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We report the dielectric anomaries at the phase transitions of Lu(Fe 2+ Fe 3+ )O 4 , which is considered as a charge and spin frustrated system. Dielectric constant shows a hump, just above the transition from the high temperature two-dimensional to the low temperature three-dimensional charge ordering. A few glassy behaviors were observed at this transition. The presence of spontaneous polarization in the low temperature phase was confirmed by the measurement of pyroelectricity. A step-wise increase of the dielectric constant with increasing temperature was observed between 150 K and 250 K as in other RFe 2 O 4 . These phenomena are explained by the presence of the polar domains with the ordering of Fe 2+ and Fe 3+ and the motions of the antiphase boundaries between them.

Velocities and attenuations of longitudinal sound waves were measured at 100 MHz for a single crystal of non-stoichiometric ErFe 2 O 4-δ , along the and axes in the hexagonal frame, below 300 K. At room temperature, velocities were 5.22× 10 3 and 5.49×10 3 m/s along the and axes, respectively. These values correspond to the elastic constants of C 11 =1.94×10 12 dyne/cm 2 and C 33 =1.75×10 12 dyne/cm 2 . Slight softening was disclosed along the axis at around 240 K. Correspondingly, a large peak was observed in the attenuation. These anomalies are attributed to the emergence of two-dimensional spin ordering in this crystal at this temperature. On the contrary, no anomaly was detected along the axis at around 240 K, with rather high attenuation level almost independent of temperature. This absence of anomalies was discussed on the existence of three kinds of magnetic domains. Development of the correlation of Er spins was suggested below 30 K.

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

The electrical resistivity, Hall effect, magnetoresistance and magnetic susceptibility of Ta 3 S 2 , Ta 2 S and Ta 6 S are reported for the first time. The Hall coefficient of Ta 3 S 2 changes its sign from positive to negative with increasing temperature. The Hall coefficient is pronouncedly dependent on magnetic field strength below 77 K. The magnetoresistance increases almost quadratically with the field strength below 77 K, and is still observed at 291 K. These results are interpreted in terms of two bands with the carrier densitiy 3.0×10 19 cm -3 of electrons and 7.9×10 17 cm -3 of holes. These results strongly suggest that Ta 3 S 2 is a semimetal with very slightly overlapping conduction and valence bands near the Fermi level. In contrast with Ta 3 S 2 , the compound Ta 2 S is a metal with hole conduction, while Ta 6 S is a metal with electron conduction.