products turned from clear blue (Thenard's blue) to dark green when sintering temperatures were below 1000 °C. Magnetic quantities, Curie constants, and Weiss temperatures show a strong dependence on the sintering temperature. These findings suggest that there are mixed valent states, i.e. Co(2+) and Co(3+), and unique cation distributions at the different crystallographic sites in the spinel structure, especially in the products sintered at lower temperatures.
[Show abstract][Hide abstract] ABSTRACT: This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO2) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO3)2) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 °C for 24 h to obtain pure monoclinic-structured ZrO2 nanocrystals. The crystallization temperature of the ZrO2 phase was also greater than 200 °C. However, the products of reactions conducted at 200 °C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 °C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO2 nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.
Journal of Supercritical Fluids The 01/2013; 85. DOI:10.1016/j.supflu.2013.11.001 · 2.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have measured magnetic susceptibility, powder X-ray diffraction
(XRD), and diffuse reflectance spectra of RBiBa2O6
(R = La, Ce, Pr, Nd, Sm, Gd, Eu, and Dy). The valences of the rare earth
elements estimated from magnetic susceptibility were trivalent except
CeBiBa2O6 and PrBiBa2O6 in
which Ce and Pr were tetravalent. In PrBiBa2O6, a
significant effect of crystal fields on magnetic susceptibility was
observed below 200 K. The (111) reflection in XRD, which is evidence for
the R3+--O--Bi5+ ordering in the double-perovskite
structure, was observed in all samples except
CeBiBa2O6. In PrBiBa2O6, a
weak (111) reflection was observed suggesting that a small amount of
Pr3+ ions were present. The band gaps estimated from their
optical spectra were approximately 0.7 eV for
CeBiBa2O6, 1.1 eV for
PrBiBa2O6, and 1.4--1.7 eV for
R3+BiBa2O6. Thus the band-gaps were
closely related to the valences of RBiBa2O6. We
discuss the relationship between these physical properties and the
photocatalytic activities previously reported.
Japanese Journal of Applied Physics 12/2012; 51(12):1802-. DOI:10.1143/JJAP.51.121802 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We succeeded in simple and rapid synthesis of surface-modified monoclinic ZrO2nanoparticles using a supercritical hydrothermal method. The precursor Zr(OH)4 was treated in the presence of various surface modifiers with carboxyl group (–COOH) in a batch-type reactor at 400 °C for 10 min. Oleic, sebacic, dodecanedioic, and 12-aminododecanoic acids were used as surface modifiers. Addition of surface modifiers resulted in smaller particle (crystallite) sizes than the unmodified nanoparticles, suggesting that there is an interaction between the surface modifiers and the nanoparticles. The reduced particle size in the presence of surface modifiers was attributed to the inhibition of the growth of the crystalline surface due to the surface modification. The FT-IR spectra revealed that the surface modifiers were attached to the surface of the nanoparticles through coordination bonds between the carboxylate group (–COO−) and the Zr ion. The FT-IR spectra also confirmed the presence of functional groups, such as methyl (–CH3), carboxyl (–COOH), and amine (–NH2), at the surface. The surface modification was also verified by the thermogravimetric analysis. The number of the surface modifiers attached to the surface of the products was about 2 molecules per nm2. The nanoparticles with carboxyl and amine surface functional groups were water dispersible; the isoelectric point shifted to low pH ranges because of the nature of the groups.
[Show abstract][Hide abstract] ABSTRACT: A simple and rapid technique was established for synthesis of ZrO2nanoparticles from zirconium ethoxide (Zr(OEt)4) and zirconium hydroxide (Zr(OH)4) as the precursors, using the sub- or supercritical hydrothermal method. The precursors were treated in a batch-type reactor with the reaction temperatures between 200 and 500 °C for 10 min. The products obtained at temperatures higher than 300 °C had mixtures of tetragonal and monoclinic phases. The higher the reaction temperature, the higher the volume fraction of the monoclinic phase. Selection of a suitable precursor is the most important point in this synthesis technique. Zr(OH)4 was found to be a better precursor for obtaining ZrO2 with a higher volume fraction of monoclinic phase than Zr(OEt)4 under the same synthesis conditions. Single phase pure monoclinic ZrO2 was successfully obtained after heat treatment at 500 °C. The growth rate of ZrO2nanoparticles obtained from Zr(OEt)4 and Zr(OH)4 was different. The products contained some chemisorbed water and/or a hydroxidegroup on the surface. Their presence was dependent on the reaction temperature and the particle size, and not on the precursor used. However, there was no significant difference among the band gaps of ZrO2 synthesized from different precursors, confirming that the type of precursor does not affect the quality as much as the crystallinity of the products.
