Evidence for spin clusters and glassy behaviour in Bi_1-xCa _xMnO₃ (x ∼ 0.875)

Detailed magnetic and transport properties of electron-doped Bi _0.125Ca_0.875 MnO₃ materials are reported. Low field magnetization measurements provide evidence for ferromagnetic Curie-Weiss behaviour at higher temperatures and an antiferromagnetic state below T _N = 109 K, which supports a ferromagnetic component. High field (-30 T ≤ H ≤ 30 T) DC magnetization measurements (5-300 K) show a strongly nonlinear field response below and far above the ordering temperature. The residual magnetic moment and coercive field, in the ordered state, are, however, exceptionally small. These results are discussed in terms of the FM coupling of spins into clusters. The possible coupling of these clusters to the AF staggered magnetization (below T_N) and to AF fluctuations (above T_N) is discussed. The magnetic effects appear to be consistent with canted antiferromagnetism, however, FM segregation is not ruled out. The high field magnetoresistance (up to 30 T) appears to be governed by a field-induced reduction in the doped carrier localization. The glassy character of the cluster magnetic response was investigated by measuring the frequency dependent AC susceptibility and time/history evolution of the low field magnetization. Glassy behaviour in the cluster response is indeed observed and is discussed. The differentiation of cluster glass versus canted-AF origins of the time/history effects is still an open issue.