Metastability in multiferroic manganites

Manganites R1−xAxMnO3, R and A being rare-earth and alkaline-earth ions respectively, show a wide variety of intriguing phenomena such as colossal magnetoresistance (CMR), multiferroicity, charge ordering, electronic phase separation, etc. Strong coupling of the spin, charge, orbital, and lattice degrees of freedom and the intense competition between the ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating phases cause very complicated phase diagrams. The exact phase depends on chemical substitution, level of doping, magnetic field, pressure, grain size, etc. La1−xCaxMnO3 is the most investigated member of the manganites family. Two phases dominate the well known phase diagram of La1−xCaxMnO3: a ferromagnetic metallic ground state for doping 0.2 < x < 0.5 and an antiferromagnetic (AFM) insulating charge order (CO) ground state for 0.5 < x < 0.9. However, the exact boundary between FM and CO/AFM phases is somehow sample dependent and it is believed that close to x = 0.5 the two dissimilar phases can coexist. In this work we show characterization of ceramic polycrystalline samples of La1-xCaxMnO3 with x = 0.5 using dc conductivity, ac susceptibility, magnetotransport and dielectric spectroscopy at low temperatures. Most importantly we find two prominent phase transitions, a high-temperature one corresponding to CO and a low-temperature transition with a FM fingerprint. Between these two features a metastable region is present both in charge as well as the spin sector. We discuss these findings in the context of phase separation and compare them with other La/Ca or Pr/Ca substitutions x.