[Show abstract][Hide abstract] ABSTRACT: We report on comprehensive measurements of the magnetic, transport, and thermal properties of the Heusler type compound Fe2+xV1−xAl at −0.05 ≤ x ≤ 020. We show that while stoichiometric Fe2VAl is a nonmagnetic semimetal (or narrow-gap semiconductor), a substitution on the nominal V-sites with the Fe atom leads to a ferromagnetic ground state above x = xc (∼0.05) with a rising Curie temperature TC and an ordered moment Ms. At x = 0.1 and 0.2, the reduced value of the ratio Ms/Peff ≪ 1, where Peff is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H,T), shows magnetism is itinerant. At a higher temperature T ∼ 60 K, a Schottky anomaly in specific heat C is indicated prominently at x > 0, while the anomaly is observable in the whole experimental range of x. At a lower temperature, an electronic component ΔC/T in specific heat shows a divergence that arises at both x < 0 and x > 0. The resistivity–temperature curve ρ(T) in the vicinity of the ferromagnetic quantum critical point, x ∼ 0.05, shows a non-Fermi liquid behavior, ρ ∼ Tn (n ∼ 1), above H > 20 kOe.
[Show abstract][Hide abstract] ABSTRACT: The MgB2 superconductor is a promising material for the coil conductors in the superconducting magnets. The anisotropy of the upper critical field of MgB2 is one of the limiting factors to the critical current density especially in the high magnetic field. The correlation between the critical current density and the microstructure of the MgB2 core part in the tape processed by the ex situ powder-in-tube method was investigated to obtain a clue for improving the critical current density values. The anisotropy in the field dependence of the critical current density was observed with respect to the angle between the applied magnetic field and the tape surface. Both the pole figure data in the x-ray diffraction analysis and the grain orientation map data in the electron back scattering pattern analysis indicated that the texture of the (001) plane occurred in the MgB2 core part of the tape quantitatively. This result suggests that the critical current density can be enhanced through the control of the microstructure of the MgB2 superconductor tapes.
IOP Conference Series Materials Science and Engineering 10/2011; 18(15). DOI:10.1088/1757-899X/18/15/152003
[Show abstract][Hide abstract] ABSTRACT: We have succeeded in the simple and rapid synthesis of the hydrophilic polymer-modified CeO2nanoparticles using a supercritical hydrothermal method. To prepare the nanoparticles, Ce(OH)4 as precursor was treated in a batch-type reactor with supercritical water in the presence of either polyvinyl alcohol (PVA) or polyacryl acid (PAA) as surface modifiers. The hydrophilic polymers attached to the surface of the CeO2nanoparticles by the coordination bond between the functional groups, such as hydroxyl (–OH) or carboxyl (–COOH), of the polymers and the Ce atoms. The amount of the attached polymers on the surface of the CeO2nanoparticles tended to increase with a decrease in the molecular weight of the polymer. The morphology and the particle size of the nanoparticles were cuboctahedral and about 20 nm, respectively. The nanoparticles were dispersed in water by virtue of the functional groups on the polymers. Notably, the ζ potential of PAA-modified CeO2nanoparticles did not become zero in the measured pH range between 3 and 11. Interestingly, the surface modification by the polymers controlled the band gap of the nanoparticles, suggesting the possibility of tuning the electronic and the optical properties of the metal oxidenanoparticle by modifying their surface with organic molecules.
[Show abstract][Hide abstract] ABSTRACT: We report measurements of the magnetic, transport, and thermal properties of the Heusler-type compound Fe2VAl0.95. We show that while stoichiometric Fe2VAl is a nonmagnetic semimetal a 5% substitution on the Al site with the 3d elements Fe and V atoms leads to a ferromagnetic ground state with a Curie temperature TC=33±3 K and a small ordered moment μs=0.12μB/Fe in Fe2VAl0.95. The reduced value of the ratio μs/μp=0.08, where μp=1.4μB/Fe is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H,T), show magnetism is of itinerant nature. The specific heat shows an unusual temperature variation at low temperatures with an enhanced Sommerfeld coefficient, γ=12 mJ K−2 mol−1. The resistivity, ρ(T), is metallic and follows a power law behavior ρ=ρ0+ATn with n≈1.5 below TC. With applying pressure, TC decreases with the rate of (1/TC)(dTC/dP)=−0.061 GPa−1. We conclude substitution on the Al site with Fe and V atoms results in itinerant ferromagnetism with a low carrier density.
