T. Kashiwagi

Construction of a novel topological frustrated system: a frustrated metal cluster in a helical space.

A novel topologically frustrated pentanuclear cluster helicate [{Cu(II)(μ-L)(3)}(2)Cu(II) (3)(μ(3)-OH)](3+) (L(-)=3,5-bis(2-pyridyl)pyrazolate) has been synthesized and characterized. This cluster has a helical arrangement of ligands around the central metal core. Dzyaloshinsky-Moriya interactions are essential components to observe a gradual magnetization and forbidden transitions of high-field/multi-frequency (HF/MF)-ESR. The origin of the magnetic anisotropy of this compound is influenced by its helical spin structure, and consequently, the Cu(5)-cluster helicate introduces a unique magnetic anisotropy. This observation is a direct evidence of the topological part of the new spin phase in a magnetic system.

Magnetic phase diagram of the frustrated S = 1 chain magnet LiCu2O2

We present the results of the magnetization and dielectric constant measurements on untwinned single crystal samples of the frustrated S=1/2 chain cuprate LiCu_2O_2. Novel magnetic phase transitions were observed. A spin flop transition of the spiral spin plane was observed for the field orientations H||a,b. The second magnetic transition was observed at H~15 T for all three principal field directions. This high field magnetic phase is discussed as a collinear spin-modulated phase which is expected for an S=1/2 nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic chain system.

Development of an ultra-low temperature multi-frequency ESR apparatus

An ultra-low temperature multi-frequency electron spin resonance (ESR) apparatus has been developed for use at a temperature down to about 200 mK by utilizing a 3He-4He compact dilution refrigerator and a vector network analyzer. The microwave frequency can be varied between 30 GHz and 700 GHz almost continuously. Either of the two kinds of copper blocks with a sample holder for typical two configurations, namely Faraday and Voigt configurations, is attached to a stainless steel block neighboring to a mixing chamber which provides ultra-low temperarures down to 160 mK. A magnetic field up to 16 T is produced with a superconducting magnet (14 T at 4.2 K and 16 T at 2.2 K). Thus, we have a potential to perform ESR measurements in a very wide frequency-field window at ultra-low temperatures.

Development of a multi-frequency ESR system with high sensitivity

We have developed a new Multi-Frequency (MF) ESR system for the frequencies between 35 to 130 GHz utilizing TE011 single mode resonators. Their sensitivities (1010spins/G at 1.5 K) are comparable to that of a conventional low frequency ESR resonator and an order of magnitude higher than that of a Fabry Perot resonator which was previously developed by us. Thanks to a newly developed precise and stable matching system, we observed for the first time MFESR spectra of a metalloprotein with an integer spin.

Evidence for carrier-induced high-TC ferromagnetism in Mn-doped GaN film

A GaN film doped with 8.2% Mn was grown by the molecular-beam-epitaxy technique. Magnetization measurements show that this highly Mn-doped GaN film exhibits ferromagnetism above room temperature. It is also revealed that the high-temperature ferromagnetic state is significantly suppressed below 10u2009K, accompanied by an increase of the electrical resistivity with decreasing temperature. This observation clearly demonstrates a close relation between the ferromagnetism with extremely high TC and the carrier transport in the Mn-doped GaN film.

High field magnetization and multi-frequency ESR in the spin-ladder compound Na2Fe2(C2O4)3(H2O)2

We have performed high field magnetization and multi-frequency electron spin resonance (ESR) measurements on single crystal and powder samples of Na2Fe2(C2O4)3(H2O)2, abbreviated as SIO. The Fe2+ ions in SIO are bridged by oxalate groups to make two-leg ladders. The temperature dependence of magnetic susceptibilities parallel and perpendicular to the leg direction show very different and anisotropic behavior. Experimental results of the isomorphous compound Na2Co2(C2O4)3(H2O)2 (SCO) are well explained by isolated magnetic dimers with an Ising-type anisotropy. Therefore, we analyze the frequency dependence of the ESR resonance fields by the same model with a fictitious spin 1 as in SCO. The agreement between experiment and calculation is satisfactorily good. Magnetic susceptibility and magnetization curves calculated with the same parameters obtained in the ESR analyses, however, do not agree well with the experimental ones. This indicates the presence of the exchange interaction between the dimers, namely the interaction along the leg.

Multi-frequency ESR studies on a Haldane magnet in a field-induced phase at ultra-low temperatures

We report the results of multi-frequency electron spin resonance (ESR) measurements on single crystals of Ni(C5H14N2)2N3(PF6) which is regarded as the one-dimensional Heisenberg antiferromagnet with spin one, namely the Haldane magnet, at very low temperatures down to about 100 mK. We observed the lowest resonance branch below about 500 mK for the field along the chain direction (H||c), which was observed previously only in an inelastic neutron scattering experiment at 30 mK. We compare the resonance branch with that calculated by a phenomenological field theory, and discuss the field dependence and the temperature sensitivity of this ESR branch.

Multi-frequency ESR in the haldane magnet NDMAP below 1 K

The S=1 quasi-one dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6), abbreviated as NDMAP, has been studied by electron spin resonance (ESR) in a magnetic field exceeding the critical field (Hc) above which this compound shows a long range order (LRO) at sufficiently low temperatures. Our recently developed ESR apparatus with a dilution refrigerator is utilized to study the spin excitations of NDMAP above Hc for H∥c, because this compound exhibits the LRO below about 1 K. The spin excitation modes above Hc observed below 1 K do not show any difference from those at 1.5 K and the lowest excitation mode observed at 30 mK in the inelastic neutron experiments above Hc was not observed at about 200 mK in this study. Accordingly, the result suggests that the lowest excitation mode is very sensitive to temperature.

Attempt to improve sensitivity in measuring diamagnetic anisotropy by increasing duration of rotational oscillation in microgravity

Technical basis to detect small diamagnetic anisotropy ΔχDIA is established in a method using microgravity μG. Sensitivity of Δχ can be improved by increasing length of measurable period of rotational oscillation of magnetically stable axis with respect to field direction. In order to achieve the above condition, position of the diamagnetic sample should be stabilized in static field area, which was realized by introducing a magnetic circuit with high field homogeneity. Function of a mobile sample stage was effective in minimizing translational motion of sample. Accordingly, magnitude of measurable anisotropy may be decreased to a level of 10-10 emu/g. ΔχDIA of materials with high crystal symmetry, namely hexagonal ice, ZnO, SiC and BaTiO3, can be obtained by the above improvement. Magnetically active property due to magnetic anisotropy may be recognized for almost all the solid materials when sensitivity reaches the above level.

Attempt to detect diamagnetic anisotropy of oxides with isotropic crystal structure by measuring its rotational oscillation in strong magnetic field

Sensitivity to detect diamagnetic anisotropy ΔχDIA of inorganic oxides was improved by increasing intensity of horizontal field from 1.6 T to 5.0 Tesla. The field induced a rotational oscillation of a magnetically stable axis of a sample, which was suspended with a thin fiber. Accurate Δχ values are obtained when restoration torque of the fiber is negligibly small compared to torque due to magnetic anisotropy. Accordingly, Δχ value at a level of 5×10-10 emu/g can be acquired, even if the mass of the sample is relatively small (0.318 g). Necessity of realizing Δχ measurement at low temperature is discussed, which realizes separation of Δχ DIA from a paramagnetic anisotropy caused by isolated magnetic ions.