Reentrant paramagnetic behavior and spin-state transition in the layered cobalt oxide GdBaCo₂O_5+δ

The structural, magnetic and transport properties of the layered cobalt oxide GdBaCo₂O_5+δ (δ = 0.45) were investigated. The extra oxygen resides on the Gd layer to form ordered octahedra along the a-axis evidenced by the doubling of the b-axis. The temperature and field dependent magnetization data of GdBaCo₂O_5+δ reveal a reentrant paramagnetic behavior at 75 K, which occurs via a paramagnetic to ferromagnetic transition at 285 K and a subsequent ferromagnetic to antiferromagnetic transition at 220 K. The complex magnetic behavior is explained in terms of the spin-state transition of the Co³⁺ ion in the octahedral site from intermediate to low spin configuration and the orbital ordering of the e_g electron in the intermediate spin state. In addition, we have found negative magneto resistance in the antiferromagnetic region, which is analogous to the giant magneto resistance reported for the multilayered magnetic film.