tailieunhanh - Báo cáo " The magnetic properties and charge-ordering state in La1-xCaxMnO3 (x = 0.46; 0.50) compounds "

The compounds of La1-xCaxMnO3- δ with x= and occupy special positions in the phase diagram of La1-xCaxMnO3-δ system due to their interesting properties and charge-ordering phase transition. The samples were prepared by a solid-state reaction method. The XPD patterns show that the samples are of a single-phase orthorhombic-perovskite structure. The chemical compositions of the samples are investigated by EDS. The concentrations of oxygen and Mn3+; Mn4+ ions have been determined by dichromate method. The charge-ordering state have been found below 150 K by magnetic and resistance measurements. This phenomenon relates to metal-insulator transition. . | VNU. JOURNAL OF SCIENCE Mathematics - Physics N01 2005 THE MAGNETIC PROPERTIES AND CHARGE-ORDERING STATE IN x COMPOUNDS Nguyen Huy Sinh Nguyen Anh Tuan Vu Thanh Mai Pham Hong Quang Nguyen Tuan Son Department of Physics College of Science VNU Abstract. The compounds of La1-xCaxMnO3-s with x and occupy special positions in the phase diagram of La1-xCaxMnO3-s system due to their interesting properties and charge-ordering phase transition. The samples were prepared by a solid-state reaction method. The XPD patterns show that the samples are of a single-phase orthorhombic-perovskite structure. The chemical compositions of the samples are investigated by EDS. The concentrations of oxygen and Mn3 Mn4 ions have been determined by dichromate method. The charge-ordering state have been found below 150 K by magnetic and resistance measurements. This phenomenon relates to metal-insulator transition. The results are discussed in competition between double exchange DE and super-exchange SE interaction. 1. Introduction Doped perovskite manganites of the form La1-xCaxMnO3-5 are mixed valence systems containing Mn3 and Mn4 ions. They exhibit colossal magnetoresistance effects CMR . Such CMR effects originate from a double exchange mechanism DE between Mn3 and Mn4 species that induces ferromagnetic correlation. Besides DE it has been found that the super-exchange interaction SE also has an important role to govern the electronic and magnetic properties of these compounds 1 2 . The compound of La1-xCaxMnO3-5 with x where one eg hole or one electron hoping between two Mn sites shows a peculiar behavior of chargeordering CO transition which takes place when the electrons become localized because of the ordering of cations of different charges on specific lattice sites. The charge-ordering state can be melted by a strongly external magnetic field and also by high pressure. This phenomenon has an origin from competition between the double .