Tomoyuki Terai

Evidence for preferential rearrangements of martensite variants by magnetic field in antiferromagnetic CoO crystal

The synchrotron high-energy x-ray diffraction provides the direct crystallographic evidence for the magnetic-field-driven preferential rearrangements of martensite multivariants in antiferromagnetic CoO crystal. When a magnetic field was incrementally applied up to 6 T on the CoO single crystal cooled below the Neel temperature, the martensite variants with the magnetization easy-axis parallel to the magnetic field direction (H) were consumed, while the variants with magnetic moments perpendicular to H were enhanced. The microscopic origin for the observation is discussed, which provides important information for understanding the magnetic-field-driven strain observed in the antiferromagnetic alloys, with a selection principle on martensite variants different from that found in the ferromagnetic shape memory alloys.

The influence of grain boundaries on the magnetoresistance in La0.7A0.3MnO3 (A = Ca, Sr) and La1.36Sr1.64Mn2O7

The influence of grain boundaries on the magnetoresistance in the perovskite manganites La 0.7 A 0.3 MnO 3 (A = Ca, Sr) and the layered perovskite manganites La 1.36 Sr 1.64 Mn 2 O 7 has been investigated by means of ac and dc magnetic susceptibility and magnetoresistance measurements. The following characteristics were found. (i) The magnetoresistance (MR) of single-crystal La 0.7 Ca 0.3 MnO 3 is almost independent of the magnetic field at low temperatures, and the MR of polycrystalline La 0.7 A 0.3 MnO 3 (A = Ca, Sr) and La 1.36 Sr 1.64 Mn 2 O 7 decreases drastically with the initial increment of the magnetic field up to 2.5 x 10 -1 T. (ii) The resistivity and the absolute value of the MR of polycrystalline La 0.7 Sr 0.3 MnO 3 increase with decreasing grain size at 4.2 K. (iii) The resistivity of these polycrystalline manganites shows a time-dependent nature, but the single-crystal form does not.

Grain size effect on martensitic transformation behavior in Fe-Ni invar alloys

We investigated the grain size effect on martensitic transformation behavior in Fe-30at.%Ni powder and ribbon specimens. The powder specimen with a particle size of 5 um does not show an athermal martensitic transformation but does show an isothermal martensitic transformation after an incubation time of about 104 s at 205 K. On the other hand, the powder specimen with a particle size of 20 um shows an athermal martensitic transformation at 150 K. The value of Ms is much lower than that of the single crystal and of bulk specimens. However, the Ms temperature of a ribbon specimen with an average grain size of 15 um is found to be almost identical to that of the single crystal and of bulk specimens. Considering these results, the athermal martensitic transformation is suppressed by the decrease in particle size if grains do not have grain boundaries.

P-ρ-T measurements of H2O up to 260 GPa under laser-driven shock loading

Pressure, density, and temperature data for H2O were obtained up to 260 GPa by using laser-driven shock compression technique. The shock compression technique combined with the diamond anvil cell was used to assess the equation of state models for the P-ρ-T conditions for both the principal Hugoniot and the off-Hugoniot states. The contrast between the models allowed for a clear assessment of the equation of state models. Our P-ρ-T data totally agree with those of the model based on quantum molecular dynamics calculations. These facts indicate that this model is adopted as the standard for modeling interior structures of Neptune, Uranus, and exoplanets in the liquid phase in the multi-Mbar range.

Stress and temperature dependence of the structure of the martensite and X-phase in Ni2MnGa

Stress and temperature dependence of the structure of the X-phase in Ni2MnGa has been investigated by neutron diffraction measurements to clarify the structural relationship among the parent (P-), intermediate (I-), martensite (M-) and X-phases. The satellite position and intensity of the X-phase differ from those of the I-phase under compressive stress, but approach those of the I-phase with increasing temperature and decreasing stress. In other words, the structure changes discontinuously with I → X transformation under compressive stress, but continuously under zero stress. On the other hand, the X → P transformation is continuous, regardless of stress or temperature. These results suggest the existence of a multicritical point for successive P → X → I transformation.

Static compression experiments for advanced coupling techniques of laser-driven dynamic compression and precompression target

Coupling laser-shock and static-compression techniques allows to generate material conditions unreachable by either single-shock or static technique alone. Static compression experiments on water were performed using a precompression cell for laser-driven shock experiments. The pressure on static compression in this work was reproductively 1.3 – 2.3 times higher than in previous works and theoretical predictions. We have also performed static compression experiments using a new anvil material (Gd3Ga5O12) to apply the cell to reflected shock compression. By coupling laser-driven reflected shock compression with precompression technique, it is possible to generate higher pressure and lower temperature range.

Significant static pressure increase in a precompression cell target for laser-driven advanced dynamic compression experiments

Laser shock compression experiments on precompressed samples offer the possibility to explore extreme material states unreachable by static or single-shock compression techniques alone. We have found significant increases in static compression pressure in a wide-opening and thin diamond precompression cell. This suggests that the precompression target is adaptable to advanced coupling techniques with laser-driven dynamic compression methods. The novel coupling techniques proposed give the potential to access outstanding material states required in planetary and condensed-matter physics.

Rearrangement of crystallographic domains driven by magnetic field in antiferromagnetic CoO

The rearrangement was investigated of crystallographic domains in the antiferromagnetic pseudo-tetragonal phase in CoO (Néel temperature: 293 K) when the domains were driven by a magnetic field. A rearrangement is generally observed in ferromagnetic shape-memory alloys. The rearrangement was found to occur at temperatures between 170 K and 293 K, but not at temperatures below 170 K. In order to determine the reason for such a difference, the shear stress driven by a magnetic field, τ mag, was calculated and compared with the shear stress required for twinning plane movement, τ req. It was found that τ mag is equal to or larger than τ req whenever the rearrangement of crystallographic domains occurs due to the application of a magnetic field, and vice versa. This observation is similar to past observations in the case of many ferromagnetic shape-memory alloys.

Structural Relation between the X-Phase and other Phases in Ni2MnGa

We have investigated stress and temperature dependences of the structure of the X-phase in Ni2MnGa to understand structural relation between the X-phase and other phases. Position and intensity of satellites of the X-phase are different from those of the intermediate (I-) phase under compressive stress, but they approach those of the I-phase with decreasing stress. That is, the structure change associated with the I → X transformation is discontinuous under a compressive stress, while it is continuous under zero stress. In addition, the transformation from the X-phase to the L21-type parent phase is continuous regardless of applied stress. These results strongly suggest the existence of multi-critical point in Ni2MnGa. On the other hand, the transformation from the X-phase to the martensite phase is discontinuous regardless of applied stress.

Magnetic field dependence of γ-α equilibrium temperature in Fe-Co alloys

We have investigated effect of magnetic field on γ(austenite)α(ferrite) equilibrium temperature T0 in Fe-xCo alloys with x = 0, 10, 20, 30 mol%. The α-phase at T0 is paramagnetism for x = 0 and 10, while it is ferromagnetism for x = 20 and 30. We have found that T0 increases almost proportional to magnetic field for the Fe-20Co and Fe-30Co alloys, while it does almost proportional to square of magnetic field for pure Fe and the Fe-10Co alloy. The increase in T0by applying magnetic field of 10 T is about 7 K for pure Fe, 18 K for the Fe- 10Co alloy, 24 K for the Fe-20Co alloy and 20 K for the Fe-30Co alloy. The results are discussed on the basis of the Clausius-Clapeyron equation.