Competing magnetic phases in mixed-valent manganese oxide perovskites

Measurement of specific heat C-p(T) below 300 K of melt-grown samples of La1-xCaxMnO3 (0less than or equal toxless than or equal to0.3) and R(0.7)A(0.3)MnO(3) (R=rare-earth,A=alkaline-earth) with room-temperature tolerance factor 0.950less than or equal totless than or equal to0.996 have been supplemented by transport and magnetic measurements. Comparison of the phase diagram of La1-xCaxMnO3 with that of La1-xSrxMnO3 and the evolution with t of the R(0.7)A(0.3)MnO(3) family illustrate the sensitivity to t of the crossover from localized to itinerant behavior of the sigma-bonding electrons and support the model of two magnetic phases in the crossover compositional range that has been used to account for the colossal magnetoresistance (CMR) phenomenon found in these oxides. A vanishing of the specific-heat anomaly at the Curie temperature T-c and the magnetic data at crossover are typical of a spin glass, and a broad hump in C-p(T) below a T-h>T-c, where there is no anomaly at the T-c signal ferromagnetic ordering within isolated pockets of a hole-rich, conductive O* minority phase at T-h. On cooling through T-N of the antiferromagnetic matrix, the spins freeze at a spin-glass temperature T-g in zero magnetic field H if the ferromagnetic phase does not percolate; the ferromagnetic phase grows in an applied H, and a modest H converts the spin glass to a bulk ferromagnet with a Curie temperature T(c)approximate toT(g), where the ferromagnetic phase grows to beyond percolation. As x increases in La1-xCaxMnO3, a ferromagnetic-insulator O-' phase having a charge ordering and a different orbital ordering than the parent O-' phase percolates below a T(g)approximate toT(c), and the minority O-' phase remains paramagnetic until it becomes antiferromagnetic below a T-M