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.
The outermost member of the 112 family LaBaCo 2 O 5.5 has been synthesized using a multistep method to stabilize its structure. Its structural study, combining neutron powder diffraction and electron microscopy, shows that its orthorhombic matrix consists of layers of corner-shared CoO 6 octahedra interconnected through CoO 5 pyramids like in the other 112 cobaltates but contains small LaBaMn 2 O 5.5 -type domains (8%) due to local oxygen displacement. Its magnetic properties and magnetic structure evidence the following features: G-type antiferromagnetic/G-type ferrimagnetic/paramagnetic with T N ) 260 K and T C ) 326 K. This cobaltate differs from other 112 cobaltates by its antiferromagnetic structure which keeps the same symmetry as its ferrimagnetic phase, probably due to the size effect of La 3+ upon crystal field. Trivalent cobalt is shown to keep the intermediate spin state in the whole temperature range from 10 to 326 K.
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