Yoichi Nishino

Phase stability of Fe3Al with addition of 3d transition elements

In the (Fe1−x Mx)3Al system, the lattice parameter of the DO3 phase decreases with composition for M = V and Cr, maintaining the single phase state up to x = 0.4 and x = 0.25, respectively, whereas it increases for M = Ti and Mn at a smaller rate than those expected from bee Fe solid solutions. Both the cases consistently indicate an enhanced cohesion of the DO3 superlattice. In particular, the phase stabilization induced by M = V, as well as by M = Ti, is closely related to a strong preference of the substituted atoms for the FeI site (with eight Fe nearest neighbors) in the DO, structure. The electrical resistivity for M = Ti, V, Cr and Mn shows an abnormal negative temperature dependence. At a temperature higher than 800 K, an inflection appears on the resistivity curves as a sign of the D03-B2 transformation. The substitution of Ti and V is particularly effective for stabilizing the DO3 phase at high temperatures up to 1200 K and above, whereas M = Mn causes a modest increase and M = Cr gives no change in the transformation temperature. The solute effect on the transformation temperature could be related to both the atomic size mismatch and the variation of electron concentration.

Electronic structure of possible 3d `heavy-fermion' compound

Recent transport, specific heat and magnetization measurements indicated that the Heusler-type compound is a candidate for a 3d heavy-fermion system. As a first step towards a detailed theoretical understanding of the observed anomalous electronic properties of this compound, we have performed first-principles electronic structure and total energy calculations for and . The calculated lattice constants and magnetic moments are in good agreement with experiments. Remarkably, we find that, unlike , Heusler-type is a nonmagnetic semimetal with a narrow pseudogap at the Fermi level. Furthermore, our calculations suggest that the observed large enhancement of electronic specific heat coefficient is largely caused by the mechanisms such as spin-fluctuations rather than electron - phonon coupling in . The relations between the existence of the Fermi surface and negative temperature dependence of the low-temperature electrical resistivity in are discussed and further experimental and theoretical work is suggested.

Strong valence fluctuation in the quantum critical heavy fermion superconductor β-YbAlB4: a hard x-ray photoemission study.

Electronic structures of the quantum critical superconductor β-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and β-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in β-YbAlB4.

Phase stability and mechanical properties of Fe3Al with addition of transition elements

Abstract The mechanical properties of D0 3 -type (Fe 1 − x M x ) 3 Al alloys with M = Ti, V, Cr, Mn and Mo have been investigated using a Vickers microhardness indentation technique. Electrical resistivity measurements demonstrate that the substitution of M = Ti, V and Mo results in a remarkable stabilization of the D0 3 structure relative to the B2 structure, probably because of the site preference of the substituents. At temperatures above 600 K, the hardness increases with increasing temperature and reaches a maximum at around 800 K. The anomalous peak in hardness extends to higher temperatures in parallel with increase in the D0 3 -B2 phase transformation temperature. In particular, the substitution of M = V leads to a significant improvement in the high-temperature strength up to 1000 K and above, maintaining the peak hardness almost constant.

Thermoelectric properties of supersaturated Re solid solution of higher manganese silicides

In this study, we developed a higher manganese silicide (HMS) that possesses a high dimensionless figure of merit ZT exceeding unity. HMSs containing a larger amount of Re than its solubility limit were prepared by the liquid quenching technique, and the obtained metastable HMSs showed good thermal stability to enable pulse current sintering at 1240 K. The lattice thermal conductivity was effectively reduced with increasing Re concentration, whereas the electron transport properties were not greatly affected. Consequently, the ZT of p-type HMS increased to 1.04 at 6 at. % Re from 0.4 of the Re-free sample.

Electronic, magnetic and transport properties of (Fe1-xVx)3Al alloys

We report electronic, magnetic and transport properties of (Fe1-x Vx )3 Al alloys with x = 0 - 0.38 and analyse the results on the basis of the measured electronic specific-heat coefficient, the Debye temperature and the magnetic stiffness constant. As the V composition increases, the electrical resistivity increases rapidly at low temperatures and the magnetization decreases significantly in parallel with a sharp reduction in the Curie temperature. In particular, the Heusler-type Fe2 VAl compound (x = 0.33) is found to be in a marginally magnetic state and to exhibit a semiconductor-like behaviour with the resistivity reaching 3000 µ cm at 2 K. Low-temperature specific heat studies demonstrate a substantial decrease in carrier concentration with the V substitution, being consistent with recent band calculations, which predict that Fe2 VAl is a nonmagnetic semimetal with a sharp pseudogap at the Fermi level. A large mass enhancement deduced from the electronic specific-heat measurements suggests that Fe2 VAl is a possible candidate for a 3d heavy-fermion system. The unusual electron transport is mainly attributed to the effect of strong spin fluctuations, in addition to the existence of very low carrier concentrations.

