Coexistence of interacting ferromagnetic clusters and small antiferromagnetic clusters in La0.5Ba0.5CoO3.

We report detailed dc magnetization and linear and nonlinear ac susceptibility measurements on the hole doped disordered cobaltite La0.5Ba0.5CoO3. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic clusters. The percolating ferromagnetic clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5) K. The non-ferromagnetic phases mainly consist of antiferromagnetic clusters with size smaller than the ferromagnetic clusters. Below the Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and a frequency dependence in the peak of ac susceptibility. These dynamical features indicate the possible coexistence of spin-glass phase along with ferromagnetic clusters similar to La(1-x)Sr(x)CoO3 (x ≥ 0.18), but the absence of field divergence in the third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La(1-x)Sr(x)CoO3 (x ≥ 0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases, which is absent in La0.5Ba0.5CoO3. Our analysis shows that the observed dynamical features in La0.5Ba0.5CoO3 may be due to progressive thermal blocking of ferromagnetic clusters, which is further confirmed by Wohlfarth's model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with τ0 ≈ 10(-9) s. This together with the existence of an AT-line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic clusters.