A lot of frustrated magnetic systems exhibit a nontrivial magnetic order, such as long-wavelength modulation, noncollinear, or noncoplanar order. The nontrivial order may pave the way for the novel magnetic function of matter. Neutron studies are necessary to determine the magnetic structures in the frustrated magnetic systems. In particular, spin-polarized neutron scattering is a useful technique for the investigation of the novel physical properties relevant to the nontrivial spin arrangement. Here some neutron studies on a multiferroic perovskite manganese oxide system are demonstrated as a typical case. The frustrated magnetic systems may also a playground of novel types of local magnetic excitations, which behave like particles in contrast to the magnetic waves. It is becoming a good challenge to study such particle-type magnetic excitations relevant to the magnetic frustration.

Spin-lattice coupling plays an important role in selecting the ground state in the geometrically frustrated magnets, since a small amount of structural distortion is sufficient to lift the ground state degeneracy and stabilize a long-range magnetic order. Ag₂CrO₂ consists of insulating triangular lattice planes of CrO₂ (Cr³⁺ ion with S=3/2), which are separated by the metallic Ag₂ layers. Interestingly, the electric transport in the Ag₂ layer is strongly affected by the magnetism in the CrO₂ layer. We performed neutron diffraction experiments on this material and found that a partially disordered (PD) state with 5 sublattices abruptly appears at T_N=24 K, accompanied by a structural distortion. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. The nonnegligible further-neighbor interactions suggest the existence of the RKKY interaction mediated by the conduction electrons. We have recently performed inelastic neutron scattering experiments and found anomalous magnetic excitations, which cannot be explained simply by the linear spin-wave theory.

The ground states of ideal one-dimensional frustrated magnets including singlet dimer, Haldane dimer, vector chiral, spin density wave, and bond Nematic states are reviewed and compared with the states realized in realistic model compounds. In the ferromagnetic frustrated chain LiCuVO₄ the observed spiral spin structure in zero field is ascribed to the classical analogue of the vector chiral state. Meanwhile the spin density wave at high field is purely quantum state induced by geometrical frustration and the nesting of spinon Fermi surface.

At sufficiently low temperatures, electron spins in normal magnets generally order into some fashion, for instance, ferromagnetic and antiferromagnetic. Geometrical frustration and/or reduced dimensionality can suppress such conventional orders, and occasionally induce unknown states of matter. This is the case for the two-dimensional (2D) triangular antiferromagnet NiGa₂S₄, in which S = 1 nickel spins do not order, instead show an exotic magnetism. We found (1) a resonant critical slowing down toward T^* = 8.5 K followed by a viscous spin liquid behavior, and (2) a spin-size dependent ground state. To elucidate (1), spin dynamics ranging from 10^-13 to 10⁰ seconds were quantitatively explored through the experimental techniques such as inelastic neutron scattering, backscattering, neutron spin echo, ac and nonlinear susceptibilities. The finding of (2) is evidenced by impurity effects. Integer spins substituted systems such as zinc and iron ions retain a quadratic temperature dependence of the magnetic specific heat as for the parent compound. However, substitutions of half-odd integer spins, cobalt and manganese ions, eventually induce a distinct behavior, indicating an importance of integer size of spins to stabilize the 2D magnetism realized in NiGa₂S₄. The article gives our experimental findings and as well as some relevant theoretical scenarios.

Ground states and magnetic excitations of spin-1/2 kagomé antiferromagnets (KAF) A₂Cu₃SnF₁₂ (A = Cs and Rb) were investigated by the magnetic measurements and neutron scattering experiments. For A = Cs, the system undergoes antiferromagnetic phase transition at T_N = 20K. On the other hand, for A = Rb, non-magnetic “pinwheel” VBS state is realized at the ground state which is assisted by the lattice distortion to form an exchange network consisted of four different magnetic interactions. The contrasting ground states between two compounds should be caused by the difference of the lattice distortion.

We report chiral magnetic ordering in ferromagnetic inorganic compounds T_1/3MS₂ (T = Cr and Mn, M = Nb and Ta) with a chiral space group of P6₃22. We measured the magnetization, powder neutron diffraction and small angle electron diffraction experiments. The experimental results indicate formation of long periodic helimagnetic ordering and chiral magnetic soliton lattice, and were in good agreement with the theoretical prediction based on the chiral magnetic soliton formation. Therefore, we propose that the magnetic ordering in ferromagnetic T_1/3MS₂ is not ferromagnetic, but chiral magnetic ordering.

Frustrated magnets often display spin-lattice coupled exotic ordered states in order to lift the degeneracy of their ground states. This implies the possibility that the magnetic ordering in the frustrated magnets can be controlled by application of uniaxial stress, which directly and anisotropically affects the lattice degree of freedom. In the present study, we have performed neutron diffraction, magnetic susceptibility and pyroelectric measurements on a frustrated magnet CuFe_1-xGa_xO₂ (x = 0, 0.018, 0.035), which also exhibits spin-driven ferroelectricity, under applied uniaxial stress up to 100 MPa, revealing that the magnetic phase transitions and ferroelectric polarization significantly depend on applied uniaxial stress.

