Cation size disorder as the crucial determinant of the unusual magnetic and electronic properties of Gd_0.5Ba_0.5CoO₃

Abstract

An investigation of three series of cobaltates of the formulae Gd_0.5−xLn_xBa_0.5 CoO_3−δ (Ln=Nd or La) and La_0.5−xNd_xBa_0.5CoO_3−δ 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 Gd_0.5Ba_0.5CoO₃, 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). Gd_0.5Ba_0.5−xSr_xCoO₃, 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−xLn'_xA_0.5−yA'_yCoO_{3 −δ} with fixed <r_A> values of 1.317 and 1.289 Å (the latter corresponding to Gd_0.5Ba_0.5CoO₃), reveal that on decreasing the size disorder, the materials become ferromagnetic with relative high T_c values. These cobaltates also exhibit size disorder controlled metal-insulator transitions, the compositions with high disorder being insulators. The properties of Dy_0.5Ba_0.5CoO₃ are consistent with the presence of size disorder in this material.