Cationic Ordering and Microstructural Effects in the Ferromagnetic Perovskite La_0.5Ba_0.5CoO₃: Impact upon Magnetotransport Properties

The synthesis and structural study of the stoichiometric perovskite La 0.5 Ba 0.5 CoO 3 have allowed three forms to be isolated. Besides the disordered La 0.5 Ba 0.5 CoO 3 and the perfectly ordered layered LaBaCo 2 O 6, a third form called nanoscale-ordered LaBaCo 2 O 6 is obtained. As evidenced by transmission electron microscopy investigations, the latter consists of 112-type 90° oriented domains fitted into each other at a nanometer scale which induce large strains and consequently local atomic scale lattice distortions. These three ferromagnetic perovskites exhibit practically the same T C ≅ 174–179 K, but differently from the other phases, the nanoscale-ordered LaBaCo 2 O 6 is a hard ferromagnet, with H C ≈ 4.2 kOe, due to the strains which may pin domain walls, preventing the reversal of the spins in a magnetic field. The magnetotransport properties of these phases show that all of them exhibit a maximum intrinsic magnetoresistance, close to 6–7% around T C under ±70 kOe, but that the ordered phase exhibits a much higher tunnelling magnetoresistance effect at low temperature of about 15% against 4% due to the grain boundary effects.