College Park, Maryland      June 6 - 10 , 2004

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TP60: Magnetic structure and magnetization density in the layered Tb(Dy)BaCo₂O_5.5

S.N. Barilo (Institute of Solid State & Semiconductor Physics, BAS, Minsk 220072, Belarus), V.P. Plakhty, Yu. P. Chernenkov (Petersburg Nuclear Physics Institute RAS - Gatchina, 188300 St. Petersburg, Russia), A. Goukassov (Laboratoire Léon Brillouin, C.E.A Saclay, 91191 Gif-sur-Yvette Cedex), S. Streule, A. Podlesnyak (Laboratory for Neutron Scattering, Paul Scherrer Institute & ETH Zurich, CH-5232 Villigen, Switzerland), S.V. Shiryaev, G.L. Bychkov (Institute of Solid State & Semiconductor Physics, BAS, Minsk 220072, Belarus), A. Furrer (Laboratory for Neutron Scattering, Paul Scherrer Institute & ETH Zurich, CH-5232 Villigen, Switzerland)

The cobaltites LnBaCo₂O_5.5 (Ln = Y,Pr-Ho) present spin-state, MI and filed-induced transitions, as well as charge and orbital ordering phenomena. This family of layered cobaltites displays alternating BaO and LnO_0.5 layers along the c-axis and alternating oxygen octahedral and pyramidal planes along the b-axis. In spite of the extensive studies, neither magnetic structure nor spin state in two different Co³⁺ sites are definitely established. Precisely oxygenated powder sample of TbBaCo₂O_5.5 and as large as 50 mm3 flux grown single crystals of Tb_0.9Dy_0.1BaCo₂O_5.45 have been used for neutron diffraction studies in the temperature range of 7-300 K using the powder diffractometer DMC of SINQ, Villigen and 5C2, 6T2 diffractometers of LLB, Saclay. The results obtained are rather different of everything, which has been published before. The following 3 phases of TbBaCo₂O_5.5 have been offered: 1) A very narrow by temperature ferrimagnetic phase in the range around 260 K (the unit cell doubled along short axis because of the spin state ordering in octahedra). The structure presents a simple ferrimagnetic with all moments are along a-axis in the octahedra and a simple AFM in pyramids. 2) At T = 230 K an AFM phase does appear. Magnetic cell is a previous one doubled along c-axis due to antiferromagnetic ordering with the wave vector k = (0,0,1/2). There is an antitranslation to the upper part of the unit cell. 3) Finally at T ~ 170K a transition to another AFM takes place. Now the unit cell is doubled along the c-axis because of the spin value ordering in pyramids. This low temperature AFM phase of a single crystal Tb_0.9 Dy_0.1BaCo₂O_5.45 have been studied by elastic neutron scattering in the kl-plane to show peaks indexed as (0,0-l/2) are purely magnetic and can be attributed to appearance below 200 K an AFM phase, consisting of ferromagnetic, AFM and nonmagnetic chains. We have also reconstructed the spin density distribution in a Tb_0.9 Dy_0.1BaCo₂O_5.45 single crystal. Statistically significant flipping ratios for 46 reflections have been measured in the field of 7 T. Refinement have been made to reveal the spin density difference between octahedral and pyramidal sites. These results should be considered as preliminary to show the spin density distribution over the different sites can be obtained for this twinned crystal.

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