Magnetic and electrical properties of well-characterized Gd0.5Ba0.5CoO2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln0.5A0.5CoO3 (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd0.5Ba0.5CoO2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.
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