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Time- and history-dependent transport phenomena in overdoped manganites La1-xCaxMnO3

La1-xCaxMnO3 belongs to the family of manganese oxides, known as manganites, with a general chemical formula R1-xAxMnO3, where R is a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. These compounds have been investigated since 1950., but a huge interest started in 1994. with the discovery of the famous colossal magnetoresistance (CMR). Besides CMR, manganites are interesting because of their very rich phase diagrams which show plenty of intriguing phenomena such as metal- insulator transition, charge ordering, multiferroicity, magnetocaloric effect, colossal thermoelectric power, phase coexistence, etc. In the case of the investigated compound La1-xCaxMnO3 the phase diagram consists of the three large parts: a paramagnetic (PM) insulating phase at high temperatures for all x, a ferromagnetic (FM) metallic ground state for 0.2 < x < 0.5 and an antiferromagnetic (AFM) charge order (CO) insulating ground state for 0.5 < x < 0.87. CMR effect is closely related to the FM phase and has maximum value at the Curie temperature. At present, it is believed that across the phase diagram there are areas where two phases can coexist, particularly close to the boundary between the two phases. Especially interesting is the coexistence between the FM metallic and CO/AFM insulating phase which is believed to be the key for understanding the CMR effect. Our recent work on magnetotransport properties of manganites thin films La1-xCaxMnO3 for x > 0.5 has shown the existence of CMR, FM and metal- insulator transition, properties characteristic for x < 0.5, which has been ascribed to the presence of FM clusters in the CO/AFM background. Here we present a continuation of this research to lower temperatures and higher magnetic fields which reveals a huge difference between field- and zero-field cooled resistivity curve and unusual time relaxation of the resistivity after application of an external magnetic field. We discuss this features in terms of the FM/AFM phase separation scenario.

manganites, colossal magnetoresistance, phase separation, glassy behavior

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