Short range ferromagnetism and spin glass state in Y0.7Ca0.3MnO3

Dynamic magnetic properties of Y0.7Ca0.3MnO3 are reported. The system appears to attain local ferromagnetic order at TSRF ≈ 70 K. Below this temperature the low field magnetization becomes history dependent, i.e. the zero field cooled (ZFC) and field cooled (FC) magnetization deviate from each other and closely logarithmic relaxation appears at our experimental time scales (0.3-104 sec). The zero field cooled magnetization has a maximum at Tf ≈ 30 K, whereas the field cooled magnetization continues to increase, although less sharply, also below this temperature. Surprisingly, the dynamics of the system shows non-equilibrium spin glass (SG) features not only below the maximum in the ZFC magnetization, but also in the temperature region between this maximum and TSRF. The aging and temperature cycling experiments show only quantitative differences in the dynamic behavior above and below the maximum in the ZFC-magnetization; similarly, memory effects are observed in both temperature regions. We attribute the high temperature behavior to the existence of clusters of short range ferromagnetic order below TSRF; the configuration evolves into a conventional spin glass state at temperatures below Tf .