Experimental studies carried out by the present author and his collaborators for last 35 years are described very briefly. Systems used in these studies are oxide conductors (transition metal brozes), high-T_c Cu oxides, Na_xCoO₂⋅yH₂O, Fe-based superconductors, quantum spin systems with spin-gap, so-called strongly correlated electron system, and so on. Throughout the studies, the neutron scattering measurements have been one of main methods both in the search for systems with new physics and/or in getting a decisive answer to important scientific issues, proving that neutron facilities and their development seem to be indispensable to future scientific works.

We have started a polarized ³He neutron spin filter project in order to increase utilization efficiency of polarized neutrons in Japan. We adopted the on-beam spin exchange optical pumping (SEOP) technique to polarize the nuclear spins of ³He atoms to keep stable polarization performance of the ³He neutron spin filter. We performed neutron beam tests of our compact in-situ SEOP neutron spin filter (NSF) system at the NOP beamline in the Japan Research Reactor No.3 (JRR-3) and the BL10 beamline in the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). The ³He gas polarization of 73% was achieved. Now we are developing a new design in-situ SEOP NSF system as an incident beam polarizer for JRR-3. We also performed a polarized neutron scattering measurements of Ni at BL10. This paper gives the present status of the development of in-situ SEOP system in J-PARC.

This article is a part of record of a number of technical issues which we had encountered at during design and construction of the world best pulse neutron source at J-PARC. Among issues in the project, three critical decisions on the mercury target, AIC de-coupler adoption, and the para hydrogen coupled moderator, are highlighted from a view point of large impact on performances of the J-PARC pulse neutron source.

Neutron holography is a relatively new method, which can offer 3D nucleus images. We performed the neutron holography experiment for the first time in Japan. In the present article, theory, experimental setup and result of PdH_0.78 sample are introduced. The observed neutron intensity distribution around the sample includes thermal diffuse scattering rather than hologram data. A pure hologram was extracted by subtracting accurately calculated diffuse scattering pattern from the observed intensity distribution, and then, nucleus images around H were successfully reconstructed. In addition to this PdH_0.78 result, I discuss perspective of neutron holography with a topic of X-ray fluorescence holography.

Magnetism of interpenetrating FeAs strips in the orthorhombic CaFe₄As₃ was examined through neutron diffraction. Incommensurate and predominantly longitudinally (||b) modulated order develops through a 2nd order phase transition at T_N ～ 90K. A 1st order transition at T₂ ～ 26K is associated with the development of components in a separate irreducible representation, locking the wave vector to 3b^*/8. The ab-initio Fermi surface features sheets separated by near the observed wave vector. However, Fermi surface nesting seems to have a limited role, instead magnetic structures could result from competing 2nd and 3rd nearest neighbor interactions in a localized spin picture.

The study of slow lattice dynamics using neutron spin echo technique on relaxor ferroelectrics is reviewed. At much higher temperature than T_C, we observed oscillating normalized intermediate scattering functions I(Q,t)/I(Q,0) for the [1-10] direction, which is ascribed to low frequency vibrational mode with ω～40μeV. Such low frequency vibrational mode suggests that flat energy surfaces exist in the free energy potential along the [1-10] direction, which could be associated with giant dielectric susceptibility in relaxor ferroelectrics near morphotropic phase boundary.

New types of neutron spin echo using Resonance Spin Flippers (RSFs) have been developed which are called Modulated IntEnsity by Zero Effort (MIEZE) and Neutron Resonance Spin Echo (NRSE). In NRSE, two RSFs replace a homogeneous static magnetic field for spin precession in the Mezei-type NSE spectrometer. In the MIEZE spectrometer, the sample is placed after the analyzer and there are no optical components between the sample and the detector. The principles of the NRSE and MIEZE techniques are introduced.

Pulsed neutron imaging can give information on crystal texture, crystallite size and strain by analyzing the Bragg edge structure of neutron transmission spectrum, and also on the bound state difference of incoherent scatterers such as hydrogen by analyzing gradient or structure of the transmission spectra. Furthermore, it has capability to identify materials and to measure temperature of an object by analyzing the resonance transmission. Here, we describe the principle of this method and some applications.

Visualization of magnetic fields is one of the most attractive applications for the neutron imaging technique. In this technique, the change of the neutron polarization due to the Larmor precession in a passage through a magnetic field can be projected in a two-dimensional image. This article provides a general introduction of this technique and shows some recent results of magnetic field imaging using polarized neutrons.