Abstract:
An investigation of three series of cobaltates of the formulae Gd(0.5-x)Ln(x)Ba(0.5) CoO3-delta (Ln = Nd or La) and La0.5-xNdxBa0.5CoO3-delta demonstrates the effects of the average radius of the A-site cations, <r(A)>, on the magnetic and electrical properties. Thus, the anomalous magnetic properties of Gd0.5Ba0.5CoO3, such as the transitions in the range 220-290 K, disappear, even with a small substitution of Nd or La in place of Gd, and a gradual evolution of ferromagnetism occurs at low temperatures (130-190 K). Gd0.5Ba0.5-xSrxCoO3, on the other hand, shows increasing ferromagnetic properties with increase in x although the <r(A)> decreases, suggesting the role of size disorder. Studies of cobaltates of the type Ln(0.5-x)Ln'(x)A(0.5-y)A(y)'CoO3-delta with fixed <r(A)> values of 1.317 and 1.289 Angstrom (the latter corresponding to Gd0.5Ba0.5CoO3) reveal that on decreasing the size disorder, the materials become ferromagnetic with relative high T, values. These cobaltates also exhibit size disorder controlled metal-insulator transitions, the compositions with high disorder being insulators. The properties of Dy0.5Ba0.5CoO3 are consistent with the presence of size disorder in this material.
