Effect of iron doping on the magnetic structure of TbBaCo2O5.5 layered perovskite

The crystal and magnetic structures of TbBaCo1.9Fe0.1O5.43 have been studied by neutron diffraction and magnetization measurements. It is shown that spontaneous magnetization develops at T-N = 305 K and decreases abruptly below 200 K. The crystal structure is described in the space group Pmmm, on a 2a(p)*a(p)*2a(p) unit cell. It was shown that the extra oxygen vacancies are located mainly in the pyramidal sublattice, thus making tetrahedra. Magnetic structure in the high-temperature magnetic phase consists of ferromagnetic and G-type antiferromagnetic components, while in the low-temperature phase only the G-type component exists with a 2a(p)*2a(p)*2a(p) magnetic unit cell. The ferromagnetic component results from canting magnetic moments of Co3+ ions which were found to be predominantly in the high-spin state in pyramids. The refined values of the magnetic moments of the Co3+ ions in the octahedra indicate a mixed high-low-spin state. It is assumed that non-collinear magnetic structure results from a concurrence between positive and negative exchange interactions as well as magnetocrystalline anisotropy in the pyramidal sublattice. Using the concept of non-collinear magnetic structure, the giant anisotropic magnetoresistive effect in layered cobaltites is explained.