[Show abstract][Hide abstract] ABSTRACT: The influence of both Mg grain size and ball milling time on the phase formation of MgB2 tapes has been studied. The finer grain sizes of Mg powder, under 45 µm, and the well-milled condition above 8 h improve the Jc value effectively. It was found that the Jc values of the MgB2 tapes were consistent with the results of thermogravimetry and differential thermal analysis and of x-ray diffraction studies, and the Mg grain sizes and the ball milling time affect the promotion of the reaction of mixed powders and the Jc value of MgB2 tape. The highest Jc value was estimated to be about 3200 A cm−2 at 4.2 K in a magnetic field of 10 T.
[Show abstract][Hide abstract] ABSTRACT: This work investigated methods of fabricating Cu-sheathed MgB2 wire, and succeeded in producing samples with enhanced Jc–B performance. Initially, MgB2/Cu wires were fabricated using either insitu or ex situ routes of the powder in tube (PIT) technique. The Jc–B performance of insitu MgB2/Cu wire is better (especially at lower magnetic fields) than that of the ex situ MgB2/Cu wire. However, the absolute Jc value was not particularly high (Jc at 6 T ≈ 3.4 × 102 Afirstname.lastname@example.org K). As the next step, we tried to fabricate MgB2/Cu wire by the double core method, which enabled us to obtain wire in which the outer and inner cores correspond to ex situ and insitu wires, respectively. In this case, the Jc–B performance is drastically enhanced and the Jc at 6 T was 3.5 times higher (≈1.2 × 103 A/cm2) than that of simple insitu MgB2/Cu wire.The double core method does not require special expensive tools, and there are many possibilities for future improvements in the wire. Therefore, we believe that this double core MgB2/Cu wire has a strong potential for the industrial fabrication of MgB2 wire with in-demand sheath materials such as Cu or Al.
Physica C Superconductivity 10/2009; 469(15):1531-1535. DOI:10.1016/j.physc.2009.05.227 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The J <sub>c</sub>- B performance of ex-situ MgB<sub>2</sub> tape is thought to depend on the grain-boundary coupling between the grains of an ex-situ MgB<sub>2</sub> core and between particles within a grain. This work investigates the influence of these area-dependent factors on the J <sub>c</sub>- B performance and the measured grain connectivity of ex-situ MgB<sub>2</sub> cores. While the influence of area factors on the J <sub>c</sub> - B performance of ex-situ MgB<sub>2</sub> tapes is small, the effect on measured grain connectivity is larger. Therefore, the relationship between the J <sub>c</sub> - B performance of ex-situ MgB<sub>2</sub> tape and grain connectivity should carefully take this phenomenon into account. Moreover, this study reveals that different ways to evaluate the grain connectivity have advantages and disadvantages for detecting the influence of the area factors. We propose an evaluation guideline for the grain connectivity of ex - situ MgB<sub>2</sub> cores in order to analyse the reasons for area-dependent changes. We believe that this work will improve our understanding of grain connectivity and allow it to be used as feedback in investigations of the fabrication of ex-situ MgB<sub>2</sub> tape with high J <sub>c</sub> - B performance.
[Show abstract][Hide abstract] ABSTRACT: We prepared Bi2212 round wires using the isothermal partial melting process, in which partial melting and solidification are carried out at the same temperature but in changing oxygen partial pressure. We found that the homogeneity of Bi2212 phase inside the wire was important for the J <sub>c</sub> performance of the wire and the homogeneity was improved by extending the heat treatment time. The J <sub>c</sub> value of 6.6times10<sup>4</sup> A/cm<sup>2</sup> at 4.2 K and 10 T were obtained. We can expect further enhancement of J <sub>c</sub> property by optimizing the heat treatment conditions and improving the homogeneity of Bi2212 filaments of the wires.
[Show abstract][Hide abstract] ABSTRACT: We report magnetism of spinel Co3O4 nanocrystals with average diameters of ∼25 and ∼100 nm under magnetic fields up to 50 kOe. The nanoparticles were controlled to be cubic in shape. In the smaller nanocrystal sample (d∼25 nm), a spontaneous magnetic moment increased steeply below the Néel point, TN=32 K of the bulk sample, whereas similar to bulk samples the larger crystal sample (d∼100 nm) exhibited a maximum in magnetization around TN. A spin-glass-like behavior in magnetization was observed at Tt=10 K at lower magnetic fields, whereas above H=30 kOe, this behavior disappeared.
Physical Review B 04/2009; 79(14):144411. DOI:10.1103/PhysRevB.79.144411 · 3.74 Impact Factor