Microstructural evolution and stability of (Fe1−xVx)3Al alloys in relation to the electronic structure

Abstract The evolution of the microstructure of (Fe 1− x V x ) 3 Al alloys in the D0 3 structure has been studied as function of composition by transmission electron microscopy. The domain size of the D0 3 structure significantly increases as V is added to the alloy. An increase in long range order parameter is also shown. This microstructural evolution is discussed in terms of the increased second nearest neighbor interactions induced by the V additions. Electronic structure calculations are also presented in this paper. The density of states shows a strong pseudogap for the Fe 2 VAl compound and thus strong hybridization induced by the alloying element. This electronic structure feature is discussed in terms of interesting physical properties of the system and in relation to the density of states, stability and properties of similar compounds [Fe 2 MAl (M=Ti, Cr, Mn)]. Electron energy loss spectra of the Al L 2–3 edge show that the strong hybridization induced by V does not affect the unoccupied Al s and d states.

Thermoelectric properties of Al?Mn?Si C40 phase containing small amount of W or Ta

The lattice thermal conductivity of the Al–Mn–Si C40 phase was effectively reduced by substituting a small amount of W for Mn while maintaining a large Seebeck coefficient |S| > 100 µV/K and a low electrical resistivity ρ < 1.7 mΩ cm. The Al–Mn–Si-based C40 phase containing a small amount of W showed essentially the same behaviors of both Seebeck coefficient and electrical resistivity as those of a W-free sample at the same carrier concentration. This experimental result was consistent with the first principles calculations that predicted the absence of impurity states in the energy gap in association with the W atom at the Mn site. The lattice thermal conductivity, on the other hand, markedly decreased owing to the heavy element impurity scattering effect of phonons. Consequently, the ZT of the n-type Al27.5Mn29.0W3.0Fe1.0Si39.5 C40 phase increased to a value that was ~4 times larger than that of the W-free sample.

Unusual electron transport in Heusler-type Fe2VAl compound

Abstract The electrical resistivity of D03-type (Fe1−xVx)3Al shows an anomalous temperature dependence characterized by a resistance maximum near the Curie point and a negative resistivity slope at higher temperatures up to 1000 K and above. In particular, the Heusler-type Fe2VAl compound is found to be in a marginally magnetic state and to exhibit a semiconductor-like behavior with the resistivity reaching 3000 μΩ cm at 2 K, in spite of the presence of a clear Fermi edge as revealed in valence-band photoemission spectra. A substantial mass enhancement deduced from electronic specific-heat measurements suggests that Fe2VAl is a possible candidate for a 3d heavy-fermion system. According to electronic structure calculations, Fe2VAl is a nonmagnetic semimetal with a narrow pseudogap at the Fermi level. The unusual electron transport is mainly interpreted in terms of the effect of strong spin fluctuations, in addition to the existence of very low carrier concentrations.

Thermoelectric properties of Heusler-type off-stoichiometric Fe2V1+xAl1−x alloys

We report the thermoelectric properties of Heusler-type off-stoichiometric Fe2V1+xAl1�x alloys. Due to the off-stoichiometric effect, which is the substitution of V/Al atoms with Al/V atoms, semiconductor-like electric resistivity behavior in Fe2VAl is changed to metallic behavior in Fe2V1+xAl1�x alloys and both positive and negative absolute Seebeck coefficients are drastically increased. The maximum thermoelectric power factor of Fe2V1+xAl1�x alloys is 4.3 × 10 �3 (x = �0.03: p-type) and 6.8 × 10 �3 WmK �2 (x = 0.05: n-type) with a peak temperature in the range 300–600 K, exceeding the values of previously reported Fe-based Heusler alloys as well as those of available thermoelectric materials such as Bi-Te semiconductors. Based on x-ray diffraction and photoemission spectroscopy results, it is thought that the maintenance of the Heusler-type (L21) crystal structure and the modification of the electronic structure due to the offstoichiometry could explain the large thermoelectric power factor and high peak temperature in Fe2V1+xAl1�x alloys.