Low Temperature Magnetoresistance and the Magnetic Phase Diagram ofLa1−xCaxMnO

Mean-field theory of charge ordering and phase transitions in the colossal-magnetoresistive manganites

“…The extended-Hubbard term in our model is crucial for the metal-insulator transitions that are associated with the loss of charge ordering seen in the experiments [15,16,20]. The magnetic field dependence of this transition is also in good accord with experiments [18,20].…”

“…For instance, at low T , Nd 0.5 Sr 0.5 MnO 3 is a charge-ordered antiferromagnet that has the complex CE structure [3] and undergoes a first-order transition to a charge-disordered ferromagnet when T is raised [2,12,18,19]. Doping-driven, first-order ferromagnet-to-antiferromagnet transitions (presumably associated with charge ordering) have also been reported for La 1−x Ca x MnO 3 [15,16], though the question of first-order phase coexistence has not been investigated sufficiently, as we discuss later. The transition from a charge-ordered antiferromagnetic insulator (AFO) to a chargedisordered (or charge-non-ordered) ferromagnetic metal (FN) can also be obtained by changing the magnetic field H ; e.g., in both Pr 0.5 Sr 0.5 MnO 3 and Nd 0.5 Sr 0.5 MnO 3 [18,20] the transition temperature T AF→F decreases rapidly with increasing H .…”

“…When LaMnO 3 is doped, ferromagnetic phases are stabilized via the double-exchange mechanism as in La 1−x Ca x MnO 3 , which is a ferromagnetic metal for 0.1 x 0.5. Further doping yields phases, often with complex structures, that are antiferromagnetic insulators [15,16]. In addition, these magnetically ordered phases can be charge ordered or charge disordered; for x = 0.5 this can be thought of as Mn 3+ ions on one sublattice and Mn 4+ ions on the other, though fluctuations often reduce such complete charge disproportionation.…”

The structural, morphological and magnetic properties of Ag-doped La0.8Ca0.2-xAgxMnO3 synthesized by sol-gel method

“…LaMnO 3 is an antiferromagnetic and an insulator materials [3]. Subtitution of La site with divalent ions leads to oxidize a Mn 3+ ion to a Mn 4+ ion and results in ferromagnetic state and metallic conductivity when the doping levels are sufficiently high [4,5]. Subtitution of La site with monovalent ions oxidizes Mn 3+ twice as many as divalent ions [6].Relation between electrical and magnetic properties was initially explained by Zener double exchange interaction, hopping electron from Mn 3+ ions to Mn 4+ ions through O 2-ions [7,8].…”

Local Lattice Distortion Caused by Short-Range Charge Ordering in Transition Metal Oxides

“…Then the metal to charge ordering transition must be accompanied with structural phase transition because inequivalent transition metal sites with different valences are arranged periodically in the charge ordered state. [1][2][3] However, in several materials including the transition metal ions with non-integer valences and non-metallic conductivities, superlattice reflection and/or structural phase transition showing charge ordering are not observed. [4,5] It is expected that because the electrons are localized only with a short-range ordering in such materials, the arrangement of the inequivalent sites with the different valences also has shortrange correlation, resulting in the absence of superlattice Bragg reflection.…”