Neutron scattering study and measurements of magnetic and dielectric properties have been studied for quasi one-dimensional spin 1/2 systems which are formed of edge-sharing CuO₄ square planes called CuO₂ ribbon chains. Due to the geometrical characteristic of the crystal structure of these systems, the nearest-neighbor exchange interaction between spins is ferromagnetic, and the second neighbor interaction is antiferromagnetic. The CuO₂ ribbon chain systems are typical examples for the frustrated quantum spin systems driven by the competing interactions. We found that LiVCuO₄ and PbCuSO₄(OH)₂ with CuO₂ ribbon chains have helical magnetic order and exhibit a ferroelectric transition with the magnetic transition, simultaneously (called multiferroic). For the CuO₂ ribbon chain systems, exotic quantum phases are theoretically predicted such as Haldane-dimer, spin-nematic, quadrupolar-order, and chiral-order phases. Rb₂Cu₂Mo₃O₁₂ with CuO₂ ribbon chains does not exhibit the long range ordering due to the quantum spin fluctuation and low dimensionality. For Rb₂Cu₂Mo₃O₁₂, the ferroelectric transition is found to be induced by applying field without magnetic transition, which is a new type ferroelectric transition triggered by the magnetism of frustrated quantum spin systems. The obtained results strongly suggest that Rb₂Cu₂Mo₃O₁₂ exhibits some exotic quantum states.

Polyrotaxane (PR) is a typical supramolecule, in which cyclic molecules are threaded onto a linear polymer chain. The cyclic molecules in PR can slide and rotate on the polymer chain. PR has been applied to a novel kind of polymer network, called “slide-ring (SR) gel”, which is prepared by cross-linking cyclic molecules belonging to different PRs. The cross-linking point of SR gels can slide along the polymer chains, and this unique structure causes the high extensibility of SR gels. We have investigated the structure and dynamics of PR at molecular level by contrast variation small-angle neutron scattering and neutron spin echo. The contrast variation methods allow us to elucidate the structure and dynamics of rings and polymers separately. Furthermore, we studied the macroscopic mechanical properties of SR gels and proposed a novel molecular theory for SR gels, which considered the alignment entropy of cyclic molecules on polymer networks.

We have investigated the lipid sorting in a binary small unilamellar vesicle (SUV) composed of cone-shaped (1, 2-dihexanoyl-sn-glycero-3-phosphocholine: DHPC) and cylinder-shaped (1, 2-dipalmitoyl-sn-glycero-3-phosphocholine: DPPC) lipids. In order to reveal the lipid sorting we adopted a contrast matching technique of small angle neutron scattering (SANS), which extracts the distribution of deuterated lipids in the bilayer quantitatively. The SANS profile of deuterated SUVs at the contrast matching condition showed a characteristic scattering profile, indicating an asymmetric distribution of cone-shaped lipids in the bilayer. The fitting of the observed SANS profile revealed that most DHPC molecules are localized in the outer leaflet, which supports that the shape of the lipid is strongly coupled with the membrane curvature. We compared the obtained asymmetric distribution of the cone-shaped lipids in the bilayer with the theoretical prediction based on the curvature energy model.

Recently, polymeric materials have been widely used in medical applications. In these cases, the surface is supposed to contact with a different phase. Thus, the polymer interfaces contacted with different phases should be studied, structurally and dynamically. Density profiles of a perdeuterated poly (methyl methacrylate) (dPMMA) film in water, hexane, methanol, ethanol, 1-propanol, and 1-buthanol were examined along the direction normal to the interface by neutron reflectivity (NR). Although these liquids are typical non-solvents for dPMMA, the liquid/polymer interfaces were diffuse in comparison with the air/polymer interface probably due to the partial dissolution of segments at the outermost region of the film. Interestingly, in water, the anomalous swollen layer was formed beneath the diffused interface. On the other hand, such a swollen layer was not observed for the dPMMA films in alcohols. This is simply because alcohol molecules deeply penetrated into the films. The amounts of the alcohols penetrated in the films could be explained in terms of the interaction parameter. The fractional amount of non-solvents at the substrate interface was higher than that in the internal region of the film. Furthermore, aggregation structure of poly (2-methoxyethyl acrylate) film, which has been used as a coating material for biomedical apparatus due to the excellent blood compatibility, was examined in water

Free programs for the maximum-entropy method (MEM) and three-dimensional visualization are introduced with emphasis on their applications to TOF neutron powder diffraction data. The importance of correcting for thermal diffuse scattering is stressed to improving the accuracy of results obtained by powder diffraction.

Neutron inelastic scattering techniques, including quasielastic and elasitic incoherent neutron scattering, provide unique tools to directly measure the protein dynamics at a picosecond time scale. Since the protein dynamics at this time scale is indispensable to the protein functions, elucidation of the protein dynamics is indispensable for ultimate understanding of the protein functions. There are two complementary directions of the protein dynamics studies: one is to explore the physical basis of the protein dynamics using "model" proteins, and the other is more biology-oriented. Examples of the studies on the protein dynamics with neutron inelastic scattering are described. The examples of the studies in the former direction include the studies on the dynamical transitions of the proteins, the relationship between the protein dynamics and the hydration water dynamics, and combined analysis of the protein dynamics with molecular dynamics simulation. The examples of the studies in the latter direction include the elastic incoherent and quasielasitc neutrons scattering studies of actin. Future prospects of the studies on the protein dynamics with neutron scattering are briefly described.

Hydrogen (H) atoms and proton (H⁺) of biomacromolecules are important for various biological phenomena. H atoms of functional groups such as -OH and -NH₂ make hydrogen bonds to contribute to folding and assembly of biomacromolecules. H⁺ ions are used for chemical reaction, electron transfer, and H⁺ concentration gradient is an energy source in cells.

X-ray crystallography is the most popular technique for structure determination of biomacromolecules, however it is very difficult to determine H atoms and almost impossible to identify H⁺ ions, because X-ray diffracts by electron cloud. On the other hand, neutron scattering length of H, H⁺, and its isotope (deuterium: D and D⁺) are comparable to those of non-hydrogen atoms in biomacromolecules. Therefore, the complementary use of X-ray and neutron crystallography provides detailed structural information of biomacromolecules including H and H⁺. So far, protein has been the main subject of biomacromolecular neutron crystallography; this technique is also called neutron protein crystallography (NPC). In this report, the basic principles of NPC are described.