Coexistence of ferromagnetic and glassy behavior in the La_0.5Sr_0.5CoO₃ perovskite compound

Dc magnetization, ac susceptibility, and zero-field-cooled relaxation measurements are carried out for a cluster glass compound of La_0.5Sr_0.5CoO₃. The temperature dependence of the magnetic properties could be distinguished into two regimes: a high-temperature regime with the time-independent parameters originating probably from the intracluster ferromagnetism, and a lower-temperature regime where the freezing of clusters takes place with a considerabe frequency dependence of a shoulder in χ^′(T) and a hump in χ^″(T). The cusp in M_ZFC(T) at T_a, as well as the high-temperature maximum in χ^′(T) are discussed in terms of an existence of the local anisotropy inside the clusters. The high-temperature maximum in χ^″(T) is interpreted in connection to the reversibility temperature, T_r. Empirically, we found T_r∝-H while T_a∝-H^0.58. We report also the long-time relaxation and the ageing phenomenon in this compound. The ageing effect is much more pronounced in the ferromagnetic state than in the cluster glass state and the system does not reach equilibrium for time scales up to 10⁴ s. The relaxation and ageing effects are attributed to the cluster growth slowed down by the presence of the frustration.