Hideyuki Ohtsuka

Structural control of Fe-based alloys through diffusional solid/solid phase transformations in a high magnetic field

Abstract A magnetic field has a remarkable influence on solid/solid phase transformations and it can be used to control the structure and function of materials during phase transformations. The effects of magnetic fields on diffusional solid/solid phase transformations, mainly from austenite to ferrite, in Fe-based alloys are reviewed. The effects of magnetic fields on the transformation temperature and phase diagram are explained thermodynamically, and the transformation behavior and transformed structures in magnetic fields are discussed.

Significant third-order optical nonlinearity enhancement of gold nanoparticle incorporated mesoporous silica thin films by magnetic field thermal treatment

Thermal treatments from 600 to 1100 °C with and without 6 T external magnetic fields were applied to the prepared gold nanoparticle (NP) incorporated mesoporous silica thin films (MSTFs). Significantly enhanced third-order optical nonlinear susceptibilities of the thin films thermally treated with the magnetic field were achieved as compared with that of those thermally treated without the magnetic field. The enhancement reached almost one order of magnitude from 700 to 800 °C (i.e. at around the melting point of the gold NPs). The homogeneous dispersion of gold NPs can be maintained up to 1000 °C under the magnetic field compared to the exaggerated growth of gold NPs thermally treated without the magnetic field. The origin for such optical nonlinearity enhancement is discussed.

Magnetic field-induced off-resonance third-order optical nonlinearity of iron oxide nanoparticles incorporated mesoporous silica thin films during heat treatment.

Highly dispersed and uniform Fe(2)O(3) nanoparticles (NPs) have been incorporated into the pore channels of SBA-15 mesoporous silica thin films (MSTFs). And such Fe(2)O(3) NPs incorporated MSTFs did not show detectable nonlinear optical (NLO) signals at off-resonance wavelength 1064 nm by Z-scan technique. However after a vacuum heat treatment at 800 degrees C for 1 h under 6 T magnetic field, the Fe(2)O(3) NPs incorporated MSTFs with very low Fe content (0.8 approximately 1.5 at.%) presented distinctive NLO signals with chi(3) value in an order of 10(-10) esu. We proposed the physical reason for the NLO property generation to be the magnetic domain orientation of the iron oxide NPs incorporated within the pore channels of the MSTFs by the magnetic field heat treatment.

Size and aspect ratio of martensite in FeNiC alloys formed under applied magnetic field and tensile elastic stress at 4.2 K

Abstract The dimensions of fully grown martensite plates in Fe27Ni0.8C and Fe31Ni0.4C alloys formed at 4.2 K under applied magnetic field (10 Tesla) and tensile elastic stress (220 MPa) are reported. The plates formed in a cluster are classified into two categories during stereological measurements: the plates whose radial growth is limited by the austenitic grain boundaries only (Type I), and the plates whose radial growth is limited by the pre-existing martensitic plates only (Type II). It is demonstrated that the formation sequence of the plates has a strong effect on the radial growth and aspect ratio. The measured aspect ratio of type I plates in the Fe27Ni0.8C alloy agree very well with an earlier prediction based on the aspect ratio measurements in the temperature range of 193 to 77K in an Fe23.2Ni2.9Mn alloy. Both these alloys are non-invar. However, in the case of Fe31Ni0.4C alloy, which is expected to be invar, the measured aspect ratio is less than the predicted value at 4.2 K.

Effects of magnetic field on shape evolution of Fe–Co particles in a Cu matrix

The effects of magnetic field on the shape evolution of ferromagnetic fcc Fe–Co particles in Cu–0.83 at.% Fe–1.37 at.% Co alloy single crystals were examined using magnetic anisotropy measurements. The Cu–Fe–Co single crystals were aged at 993 K for 2 h to 24 h under a magnetic field of 10 T parallel to either the [001] or [011] direction. The magnetic anisotropy was examined by measuring magnetic torque around the (100) plane. It was found that the fcc Fe–Co particles are elongated in the direction parallel to the magnetic field. Furthermore, the elongation along [001] is more remarkable than that along [011]. The results are explained quantitatively by considering the minimization of the sum of the interface energy, elastic strain energy and magnetostatic energy of spheroidal particles.