We report on the investigation of magnetic, magnetoresistive and IR-spectra of the single crystals of (Tb-Dy)BaCo₂O_5.5 grown by the spontaneous crystallization method. This layered perovskite exhibits a very rich phase diagram including two different paramagnetic phases with MI transition at 340K as well as ferromagnetic and antiferromagnetic phases. The strictly pronounced anisotropy of magnetization and resistivity was observed at the temperatures below T_c. The relationship of R_ab / R_c varies from 1 to 20 for T = 260÷100K and MR effect value exceeds 10% at 100K in 1T. Infrared absorption(transmission) spectra were measured between 300-4000 cm^-1 using IR Fourier-spectrophotometer Nikolet-460 (Varian, USA). The IR-spectra are sensitive to the O-Co-O bonding length and angles. The frequencies of bending and stretching modes are connected with 250-420 cm^-1 and 400-800 cm^-1, respectively. We assume that the display of a few splitting components in the system of the bending mode (377 and 429 cm^-1) as well as in the system of the stretching mode (522, 580, 634 and 664 cm^-1) are due to the existence of different local distortions of the fragments of the single crystal matrix owing to the presence of (Tb-Dy) rare earth ions.

The study is partly supported by BRFFI F05-129 and NATO-linkage PSTCLG 979369 grants.

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